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Anatomy of the mouth and related structures
THE PHENOMENON OF FUNCTION DENTURE PROSTHODONTICS
ANATOMY Part II.
OF THE Musculature
MOUTH
AND
RELATED
IN
COMPLETE
STRUCTURES
of Expression*
ALEXANDER L. MARTONE, D.D.S., M.Sc.,” * AND LINDEN F. EDWARDS, PH.D.*” * Norfolk, Vu., and Columbus, Ohio
of facial expression in man indicate that there is practically no facial expression in the newborn infant. It becomesmore manifold and definite as the infant’s consciousand intelligent reactions to the surrounding world increase. Throughout childhood, emotional reactions remain unmasked, making it possible to interpret the true meaning of expressions in the child’s face. As the individual matures, with experience and education, he learns to control facial expression. This ability to conceal emotions makes it more difficult to read the faces of adults. TUDIES OF THE EVOLUTION
S
Huberl states : “In man facial expression has finally reached marvelous perfection. This phenomenon may be attributed to further differentiation of the ‘mimetic musculature’ in close correlation with higher evolution of the central nervous mechanism, particularly with further ’ development of the facial area in the motor cortex and through elaboration of association centers.” Since it is true that human beings engage in the actions of speech and facial expression to a greater degree than they do in mastication and since dentists must be concerned with these activities, a knowledge of the intricate musculature responsible for these movements is essential. Dental teachers and authors have stressed the importance of understanding the muscles of mastication, and rightfully so. However, this emphasis should not result in a disregard or lack of understanding of the other musclesthat play vital roles in the complex organism requiring dental treatment. For this reason, particular attention will be devoted in this consideration of myology to the intricate and delicate muscles responsible for facial expression and speech. *Part I, J. PROS.DEN. 11:1009-1018, 1961. **Associate, Departments of Prosthodontics and Anatomy, Medical College of Virginia. ***Professor, Department of Anatomy, Ohio State University. 4
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PHENOMEKON
GRO\\rTIl
AND
OF
FUNCTION
Ilr;
COMPLETE
DEPI‘TURE
PROSTIIODOiXTICS
5
DEVELOPMENT
The muscles of the mouth and related structures are derived from the mesoderm of the branchial arches (Fig. 1). About the sixth week of embryonal life, the muscles of these arches, which are innervated by certain cranial nerves, undergo modification and migration hut retain their original cranial nerve supply. The muscles of mastication, the tensor veli palatini, tensor tympani, anterior belly of the digastric, and mylohyoid, are derived from the first branchial arch and are supplied by the mandibular branch of the trigeminal (Vth cranial) nerve. The second or hyoid arch, innervated l)y the facial (\-11th cranial) nerve, furnishes the muscles of expression, the posterior belly of the digastric, the stylolkyoid, the stapetlius, and the auricular muscles. The mesoderm of the third arch furnishes the material for the stylopharyngeus muscle and is innervated by the glossopharyngeal (IXth cranial) nerve. From the fourth and fifth arches develop the pharyngeal, laryngeal, and the other palatal muscles which are supplied by the vagus (AXth cranial) and the cranial portion of the spinal accessory (XIth cranial) nerves. The muscles of the tongue, however, are derived from occipital myotomes and are supplied by the hypoglossal (SIIth cranial ) nerve. I’IIYSICAL
The basic units of a voluntary skeletal striated muscle are the muscle fibers which are elongated and cylindric in shape, bound together by connective tissue and supplied by nerves, blood, and lymph vessels. Each fiber is surrounded by a connective tissue sheath or capsule which insulates the fibers so that each may ir?clividually contract without activating other fibers. There are many nuclei intermingled in the alternating light and dark bands on their surfaces. The fibers are bound together in bundles by a delicate connective tissue sheath, and their length is generally correlated with the length of the muscle. Striated muscle fibers do not increase in number after birth. The enlargement of the muscle after birth is due to the increase in the diameter of its fibers. Because of the lack of power of regeneration, injured muscles do not repair easily and smoothly, but repair with connective tissue, leaving an irregular scar. PHYSIOLOGIC
CONSIDERrlTIOKS
Muscles are very excitable and respond readily to stimuli. They are activated by impulses which pass to them by way of motor nerve fibers. The phenomenon of transmission of an impulse from a motor heuron to a muscle fiber at the motor end plate is similar to the mechanism of the transmission of impulses across a synapse. A nerve fiber supplies several muscle fibers, the number being quite variable (100 to 200 j. This constitutes a motor unit, and the number of functioning motor units depends largely upon the physiologic demands placed on a muscle. The more highly specialized the muscular activity, the greater the number of motor units in proportion to the muscle fibers. For example, specialization of muscular activity of the face and tongue is of a very high degree and complicated.
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MARTONE
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EDWARDS
J. Pros. Jan.-Feb.,
Den. 1962
In this area, the muscle fibers are relatively very few and small. For delicate and intricate movements such as those in speech and facial expression to take place, small groups of fibers must be activated without involving other groups. There are two types of muscular contraction, isotonic and isometric. In isotonic (is0 meaning same, tonic meaning tension) contraction, tension remains constant while the length of the fibers shortens. For example, stimulation of the masseter muscle while the teeth are not in occlusion will result in a shortening of the muscle fibers. Conversely, in isometric (iso meaning same, metric meaning length) contraction, tension increases, but the length of the fiber remains the same. For example, when the teeth are in centric occlusion, further stimulation of the masseter musclewill not permit the fibers to become shorter. Under normal circumstances, musclesare in a state of slight tension or partial contraction (tonus) all of the time. When the person is in a nornial standing or sitting position, the mandible is maintained in a physiologic rest position. The masseter, temporal, and internal pterygoid muscles are in a partially contracted state, suspending the mandible in a balanced position against the pull of gravity, the weight of the mandible and supportive structures, and the partial contraction and tension of the inframandibular muscles. Facial Movemmts.-Facial movements are the result of the automatic, coordinated effort of many muscles in different functional capacities executed in the cerebral cortex. Movement occurs through paired muscular activity. In order for a muscle to contract, its opponents must relax. Individual muscles may function by being (1) prime movers, (2) antagonists, (3) fixators, or (4) synergists. Prime movers are those actually responsible for the movement, whereas antagonists are those muscles which produce opposing movements. Thus, in closing the mouth, the inframandibular muscles relax and the three closing muscles of mastication contract; in opening of the mouth, the reverse takes place. Fixation musclesestablish a stable basisby steadying a structure. For example, the infrahyoid muscles steady the hyoid bone so that the suprahyoid muscles may contract the lowering mandible. A synergist (meaning “to work together”) muscle aids the prime mover in its action. For example, the internal pterygoid muscle is a synergist of the masseter musclein that it helps it to elevate the mandible. Fundamentally, any functional movement is a result of muscular contraction. If the muscular tension is extreme, it may be desirablcefor this tension to be reduced. This is frequently the case in training a student in voice or speech. However, advising him to relax may produce a result directly opposite to the desired one, for in an effort to relax, the performer may actually increase the tension. He is striving to obtain a balanced prime mover-antagonist musculature relationship. The accuracy, control, and quality of the voice are improved in a like degree as this balance is attained. This same principle of muscle balance can be applied in recording maxillomandibular relationships in dental treatment procedures. P&ciples of Muscular Contraction.-There are several basic physiologic principles of muscular contraction : (1) musclesare excitable and respond to changes in the stimuli ; (2) a muscle always pulls and never pushes; (3) the direction of
>;;F’,‘r:”
PHENOMENON
OF FUNCTION
IN COMPLETE
DENTURE
PROSTHODONTICS
7
---
larynx, safl palate
Fig. I.-Motor and proprioceptive innervation of the muscles of mastication, expression, soft palate, larynx, pharynx, and tongue: (A) Mesencephalic nucleus; (13) motor nucleus of mandibular nerve; (C) motor nucleus of facial nerve; (D) motor nucleus (ambiguus) of glossopharyngeal and vagus nerves; (E) hypoglossal nucleus; (3’~ sensory nucleus of trigeminal nerve; (G) sensory (solitary) nUCleUs of facial, glossopharyngeal, and vagus nerves; IDP) corticohulbar and corticospinal tract; (EP) extrapyramidal tract; (Bas. gang.) basal ganglia. Inset A, A lateral view of the branchial arches, the branchial musculature, and the cranial nerve supply (in Roman numerals) of each. Inset B, A dorsal view of the pharyngeal floor showing the branchial arches (in Arabic numerals) sectioned along the horizontal line (Xl in inset A.
J, Pros.
Jan.-Feb.,
A.
B.
Fig.
2.-The
complex interplay saying “lone.”
of structures during speech. A, Subject saying “pooh.” C, Profile view of subject producing vowel sound a.
B, Subject
Deii.
1962
\~olume
,Yumher
I.!
1
I’HENOMEXON
OF
FUNCTION
IN
COMPLETE
DENTC;RE
PROSTHODONTICS
0
action of a muscle is determined by the direction of the fibers at the time of contraction : (4) the initial length of the fiber determines the force with which the fiber will contract; (5) muscles obey the “all or none” law-when a single muscle fiber is activated, it either contracts completely or not at all : (6 ) muscles are in a state of continuous partial contraction or tonus even when they are at rest ; (7 j the amount of power produced by a muscle is determined by the number of fihers contracting at a given time ; (8) d i ff erent muscle fibers have different thresholds of stimulation. NEUROl.OGIC
CONSIDERATIONS
With reference to the physiologic considerations of muscles and the role they play in the production of facial expression, speech, and mandibular rest position, some mention shouId be made of the nature of the neurologic mechanism involved. Motor fibers within the nerves which supply the muscles of the tongue and those derived from the mesoderm of the pharyngeal arches represent axons of motor neurons whose cell bodies are located in motor nuclei of these nerves within the brain stem (Fig. 1). In addition, these nerves, with the exception of the hypoglossal, also contain sensory fibers which represent dendrites of cell bodies that are located within the ganglion of each nerve. These sensory fibers are distributed to various sense organs, including those of touch, pressure, temperature, pain, and proprioception. The latter consist of sense organs located within muscles, tendons, and ligaments and are stimulated whenever these structures are stretched. Thus, a muscle and its tendon are stretched whenever its antagonists contract, and ligaments are made more or less tense when joints undergo movements. The cell bodies of the proprioceptive fibers within the mandibular nerve are regarded as being located in the “mesencephalic nucleus” within the midbrain. However, the location of the neuron cell bodies of proprioceptive fibers in the facial, glossopharyngeal, and vagus nerves is uncertain. Obviously, the nerves supplying the tongue and facial muscles and those of the larynx, pharynx, and soft palate would be expected to contain proprioceptive fibers for the control of the mechanism involved in articulate speech. In regard to the proprioceptive innervation of the facial muscles, it has been suggested that these muscles are supplied by fibers from the mesencephalic nucleus via branches of the trigeminal nerve, since numerous connections occur between these and motor branches of the facial nerve on the face. Anatomic and physiologic evidence, however, is lacking to support this contention. The only other logical possibility is that the cell bodies of such fibers are located in the geniculate ganglion of the facial nerve, a concept for which some clinical evidence exists, notably in surgical casesinvolving ablation of the sensory root of the trigeminal nerve following which deep sensibility of the facial muscles \vas n(lt impaired or lost. Although the source of the proprioceptive fibers supposedly contained in the glossopharyngeal and vagus nerves, which supply the muscles of the larynx, pharynx, and soft palate, is unknown, it is conjectured that their cell bodies are located within the ganglia of these nerves as are those of other sensory fibers contained in these nerves.
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MARTONE
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Den. 1962
Although proprioceptors have been demonstrated histologically in the muscles of the tongue, the peripheral pathway and location of the cell bodies of proprioceptive fibers are hypothesized. Inasmuch as the hypoglossal nerve lacks a ganglion and is considered a purely motor nerve and since it communicates with the glossopharyngeal and lingual nerves, the opinion is held that these nerves serve to provide proprioceptive fibers to the tongue muscles. Impulses transmitted by proprioceptive fibers upon reaching the brain stem may be relayed to the cerebral cortex, where they arouse a sensation, or directly to motor nuclei of the cranial nerves, thus becoming converted into motor impulses. These, in turn, are conducted centrifugally by motor fibers to the muscles, which respond by involuntary or automatic contraction. Such a response is commonly called a reflex action, a notable example of which is the stretch reflex. Muscle Tonus.-The motor portion of the reflex mechanism usually consists of a relatively few motor units, or muscle fibers. The phenomenon of muscle tonus is regarded as being due to the reflex functioning of a relatively few motor units, which work in relays, thus resulting in a state of constant mild contraction of the muscle, which is finely controlled with a minimum of fatigue. The degree of muscle tonus varies. It is diminished by sleep, it is reduced by fatigue, and it is influenced by emotional excitement. Thus, feelings of joy and hope augment the tonus of the facial muscles and find expression in smiling or laughing, whereas, grief and worry depress facial tonus, giving rise to the wellknown “down in the mouth” appearance. It is evident, therefore, that the state of muscle relaxation is a relative one, inasmuch as a skeletal muscle though apparently at rest is always in a state of tonus or constant mild contraction, except when a patient is under general anesthesia, when the nerve to a muscle is sectioned, or when curare is administered. Stretch Reflex and Posture.-Muscle tonus is a particularly significant feature of the antigravity muscles which serve to maintain upright posture, including balancing of the head and suspension of the mandible when in rest position. In the latter position, the pull of gravity on the mandible along with the tonus or tension of its depressors stimulates the proprioceptors in the temporomandibular joints and elevators of the mandible, resulting in a stretch reflex or tonus of the latter muscles. The tonus or mild constant contraction of the elevators of the mandible in turn puts the depressors in tension, and like a feedback arrangement, the stretch reflex thus set up creates tonus in the latter muscles. The end result of this neuromuscular mechanism is a balance in tonus between the elevators and depressors and, consequently, the suspension of the mandible in mid-air as it were (see Figs. 14, 15, and 16). It should be emphasized that not all of the proprioceptive impulses upon entering the brain stem participate in the stretch reflex. Some of these impulses are relayed to the cerebellum, while others are transferred by way of the thalamus to the postcentral gyrus of the cerebral hemisphere where they arouse a kinesthetic sensation commonly called body sense, which is one’s awareness of position in space. From this gyrus, these impulses along with others such as touch, pressure, temperature, and pain, which have also reached this level of consciousness or sensation, are relayed to the voluntary motor area (the precentral gyrus (of the
v,;‘;pe;;’
PHENOMENON
OF FUNCTION
IN
COMPLETE
DENTURE
PROSTHODONTICS
11
cerebral hemisphere), where pyramidal cell bodies of upper (voluntary) motor neurons are located and where the impulses are converted into voluntary, willful, or intentional motor impulses. Axons of these motor neurons descend in two paths, one of which is called pyramidal and the other extrapyramidal. Axons of the pyramidal pathway are divisible into corticospinal fibers which descend and synapse with motor neurons of spinal nerves in the ventral gray horn of the spinal cord and into corticobulbar fibers which descend and synapse with motor neurons in motor nuclei of cranial nerves in the brain stem. The impulses transmitted by these two sets of fibers to lower motor neurons are conducted by the latter to skeletal muscles, which respond by active voluntary contraction, during which a relatively large number of motor units function. This type of response is diametrically opposite to involuntary reflex action, in that movements produced by active muscular contraction are volitional, or, as it is commonly expressed, under control of the will, It is not to be implied that these active, willful muscular contractions are never automatic or habitual, since it is well known that such activities as walking, feeding one’s self, mastication, deglutition, smiling, frowning, speech, and even driving a car become habitual and automatic but are under voluntary control. Details of all the connections of the fibers making up the extrapyramidal path are not firmly established, and neither are the functions of this system. It is known to consist of several links of neurons, which relay voluntary motor impulses to motor nuclei of cranial and spinal nerves, resulting in mass, coordinated, synergic movements, as opposed to isolated movements of individual muscles. One of the links in this system is called the corticopontocerebellar tract; it extends from the voluntary motor cortex to the pons and then to the cerebellum. Since the cerebellum also receives proprioceptive impulses, which subsequently leave this portion
4.
Fig.
3.-The
polyfunctional parent
pyramid. in profile
A, Outlined on view of surface
hemisection anatomy
cut of of a patient.
head
and
neck.
B,
Ap.
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MARTONE
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EDWARDS
J. Pros. JawFeb.,
Den. 1962
of the brain as motor impulses that are relayed to lower motor neurons, the function of the cerebellum and of the corticopontocerebellar tract is regarded as that of a regulating influence on muscle tonus, equilibrium, and coordination of muscular movements. Although muscle tonus is a reflex phenomenon, it is somewhat under cortical control, and, moreover, it disappears in the absence or loss of function of the cerebellum. ANATOMY
IN
ACTION
In an initial study of anatomy, the student, of necessity, learns structures in a sequence of groupings or classifications based on either areas of location or separate functional activities, and his thinking, consequently, is in terms of single actions of individual structures. Until he attains an understanding of the complex interplay of various structures that must occur to produce a single movement, he is without a concept of true functional anatomy. It is the knowledge of this interrelated and interdependent anatomic activity that provides the real basis for the practical application of anatomy to clinical problems. The considerations which will follow, therefore, may depart from customary textbook classifications and sequence when it is more advantageous to think in terms of group actions which may not fall into specific areas or divisions previously learned. Arbitrary demarcations of “head” and .“neck” cannot be adhered to if one is to attempt to conceive of what occurs when a simple sound is uttered or the act of swallowing takes place (Fig. 2). lnterpluy of Muscle and Bone.--For any facial movements, there must be a frame; that frame is the skull. In the anatomic region concerned with these movements, there exist a fixed bone region and a movable bone region. The two points of anchorage are the anterior part of the cranium and the sternum, and between are synergistic groups of muscles that move the skin and bone to alter expression. Eetween these anchors, the mandible and the hyoid bone are suspended. The two condyles of the mandible articulate with the cranium in the mandibular fossae of the temporal bones. These articulations provide certain leverage advantages as well as control and limitation of mandibular movement, Further limitation is placed on the movements of the mandible by the teeth and by muscle attachments. The hyoid bone, smaller than the mandible but of a similar shape, is situated in front of the throat at the root of the tongue. It is called the tongue bone because it supports the tongue and serves as an origin of attachment for some of its muscles. It is not articulated with any other bone. Its movements are dependent upon muscular activity. The pyramidal area housing these two movable bones, their articulators, and the muscle attachments is of extreme significance because of the multiple functions which take place in whole or in part therein. Some of these functions are of a vegetative nature, common to all animals, such as breathing, mastication, and deglutition, while others, equally highly specialized, are characteristic of man, such as speech and facial expression. Thus, this inverted pyramid, extending from the sternum to the tip of the nose and then to the outer orifices of the ears, following downward along the anterior border of the sternocleidomastoideus muscle, represents an anatomic region of major concern in the study of multifunctions. Within it lies the
MARTONE MUSCULATURE
AND
EDWARDS OF EXPRESSION
A.
J. PROS. I JAN.-FEB.,
MUSCULATURE
0F EXPRESSION
c.
D.
Fig. 4.-Anatomic dissections of muscles of facial expression showing their vari, ability in the different specimens. A, The face of this cadaver, prior to dissection, was thin and h: id a w clldeveloped musculature. Dissection revealed extremely coarse and cordlike muscle fibe ‘r‘s wh ich were relatively fat free. The muscles were easily dissected and stood out distinctly E, F ‘ew variations were evident in this specimen, which yielded a dramatic dissection because Of ear ;ilg distinguishable fibers, a minimum of adipose tissue, and brilliance of color in the must les. C, In this specimen, many variations were noted. Muscle fibers of various muscles were clos ely in termingled and intermeshed, making identification difficult. Fihers were generally pale, u rith m inimum color Contrast. D, Fibers in this specimen were unusually delicate and fine and were SO closely interspersed with adipose tissue, equally delicate, that dissection had to be F lerforn netl under high magnification.
;$hm&~
PHENO&lENON
0~
FUNCTION
IN
COMPLETE
DENTURE
PROSTHODONTICS
13
oral cavity, the operating field of the dentist (Fig. 3). Subsequent reference will be made to this polyfunctional pyramid since it is the focal point of interest in this article and those which follow. I;a&l Expression.-With this brief description of the underlying framework, or lvhat possibly may be termed the stage setting, we are now prepared to view the true drama of expression as portrayed by the prime actors-the muscles of facial expression. In writing of facial expression, Charles Darwin? has said, “The movements of expression give vividness and energy to our spoken words. They reveal the thoughts and intentions of others more truly than do words, which may be falsified.” Since the movements of facial expression become most evident when the individual is in an emotional state, several emotions will be analyzed from the standpoint of the facial expressions which reflect them and the muscular activity responsible for their production. Similar treatment will be accorded the activity of these muscles in the production of certain significant speech sounds. In this way, our thinking regarding this musculature is not in terms of mere insertion or origin but rather in relation to the phenomenon of function as it is correlated in the activities of speech and facial expression. Tile Facial Muscles of Expression.-The muscles of this unique group are unlike other skeletal muscles in that they lack fascial sheaths and are inserted into the skin. The absence of fasciae around these facial muscles, coupled with intricate, delicate movements of very small muscle bundles, makes possible the manifold combinations of synergistic movements independent of the rest of the muscle or within the complexities of groups of muscles, during elaborate facial expression. These muscles are further characterized by the insertion of delicate fiber and tendon attachments into the skin and mucosa of the lips and by their rich innervation and blood “LIpply. Dissection of these muscles is verv2 difficult because of their variability in each individual (Fig. 4). They are variable in shape, size, amount of development, and color. Moreover, they are difficult to separate because of the intricate fusing and exchange of bundles of muscle fibers. Thin elastic tendons, continuous with the fibers, insert into the subcutaneous tissue of the cLLtis itself in lines or in small areas. These muscles are directly subjacent to the freely movable skin. When the! contract, the elastic skin is folded at right angles to the direction of the pull of the m~~scles (Fig. 5 ) . By repeated contraction of the mLLscles that are inserted directly into the cutis, wrinkles develop into grooves or dimples. These grooves become deeper with advancing age because of the loss of elasticity of the skin. For the purposes of description, the muscles of facial expression may be divicled into the following groups : (1) the mLtscles of the forehead, (2) the muscles of the eyelids and eyebrows, (3 ) the muscles of the nose, and (4) the muscles of the lips and cheeks. Af~tscles of the fouelzead: The frontalis is a large, flat quadrilateral muscle having no bony attachment. It originates from the galea aponeurotica of the scalp
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MARTONE
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EDWARDS
J. Pros. Den. Jan.-Feb., 1962
and is inserted into the skin ,under the eyebrows and the skin over the root of the nose. Its broad fibers blend in with other muscle fibers in the orbital region. The contraction of this muscle is responsible for the transverse lines of the forehead and the raising of the eyebrows, giving an expression of surprise, amazement, or fear. Muscles of the eyelids and eyebrows: The orbicularis oculi surrounds the eye, and its contraction closes the eyelid, pulls down the eyebrow, and raises the cheek. The latter two actions are responsible for the “crow feet” wrinkles in the corner of the eye. The corrugator is the chief muscle controlling the movements of the eyebrow. A supercilious look of an individual is the result of the contraction of this muscle. The eyebrows are pulled downward and medially, resulting in vertical wrinkles of the forehead. Muscles of the nose region: The four muscles of the nose are very delicate and feeble (Fig. 6). They are the procerus, nasalis, dilatoris naris, and depressor septi nasi.
Fig. &-Folds and grooves that develop perpendicularly to the direction of the pull of the muscles which produce them: (00~) orbicularis oculi responsible for the infraorbital folds (IF); (2) zygomaticus which produces the nasolabial sulcus (NS); (P) the platysma, most superficial of the neck muscles.
“N;;;e;‘l”
Fig.
PHENOMENON
6.-Dissection
OF FUNCTION
IN COMPLETE
of the muscles of the nose region: naris; (DSN) depressor
DENTURE
(Pr) septi
procerus; nasi.
15
PROSTHODONTICS
(V)
nasalis;
fDX)
dilatoris
The procerus has its origin in the bridge of the nose, inserting into the skin between the eyebrows, the glabella. When it contracts, it tends to pull the eyebrows downward, and wrinkles occur over the bridge of the nose. The nasalis muscle draws the wing of the nose toward the septum. The nares of the nose are compressed by this action, and this compression may be observed during the crying of an infant or the utterance of certain speech sounds. The nostrils are dilated by the action of the two dilatores naris, and the depressor septi nasi draws the septum downward, flattening the philtrum of the maxillary lip and narrowing the nostril. Muscles of the lips and cheeks: Emotion of Joy.-Joy or happiness may be reflected in individuals in varying degrees, depending upon the intensity of the emotion experienced and the emotional level of the person. The range may be from the quiet warmth of a smile to uproarious laughter, but in either expression, the muscles of the lips and cheeks play a dominant role, and their action is frequently accompanied by a brightening or lighting up of the eyes. Smiling.-In general, the movements associated with a smile are lifting or
MARTONE
AND
EDWARDS
A.
B.
--
I. CALMNESS
i. SADNESS
Fig. 7.-A patient expressing (A) the three interpreting de Superville’s three fundamental Masher, H. D.: Laryngoscope 61:1-38, 1951.)
emotions expressions
3. GAIETY INCONSTANCY reflected in Masher’s of the face (B).
line drawings (Modified from
raising motions, producing upward lines which are characteristic of mirth (Fig. 7). A smile may be slight and of momentary duration, or it may be intense and prolonged. It may be unaccompanied by sound or it may be a facial overtone during speech, resulting in the modifications of certain speech sounds.
;$$;;2
PHENOMENON
OF FUNCTION
IN
COMPLETE
DENTURE
PROSTHODONTICS
17
In smiling, there is contraction of the zygomaticus (antagonist : orbicularis oris) which draws the modiolus and the angle of the mouth upward and backward. The quadratus labii superioris (antagonist : orbicularis oris) elevates the maxillary lip, the corner of the mouth, and the ala nasi. The risorius (antagonist : orbicularis oris) works in synergistic action with the buccinator and draws the angle of the mouth backward, producing a grinning expression which may not be of pleasant quality. Its action is associated with smiling and speech but not laughter (Fig. 8). As these muscles contract in varying degrees, they exert a pull on the angles of the mouth, resulting in the relaxing of corresponding continuous fibers of the orbicularis oris muscle, the sphincter control of the mouth. The degree of opening varies with the individual or the extent of the expression of the emotion. The fibers of the orbicularis oris muscle occupy the entire width of the lips. They extend from the angles of the mouth and run medially across the midline to insert into connective tissue and skin in the ridge area of the philtrum and the septum of the nose. The bulk of the orbicularis oris muscle is made up of a continuation of the fibers of the buccinator muscle as well as all of the muscles that insert into the lips. The arrangement and movement of the fibers are similar in both lips. The convergence of the fibers at the angle of the mouth is known as the modiolus. A smile may terminate with the face returning to a state of repose and the modiolus assuming its neutral position. This termination is brought about by the simultaneous contraction of the orbicularis oris muscle and the relaxing of all the muscles contributing to the smiling expression. When the face is in repose, the mandible is in a physiologic rest position (see Figs. 14, 15, and 16). As smiling occurs, the elevating actions of the smiling musculature, working synergistically with the three closing muscles of mastication, raise the mandible and retrude it slightly toward the vertical dimension of occlusion, diminishing the interocclusal distance. Laughing.-The smile, instead of terminating, may progress to accelerated proportions in the activity of laughter, which is characterized by the opening of the mouth and separation of the teeth. Laughter may occur instantaneously and to the full extent of the limitations of the musculature involved in this facial expression, or it may develop progressively as the emotion is built up, having its origin in the smile. Lightoller,3 in describing laughter, says that the modioli are drawn cranially and laterally, and the maxillary lip forms a straight or somewhat orally convex line stretching from modiolus to modiolus. He suggests that the maxillary teeth are exposed as far laterally as the first molars and cranially as far as the gingivae, and even this may be exposed. The distance between the nasal septum and the red marking of the maxillary lip is very much decreased, and the nasolabial fold is deepened, concaved orally, and extended downward to the rima oris, its cranial portion becoming more horizontal in direction. The mandibular lip is bowed downward, with marked oral concavity, but the mandibular teeth are only slightly exposed or may not be seen. This semilunar outline of the mandibular lip results from (1) the cranial position of the modiolus, (2) the caudal position of the mandible, and (3) activities of the inferior labial tractors. The muscle activities which
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MARTONE
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J. Pros. Den. Jan.-Feb., 1962
Fig. S.-Dissection of the following muscles of facial expression: (00~) orbicularis oculi; (Pr) procerus; (N) nasalis; (ON) dilatoris naris; (DSN) depressor septi nasi; (QLS) quadratus labii superior-is; (ah) angular head; (ih) infraorbital head; (.%h) zygomatic head; (2) zygomaticus; (C) caninus; (00~) orbicularis oris; (R) risorius; (B) buccinator; (T) triangularis; (QLZ) quadratus labii inferioris; (kfo) the modiolus.
cause laughter are the dominance of the zygomaticus and the resistance of the triangularis in modiolar activity ; in labial activity, the superior and inferior labial tractors are dominant, and the orbicularis oris muscle is completely and involuntarily inhibited. It is this inhibition which is the basis for true laughter. Emotion of Distress .-In distress, the mouth is firmly closed (see Fig. 7). The rima oris is reduced in size to a narrow, cranially bowed slit with the corners of the mouth extending downward. This is caused by (1) the tensing of the muscles inserted into the modioli which fixes the modioli and (2) the pushing of the mandibular lip under and against the maxillary lip by the superior and inferior portions of the orbicularis oris muscle. The mandibular lip becomes broader laterally and, in so doing, produces creases that continue into the curved depression of the rima oris. This action makes it appear that the angle of the mouth has been dragged downward and laterally. Lightoller 3 believes that when the rima oris is closed, the angle of the mouth is not depressed. He further states that by stiffening the mandibular lip with the mentalis muscle, emotion can be controlled. This is an antithesis to the old adage of controlling emotion by “keeping a stiff upper lip.”
“lume l21 Number SUPERFICIAL
PHENOMENON MUSCLES
OF FUNCTION OF THE
IN
COMPLETE
DENTURE
19
PROSTHODONTICS
NECK
Acting with the superficial facial muscles in expression and related functions are the superficial muscles of the neck. These muscles can be divided into two groups: (1) the inframandibular muscles, made up of the suprahyoid and infrahyoid muscles, and (2) the posterior cervical muscles.
E
Fig. 9.-Dissection of suprahyoid muscles, frontal view: digastric, posterior belly; (5%) stylohyoid; (Mh) mylohyoid. Fig. IO.-Dissection of suprahyoid muscles, left lateral (Sh) stylohyoid; (&fh) mylohyoid.
(D,a)
view:
Fig.
9
digastric, (D,a)
anterior
digastric,
belly; anterior
(D,p)
belly;
20
MARTONE
AND
J. Pros. Den. Jan.-Feb., 1962
EDWARDS
Fig.
Fig.
tric,
11
12
Fig. 11.- Dissection of suprahyoid muscles, right lateral view: (Sh) stylohyoid; posterior belly; (D,aj digastric, anterior belly; (Mh) mylohyoid. Fig. XL-Dissection of infrahyoid muscles: (l’h) thyrohyoid; (Oh) omohyoid; (Sth)
(D,p)
digas-
sternohyoid.
;f;Te;;’
PHENOMENON
OF FUNCTION
IN COMPLETE
DENTURE
21
PROSTHODONTICS
Inframandibular Muscles.-These muscles suspend the hyoid bone, lower the mandible, and have prosthodontic significance in relation to physiologic rest position, speech, facial expression, mastication, and deglutition. The most superficial of the neck muscles, the platysma, covers the side of the neck. It originates in the deltoideus and pectoralis muscle regions and inserts into the inferior border of the mandible and the angle of the mouth (see Fig. 5). The suprahyoid muscles consist of the digastric, stylohyoid, mylohyoid, and geniohyoid muscles (Figs. 9, 10, and 11). They are depressors of the mandible and elevators of the hyoid bone. The infrahyoid muscles are the sternohyoid, sternothyroid, omohyoid, and thyrohyoid muscles (Figs. 12 and 13). They depress the hyoid bone and larynx and fix the hyoid bone so that the suprahyoid muscles may act upon the mandible. These are voluntary muscles but act reflexively and, during mandibular rest position, are in balance with the supramandibular muscles. The interocclusal distance is due to the equal tonicity of the supramandibular and the inframandibular muscles. As compared with the closing muscles of the mandible, these are very small and weak. They are used for rapid movements and those of long duration. In old age, the supramandibular muscles increase in tonicity, while the inframandibular muscles decrease.
Fig. la.-Dissection tracted: (8th) sternohyoid;
of infrahyoid muscles (Oh) omohyoid; (2%)
with the thyrohyoid;
sternohyoid and omohyoid (5%) sternothyroid.
muscles
re-
MARTONE
AND
EDWARDS
Fig. 14.-The head in muscular balance; (H) hyoid bone; (VC) vertebral column; (1) craniomandibular muscles; (2) suprahyoid muscles; (3) infrahyoid muscles; (4) posterior cervical muscles. (Diagram at lower right modified from Sarnat, B. G., editor: The Temporumandibular Joint, Springfield, Ill., 1951, Charles C Thomas, Publisher, p. 114.)
Posterior Cervical Muscles.--The head rests upon the atlas, the uppermost vertebra of the spinal column, in positional unbalance because the center of gravity of the head is not directly over the atlas, but anterior to it, in the approximate region of the temporomandibular articulation. The weight of the projecting mandible and teeth suspended from this region lies about in the center of the entire polyfunctional pyramid, thus contributing to this unbalance. The structures responsible for maintaining the head in balance are the posterior cervical muscles working antagonistically to the supramandibular and inframandibular muscles (Figs. 14, 1.5, and 16). Thus, if one falls asleep in an upright position, and the musculature controlling this balance relaxes, gravity pulls the head forward and the mandible down.
;$‘F”r’l”
PHENOMENON
OF FUNCTION
IN
COMPLETE
DENTURE
PROSTHODONTICS
23
The posterior cervical muscles function continuously approximately two-thirds of each day, maintaining the heavy head in balance. They are also responsible for the many and delicate positionings of the head associated with the activities of expression, eating, breathing, vision, hearing, gait, and posture. Factors Contributing to Postural Change.-The physiologic factors responsible for postural changes which come about through aging take place gradually by the “giving way” of the long-laboring posterior cervical muscles. In addition to loss of tonicity, other dominant forces responsible for the weakening of these muscles include (1) the excessive weight supported by these muscles because the center of gravity of the head is anterior to the atlas, (2) accentuation of the secondary curvature of the cervical vertebrae, (3) the greater leverage factor of the complex mechanism of smaller anterior muscles serving as the antagonist, including the prevertebral muscles, and (4) habitual and occupational activities. Prosthodontically, little concern is evidenced for these muscles as contrasted with the interest displayed in their antagonists. This is unfortunate. The prosthodontic implications which are a result of postural changes require a dynamic concept as opposed to a static concept in the clinical treatment of patients. MOVEMENTS
OF
THE
WANDIBLE
Varying movements of the mandible take place and in differing degrees, depending upon the specific function being performed, the habits of the individual, and his emotional state. The mandible thus assumes a role of main actor in the drama of action, and it plays a supporting role for the denture. It becomes essential, therefore, for the dentist to be fully aware not only of the basic mandibular movements and positions but also of functional patterns of movements, in which variations may occur because of emotional state or physiologic needs. These varying patterns may be noted in either extent of movement or rate of speed at which it
TABLE
I.SOMEVARIATIONSIN DURING
MANDIBULAR MOVEMENTS DIFFERENTFUNCTIONS
_-.___~ -
OCCURRING
-
VARIATION _ ~__ _---.
MASTICATORYMOVEMENTS
NONMASTICATORYMOVEMENTS
-In occlusion
Not
in occlusion
Rate and duration of movements
Slower than nonmasticatory; limited, less than 2 hours per day
More
rapid mittent,
Strength of movements
Powerful
Delicate
Variations
Limited by individual masticatory mechanism but adapted to cope with difference of texture in boluses of food
Greater number of variations than in mastication because of greater number of muscles involved and the highly individual responses to multiple emotions or speech sounds
Position
of teeth
than masticatory; 14 to 16 hours
interper day
24
Fig.
MARTONE
15.-The
mandible
lowered
AND
J. Pros. Den. Jan.-F&., 1962
EDWARDS
by contraction of the inframandibular of the supramandibular muscles.
muscles
and
relaxation
takes place. Such motion patterns are dislodging forces which must be anticipated in the design of any appliance that must absorb them and still maintain a degree of stability and retention. The vertical tug of war which takes place in the opening and closing movements of the mandible is produced by the elevators of the mandible (the temporalis, masseter, and internal pterygoid) and the depressors of the mandible (the geniohyoid, mylohyoid, and digastric) (see Figs. 14, 15, and 16). The external pterygoid muscles move the mandible forward, while the posterior fibers of the temporalis retrude the mandible. The external pterygoid and the elevators opposite them produce the side to side or lateral movements. These movements are basic and are governed neurologically by a central regulator which receives impulses through the various physiologic reflex arcs.
,v;‘;‘=&:,e,2
Fig.
l&-The
PHENOMENON
OF FUNCTION
IN COMPLETE
DENTURE
PROSTHODONTICS
head in positional unbalance because of the relaxation of the controlling ture which allows gravity to pull the head forward and downward.
25
muscula-
Mandibular movements are limited and modified by the temporomandibular articulations posteriorly and the teeth anteriorly. The mandible assumes various positions which serve as starting, limiting, and returning points in its complex physiologic activities. Those on a vertical plane are physiologic rest position and vertical dimension of occlusion. Those on a horizontal plane are centric relation (which also has a vertical component) and right and left lateral and protrusive positions. All of these mandibular movements and positions occur within the perimeter of extreme movements which the mandible is capable of performing.4 Certain variations in mandibular movements that occur during different functions are indicated in Table I. Nonmasticatory Movements.-Too often, dentists, in thinking of function, think only in terms of mastication. As a result of this, musculature considerations
26
MARTONE
AND
EDWARDS
J. Pros. Den. Jan.-Feb., 1962
Fig. 17.-Dissection showing continuity of fibers of the caninus muscle (C) and the train gularis muscle (T). This continuity indicates that the muscles can be considered as one muscle having its origin in fixed bone (maxilla) and inserting into movable bone (mandible).
are frequently limited to those of the “muscles of mastication.” The musculature responsible for mandibular movements may be divided into the supramandibular (craniomandibular muscles and muscles of facial expression) and inframandibular groups. Clinical analyses of muscular activity indicate an association between the type of stimulus and the sequence of events which follow. If the stimulus is in the form of a bolus of food entering the mouth, the muscles of mastication go into play, accompanied by various lip, cheek, and tongue activities designed to assist in the eating process. However, if the stimulus is such as to produce a speech sound or a facial expression in the mouth region, the muscles responsible for producing the sound or expressing the emotion coordinate with the muscles of mastication and serve to guide the mandible into the proper position. Attempts to analyze these various functional mandibular movements led to multiple dissections of the facial muscles of the mouth region from the standpoint of their strength, locations, and direction and extent of fibers, with a view to determining whether or not they may fulfill these functions.
;;$$y
PHENOMENON
OF FUNCTION
IN
COMPLETE
DENTURE
PROSTHODONTICS
27
Repeated dissections show such a continuity of fibers between the muscles elevating the lips and those depressing them that these muscles can well be thought of in terms of one muscle attached to fixed bone (the maxilla) and to movable bone (the mandible). An example of this unity is apparent in Fig. 17, which shows the dissection of the caninus and triangularis muscles. Other radial components, running through the modioli, reflect this same unity. It may well be that in producing the lip movements of elevation and depression, some muscles of facial expression may also serve as initiators of the delicate mandibular movements found in speech and facial expression or as a trigger musculature which sets into play the more powerful craniomandibular muscles when their action is required.
The complex interplay of bone and musculature becomes apparent in even a limited analysis of facial expression. The complexity continues in any attempt to analyze further functions of speech, respiration, mastication, and deglutition, all of which are processes occurring in the arena of the pyramid previously delineated. Analyses of functions of facial expression and speech and the study of dissections of the muscles of facial expression indicate that certain of these muscles, acting as combined units and attached to fixed and movable bone, may play a role in mandibular movements associated with the functions of speech and facial expression. These functions and their interassociations will be considered in the next article of this series. REFERENCES
1. Huber, E.: Evolution of Facial Musculature and Facial Expression, Baltimore, 1931, Johns Hopkins Press, p. 151. 2. Darwin, C. : Expression of Emotions in Man and Animals, New York, 1898, D. Appleton and Company, pp. 364-365. 3. Lightoller., G. H. S.: The Action of the M. Mentalis in the Expression of the Emotion of Distress, J. Anat. 62:319-332, 1927-28. 4. Posselt, U. : Movement Areas of the Mandible, J. PROS. DEN. 7:376, 381, 1957. 909 NORFOLK MEDICAL TOWER NORFOLK 7, VA. DEPARTMENT OF ANATOMY OHIO STATE UNIVERSITY COLUMBUS 10, OHIO
ANATOMY Part II.
OF THE Musculature
MOUTH
AND
RELATED
IN
COMPLETE
STRUCTURES
of Expression*
ALEXANDER L. MARTONE, D.D.S., M.Sc.,” * AND LINDEN F. EDWARDS, PH.D.*” * Norfolk, Vu., and Columbus, Ohio
of facial expression in man indicate that there is practically no facial expression in the newborn infant. It becomesmore manifold and definite as the infant’s consciousand intelligent reactions to the surrounding world increase. Throughout childhood, emotional reactions remain unmasked, making it possible to interpret the true meaning of expressions in the child’s face. As the individual matures, with experience and education, he learns to control facial expression. This ability to conceal emotions makes it more difficult to read the faces of adults. TUDIES OF THE EVOLUTION
S
Huberl states : “In man facial expression has finally reached marvelous perfection. This phenomenon may be attributed to further differentiation of the ‘mimetic musculature’ in close correlation with higher evolution of the central nervous mechanism, particularly with further ’ development of the facial area in the motor cortex and through elaboration of association centers.” Since it is true that human beings engage in the actions of speech and facial expression to a greater degree than they do in mastication and since dentists must be concerned with these activities, a knowledge of the intricate musculature responsible for these movements is essential. Dental teachers and authors have stressed the importance of understanding the muscles of mastication, and rightfully so. However, this emphasis should not result in a disregard or lack of understanding of the other musclesthat play vital roles in the complex organism requiring dental treatment. For this reason, particular attention will be devoted in this consideration of myology to the intricate and delicate muscles responsible for facial expression and speech. *Part I, J. PROS.DEN. 11:1009-1018, 1961. **Associate, Departments of Prosthodontics and Anatomy, Medical College of Virginia. ***Professor, Department of Anatomy, Ohio State University. 4
;;‘;rr:‘l-7
PHENOMEKON
GRO\\rTIl
AND
OF
FUNCTION
Ilr;
COMPLETE
DEPI‘TURE
PROSTIIODOiXTICS
5
DEVELOPMENT
The muscles of the mouth and related structures are derived from the mesoderm of the branchial arches (Fig. 1). About the sixth week of embryonal life, the muscles of these arches, which are innervated by certain cranial nerves, undergo modification and migration hut retain their original cranial nerve supply. The muscles of mastication, the tensor veli palatini, tensor tympani, anterior belly of the digastric, and mylohyoid, are derived from the first branchial arch and are supplied by the mandibular branch of the trigeminal (Vth cranial) nerve. The second or hyoid arch, innervated l)y the facial (\-11th cranial) nerve, furnishes the muscles of expression, the posterior belly of the digastric, the stylolkyoid, the stapetlius, and the auricular muscles. The mesoderm of the third arch furnishes the material for the stylopharyngeus muscle and is innervated by the glossopharyngeal (IXth cranial) nerve. From the fourth and fifth arches develop the pharyngeal, laryngeal, and the other palatal muscles which are supplied by the vagus (AXth cranial) and the cranial portion of the spinal accessory (XIth cranial) nerves. The muscles of the tongue, however, are derived from occipital myotomes and are supplied by the hypoglossal (SIIth cranial ) nerve. I’IIYSICAL
The basic units of a voluntary skeletal striated muscle are the muscle fibers which are elongated and cylindric in shape, bound together by connective tissue and supplied by nerves, blood, and lymph vessels. Each fiber is surrounded by a connective tissue sheath or capsule which insulates the fibers so that each may ir?clividually contract without activating other fibers. There are many nuclei intermingled in the alternating light and dark bands on their surfaces. The fibers are bound together in bundles by a delicate connective tissue sheath, and their length is generally correlated with the length of the muscle. Striated muscle fibers do not increase in number after birth. The enlargement of the muscle after birth is due to the increase in the diameter of its fibers. Because of the lack of power of regeneration, injured muscles do not repair easily and smoothly, but repair with connective tissue, leaving an irregular scar. PHYSIOLOGIC
CONSIDERrlTIOKS
Muscles are very excitable and respond readily to stimuli. They are activated by impulses which pass to them by way of motor nerve fibers. The phenomenon of transmission of an impulse from a motor heuron to a muscle fiber at the motor end plate is similar to the mechanism of the transmission of impulses across a synapse. A nerve fiber supplies several muscle fibers, the number being quite variable (100 to 200 j. This constitutes a motor unit, and the number of functioning motor units depends largely upon the physiologic demands placed on a muscle. The more highly specialized the muscular activity, the greater the number of motor units in proportion to the muscle fibers. For example, specialization of muscular activity of the face and tongue is of a very high degree and complicated.
6
MARTONE
AND
EDWARDS
J. Pros. Jan.-Feb.,
Den. 1962
In this area, the muscle fibers are relatively very few and small. For delicate and intricate movements such as those in speech and facial expression to take place, small groups of fibers must be activated without involving other groups. There are two types of muscular contraction, isotonic and isometric. In isotonic (is0 meaning same, tonic meaning tension) contraction, tension remains constant while the length of the fibers shortens. For example, stimulation of the masseter muscle while the teeth are not in occlusion will result in a shortening of the muscle fibers. Conversely, in isometric (iso meaning same, metric meaning length) contraction, tension increases, but the length of the fiber remains the same. For example, when the teeth are in centric occlusion, further stimulation of the masseter musclewill not permit the fibers to become shorter. Under normal circumstances, musclesare in a state of slight tension or partial contraction (tonus) all of the time. When the person is in a nornial standing or sitting position, the mandible is maintained in a physiologic rest position. The masseter, temporal, and internal pterygoid muscles are in a partially contracted state, suspending the mandible in a balanced position against the pull of gravity, the weight of the mandible and supportive structures, and the partial contraction and tension of the inframandibular muscles. Facial Movemmts.-Facial movements are the result of the automatic, coordinated effort of many muscles in different functional capacities executed in the cerebral cortex. Movement occurs through paired muscular activity. In order for a muscle to contract, its opponents must relax. Individual muscles may function by being (1) prime movers, (2) antagonists, (3) fixators, or (4) synergists. Prime movers are those actually responsible for the movement, whereas antagonists are those muscles which produce opposing movements. Thus, in closing the mouth, the inframandibular muscles relax and the three closing muscles of mastication contract; in opening of the mouth, the reverse takes place. Fixation musclesestablish a stable basisby steadying a structure. For example, the infrahyoid muscles steady the hyoid bone so that the suprahyoid muscles may contract the lowering mandible. A synergist (meaning “to work together”) muscle aids the prime mover in its action. For example, the internal pterygoid muscle is a synergist of the masseter musclein that it helps it to elevate the mandible. Fundamentally, any functional movement is a result of muscular contraction. If the muscular tension is extreme, it may be desirablcefor this tension to be reduced. This is frequently the case in training a student in voice or speech. However, advising him to relax may produce a result directly opposite to the desired one, for in an effort to relax, the performer may actually increase the tension. He is striving to obtain a balanced prime mover-antagonist musculature relationship. The accuracy, control, and quality of the voice are improved in a like degree as this balance is attained. This same principle of muscle balance can be applied in recording maxillomandibular relationships in dental treatment procedures. P&ciples of Muscular Contraction.-There are several basic physiologic principles of muscular contraction : (1) musclesare excitable and respond to changes in the stimuli ; (2) a muscle always pulls and never pushes; (3) the direction of
>;;F’,‘r:”
PHENOMENON
OF FUNCTION
IN COMPLETE
DENTURE
PROSTHODONTICS
7
---
larynx, safl palate
Fig. I.-Motor and proprioceptive innervation of the muscles of mastication, expression, soft palate, larynx, pharynx, and tongue: (A) Mesencephalic nucleus; (13) motor nucleus of mandibular nerve; (C) motor nucleus of facial nerve; (D) motor nucleus (ambiguus) of glossopharyngeal and vagus nerves; (E) hypoglossal nucleus; (3’~ sensory nucleus of trigeminal nerve; (G) sensory (solitary) nUCleUs of facial, glossopharyngeal, and vagus nerves; IDP) corticohulbar and corticospinal tract; (EP) extrapyramidal tract; (Bas. gang.) basal ganglia. Inset A, A lateral view of the branchial arches, the branchial musculature, and the cranial nerve supply (in Roman numerals) of each. Inset B, A dorsal view of the pharyngeal floor showing the branchial arches (in Arabic numerals) sectioned along the horizontal line (Xl in inset A.
J, Pros.
Jan.-Feb.,
A.
B.
Fig.
2.-The
complex interplay saying “lone.”
of structures during speech. A, Subject saying “pooh.” C, Profile view of subject producing vowel sound a.
B, Subject
Deii.
1962
\~olume
,Yumher
I.!
1
I’HENOMEXON
OF
FUNCTION
IN
COMPLETE
DENTC;RE
PROSTHODONTICS
0
action of a muscle is determined by the direction of the fibers at the time of contraction : (4) the initial length of the fiber determines the force with which the fiber will contract; (5) muscles obey the “all or none” law-when a single muscle fiber is activated, it either contracts completely or not at all : (6 ) muscles are in a state of continuous partial contraction or tonus even when they are at rest ; (7 j the amount of power produced by a muscle is determined by the number of fihers contracting at a given time ; (8) d i ff erent muscle fibers have different thresholds of stimulation. NEUROl.OGIC
CONSIDERATIONS
With reference to the physiologic considerations of muscles and the role they play in the production of facial expression, speech, and mandibular rest position, some mention shouId be made of the nature of the neurologic mechanism involved. Motor fibers within the nerves which supply the muscles of the tongue and those derived from the mesoderm of the pharyngeal arches represent axons of motor neurons whose cell bodies are located in motor nuclei of these nerves within the brain stem (Fig. 1). In addition, these nerves, with the exception of the hypoglossal, also contain sensory fibers which represent dendrites of cell bodies that are located within the ganglion of each nerve. These sensory fibers are distributed to various sense organs, including those of touch, pressure, temperature, pain, and proprioception. The latter consist of sense organs located within muscles, tendons, and ligaments and are stimulated whenever these structures are stretched. Thus, a muscle and its tendon are stretched whenever its antagonists contract, and ligaments are made more or less tense when joints undergo movements. The cell bodies of the proprioceptive fibers within the mandibular nerve are regarded as being located in the “mesencephalic nucleus” within the midbrain. However, the location of the neuron cell bodies of proprioceptive fibers in the facial, glossopharyngeal, and vagus nerves is uncertain. Obviously, the nerves supplying the tongue and facial muscles and those of the larynx, pharynx, and soft palate would be expected to contain proprioceptive fibers for the control of the mechanism involved in articulate speech. In regard to the proprioceptive innervation of the facial muscles, it has been suggested that these muscles are supplied by fibers from the mesencephalic nucleus via branches of the trigeminal nerve, since numerous connections occur between these and motor branches of the facial nerve on the face. Anatomic and physiologic evidence, however, is lacking to support this contention. The only other logical possibility is that the cell bodies of such fibers are located in the geniculate ganglion of the facial nerve, a concept for which some clinical evidence exists, notably in surgical casesinvolving ablation of the sensory root of the trigeminal nerve following which deep sensibility of the facial muscles \vas n(lt impaired or lost. Although the source of the proprioceptive fibers supposedly contained in the glossopharyngeal and vagus nerves, which supply the muscles of the larynx, pharynx, and soft palate, is unknown, it is conjectured that their cell bodies are located within the ganglia of these nerves as are those of other sensory fibers contained in these nerves.
10
MARTONE
AND
EDWARDS
J. Pros. Jan..Feb.,
Den. 1962
Although proprioceptors have been demonstrated histologically in the muscles of the tongue, the peripheral pathway and location of the cell bodies of proprioceptive fibers are hypothesized. Inasmuch as the hypoglossal nerve lacks a ganglion and is considered a purely motor nerve and since it communicates with the glossopharyngeal and lingual nerves, the opinion is held that these nerves serve to provide proprioceptive fibers to the tongue muscles. Impulses transmitted by proprioceptive fibers upon reaching the brain stem may be relayed to the cerebral cortex, where they arouse a sensation, or directly to motor nuclei of the cranial nerves, thus becoming converted into motor impulses. These, in turn, are conducted centrifugally by motor fibers to the muscles, which respond by involuntary or automatic contraction. Such a response is commonly called a reflex action, a notable example of which is the stretch reflex. Muscle Tonus.-The motor portion of the reflex mechanism usually consists of a relatively few motor units, or muscle fibers. The phenomenon of muscle tonus is regarded as being due to the reflex functioning of a relatively few motor units, which work in relays, thus resulting in a state of constant mild contraction of the muscle, which is finely controlled with a minimum of fatigue. The degree of muscle tonus varies. It is diminished by sleep, it is reduced by fatigue, and it is influenced by emotional excitement. Thus, feelings of joy and hope augment the tonus of the facial muscles and find expression in smiling or laughing, whereas, grief and worry depress facial tonus, giving rise to the wellknown “down in the mouth” appearance. It is evident, therefore, that the state of muscle relaxation is a relative one, inasmuch as a skeletal muscle though apparently at rest is always in a state of tonus or constant mild contraction, except when a patient is under general anesthesia, when the nerve to a muscle is sectioned, or when curare is administered. Stretch Reflex and Posture.-Muscle tonus is a particularly significant feature of the antigravity muscles which serve to maintain upright posture, including balancing of the head and suspension of the mandible when in rest position. In the latter position, the pull of gravity on the mandible along with the tonus or tension of its depressors stimulates the proprioceptors in the temporomandibular joints and elevators of the mandible, resulting in a stretch reflex or tonus of the latter muscles. The tonus or mild constant contraction of the elevators of the mandible in turn puts the depressors in tension, and like a feedback arrangement, the stretch reflex thus set up creates tonus in the latter muscles. The end result of this neuromuscular mechanism is a balance in tonus between the elevators and depressors and, consequently, the suspension of the mandible in mid-air as it were (see Figs. 14, 15, and 16). It should be emphasized that not all of the proprioceptive impulses upon entering the brain stem participate in the stretch reflex. Some of these impulses are relayed to the cerebellum, while others are transferred by way of the thalamus to the postcentral gyrus of the cerebral hemisphere where they arouse a kinesthetic sensation commonly called body sense, which is one’s awareness of position in space. From this gyrus, these impulses along with others such as touch, pressure, temperature, and pain, which have also reached this level of consciousness or sensation, are relayed to the voluntary motor area (the precentral gyrus (of the
v,;‘;pe;;’
PHENOMENON
OF FUNCTION
IN
COMPLETE
DENTURE
PROSTHODONTICS
11
cerebral hemisphere), where pyramidal cell bodies of upper (voluntary) motor neurons are located and where the impulses are converted into voluntary, willful, or intentional motor impulses. Axons of these motor neurons descend in two paths, one of which is called pyramidal and the other extrapyramidal. Axons of the pyramidal pathway are divisible into corticospinal fibers which descend and synapse with motor neurons of spinal nerves in the ventral gray horn of the spinal cord and into corticobulbar fibers which descend and synapse with motor neurons in motor nuclei of cranial nerves in the brain stem. The impulses transmitted by these two sets of fibers to lower motor neurons are conducted by the latter to skeletal muscles, which respond by active voluntary contraction, during which a relatively large number of motor units function. This type of response is diametrically opposite to involuntary reflex action, in that movements produced by active muscular contraction are volitional, or, as it is commonly expressed, under control of the will, It is not to be implied that these active, willful muscular contractions are never automatic or habitual, since it is well known that such activities as walking, feeding one’s self, mastication, deglutition, smiling, frowning, speech, and even driving a car become habitual and automatic but are under voluntary control. Details of all the connections of the fibers making up the extrapyramidal path are not firmly established, and neither are the functions of this system. It is known to consist of several links of neurons, which relay voluntary motor impulses to motor nuclei of cranial and spinal nerves, resulting in mass, coordinated, synergic movements, as opposed to isolated movements of individual muscles. One of the links in this system is called the corticopontocerebellar tract; it extends from the voluntary motor cortex to the pons and then to the cerebellum. Since the cerebellum also receives proprioceptive impulses, which subsequently leave this portion
4.
Fig.
3.-The
polyfunctional parent
pyramid. in profile
A, Outlined on view of surface
hemisection anatomy
cut of of a patient.
head
and
neck.
B,
Ap.
12
MARTONE
AND
EDWARDS
J. Pros. JawFeb.,
Den. 1962
of the brain as motor impulses that are relayed to lower motor neurons, the function of the cerebellum and of the corticopontocerebellar tract is regarded as that of a regulating influence on muscle tonus, equilibrium, and coordination of muscular movements. Although muscle tonus is a reflex phenomenon, it is somewhat under cortical control, and, moreover, it disappears in the absence or loss of function of the cerebellum. ANATOMY
IN
ACTION
In an initial study of anatomy, the student, of necessity, learns structures in a sequence of groupings or classifications based on either areas of location or separate functional activities, and his thinking, consequently, is in terms of single actions of individual structures. Until he attains an understanding of the complex interplay of various structures that must occur to produce a single movement, he is without a concept of true functional anatomy. It is the knowledge of this interrelated and interdependent anatomic activity that provides the real basis for the practical application of anatomy to clinical problems. The considerations which will follow, therefore, may depart from customary textbook classifications and sequence when it is more advantageous to think in terms of group actions which may not fall into specific areas or divisions previously learned. Arbitrary demarcations of “head” and .“neck” cannot be adhered to if one is to attempt to conceive of what occurs when a simple sound is uttered or the act of swallowing takes place (Fig. 2). lnterpluy of Muscle and Bone.--For any facial movements, there must be a frame; that frame is the skull. In the anatomic region concerned with these movements, there exist a fixed bone region and a movable bone region. The two points of anchorage are the anterior part of the cranium and the sternum, and between are synergistic groups of muscles that move the skin and bone to alter expression. Eetween these anchors, the mandible and the hyoid bone are suspended. The two condyles of the mandible articulate with the cranium in the mandibular fossae of the temporal bones. These articulations provide certain leverage advantages as well as control and limitation of mandibular movement, Further limitation is placed on the movements of the mandible by the teeth and by muscle attachments. The hyoid bone, smaller than the mandible but of a similar shape, is situated in front of the throat at the root of the tongue. It is called the tongue bone because it supports the tongue and serves as an origin of attachment for some of its muscles. It is not articulated with any other bone. Its movements are dependent upon muscular activity. The pyramidal area housing these two movable bones, their articulators, and the muscle attachments is of extreme significance because of the multiple functions which take place in whole or in part therein. Some of these functions are of a vegetative nature, common to all animals, such as breathing, mastication, and deglutition, while others, equally highly specialized, are characteristic of man, such as speech and facial expression. Thus, this inverted pyramid, extending from the sternum to the tip of the nose and then to the outer orifices of the ears, following downward along the anterior border of the sternocleidomastoideus muscle, represents an anatomic region of major concern in the study of multifunctions. Within it lies the
MARTONE MUSCULATURE
AND
EDWARDS OF EXPRESSION
A.
J. PROS. I JAN.-FEB.,
MUSCULATURE
0F EXPRESSION
c.
D.
Fig. 4.-Anatomic dissections of muscles of facial expression showing their vari, ability in the different specimens. A, The face of this cadaver, prior to dissection, was thin and h: id a w clldeveloped musculature. Dissection revealed extremely coarse and cordlike muscle fibe ‘r‘s wh ich were relatively fat free. The muscles were easily dissected and stood out distinctly E, F ‘ew variations were evident in this specimen, which yielded a dramatic dissection because Of ear ;ilg distinguishable fibers, a minimum of adipose tissue, and brilliance of color in the must les. C, In this specimen, many variations were noted. Muscle fibers of various muscles were clos ely in termingled and intermeshed, making identification difficult. Fihers were generally pale, u rith m inimum color Contrast. D, Fibers in this specimen were unusually delicate and fine and were SO closely interspersed with adipose tissue, equally delicate, that dissection had to be F lerforn netl under high magnification.
;$hm&~
PHENO&lENON
0~
FUNCTION
IN
COMPLETE
DENTURE
PROSTHODONTICS
13
oral cavity, the operating field of the dentist (Fig. 3). Subsequent reference will be made to this polyfunctional pyramid since it is the focal point of interest in this article and those which follow. I;a&l Expression.-With this brief description of the underlying framework, or lvhat possibly may be termed the stage setting, we are now prepared to view the true drama of expression as portrayed by the prime actors-the muscles of facial expression. In writing of facial expression, Charles Darwin? has said, “The movements of expression give vividness and energy to our spoken words. They reveal the thoughts and intentions of others more truly than do words, which may be falsified.” Since the movements of facial expression become most evident when the individual is in an emotional state, several emotions will be analyzed from the standpoint of the facial expressions which reflect them and the muscular activity responsible for their production. Similar treatment will be accorded the activity of these muscles in the production of certain significant speech sounds. In this way, our thinking regarding this musculature is not in terms of mere insertion or origin but rather in relation to the phenomenon of function as it is correlated in the activities of speech and facial expression. Tile Facial Muscles of Expression.-The muscles of this unique group are unlike other skeletal muscles in that they lack fascial sheaths and are inserted into the skin. The absence of fasciae around these facial muscles, coupled with intricate, delicate movements of very small muscle bundles, makes possible the manifold combinations of synergistic movements independent of the rest of the muscle or within the complexities of groups of muscles, during elaborate facial expression. These muscles are further characterized by the insertion of delicate fiber and tendon attachments into the skin and mucosa of the lips and by their rich innervation and blood “LIpply. Dissection of these muscles is verv2 difficult because of their variability in each individual (Fig. 4). They are variable in shape, size, amount of development, and color. Moreover, they are difficult to separate because of the intricate fusing and exchange of bundles of muscle fibers. Thin elastic tendons, continuous with the fibers, insert into the subcutaneous tissue of the cLLtis itself in lines or in small areas. These muscles are directly subjacent to the freely movable skin. When the! contract, the elastic skin is folded at right angles to the direction of the pull of the m~~scles (Fig. 5 ) . By repeated contraction of the mLLscles that are inserted directly into the cutis, wrinkles develop into grooves or dimples. These grooves become deeper with advancing age because of the loss of elasticity of the skin. For the purposes of description, the muscles of facial expression may be divicled into the following groups : (1) the mLtscles of the forehead, (2) the muscles of the eyelids and eyebrows, (3 ) the muscles of the nose, and (4) the muscles of the lips and cheeks. Af~tscles of the fouelzead: The frontalis is a large, flat quadrilateral muscle having no bony attachment. It originates from the galea aponeurotica of the scalp
14
MARTONE
AND
EDWARDS
J. Pros. Den. Jan.-Feb., 1962
and is inserted into the skin ,under the eyebrows and the skin over the root of the nose. Its broad fibers blend in with other muscle fibers in the orbital region. The contraction of this muscle is responsible for the transverse lines of the forehead and the raising of the eyebrows, giving an expression of surprise, amazement, or fear. Muscles of the eyelids and eyebrows: The orbicularis oculi surrounds the eye, and its contraction closes the eyelid, pulls down the eyebrow, and raises the cheek. The latter two actions are responsible for the “crow feet” wrinkles in the corner of the eye. The corrugator is the chief muscle controlling the movements of the eyebrow. A supercilious look of an individual is the result of the contraction of this muscle. The eyebrows are pulled downward and medially, resulting in vertical wrinkles of the forehead. Muscles of the nose region: The four muscles of the nose are very delicate and feeble (Fig. 6). They are the procerus, nasalis, dilatoris naris, and depressor septi nasi.
Fig. &-Folds and grooves that develop perpendicularly to the direction of the pull of the muscles which produce them: (00~) orbicularis oculi responsible for the infraorbital folds (IF); (2) zygomaticus which produces the nasolabial sulcus (NS); (P) the platysma, most superficial of the neck muscles.
“N;;;e;‘l”
Fig.
PHENOMENON
6.-Dissection
OF FUNCTION
IN COMPLETE
of the muscles of the nose region: naris; (DSN) depressor
DENTURE
(Pr) septi
procerus; nasi.
15
PROSTHODONTICS
(V)
nasalis;
fDX)
dilatoris
The procerus has its origin in the bridge of the nose, inserting into the skin between the eyebrows, the glabella. When it contracts, it tends to pull the eyebrows downward, and wrinkles occur over the bridge of the nose. The nasalis muscle draws the wing of the nose toward the septum. The nares of the nose are compressed by this action, and this compression may be observed during the crying of an infant or the utterance of certain speech sounds. The nostrils are dilated by the action of the two dilatores naris, and the depressor septi nasi draws the septum downward, flattening the philtrum of the maxillary lip and narrowing the nostril. Muscles of the lips and cheeks: Emotion of Joy.-Joy or happiness may be reflected in individuals in varying degrees, depending upon the intensity of the emotion experienced and the emotional level of the person. The range may be from the quiet warmth of a smile to uproarious laughter, but in either expression, the muscles of the lips and cheeks play a dominant role, and their action is frequently accompanied by a brightening or lighting up of the eyes. Smiling.-In general, the movements associated with a smile are lifting or
MARTONE
AND
EDWARDS
A.
B.
--
I. CALMNESS
i. SADNESS
Fig. 7.-A patient expressing (A) the three interpreting de Superville’s three fundamental Masher, H. D.: Laryngoscope 61:1-38, 1951.)
emotions expressions
3. GAIETY INCONSTANCY reflected in Masher’s of the face (B).
line drawings (Modified from
raising motions, producing upward lines which are characteristic of mirth (Fig. 7). A smile may be slight and of momentary duration, or it may be intense and prolonged. It may be unaccompanied by sound or it may be a facial overtone during speech, resulting in the modifications of certain speech sounds.
;$$;;2
PHENOMENON
OF FUNCTION
IN
COMPLETE
DENTURE
PROSTHODONTICS
17
In smiling, there is contraction of the zygomaticus (antagonist : orbicularis oris) which draws the modiolus and the angle of the mouth upward and backward. The quadratus labii superioris (antagonist : orbicularis oris) elevates the maxillary lip, the corner of the mouth, and the ala nasi. The risorius (antagonist : orbicularis oris) works in synergistic action with the buccinator and draws the angle of the mouth backward, producing a grinning expression which may not be of pleasant quality. Its action is associated with smiling and speech but not laughter (Fig. 8). As these muscles contract in varying degrees, they exert a pull on the angles of the mouth, resulting in the relaxing of corresponding continuous fibers of the orbicularis oris muscle, the sphincter control of the mouth. The degree of opening varies with the individual or the extent of the expression of the emotion. The fibers of the orbicularis oris muscle occupy the entire width of the lips. They extend from the angles of the mouth and run medially across the midline to insert into connective tissue and skin in the ridge area of the philtrum and the septum of the nose. The bulk of the orbicularis oris muscle is made up of a continuation of the fibers of the buccinator muscle as well as all of the muscles that insert into the lips. The arrangement and movement of the fibers are similar in both lips. The convergence of the fibers at the angle of the mouth is known as the modiolus. A smile may terminate with the face returning to a state of repose and the modiolus assuming its neutral position. This termination is brought about by the simultaneous contraction of the orbicularis oris muscle and the relaxing of all the muscles contributing to the smiling expression. When the face is in repose, the mandible is in a physiologic rest position (see Figs. 14, 15, and 16). As smiling occurs, the elevating actions of the smiling musculature, working synergistically with the three closing muscles of mastication, raise the mandible and retrude it slightly toward the vertical dimension of occlusion, diminishing the interocclusal distance. Laughing.-The smile, instead of terminating, may progress to accelerated proportions in the activity of laughter, which is characterized by the opening of the mouth and separation of the teeth. Laughter may occur instantaneously and to the full extent of the limitations of the musculature involved in this facial expression, or it may develop progressively as the emotion is built up, having its origin in the smile. Lightoller,3 in describing laughter, says that the modioli are drawn cranially and laterally, and the maxillary lip forms a straight or somewhat orally convex line stretching from modiolus to modiolus. He suggests that the maxillary teeth are exposed as far laterally as the first molars and cranially as far as the gingivae, and even this may be exposed. The distance between the nasal septum and the red marking of the maxillary lip is very much decreased, and the nasolabial fold is deepened, concaved orally, and extended downward to the rima oris, its cranial portion becoming more horizontal in direction. The mandibular lip is bowed downward, with marked oral concavity, but the mandibular teeth are only slightly exposed or may not be seen. This semilunar outline of the mandibular lip results from (1) the cranial position of the modiolus, (2) the caudal position of the mandible, and (3) activities of the inferior labial tractors. The muscle activities which
18
MARTONE
AND
EDWARDS
J. Pros. Den. Jan.-Feb., 1962
Fig. S.-Dissection of the following muscles of facial expression: (00~) orbicularis oculi; (Pr) procerus; (N) nasalis; (ON) dilatoris naris; (DSN) depressor septi nasi; (QLS) quadratus labii superior-is; (ah) angular head; (ih) infraorbital head; (.%h) zygomatic head; (2) zygomaticus; (C) caninus; (00~) orbicularis oris; (R) risorius; (B) buccinator; (T) triangularis; (QLZ) quadratus labii inferioris; (kfo) the modiolus.
cause laughter are the dominance of the zygomaticus and the resistance of the triangularis in modiolar activity ; in labial activity, the superior and inferior labial tractors are dominant, and the orbicularis oris muscle is completely and involuntarily inhibited. It is this inhibition which is the basis for true laughter. Emotion of Distress .-In distress, the mouth is firmly closed (see Fig. 7). The rima oris is reduced in size to a narrow, cranially bowed slit with the corners of the mouth extending downward. This is caused by (1) the tensing of the muscles inserted into the modioli which fixes the modioli and (2) the pushing of the mandibular lip under and against the maxillary lip by the superior and inferior portions of the orbicularis oris muscle. The mandibular lip becomes broader laterally and, in so doing, produces creases that continue into the curved depression of the rima oris. This action makes it appear that the angle of the mouth has been dragged downward and laterally. Lightoller 3 believes that when the rima oris is closed, the angle of the mouth is not depressed. He further states that by stiffening the mandibular lip with the mentalis muscle, emotion can be controlled. This is an antithesis to the old adage of controlling emotion by “keeping a stiff upper lip.”
“lume l21 Number SUPERFICIAL
PHENOMENON MUSCLES
OF FUNCTION OF THE
IN
COMPLETE
DENTURE
19
PROSTHODONTICS
NECK
Acting with the superficial facial muscles in expression and related functions are the superficial muscles of the neck. These muscles can be divided into two groups: (1) the inframandibular muscles, made up of the suprahyoid and infrahyoid muscles, and (2) the posterior cervical muscles.
E
Fig. 9.-Dissection of suprahyoid muscles, frontal view: digastric, posterior belly; (5%) stylohyoid; (Mh) mylohyoid. Fig. IO.-Dissection of suprahyoid muscles, left lateral (Sh) stylohyoid; (&fh) mylohyoid.
(D,a)
view:
Fig.
9
digastric, (D,a)
anterior
digastric,
belly; anterior
(D,p)
belly;
20
MARTONE
AND
J. Pros. Den. Jan.-Feb., 1962
EDWARDS
Fig.
Fig.
tric,
11
12
Fig. 11.- Dissection of suprahyoid muscles, right lateral view: (Sh) stylohyoid; posterior belly; (D,aj digastric, anterior belly; (Mh) mylohyoid. Fig. XL-Dissection of infrahyoid muscles: (l’h) thyrohyoid; (Oh) omohyoid; (Sth)
(D,p)
digas-
sternohyoid.
;f;Te;;’
PHENOMENON
OF FUNCTION
IN COMPLETE
DENTURE
21
PROSTHODONTICS
Inframandibular Muscles.-These muscles suspend the hyoid bone, lower the mandible, and have prosthodontic significance in relation to physiologic rest position, speech, facial expression, mastication, and deglutition. The most superficial of the neck muscles, the platysma, covers the side of the neck. It originates in the deltoideus and pectoralis muscle regions and inserts into the inferior border of the mandible and the angle of the mouth (see Fig. 5). The suprahyoid muscles consist of the digastric, stylohyoid, mylohyoid, and geniohyoid muscles (Figs. 9, 10, and 11). They are depressors of the mandible and elevators of the hyoid bone. The infrahyoid muscles are the sternohyoid, sternothyroid, omohyoid, and thyrohyoid muscles (Figs. 12 and 13). They depress the hyoid bone and larynx and fix the hyoid bone so that the suprahyoid muscles may act upon the mandible. These are voluntary muscles but act reflexively and, during mandibular rest position, are in balance with the supramandibular muscles. The interocclusal distance is due to the equal tonicity of the supramandibular and the inframandibular muscles. As compared with the closing muscles of the mandible, these are very small and weak. They are used for rapid movements and those of long duration. In old age, the supramandibular muscles increase in tonicity, while the inframandibular muscles decrease.
Fig. la.-Dissection tracted: (8th) sternohyoid;
of infrahyoid muscles (Oh) omohyoid; (2%)
with the thyrohyoid;
sternohyoid and omohyoid (5%) sternothyroid.
muscles
re-
MARTONE
AND
EDWARDS
Fig. 14.-The head in muscular balance; (H) hyoid bone; (VC) vertebral column; (1) craniomandibular muscles; (2) suprahyoid muscles; (3) infrahyoid muscles; (4) posterior cervical muscles. (Diagram at lower right modified from Sarnat, B. G., editor: The Temporumandibular Joint, Springfield, Ill., 1951, Charles C Thomas, Publisher, p. 114.)
Posterior Cervical Muscles.--The head rests upon the atlas, the uppermost vertebra of the spinal column, in positional unbalance because the center of gravity of the head is not directly over the atlas, but anterior to it, in the approximate region of the temporomandibular articulation. The weight of the projecting mandible and teeth suspended from this region lies about in the center of the entire polyfunctional pyramid, thus contributing to this unbalance. The structures responsible for maintaining the head in balance are the posterior cervical muscles working antagonistically to the supramandibular and inframandibular muscles (Figs. 14, 1.5, and 16). Thus, if one falls asleep in an upright position, and the musculature controlling this balance relaxes, gravity pulls the head forward and the mandible down.
;$‘F”r’l”
PHENOMENON
OF FUNCTION
IN
COMPLETE
DENTURE
PROSTHODONTICS
23
The posterior cervical muscles function continuously approximately two-thirds of each day, maintaining the heavy head in balance. They are also responsible for the many and delicate positionings of the head associated with the activities of expression, eating, breathing, vision, hearing, gait, and posture. Factors Contributing to Postural Change.-The physiologic factors responsible for postural changes which come about through aging take place gradually by the “giving way” of the long-laboring posterior cervical muscles. In addition to loss of tonicity, other dominant forces responsible for the weakening of these muscles include (1) the excessive weight supported by these muscles because the center of gravity of the head is anterior to the atlas, (2) accentuation of the secondary curvature of the cervical vertebrae, (3) the greater leverage factor of the complex mechanism of smaller anterior muscles serving as the antagonist, including the prevertebral muscles, and (4) habitual and occupational activities. Prosthodontically, little concern is evidenced for these muscles as contrasted with the interest displayed in their antagonists. This is unfortunate. The prosthodontic implications which are a result of postural changes require a dynamic concept as opposed to a static concept in the clinical treatment of patients. MOVEMENTS
OF
THE
WANDIBLE
Varying movements of the mandible take place and in differing degrees, depending upon the specific function being performed, the habits of the individual, and his emotional state. The mandible thus assumes a role of main actor in the drama of action, and it plays a supporting role for the denture. It becomes essential, therefore, for the dentist to be fully aware not only of the basic mandibular movements and positions but also of functional patterns of movements, in which variations may occur because of emotional state or physiologic needs. These varying patterns may be noted in either extent of movement or rate of speed at which it
TABLE
I.SOMEVARIATIONSIN DURING
MANDIBULAR MOVEMENTS DIFFERENTFUNCTIONS
_-.___~ -
OCCURRING
-
VARIATION _ ~__ _---.
MASTICATORYMOVEMENTS
NONMASTICATORYMOVEMENTS
-In occlusion
Not
in occlusion
Rate and duration of movements
Slower than nonmasticatory; limited, less than 2 hours per day
More
rapid mittent,
Strength of movements
Powerful
Delicate
Variations
Limited by individual masticatory mechanism but adapted to cope with difference of texture in boluses of food
Greater number of variations than in mastication because of greater number of muscles involved and the highly individual responses to multiple emotions or speech sounds
Position
of teeth
than masticatory; 14 to 16 hours
interper day
24
Fig.
MARTONE
15.-The
mandible
lowered
AND
J. Pros. Den. Jan.-F&., 1962
EDWARDS
by contraction of the inframandibular of the supramandibular muscles.
muscles
and
relaxation
takes place. Such motion patterns are dislodging forces which must be anticipated in the design of any appliance that must absorb them and still maintain a degree of stability and retention. The vertical tug of war which takes place in the opening and closing movements of the mandible is produced by the elevators of the mandible (the temporalis, masseter, and internal pterygoid) and the depressors of the mandible (the geniohyoid, mylohyoid, and digastric) (see Figs. 14, 15, and 16). The external pterygoid muscles move the mandible forward, while the posterior fibers of the temporalis retrude the mandible. The external pterygoid and the elevators opposite them produce the side to side or lateral movements. These movements are basic and are governed neurologically by a central regulator which receives impulses through the various physiologic reflex arcs.
,v;‘;‘=&:,e,2
Fig.
l&-The
PHENOMENON
OF FUNCTION
IN COMPLETE
DENTURE
PROSTHODONTICS
head in positional unbalance because of the relaxation of the controlling ture which allows gravity to pull the head forward and downward.
25
muscula-
Mandibular movements are limited and modified by the temporomandibular articulations posteriorly and the teeth anteriorly. The mandible assumes various positions which serve as starting, limiting, and returning points in its complex physiologic activities. Those on a vertical plane are physiologic rest position and vertical dimension of occlusion. Those on a horizontal plane are centric relation (which also has a vertical component) and right and left lateral and protrusive positions. All of these mandibular movements and positions occur within the perimeter of extreme movements which the mandible is capable of performing.4 Certain variations in mandibular movements that occur during different functions are indicated in Table I. Nonmasticatory Movements.-Too often, dentists, in thinking of function, think only in terms of mastication. As a result of this, musculature considerations
26
MARTONE
AND
EDWARDS
J. Pros. Den. Jan.-Feb., 1962
Fig. 17.-Dissection showing continuity of fibers of the caninus muscle (C) and the train gularis muscle (T). This continuity indicates that the muscles can be considered as one muscle having its origin in fixed bone (maxilla) and inserting into movable bone (mandible).
are frequently limited to those of the “muscles of mastication.” The musculature responsible for mandibular movements may be divided into the supramandibular (craniomandibular muscles and muscles of facial expression) and inframandibular groups. Clinical analyses of muscular activity indicate an association between the type of stimulus and the sequence of events which follow. If the stimulus is in the form of a bolus of food entering the mouth, the muscles of mastication go into play, accompanied by various lip, cheek, and tongue activities designed to assist in the eating process. However, if the stimulus is such as to produce a speech sound or a facial expression in the mouth region, the muscles responsible for producing the sound or expressing the emotion coordinate with the muscles of mastication and serve to guide the mandible into the proper position. Attempts to analyze these various functional mandibular movements led to multiple dissections of the facial muscles of the mouth region from the standpoint of their strength, locations, and direction and extent of fibers, with a view to determining whether or not they may fulfill these functions.
;;$$y
PHENOMENON
OF FUNCTION
IN
COMPLETE
DENTURE
PROSTHODONTICS
27
Repeated dissections show such a continuity of fibers between the muscles elevating the lips and those depressing them that these muscles can well be thought of in terms of one muscle attached to fixed bone (the maxilla) and to movable bone (the mandible). An example of this unity is apparent in Fig. 17, which shows the dissection of the caninus and triangularis muscles. Other radial components, running through the modioli, reflect this same unity. It may well be that in producing the lip movements of elevation and depression, some muscles of facial expression may also serve as initiators of the delicate mandibular movements found in speech and facial expression or as a trigger musculature which sets into play the more powerful craniomandibular muscles when their action is required.
The complex interplay of bone and musculature becomes apparent in even a limited analysis of facial expression. The complexity continues in any attempt to analyze further functions of speech, respiration, mastication, and deglutition, all of which are processes occurring in the arena of the pyramid previously delineated. Analyses of functions of facial expression and speech and the study of dissections of the muscles of facial expression indicate that certain of these muscles, acting as combined units and attached to fixed and movable bone, may play a role in mandibular movements associated with the functions of speech and facial expression. These functions and their interassociations will be considered in the next article of this series. REFERENCES
1. Huber, E.: Evolution of Facial Musculature and Facial Expression, Baltimore, 1931, Johns Hopkins Press, p. 151. 2. Darwin, C. : Expression of Emotions in Man and Animals, New York, 1898, D. Appleton and Company, pp. 364-365. 3. Lightoller., G. H. S.: The Action of the M. Mentalis in the Expression of the Emotion of Distress, J. Anat. 62:319-332, 1927-28. 4. Posselt, U. : Movement Areas of the Mandible, J. PROS. DEN. 7:376, 381, 1957. 909 NORFOLK MEDICAL TOWER NORFOLK 7, VA. DEPARTMENT OF ANATOMY OHIO STATE UNIVERSITY COLUMBUS 10, OHIO