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The effect of jaw opening on the sarcomere length of the masseter and temporal muscles of the rat
THE EFFECT OF JAW OPENING ON THE SARCOMERE LENGTH OF THE MASSETER AND TEMPORAL MUSCLES OF THE RAT S. H. NORDSTROM.M. BISHOP* and R. YEMM*
Departmrnt of Oral Biology. Facult!
of Dentistry,
University
of Alberta.
Edmonton.
Canada
Summary-Sarcomere lengths of the masseter and temporal muscles of the rat were measured. b! conventional histological techniques, at varying degrees of stretch. It was found that the anterior fibres of the deep masseter muscle exhibit more lengthening of the sarcomeres as the mouth is opened, the posterior fibres and those of part of the temporal muscle being extended to a lesser extent. Since sarcomere length determines the ability of skeletal muscle to develop active tension. these results ma> indicate a variation in the relative effectiveness of the muscles as jaw position varies in normal function.
METHODS
IN TRODL’CTION
In a recent study (Nordstrom and Yemm. 1972). the sarcomcrr length of the anterior fibres of the deep belly of the rat mass&r muscle was determined. Conventional histological techniques were used, and shrinkage of the muscle during processing was controlled. The sarcomere length for eight animals ranged between 2.0 and 2.23 ;lm when the teeth were in occlusion. In two additional animals, it was shonn that the sarcomeres were greatly cutended by a wide opening of the mouth. The efcct of opening the mouth upon fibres of the masseter muscle is likely to vary from one part of the muscle to another because of the geometry of the muscle and temporomandibular joint system. The complexity of this system renders it difficult to predict the effect upon sarcomere length. It is also possible that. if different degrees of stretching occur between anterior and posterior fibres, there might be some position in which the sarcomere length is most uniform. Sarcomere length of skeletal muscle is generally held to be of importance in determining the ability of a muscle fibre to develop tension (e.g. Edman. 1966: Gordon. Huxley and Julian. 1966: Gu! ton. 1971).
The techniques employed were essentially similar to those of the earlier study (Nordstrom and Yemm. 1972). The experiments were performed on a total of I4 male Wistar rats, weighing from 300 to 380 g. Urethane anaesthesia was used at a dosage of 1500 mg/kg body weight. In each cast the animal was mounted, as in previous experiments. on a moving platform so that reference points could be visuah7ed In turn through a dissectmg mlcroscope placed vertically above. The head was placed on 115 side (right side uppermost), using ear bars to locate the posterior part of the head. Rotation about the ear bars was prevented b! means of a stainless steel pin (0,015 in. JM orthodontic Mire) inserted into a hole (0426 in dia) drilled transbcrsely through the upper incisors. and passed through a yoke. The system is depicted in Fig. I. and replaced the intraoral bar fixation of the earlier experiments: it provided improved fixation and eliminated any risk of interference with the occlusion of the teeth.
Regional variation of the sarcomerc length within a muscle. as the latter changes length in its operating range, would affect the form of its length tension curve;
at a given muscle length. some fibres might be at their optimum length, while others are over- or under-extended. A further study has therefore been performed in which results have been obtained to enable a comparison of the sarcomere length of anterior and posterior fibres of the deep belly of the masseter muscle. at varying jaw positions. A secondary objective of the study was to obtain preliminary data about changes in sarcomere length of the temporal muscle with similar variation in mouth opening. * Present address: Bristol. England.
University
of Bristol
Dental
School.
Fig. I. The method of location of the anterior part of the rat’s head to prevent rotation about the ear bars. .A stxinlcss steel pin. 0425 in. dia. was pussed through a J okc. attach4 to the mounting platform. and through ‘I ho1c drilled through the upper mcisor teeth (C’) \\irh :I 0~026 in dia IL\I\I dl 111of the type supplied for insertion of pms for rctwtinn of
152
S. H. Nordstrom,
M. Bishop
and
R. Ycmm
The samples were embedded. sectioned at 3 5 ,ttm. taking care to cut longitudmal sections of the muscle fibres. The sections were stained with Gomori’s trichrome. omitting the nuclear stain. Counts were made at x 400 magnification. An ocular micrometer scale. calibrated length 0.08 mm. was used and 25 counts of the number of sarcomeres rn this length were
for each masseter muscle sample. using at least 15secThe counting was carried out by three investigators, each performing the same number of counts on each sample. made tions.
For the temporal muscle. IO counts performed by one individual.
were
made.
all
being
RESULTS
Fig. 2. A diagram to show the surface of masseter muscles. The superficial
shown reflected. The anterior taken
from
region
sample
outline of the samples masseter muscle is of masseter
A. and the posterior sample
muscle
was from B.
The right masseter muscle was exposed and its anterior superficial belly reflected revealing the main bulk of the muscle. Two reference points on the muscle surface, in line with the fibres. were formed by passing fine wires into the exposed surface 5 8 mm apart. Losing a microelcctrode manipulator to move the platform carrying the animal, these reference points and the tips of upper and lower right incisor teeth were aligned in turn with a fine spot in the eyepiece of the stationary provided co-ordinates
microscope. of each
The manipulator position, which
The measurement of reference points after fixation, dehydration and clearing
before and of the head
showed that shortening of the masseter muscle took place during the processing stages. This was not a result of an approximation of the jaws. The average shrinkage was 6.5 per cent; the individual values varied and are presented in the tables. From the counts for each muscle sample, the mean number of sarcomeres in 0.08 mm was calculated. For the masseter muscle samples. the mean sarcomere length was obtained, allowing for the processing shrinkage for that animal. The results are presented in
scales
were recorded and from which the distance in the horizontal plane between any two reference points could be calculated. For five animals,
used
to study
sarcomere
length of the was held and incisors were recorded. In the remaining nine experiments. where the mouth was to be opened. the first stage was to record the co-ordinates for the incisor tips with the teeth in occlusion. A rigid block of the desired sire was then inserted between the incisors. and both muscle and incisor tooth reference points were recorded. Following these recordings. the animal was killed. decapitated and 111~’entire head fixed m Zenkers tixative for 18 22 hr. During this period the lower jaw was retamed etthcr with the teeth in occlusion or in the open position in which measurements had been taken. In 1 I of the 14 animals. the temporal muscle was exposed by skin reflection prior to fix-
muscle with the teeth in occlusion. the mouth closed while the co-ordinates for the wire markers
2.93 + 0.56 2.65 k O-33 3 02 * 0.75 ,XX_+OI? I 97 + 0 34 2.0x + 0.24 2.54 + O.JO 2 86 _+o-53 I 90 * 0 29 I,86 + 0.20 2-32 * 0 24 2 2? * 0 22
In.5 14
2 52 * 0.26
- -
2 73 * 0 27
I65 143 16-5 0 0
147 I42 0 0 82 9.0 13~0 14 0
ation. The head was subsequently dehydrated. and cleared in butanol. It was then replaced on the platform, and repeat measurements taken. These enabled calculation of any reduction in separation of the wire markers, and hence the extent of shrinkage. They also provided a means of detcrmining whether jaw position had been stable during the precedrng processes. Samples of the masseter and temporal muscles were taken for examination. For the masseter the approximate surface representation of the anterior and posterior samples is shown in Fig. 2. According to the anatomical description of Greene (1935), the anterior masseter sample contained fibres of the anterior deep belly of the muscle, while the posterior sample included fibres of posterior superficial and deep bellies. The sample for the temporal muscle was taken from the centre of the muscle. midway between origin and insertion.
ShrInkage at lixat1on (“,,I Y2 x.0
105 74 74 I4-4 02 61 50 61 hY 4x 25 li
Mean rarc”mrr~ length iZ SD ipll 2 57 + 0 .52
I65
2 29 + 0.37 2.31 * (1.4I 213*02x 2-06 k 0 14 2 22 + 0.26 I.91 *o-14 2IZiO37 2.05 i W53 I.98 i 0 I? ?.I8 + 0 25 2 22 + 0 2Y
143 I65 0 0 0 147 14.2 0 0 x2 Y0
Sarc~~tn~rc length of rat massetrr
and temporal
muscles
b =0,06 C” I.91 r=O 96 P 10 001
E
f
? a,
Tables 1 and 2. together with the degree of opening imposed in each cast and the value of two standard deviations for the final sarcomere lengths. Since no attempt was made to measure shrinkage of the temporal muscle, we have assumed that the processing stages affect the two muscles in a similar way and have employed the correction factor appropriate to the masseter. The results obtained from the temporal muscle samples are shown in Table 3. Inspection of the tables will show that the sarcomere length increased as mouth opening was imposed. The relationship is shown graphically for each series of results in Fig. 3. Lines have been fitted statistically to the points. assuming that the relationship is linear. The correlation coefficients are high (anterior masseter, 0.96; posterior masseter. 0.47; temporalis, 0.87). The fitted lines from the separate graphs are superimposed in Fig. 4. The anterior fibres of the masseter show a much greater increase in sarcomere length as the mouth is opened than do either the posterior fibres of that muscle or the fibres of the temporal muscle. The posterior fibrcs of the masscter muscle were found in o\ery case to exhibit somewhat greater sarcomcre lengths than the anterior fibres. when the teeth were in occulsion. The mean difference between the two results was 0.15 /Irn (range 0.09%0,25 pm). A twotailed 1 test of the two series of counts showed that this difference was highly significant (P < 0.001). The sarcomere length of the temporal muscle at the length of occlusion was close to that found for the anterior fibres of the masseter. and alua\s shorter than that obtained for the posterior masseter. However, this difference must bc interpreted with caution in view of the assumption made in correcting the temporal muscle results for shrinkage.
b=OOl c=2.08 r=O47 PC0.I
20.:
E al
25
C
I
‘“.:/ 1.5
Comparison of sarcomere lengths in diRerent jaw positions by the present method, in which a different animal is used to obtain each point on a graph, is only valid if sarcomcre length is consistent for all animals at any given .jaw position. That this is approximately true for the rat muscles tested is shown by the generally
r =0437 P < 0001
1 0
5
Mouth
IO
opening,
I5
20
mm
Fig. 3. Graphic representation of the relationship between sarcomere length and mouth opening. (A) anterior fibres of the masseter muscle; (B) posterior fibres of the masseter; (C) temporal muscle sample. In each case, the line drawn has been fitted statistically, assuming that there is a linear relationship between the variables. On each graph the following values are given for the fitted line: slope (b): intercept(c): correlationcoefficient( probability ofthe observed correlation coefficient occurring by change (P).
l50
DISCLSSIOV
b=O02 C’ l-89
5
Mouth
IO
opening,
15
mm
Fig. 4. The fitted lines of the three graphs in the preceding figure superimposed. The slope for the anterior masseter (A) is the steepest. The intercept for the posterior fibres of the masseter (B) is higher on the vertical axis than those for the anterior fibres and temporal muscle (C). The difference in intercept between anterior and posterior masseter samples is statistically significant.
S. H. Nordstrom, M Bishop and R. Yemm
154
close clustering of the po nts on the graphs of Fig. 3. For instance. the range elf sarcomere lenth demonstrated in the anterior masseter fibres with the teeth in occlusion was, for five animals, l+S2,08 pm. In addition, these figures are similar to the values obtained in our earlier study (Nordstrom and Yemm. 1972) in which the range was 2.Ob2.23 pm. The error between the two studies may be in part due to the use of slightly lighter rats in the more recent experiments. It has been reported (Goldspink, 1968) that sarcomere length increases as growth takes place. Errors in the slopes of the fitted lines (Fig. 4) are unlikely to arise from systematic variations in the degree of shrinkage of the muscle with the amount of stretch imposed by the mouth open positions. since such shrinkage was measured and allowance made. In any case, inspection of the percentage reductions in length during processing (Table 1) will show that shrinkage appears to be independent of jaw position. The line for the temporal muscle may be misplaced if there were consistently greater or lesser degrees of shrinkage. but the slope would be unaffected in this case. As indicated by the fitted lines of Fig. 4. the posterior fibres of the masseter were stretched less by a given open position than the anterior fibres. It is reasonable to suppose that comparison of the extreme anterior and posterior fibres would reveal an even more marked difference in extension than seen in these experiments where such fibres were excluded. Since the length of skeletal muscle fibres has a profound effect upon their ability to develop active tension (Edman. 1966; Gordon rt LL/.,1966; Rack and Westbury, 1969). the demonstrated differential extension of the fibres of the rat masseter probably indicates a change in the relative efficiency of the fibres as jaw position changes. The sarcomeres
of the part of the temporal muscle sampled also appeared to be extended to a lesser extent than those of the anterior fibres of the masseter. Our sample included a large proportion of the tibres of the muscle as it narrows towards its insertion on the coronoid process, and is probably representative of the muscle as a whole. It may be. therefore. that a difference exists in the optimum working length for the two muscles, or at least a difference in the rate at which efficiency changes with jaw position.
As far as can be ascertained, the only study in which the relationship between sarcomere length and tension development has been studied for mammalian skeletal muscle is that of Rack and Westbury (1969) for the soleus muscle of the cat. It was found that peak active tension is generated at a sarcomere length of 2% pm. In the absence of specific evidence of the contractile properties of rat muscle fibres of the appropriate type (cat soleus is a slow twitch muscle), immediate assessment of the present findings was not possible. We have therefore conducted further experiments in which the masseter muscle was stimulated and the tension recorded. A report of these experiments is in course of preparation. In it we shall show that peak tension is developed by the masseter at a jaw position significantly open from that of occlusion. ilcknowlrdgenlrnts-Thanks are due to Mr. D. Carmel for assistance with the histological techniques. We also wish to thank Mr. G. James and Mr. K. Robbins. both of the Unl\rrsit) of Bristol. for preparing the Illustrations. The study was supported bq the Medical Research Council of Canada.
REFERENCES Edman K. A. P. 1966. The relation between sarcomere length and active tension in isolated semitendinosus fibres of the froe. J. Phvsiof.. Land. 183. 407-117. Goldspink E. 1966. Sarcomere length during post-natal growth of mammalian muscle fibres. J. Cell Sci. 3, S39548. Gordon A. M., Huxley A. F. and Julian F. J. 1966. The variation in isometric tension with sarcomere length in vertebrate muscle fibres. J. Physiol., Lond. 184, 170-192. Greene E. C. 1935. Anatomy of the rat. Trans. Am. Phil. Sot. 21,33-34. Guyt0nA.C. 1971. &sic HL[I~U!I Plt~.sioloy~:!\‘~~~~t~f FK!I~trorr tri~tl ,&~~,c/~tr,~isrr~\ of Di.xw.w. Chap. 3. pp. 75-76. Saunders, Philadelphia. Nordstrom S. H. and Yemm R. 1972. Sarcomere length in the masseter muscle of the rat. Archs oral Biol. 19, 895902. Rack P. M. H. and Westbury D. R. 1969. The effects of length and stimulus rate on tension in the isometric cat soleus muscle. J. Physiol.. Lortd. 204,443-460.
R&urn& Lcs longueul-s de5 sarcom&e> des muscles temporaux et massetirins du rat ont 6tk mesurks par des techniques histologiques con\~ntionclles. ;t divers $tadcs d’etiremcnt. Lcs iihres antirieures du muscle massL:rcr profond prescntent un allong~mcnt dcs aarcom&s plus important lorsque la bouche est ou\erte. lea fibrcs postCrieures ct ccllc\ d‘unc pal-tic du muscle temporal L;tant moins dtirtes. Etant donni quc la longucur du snrcomkrc dCterminc la proprit;tl; du muscle pour dtivelopper une tension active. ces vhultats semblent lndiquer une variation dans I’efficacitt: relative des muscles, lorsque la position des maxillaires varle pendant le fonctionnement normal.
Mit H~lfc kcw\i’ntloncllrllli\tolo~l,cllclMetllNh2ll \I urde Lltltc’l- ~~l-s~llKdcncn Streckungsgradcn die Lange dcr Snrkomcrc dcr M. mahhcter und tcmporalis he1 det- Rattc gemcssen. Die anterioren Fasern dcs tiefcn .Anteils dcs M. ma\seter aeisrn bei der Mund6ffnung clne gr6i3ere Drhnung der Sarkomere auf. wiihrmd die posterioren Fasern und diejenige eines Teiles des M. temporalis wsniger stark gcdehnt waren. Da die Lange der Sarkomere fiir die Befihigung eines Skelettmuskels Lur aktiven Anspannung van Bedeutung ist. bcdeuten diese Ergebnisse. dal3 sich die relative Wirksamkeit dcr Muskeln ahhiingig \on der untrr~chledlichen Kicfcrlage bei normalcr Funktion unterschiedlich \erhdlt. Zusammenfassung
Departmrnt of Oral Biology. Facult!
of Dentistry,
University
of Alberta.
Edmonton.
Canada
Summary-Sarcomere lengths of the masseter and temporal muscles of the rat were measured. b! conventional histological techniques, at varying degrees of stretch. It was found that the anterior fibres of the deep masseter muscle exhibit more lengthening of the sarcomeres as the mouth is opened, the posterior fibres and those of part of the temporal muscle being extended to a lesser extent. Since sarcomere length determines the ability of skeletal muscle to develop active tension. these results ma> indicate a variation in the relative effectiveness of the muscles as jaw position varies in normal function.
METHODS
IN TRODL’CTION
In a recent study (Nordstrom and Yemm. 1972). the sarcomcrr length of the anterior fibres of the deep belly of the rat mass&r muscle was determined. Conventional histological techniques were used, and shrinkage of the muscle during processing was controlled. The sarcomere length for eight animals ranged between 2.0 and 2.23 ;lm when the teeth were in occlusion. In two additional animals, it was shonn that the sarcomeres were greatly cutended by a wide opening of the mouth. The efcct of opening the mouth upon fibres of the masseter muscle is likely to vary from one part of the muscle to another because of the geometry of the muscle and temporomandibular joint system. The complexity of this system renders it difficult to predict the effect upon sarcomere length. It is also possible that. if different degrees of stretching occur between anterior and posterior fibres, there might be some position in which the sarcomere length is most uniform. Sarcomere length of skeletal muscle is generally held to be of importance in determining the ability of a muscle fibre to develop tension (e.g. Edman. 1966: Gordon. Huxley and Julian. 1966: Gu! ton. 1971).
The techniques employed were essentially similar to those of the earlier study (Nordstrom and Yemm. 1972). The experiments were performed on a total of I4 male Wistar rats, weighing from 300 to 380 g. Urethane anaesthesia was used at a dosage of 1500 mg/kg body weight. In each cast the animal was mounted, as in previous experiments. on a moving platform so that reference points could be visuah7ed In turn through a dissectmg mlcroscope placed vertically above. The head was placed on 115 side (right side uppermost), using ear bars to locate the posterior part of the head. Rotation about the ear bars was prevented b! means of a stainless steel pin (0,015 in. JM orthodontic Mire) inserted into a hole (0426 in dia) drilled transbcrsely through the upper incisors. and passed through a yoke. The system is depicted in Fig. I. and replaced the intraoral bar fixation of the earlier experiments: it provided improved fixation and eliminated any risk of interference with the occlusion of the teeth.
Regional variation of the sarcomerc length within a muscle. as the latter changes length in its operating range, would affect the form of its length tension curve;
at a given muscle length. some fibres might be at their optimum length, while others are over- or under-extended. A further study has therefore been performed in which results have been obtained to enable a comparison of the sarcomere length of anterior and posterior fibres of the deep belly of the masseter muscle. at varying jaw positions. A secondary objective of the study was to obtain preliminary data about changes in sarcomere length of the temporal muscle with similar variation in mouth opening. * Present address: Bristol. England.
University
of Bristol
Dental
School.
Fig. I. The method of location of the anterior part of the rat’s head to prevent rotation about the ear bars. .A stxinlcss steel pin. 0425 in. dia. was pussed through a J okc. attach4 to the mounting platform. and through ‘I ho1c drilled through the upper mcisor teeth (C’) \\irh :I 0~026 in dia IL\I\I dl 111of the type supplied for insertion of pms for rctwtinn of
152
S. H. Nordstrom,
M. Bishop
and
R. Ycmm
The samples were embedded. sectioned at 3 5 ,ttm. taking care to cut longitudmal sections of the muscle fibres. The sections were stained with Gomori’s trichrome. omitting the nuclear stain. Counts were made at x 400 magnification. An ocular micrometer scale. calibrated length 0.08 mm. was used and 25 counts of the number of sarcomeres rn this length were
for each masseter muscle sample. using at least 15secThe counting was carried out by three investigators, each performing the same number of counts on each sample. made tions.
For the temporal muscle. IO counts performed by one individual.
were
made.
all
being
RESULTS
Fig. 2. A diagram to show the surface of masseter muscles. The superficial
shown reflected. The anterior taken
from
region
sample
outline of the samples masseter muscle is of masseter
A. and the posterior sample
muscle
was from B.
The right masseter muscle was exposed and its anterior superficial belly reflected revealing the main bulk of the muscle. Two reference points on the muscle surface, in line with the fibres. were formed by passing fine wires into the exposed surface 5 8 mm apart. Losing a microelcctrode manipulator to move the platform carrying the animal, these reference points and the tips of upper and lower right incisor teeth were aligned in turn with a fine spot in the eyepiece of the stationary provided co-ordinates
microscope. of each
The manipulator position, which
The measurement of reference points after fixation, dehydration and clearing
before and of the head
showed that shortening of the masseter muscle took place during the processing stages. This was not a result of an approximation of the jaws. The average shrinkage was 6.5 per cent; the individual values varied and are presented in the tables. From the counts for each muscle sample, the mean number of sarcomeres in 0.08 mm was calculated. For the masseter muscle samples. the mean sarcomere length was obtained, allowing for the processing shrinkage for that animal. The results are presented in
scales
were recorded and from which the distance in the horizontal plane between any two reference points could be calculated. For five animals,
used
to study
sarcomere
length of the was held and incisors were recorded. In the remaining nine experiments. where the mouth was to be opened. the first stage was to record the co-ordinates for the incisor tips with the teeth in occlusion. A rigid block of the desired sire was then inserted between the incisors. and both muscle and incisor tooth reference points were recorded. Following these recordings. the animal was killed. decapitated and 111~’entire head fixed m Zenkers tixative for 18 22 hr. During this period the lower jaw was retamed etthcr with the teeth in occlusion or in the open position in which measurements had been taken. In 1 I of the 14 animals. the temporal muscle was exposed by skin reflection prior to fix-
muscle with the teeth in occlusion. the mouth closed while the co-ordinates for the wire markers
2.93 + 0.56 2.65 k O-33 3 02 * 0.75 ,XX_+OI? I 97 + 0 34 2.0x + 0.24 2.54 + O.JO 2 86 _+o-53 I 90 * 0 29 I,86 + 0.20 2-32 * 0 24 2 2? * 0 22
In.5 14
2 52 * 0.26
- -
2 73 * 0 27
I65 143 16-5 0 0
147 I42 0 0 82 9.0 13~0 14 0
ation. The head was subsequently dehydrated. and cleared in butanol. It was then replaced on the platform, and repeat measurements taken. These enabled calculation of any reduction in separation of the wire markers, and hence the extent of shrinkage. They also provided a means of detcrmining whether jaw position had been stable during the precedrng processes. Samples of the masseter and temporal muscles were taken for examination. For the masseter the approximate surface representation of the anterior and posterior samples is shown in Fig. 2. According to the anatomical description of Greene (1935), the anterior masseter sample contained fibres of the anterior deep belly of the muscle, while the posterior sample included fibres of posterior superficial and deep bellies. The sample for the temporal muscle was taken from the centre of the muscle. midway between origin and insertion.
ShrInkage at lixat1on (“,,I Y2 x.0
105 74 74 I4-4 02 61 50 61 hY 4x 25 li
Mean rarc”mrr~ length iZ SD ipll 2 57 + 0 .52
I65
2 29 + 0.37 2.31 * (1.4I 213*02x 2-06 k 0 14 2 22 + 0.26 I.91 *o-14 2IZiO37 2.05 i W53 I.98 i 0 I? ?.I8 + 0 25 2 22 + 0 2Y
143 I65 0 0 0 147 14.2 0 0 x2 Y0
Sarc~~tn~rc length of rat massetrr
and temporal
muscles
b =0,06 C” I.91 r=O 96 P 10 001
E
f
? a,
Tables 1 and 2. together with the degree of opening imposed in each cast and the value of two standard deviations for the final sarcomere lengths. Since no attempt was made to measure shrinkage of the temporal muscle, we have assumed that the processing stages affect the two muscles in a similar way and have employed the correction factor appropriate to the masseter. The results obtained from the temporal muscle samples are shown in Table 3. Inspection of the tables will show that the sarcomere length increased as mouth opening was imposed. The relationship is shown graphically for each series of results in Fig. 3. Lines have been fitted statistically to the points. assuming that the relationship is linear. The correlation coefficients are high (anterior masseter, 0.96; posterior masseter. 0.47; temporalis, 0.87). The fitted lines from the separate graphs are superimposed in Fig. 4. The anterior fibres of the masseter show a much greater increase in sarcomere length as the mouth is opened than do either the posterior fibres of that muscle or the fibres of the temporal muscle. The posterior fibrcs of the masscter muscle were found in o\ery case to exhibit somewhat greater sarcomcre lengths than the anterior fibres. when the teeth were in occulsion. The mean difference between the two results was 0.15 /Irn (range 0.09%0,25 pm). A twotailed 1 test of the two series of counts showed that this difference was highly significant (P < 0.001). The sarcomere length of the temporal muscle at the length of occlusion was close to that found for the anterior fibres of the masseter. and alua\s shorter than that obtained for the posterior masseter. However, this difference must bc interpreted with caution in view of the assumption made in correcting the temporal muscle results for shrinkage.
b=OOl c=2.08 r=O47 PC0.I
20.:
E al
25
C
I
‘“.:/ 1.5
Comparison of sarcomere lengths in diRerent jaw positions by the present method, in which a different animal is used to obtain each point on a graph, is only valid if sarcomcre length is consistent for all animals at any given .jaw position. That this is approximately true for the rat muscles tested is shown by the generally
r =0437 P < 0001
1 0
5
Mouth
IO
opening,
I5
20
mm
Fig. 3. Graphic representation of the relationship between sarcomere length and mouth opening. (A) anterior fibres of the masseter muscle; (B) posterior fibres of the masseter; (C) temporal muscle sample. In each case, the line drawn has been fitted statistically, assuming that there is a linear relationship between the variables. On each graph the following values are given for the fitted line: slope (b): intercept(c): correlationcoefficient( probability ofthe observed correlation coefficient occurring by change (P).
l50
DISCLSSIOV
b=O02 C’ l-89
5
Mouth
IO
opening,
15
mm
Fig. 4. The fitted lines of the three graphs in the preceding figure superimposed. The slope for the anterior masseter (A) is the steepest. The intercept for the posterior fibres of the masseter (B) is higher on the vertical axis than those for the anterior fibres and temporal muscle (C). The difference in intercept between anterior and posterior masseter samples is statistically significant.
S. H. Nordstrom, M Bishop and R. Yemm
154
close clustering of the po nts on the graphs of Fig. 3. For instance. the range elf sarcomere lenth demonstrated in the anterior masseter fibres with the teeth in occlusion was, for five animals, l+S2,08 pm. In addition, these figures are similar to the values obtained in our earlier study (Nordstrom and Yemm. 1972) in which the range was 2.Ob2.23 pm. The error between the two studies may be in part due to the use of slightly lighter rats in the more recent experiments. It has been reported (Goldspink, 1968) that sarcomere length increases as growth takes place. Errors in the slopes of the fitted lines (Fig. 4) are unlikely to arise from systematic variations in the degree of shrinkage of the muscle with the amount of stretch imposed by the mouth open positions. since such shrinkage was measured and allowance made. In any case, inspection of the percentage reductions in length during processing (Table 1) will show that shrinkage appears to be independent of jaw position. The line for the temporal muscle may be misplaced if there were consistently greater or lesser degrees of shrinkage. but the slope would be unaffected in this case. As indicated by the fitted lines of Fig. 4. the posterior fibres of the masseter were stretched less by a given open position than the anterior fibres. It is reasonable to suppose that comparison of the extreme anterior and posterior fibres would reveal an even more marked difference in extension than seen in these experiments where such fibres were excluded. Since the length of skeletal muscle fibres has a profound effect upon their ability to develop active tension (Edman. 1966; Gordon rt LL/.,1966; Rack and Westbury, 1969). the demonstrated differential extension of the fibres of the rat masseter probably indicates a change in the relative efficiency of the fibres as jaw position changes. The sarcomeres
of the part of the temporal muscle sampled also appeared to be extended to a lesser extent than those of the anterior fibres of the masseter. Our sample included a large proportion of the tibres of the muscle as it narrows towards its insertion on the coronoid process, and is probably representative of the muscle as a whole. It may be. therefore. that a difference exists in the optimum working length for the two muscles, or at least a difference in the rate at which efficiency changes with jaw position.
As far as can be ascertained, the only study in which the relationship between sarcomere length and tension development has been studied for mammalian skeletal muscle is that of Rack and Westbury (1969) for the soleus muscle of the cat. It was found that peak active tension is generated at a sarcomere length of 2% pm. In the absence of specific evidence of the contractile properties of rat muscle fibres of the appropriate type (cat soleus is a slow twitch muscle), immediate assessment of the present findings was not possible. We have therefore conducted further experiments in which the masseter muscle was stimulated and the tension recorded. A report of these experiments is in course of preparation. In it we shall show that peak tension is developed by the masseter at a jaw position significantly open from that of occlusion. ilcknowlrdgenlrnts-Thanks are due to Mr. D. Carmel for assistance with the histological techniques. We also wish to thank Mr. G. James and Mr. K. Robbins. both of the Unl\rrsit) of Bristol. for preparing the Illustrations. The study was supported bq the Medical Research Council of Canada.
REFERENCES Edman K. A. P. 1966. The relation between sarcomere length and active tension in isolated semitendinosus fibres of the froe. J. Phvsiof.. Land. 183. 407-117. Goldspink E. 1966. Sarcomere length during post-natal growth of mammalian muscle fibres. J. Cell Sci. 3, S39548. Gordon A. M., Huxley A. F. and Julian F. J. 1966. The variation in isometric tension with sarcomere length in vertebrate muscle fibres. J. Physiol., Lond. 184, 170-192. Greene E. C. 1935. Anatomy of the rat. Trans. Am. Phil. Sot. 21,33-34. Guyt0nA.C. 1971. &sic HL[I~U!I Plt~.sioloy~:!\‘~~~~t~f FK!I~trorr tri~tl ,&~~,c/~tr,~isrr~\ of Di.xw.w. Chap. 3. pp. 75-76. Saunders, Philadelphia. Nordstrom S. H. and Yemm R. 1972. Sarcomere length in the masseter muscle of the rat. Archs oral Biol. 19, 895902. Rack P. M. H. and Westbury D. R. 1969. The effects of length and stimulus rate on tension in the isometric cat soleus muscle. J. Physiol.. Lortd. 204,443-460.
R&urn& Lcs longueul-s de5 sarcom&e> des muscles temporaux et massetirins du rat ont 6tk mesurks par des techniques histologiques con\~ntionclles. ;t divers $tadcs d’etiremcnt. Lcs iihres antirieures du muscle massL:rcr profond prescntent un allong~mcnt dcs aarcom&s plus important lorsque la bouche est ou\erte. lea fibrcs postCrieures ct ccllc\ d‘unc pal-tic du muscle temporal L;tant moins dtirtes. Etant donni quc la longucur du snrcomkrc dCterminc la proprit;tl; du muscle pour dtivelopper une tension active. ces vhultats semblent lndiquer une variation dans I’efficacitt: relative des muscles, lorsque la position des maxillaires varle pendant le fonctionnement normal.
Mit H~lfc kcw\i’ntloncllrllli\tolo~l,cllclMetllNh2ll \I urde Lltltc’l- ~~l-s~llKdcncn Streckungsgradcn die Lange dcr Snrkomcrc dcr M. mahhcter und tcmporalis he1 det- Rattc gemcssen. Die anterioren Fasern dcs tiefcn .Anteils dcs M. ma\seter aeisrn bei der Mund6ffnung clne gr6i3ere Drhnung der Sarkomere auf. wiihrmd die posterioren Fasern und diejenige eines Teiles des M. temporalis wsniger stark gcdehnt waren. Da die Lange der Sarkomere fiir die Befihigung eines Skelettmuskels Lur aktiven Anspannung van Bedeutung ist. bcdeuten diese Ergebnisse. dal3 sich die relative Wirksamkeit dcr Muskeln ahhiingig \on der untrr~chledlichen Kicfcrlage bei normalcr Funktion unterschiedlich \erhdlt. Zusammenfassung