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Anatomy of the olfactory system
Handbook of Clinical Neurology, Vol. 164 (3rd series) Smell and Taste R.L. Doty, Editor https://doi.org/10.1016/B978-0-444-63855-7.00002-2 Copyright © 2019 Elsevier B.V. All rights reserved
Chapter 2
Anatomy of the olfactory system TIMOTHY D. SMITH1* AND KUNWAR P. BHATNAGAR2 School of Physical Therapy, Slippery Rock University, Slippery Rock, PA, United States
1 2
Anatomical Sciences and Neurobiology, University of Louisville School of Medicine, Louisville, KY, United States
Abstract Of the principal sensory systems (vision, olfaction, taste, hearing, and balance), olfaction is one of the oldest. This ubiquitous system has both peripheral and central subdivisions. The peripheral subdivision is comprised of the olfactory epithelium and nerve fascicles, whereas the central subdivision is made up of the olfactory bulb and its central connections. Humans lack the “accessory olfactory system” of many other mammals, exhibiting only a nonfunctioning vestige of its peripheral element, the vomeronasal organ. Compared to most mammals, major elements of the human olfactory system are reduced; for example, humans have fewer turbinates than many mammals, and their olfactory epithelia are found only on one or two of these structures and their adjacent surfaces. Nonetheless, humans retain a full complement of functional cellular elements including a regenerating population of olfactory sensory neurons. These neurons extend long ciliary processes into the mucus that form a mat of cilia on which the odorant receptors are located. The olfactory sensory neurons send their axons directly to synapse within the olfactory bulb. Mitral and tufted cells then relay impulses from the bulb to other brain regions. This chapter describes the general anatomy and microanatomy of the olfactory system.
INTRODUCTION The human sense of smell is sometimes celebrated (Piesse, 1857) and, at other times, denigrated as a reduced, less useful version of the powerful tracking sense that some of our fellow mammals enjoy (Turner, 1891). To some degree this is due to the seemingly less elaborate anatomy of the human olfactory system. However, appearances deceive. While the gross anatomy relating to olfaction differs in humans from that of many other mammals, the human olfactory system is essentially equivalent at the microanatomical level. In this chapter, we review the major elements of the human olfactory pathway, including the olfactory epithelium, the bundles of olfactory receptor axons that project from the epithelium into the olfactory bulb, the olfactory bulb, and the connections to central brain structures that collectively make up the central olfactory structures, commonly referred to as the olfactory cortex. The latter structures and their connections are discussed in detail in other chapters of the volume.
STRUCTURES OF THE PERIPHERAL OLFACTORY SYSTEM Osseous support for the nasal cavity and associated soft tissues In skeletal anatomy, the human olfactory apparatus begins with structures constituting the boundaries of the nasal cavity (Fig. 2.1; Clerico et al., 2003; Márquez et al., 2015). The piriform aperture is the opening into the nasal cavity (Fig. 2.2). Within this cavity, two bony elements form a midline partition that divides it into right and left nasal fossae. This osseous nasal septum is composed of the perpendicular plate of the ethmoid and the vomer bone; these articulate with septal cartilage anteriorly. The lateral wall is the most complex boundary of the nasal fossa, being formed by parts of the maxillary, ethmoid, lacrimal, and palatine bones. This wall takes on added complexity in that it communicates with the paranasal cavities. The cribriform plate of the anterior cranial base is the roof of each nasal fossa, while the palate forms the floor (Fig. 2.2).
*Correspondence to: Timothy D. Smith, School of Physical Therapy, Slippery Rock University, Slippery Rock, PA, United States. Tel: +1-724-738-2885, Fax: +1-724-738-2113, E-mail: [email protected]
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T.D. SMITH AND K.P. BHATNAGAR Figs. 2.1 and 2.2). The middle and superior nasal turbinates are part of the lateral mass of the ethmoid bone. A fourth fold (the “supreme” nasal concha) is also present in some individuals (Schaeffer, 1920). The turbinates serve not only to filter, warm, and moisten inspired air but also provide the scaffold for the olfactory epithelium and airflow patterns that send odor-laden air to its receptor surface. In addition, they divide the nasal airways into three compartments, called meatuses. Each meatus is located lateral to its respective turbinate (inferior, middle, superior; Fig. 2.1).
Microanatomy of the nasal mucosa
Fig. 2.1. CT scan slice showing the human nasal cavity in a coronal plane. This is a relatively posterior level of the nasal cavity in which all three turbinates are visible. Olfactory epithelium is often restricted to the lateral surface of the superior nasal turbinate (SC), the adjacent level of the nasal septum, and the roof of the nasal cavity in the same region. IC, inferior nasal turbinate; IM, inferior meatus; MC, middle nasal turbinate; MS, maxillary sinus; NS, nasal septum; OF, olfactory fossa; Or, orbit. Image courtesy of Dr. Samuel Ma´rquez.
Soft tissue structures transform the osseous nasal cavity into bilateral spaces that lead to the pharynx posteriorly. These nasal passageways are the first (proximal) part of the upper respiratory tract. The piriform aperture is elaborated by cartilaginous support structures that house the downwardly oriented anterior nares. The anterior nares open to a dilated vestibule on each side. Separating the right and left vestibules is the columella, the fleshy anteriormost extension of the nasal septum (Fig. 2.2). The majority of the nasal septum, a mucosalined partition supported internally by bone and cartilage, is found within the main nasal chamber. The nasal passages end at the choanae (or posterior nares), paired apertures that transmit air from the nasal fossae into the nasopharynx (Fig. 2.2). The nasal choanae are divided by the posterior end of the nasal septum, which is supported internally by the vomer bone. The vestibule may be thought of as a simple outer chamber that communicates posteriorly into the nasal cavity proper; this occurs approximately at an internal ridge called the limen nasi, which corresponds roughly to the bony margin of the piriform aperture. In the nasal cavity proper, the air passages become more complex. The nasal turbinates (conchae, turbinals) are projections from the lateral walls that extend into the nasal passages. The inferior nasal turbinate articulates with the maxillary bone (a basis for its alternate name—maxilloturbinal;
In other mammals, nasal skeletal structure may be used to infer some physiologic specializations. For example, the maxilloturbinal (¼ inferior nasal turbinate) is highly branched or scrolled in various mammals, reaching great complexity in mammals with special homeothermic demands (e.g., aquatic species; Van Valkenburgh et al., 2011). The maxilloturbinal is often used as a quantifiable proxy for respiratory adaptations, although recent research has shown that other turbinals may have “mixed functions” for respiratory air-conditioning as well as olfaction. Thus, the actual functional anatomy is mostly dictated by the distribution of specific types of nasal mucosa. The nasal fossa is lined with four types of epithelia: stratified, respiratory, transitional, and olfactory (Harkema et al., 1987, 2006; Fig. 2.3). Stratified epithelium typically lines regions of the nasal fossa in which fluid transport is passive, such as the floor of the nasal cavity. Respiratory epithelium in the nasal fossa has numerous motile cilia, which propel mucus toward the pharynx. At least four other cell types are present within this epithelium, such as nonciliated, goblet (mucus-producing), secretory (small granule), and basal cells (Harkema et al., 1987). Transitional epithelium is a nonstratified type that occurs as a narrow strip between stratified squamous and respiratory epithelium (Harkema et al., 1987, 2006). Olfactory epithelium possesses nonmotile cilia involved in chemoreception, and is typically distributed dorsally in monkeys, the closest relatives of humans that have been quantitatively studied (Harkema et al., 1987; Smith et al., 2014a), as well as humans (see Clerico et al., 2003). The majority of the human nasal mucosa is nonolfactory. Regionally, the vestibule is simply lined with facial skin, including large hairs (vibrissae). After passing the limen nasi, nonkeratinized stratified squamous epithelium is found, followed by a narrow strip of transitional epithelium. Respiratory mucosa covers most of the remaining nasal fossa, although stratified squamous epithelium lines the floor. Olfactory mucosa covers the “roof” of the nasal cavity along surfaces that are ventral to the cribriform
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Fig. 2.2. Osseous and soft tissue structures that delimit the nasal cavity. (A) The anterior nares from below, also illustrating the nasal valve (Va) and columnella (C). (B) After passing through the anterior nares, air enters the vestibule (Ve) and then into the main chamber, where three pairs of turbinates are shown on lateral nasal wall. The superior turbinate (SC) consistently has olfactory mucosa. Air exits the main nasal chamber through the choana (arrow is passing through the choana on the right side). (C) A posterior view of the paired choanae (Ch). Note that turbinates can be seen inside the main nasal chamber. (D and E) Osseous boundaries of the nasal cavity are shown for lateral (D) and medial (E) walls. The maxilla (M), lacrimal, palatine (P), inferior nasal turbinates (IC), and ethmoid bone form the lateral wall. The superior and middle nasal turbinates (SC, MC) are projections of the ethmoid bone. The perpendicular plate of the ethmoid bone (PP) helps to form the medial boundary of each nasal cavity. It articulates inferiorly with the vomer bone (V) and anteriorly with the septal cartilage (SC). The location of the vestigial vomeronasal organ (VNO) is indicated. FS, frontal sinus; OB, olfactory bulb; OA, opening of auditory tube; OC, oral cavity; U, uvula. Modified after Smith, T.D., Eiting, T.P., Bhatnagar, K.P., 2015. Anatomy of the nasal passages in mammals. In: Doty, R.L. (Ed.), Handbook of olfaction and gustation, third ed. John Wiley and Sons, Inc., Hoboken, NJ, 37–62.
plate. It also generally covers the superior nasal turbinate and may extend over the middle nasal turbinate (Fig. 2.3). The septal mucosa adjacent to the superior turbinate also bears olfactory mucosa (Read, 1908; Leopold et al., 2000; see Clerico et al., 2003, regarding variations). A portion of the middle nasal turbinate may bear olfactory mucosa, though this varies (Fig. 2.3). The olfactory mucosa, comprising the surface neurosensory epithelium and supporting the lamina propria, has distinguishing characteristics throughout its depth. It is layered in the following order beginning from the nasal cavity: mucus layer surrounding the free processes (cilia) of olfactory sensory neurons (OSNs), microvillus of the supporting cells, openings of the mucus gland ducts, cell bodies within the olfactory epithelium, and the basal lamina. The thick lamina propria comprises connective tissue with glandular elements, olfactory nerve fascicles, blood and lymphatic vessels, and autonomic and terminal nerve elements. The olfactory epithelium is thicker than the adjacent respiratory epithelium by several orders of magnitude (Fig. 2.4)
and has more numerous rows of cells bodies (see following text). The lamina propria is also distinctive from that of the respiratory mucosa. Olfactory mucosa has distinctive glands and more numerous large bundles of axons. Bowman’s glands have a branched tubuloalveolar morphology and manufacture mucus secretions that are transmitted to the surface via a single duct (Fig. 2.4). The secretions protect the epithelium and its apical processes. It is further supposed that the secretions facilitate odorant access to receptor sites on olfactory cilia (Farbman, 1992). Nervous elements include collections of unmyelinated axons emanating from the base of the olfactory neuroepithelium. Together the characteristics of the epithelium and lamina propria make the olfactory mucosa distinctive by light microscopy (Fig. 2.4). The olfactory epithelium (Fig. 2.5) is composed of the bipolar OSNs, sustentacular (supporting) cells, microvillar cells, and basal cells, with olfactory gland ducts, autonomic nerves and blood vessels. The OSNs are large, oftentimes extending through the entire height of the epithelium (Figs. 2.4D and 2.5). The olfactory
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Fig. 2.3. Lateral nasal wall of adult human with olfactory (blue) and nonolfactory mucosa indicated. The location is based on a biopsy survey and literature review provided by Clerico et al. (2003). On the lateral nasal wall, part of the superior nasal turbinate (SC) and the superior extent of the middle nasal turbinate (MC) commonly bear olfactory mucosa. The extent to which it extends over the remainder of the superior turbinate varies (light blue). There is also olfactory mucosa on the roof of the nasal cavity, which is opposite the cribiform plate (CP), upon which the olfactory bulb (OB) rests, and finally the nasal septum (not shown) bears olfactory mucosa, approximately opposite to that shown here for the lateral wall. Na, anterior nares; Sph, sphenoid bone.
neuroepithelium is 60–80 mm thick (Morrison and Constanzo, 1992) and covers about 5 cm2 of the posterodorsal nasal fossa (Standring, 2005, p. 571). Electron microscopic studies offer a finer picture of human olfactory epithelium, which generally resembles that of other vertebrates (Morrison and Constanzo, 1990; Morrison and Constanzo, 1992; Menco and Morrison, 2003). The neuronal body is globose with a round nucleus with just a peripheral rim of cytoplasm. There are multiple cell types, most notably the OSNs. OSNs have unbranched dendrites that project to the apical surface of the epithelium, and axons that project to the basal aspect of the epithelium, where they project into the lamina propria (Fig. 2.5). Mitochondria are concentrated within the cytoplasm of the dendrites. The dendrites terminate as 1–2 mm thick swellings called olfactory knobs. These may extend beyond the apical border into the nasal cavity. From each olfactory knob, 10–25 olfactory cilia extend further, forming a dense tangle of cell processes coated with mucus. Although they contain some microtubules, these processes are nonmotile projections with transmembrane odorant receptors (Dennis et al., 2015). Each OSN expresses just one of the numerous odorant receptor proteins (Buck and Axel, 1991).
Fig. 2.4. Olfactory mucosa of an adult common marmoset stained with Gomori trichrome procedure (A, B), and periodic acidSchiff (C). The section in (D) is prepared to show immunoreactivity to neuron-specific beta tubulin (D). The olfactory mucosa is composed of the olfactory epithelium (OE) and the deeper connective tissue, the lamina propria. The lamina propria of the olfactory epithelium has bundles of OSN axons (ON, or fila olfactoria) and Bowman’s glands (BG). Nuclei of OSNs are indicated by arrowheads. Bowman’s glands produce neutral (see C, see magenta stain) or acidic mucins. Note that the beta tubulin immunohistochemistry emphasizes the olfactory epithelium indicated by presence of OSNs (arrowheads) and that the respiratory epithelium (RE) has short cilia (small arrows). Scale bars: A, 50 mm; B–D, 20 mm.
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Fig. 2.5. (A) Schematic diagram showing olfactory sensory neurons indicated in differing stages of maturity. Three of the cell types are represented, including supporting cells (SC), basal cells (BC), and sensory neurons. The mature sensory neurons (shaded light gray) span the entire distance of the epithelium from the base to the apex. At the apex, dendritic knobs protrude and emit elongated cilia that join other processes and secreted mucus in the mucociliary layer (MC). In this medium odorants may attach to olfactory receptor proteins found on the cilia. At the basal membrane are mature sensory neurons sending their axons into the lamina propria, where they join others in fascicles of the olfactory nerves (OF). Some sensory neurons located near the basal side are immature (dark gray). As yet, these cells lack dendrites that are long enough to reach the apex and are still undergoing axonogenesis. (B) Micrograph of olfactory epithelium in a nonhuman primate (Otolemur garnettii) prepared using immunohistochemistry to Growth-Associated Protein 43 (GAP43), a neuronal marker that identifies OSNs during axonogenesis. Note the GAP43 + OSNs (arrows) are close to the basal side of the epithelium. Scale bar, 20 mm. Redrawn after Farbman, A.I., 1992. Cell biology of olfaction. Cambridge University Press, Cambridge.
The receptor cell axons are unmyelinated and extremely thin (0.1–0.7 mm) in diameter. Within the epithelium, these axons intermingle as intraepithelial fascicles and, after they pass through the basement membrane, combine into larger bundles of axons (fila olfactoria). The fila olfactoria are readily observable by light microscopy within the lamina propria (Fig. 2.4). After piercing the basement membrane, the fila olfactoria are covered with olfactory ensheathing (glial) cells. These neural crest-derived cells likely promote axonogenesis (Forni et al., 2011; Katoh et al., 2011; Mackay-Sim and St John, 2011), presumably after stress/injury or for newly differentiating OSNs. After traversing the lamina propria the fila olfactoria enter the anterior cranial fossa through the cribriform plate of ethmoid spreading over the entire ventral surface of the olfactory bulb (Fig. 2.6; Bhatnagar and Kallen, 1974; Bhatnagar et al., 1987). Other columnar cells in the olfactory epithelium are supporting cells and microvillar cells. The supporting cells have narrow basal processes, intermingling with OSN axons. The oval nuclei form a prominent row closer to the apical border than those of other cells types. Short microvilli project from the broader apical end of the cell. The apical cytoplasm of supporting cells contains mitochondria, free ribosomes, Golgi apparatus, and vesicles, thus suggesting a secretory function, although they are not considered the source of the mucus coating at the olfactory surface. In a recent review, Dennis et al. (2015) suggested these cells offer structural support with
protective functions. The microvillar cells are neuronlike, but are as yet of unknown function (Morrison and Constanzo, 1990). Basal cells of the olfactory epithelium are morphologically differentiated into two types, horizontal and globose basal cells. The former are considered stem cells and divide slowly (Holbroook et al., 1995). The globose basal cells give rise to new OSNs which are in constant regeneration (Mackay-Sim, 2010). The stages of neuron regeneration and degeneration are seen in the basal regions of the olfactory epithelium. During differentiation, the axons develop first, sprouting through the basement membrane (Fig. 2.5). In most mammals, a specialized chemosensory epithelial organ is found near the base of the nasal septum anteriorly. This structure, the vomeronasal organ (VNO), is present bilaterally and often encapsulated by cartilage or bone. Most frequently, the VNO is an epithelial tube lined with two types of epithelia: chemosensory and receptor-free ciliated epithelia (Smith and Bhatnagar, 2017). Vomeronasal chemoreception in mammals has been most clearly linked to sociosexual communication as well as aggression (Wysocki et al., 1991). However, in humans and other catarrhine primates (Old World Monkeys and apes), a growing consensus has emerged that the organ is absent or vestigial in adults (Bhatnagar and Smith, 2001, 2006; Witt et al., 2002; Wyatt, 2015). During prenatal development, the human VNO has sensory neurons transiently (Takami et al., 2016). It may also be part of a migration route for luteinizing
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Fig. 2.6. Sagittal histological sections in two perinatal mammals, showing the fila olfactoria (FO) through openings (olfactory foramina) in the cribriform plate (CP). At the top is a mammal that possesses a complex nasal cavity (A, B: a flying lemur, Cynocephaus volans). Note the enlarged nasal airways (NA) that project toward the cranial base. In an anthropoid primate (C, D: marmoset, Callithrix jacchus), as in humans, the nasal airways are truncated posteriorly due to the convergent orbits. Note that the olfactory bulb of the macrosmatic flying lemur is larger with more numerous axon bundles penetrating the cribriform plate compared to the marmoset. Scale bars, A, 2 mm; B, D, 0.5 mm; C, 1 mm.
hormone-releasing hormone neurons that develop peripherally and migrate to the brain (Boehm and Gasser, 1993). Smith et al. (2014b) termed the human VNO a “chronological” vestige, because it fulfills this function during prenatal ontogeny and then becomes a vestigial epithelial tube.
Physiology of the nasal walls as relates to olfactory function The turbinates on the lateral nasal wall are significant to nasal air movements as well as to the cleansing and warming of the inspired air. The inferior nasal turbinate (or maxilloturbinal) resides in the ventral air space between the anterior nares and the choanal apertures posteriorly. Much inhaled air passes across this turbinate, which is covered only with nonolfactory epithelium. In particular, this turbinate and most or all of the middle nasal turbinate bears respiratory mucosa, a ciliated epithelium with a highly vascular lamina propria (Harkema et al., 1987). Air is humidified and warmed by this mucosa, prior to its passage to the lower respiratory airways. The surface area of respiratory mucosa can be quite complex in other mammals, especially in environments that put in place extreme thermoregulatory
demands (Van Valkenburgh et al., 2011). Olfactory mucosa is more restricted, as discussed earlier (Fig. 2.3). In a comparative perspective, many mammals have a cul-de-sac in the nasal cavity that is dedicated for olfactory function (Fig. 2.5). In this region, called the olfactory recess, air movements are slowed to enhance odorant access by the olfactory epithelium (Craven et al., 2007; Eiting et al., 2014; Smith et al., 2015). Humans and all other anthropoid primates lack this dedicated olfactory region (Fig. 2.6). This recess is one anatomical feature used to denote mammals with a nasal architecture that is optimized for olfaction, the “macrosmatic” mammals such as canids (see Craven et al., 2010). Although there is increasing evidence for the great importance of olfaction to the macrosmatic (“keen-scented”) mammals (see Van Valkenburgh et al., 2014, for review), this does not render olfaction insignificant in other mammals, including humans (Schaal and Porter, 1991; Laska et al., 2000; Smith and Bhatnagar, 2004). There is little doubt that the simpler anatomical arrangement (fewer turbinals, no olfactory recess) in humans and other anthropoids relates to markedly different airflow patterns in the two groups. Human nasal airflow, for instance, occurs in an arc flowing first
ANATOMY OF THE OLFACTORY SYSTEM dorsally and then turning ventrally toward the choanae. Dogs and many other mammals, in contrast, have separate streams of air dedicated mainly for air-conditioning or olfaction (Craven et al., 2007, 2010). In addition, most mammals can increase the percentage of air that reaches the olfactory region by forcefully inhaling air (sniffing) (Kimbell et al., 1997; Yang et al., 2007; Craven et al., 2010). Based on rodent models, the nature of receptivity of the olfactory epithelium may not be identical across the entire nasal airway. For example, the OSNs located along the more central ethmoturbinals project axons to spatially distinct portions of the main olfactory bulb compared to those along the frontoturbinals (Miyamichi et al., 2005). These regions may correspond to different odorant absorption properties, suggesting a structure– function relationship between the internal nasal anatomy and odorant absorption (Schoenfeld and Cleland, 2005; Zhao and Dalton, 2007). The bearing of this on the far simpler human nasal anatomy, if any, is as yet unexplored. Since entire regions of the nasal cavity have been lost or displaced during evolution of humans and their closest relatives (Smith et al., 2014a,b), it seems possible that sensitivity to a certain range of odorants may have also been lost.
THE CENTRAL OLFACTORY APPARATUS The communication of the cranial cavity with the nasal cavity and the central olfactory apparatus The anterior cranial fossa is a depression in the cranial base, which primarily supports the large frontal lobes of the cerebral hemispheres. However, in the center of the anterior cranial fossa are small depressions, the olfactory fossae, which support the olfactory bulbs (Figs. 2.1, 2.6, and 2.7). The bulbs represent the first part of the
Fig. 2.7. Schematic view of the human olfactory bulb receiving fila olfactoria (yellow). The bone is represented in black. CP, cribriform plate; OB, olfactory bulb; SNC, superior nasal concha.
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olfactory system within the brain itself. The floor of each olfactory fossa is the cribriform plate of the ethmoid bone, a thin lamina of bone that is perforated by numerous olfactory foramina, which transmit olfactory nerves. The sides of the olfactory fossa are the raised surfaces of the frontal bone, which are convex over the orbits. The olfactory foramina have generated considerable interest as osteological correlates in mammals relating to the olfactory axons that traverse the cribriform plate (Bhatnagar and Kallen, 1974, 1975; Bird et al., 2014). The axons of the fila olfactoria are unmyelinated. They pierce the cribriform plate to enter the olfactory bulb and form Layer I (the olfactory nerve fiber layer; Fig. 2.8). In humans, these foramina have a documented age-related decrease in number, that is, they apparently ossify as olfactory nerves decrease in number (Kalmey et al., 1998; Doty and Kamath, 2014). An age-related decrease in neuronal number has also been reported in the olfactory bulb (Samaulhaq et al., 2008).
THE OLFACTORY BULB Of the principal sensory systems (vision, olfaction, taste, hearing and balance), the sense of smell is one of the oldest. It has both peripheral and central subdivisions. Most mammals also possess an accessory olfactory bulb; it is absent in humans, Old World monkeys, fruit bats, and some other mammals (Bhatnagar et al., 1987; Bhatnagar and Smith, 2006). The accessory olfactory bulb is absent in humans even though vestiges of the peripheral receptor structure, the VNO, are present in the nasal cavity (see earlier text). The peripheral olfactory system comprises the olfactory epithelium and nerve fascicles. The central olfactory system can be described in one word, namely rhinencephalon. This term applies to the olfactory brain, especially to those structures that receive projections from the olfactory epithelium via the olfactory bulb (Brodal, 1963). The rhinencephalon, also known as the paleopallium or primitive olfactory lobe (Valverde, 1965), includes the olfactory bulb, tract, tubercle, lateral, oftentimes an intermediate, and medial striae, anterior olfactory nucleus, parts of the amygdaloid complex parts of the prepiriform cortex, the archipallium. The oldest cortical structures include the hippocampal formation, the dentate gyrus, the fasciolar gyrus, and the indusium griseum (supracallosal gyrus) (Standring, 2005). Most importantly, unlike other sensory systems, the afferent olfactory projections bypass the thalamus in reaching the olfactory cortex. The mammalian olfactory bulb is described as a six-layered structure (Shepherd, 1972; Farbman, 1992). It is stated that the olfactory fascicles/fila enter the tip of the olfactory bulb. Although this may be true of mammals with rostrally positioned olfactory foramina, this is not the case
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Fig. 2.8. (A) Schematic representation of the olfactory bulb showing the six layers. The same layers are also numbered in a micrograph of an adult human olfactory bulb (B). From the exterior, these are numbered layers I through VI. Layer I contains axons of olfactory sensory neurons (OSN) emanating from the olfactory epithelium. These axons synapse in layer II, within roughly spherical bodies containing OSN synapses with dendrites of mitral (MC) and tufted cells (TC). Glomeruli (Gl) are magnified in (C), where periglomerular cells are seen surrounding them (small arrows). Layer III is the external plexiform layer containing the main and secondary dendrites of mitral cells and tufted cells as well as the cell bodies of some tufted cells (TC). Layer IV is the mitral cell body. At higher magnification (C), one sees a row of large nuclei (M). Layer V is the internal plexiform layer containing mitral cell axons, recurrent and deep collaterals of mitral and tufted cells, and granule cells with their spine-covered processes. Layer VI contains granule cell columns. The cell bodies are in this layer, while their dendrites project to other layers. Panel (A): Modified after Smith, T.D., Bhatnagar, K.P., 2004. Microsmatic primates: Reconsidering how and when size matters. Anat Rec 279: 24–31. Scale bars: A, 250 mm; B, 100 mm.
in humans. The olfactory nerve fibers enter the ventral aspect of the olfactory bulb on the entire length of the nearly 11 mm long olfactory bulb (Bhatnagar et al., 1987). There are as many olfactory nerves (and not just one nerve) as there are perforations (a few let pass vascular branches) in the cribriform plate (Bhatnagar and Kallen, 1974; Bhatnagar et al., 1987). The perforations are not uniform. That results in the olfactory bulb tissue bulging toward the nasal fossa. The six layers (Fig. 2.8) are as follows: Layer I, the most superficial, is formed by the olfactory nerve fascicles (Cranial nerve I) after they pierce the cribriform plate to enter the olfactory bulb. This layer tends to be thickest ventrally where relatively more axonal bundles approach the bulb (Shepherd, 1972). Layer II, the glomerular layer, comprises of tiers of glomeruli, which are synaptic fields for the centrifugal and centripetal projections to the olfactory bulb. Glomeruli are round, ball-like structures that receive inputs from
the olfactory epithelium, mitral, and tufted cells of layers IV and III, and periglomerular cells. The periglomerular cells occur in a network that surrounds the entire glomeruli (Shepherd, 1972). Layer III, the external plexiform layer, carries the main and secondary dendrites of mitral cells and tufted cells. Displaced mitral and tufted cells, primarily the latter and their processes, are also seen in this layer (Bhatnagar et al., 1987). Extrinsic fibers from the telencephalon also enter this layer. Layer IV, the mitral cell layer, contain the cell bodies of mitral cells. Each mitral cell emits a principal dendrite to a glomerulus, secondary dendrites to the external plexiform layer, and an axon to the olfactory tract. A few granule cells are also found in layer IV. Mitral cells of the olfactory bulb are the largest neurons of the olfactory bulb, and for that reason they are able to send projections to distant cortical regions. This layer of cell bodies occurs in a narrow band, typically densely stained (Fig. 2.8).
ANATOMY OF THE OLFACTORY SYSTEM Layer V, the internal plexiform layer, contains mitral cell axons, recurrent and deep collaterals of mitral and tufted cells, and granule cells with their spine-covered processes. Granule cells are typical in not having an axon. Two principal spine-bearing dendrites of these cells ramify terminating in the external plexiform layer. Layer VI contains granule cell columns and projections of the olfactory bulbar axons described earlier. Granule cells are the most numerous of the cell types in the bulb (Farbman, 1992). The cell bodies are in this layer, while their dendrites project to other layers. Bulbar cells have functions related to transmission or to modulation. Mitral and tufted cells are those with projections to other brain regions. In mammals, including humans, each of these cells has a single primary dendrite that is associated with a particular glomerulus. Each mitral and tufted cell has secondary or collateral dendrites that project into the external plexiform layer. In rodents, two types of mitral cells have been identified (Orona et al., 1984), one of which is more responsive to OSN stimulation (Wellis et al., 1989). Tufted cells are more diverse than mitral cells and occur at multiple levels throughout the thickness of the external plexiform layer (Farbman, 1992). External and middle tufted cells have restricted projections within the cortex. The internal tufted cells, like mitral cells, project more extensively, including to deeper cortical levels (Nagayama et al., 2010). Malz et al. (2000) observed that diverse mitral and tufted cells communicate with one another and, thus, function distinctly within a laminar framework of the olfactory bulb. In other words, there are functional subclasses of these neurons. Periglomerular and granule cells modulate the transmission of olfactory signals. Periglomerular cells cover the glomerulus and have reciprocal dendrodendritic synapses with tufted and mitral cells. Granule cells lack axons, but have multiple branched dendrites. Many of the synapses with tufted cells and mitral cells occur in the external plexiform layer. Thus, the cells involved in signal projection have dendritic synapses in two parts of the bulb with these two cell types. These cells may refine olfactory reception. For example, lateral inhibition may take place in the glomerular layer with periglomerular cells of a particular glomerulus, inhibiting responses in neighboring glomeruli. It is thought that at high enough concentrations, a particular odorant might invoke this response. This may enhance odorant detection by reducing the signal to noise ratio (Farbman, 1992). In this model, a central glomerulus is a source of strong excitation and is transmitted by a mitral cell, and the surrounding field of glomeruli is inhibited (dense receptive field hypothesis). An alternative model has been discussed for olfaction (sparse receptive field hypothesis; Fantana et al., 2008), and this continues to
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be a point of discussion based on rodent models for olfactory function (Lehmann et al., 2016). Neurons of the olfactory cortex and anterior olfactory nucleus provide centrifugal projections to the olfactory bulb. Other inputs to this region come from neurons in the diagonal band of Broca, to the granule cell layer and the glomerular layer. The olfactory tract continues along the olfactory sulcus on the orbital surface of the frontal lobe. The neurons of the granule cell layer constitute the anterior olfactory nucleus continuing into the lateral, intermediate, and medial striae and olfactory trigone to the prepiriform cortex, the anterior perforated substance, and precommissural septal regions (see Truex and Carpenter’s Neuroanatomy, 1969, Fig. 3.15, p. 46). An intermediate stria sometimes forms leading into an olfactory tubercle. These striae are thinly covered with the gray matter of the corresponding olfactory gyri. The largest cortical olfactory area is the piriform cortex. Other areas such as the anterior olfactory nucleus, entorhinal and insular cortex and amygdale and areas of limbic lobe also receive direct inputs from the olfactory bulb, without any relay in the thalamus. The connectivity beyond the olfactory bulb is not so well understood, nor have such studies been undertaken in humans. Modern imaging techniques are beginning to provide some functional information of higher level communications (Savic, 2005).
SUMMARY The olfactory system has peripheral and central subdivisions. The peripheral olfactory system includes the olfactory epithelium and nerve fascicles, and the central olfactory system corresponds to the olfactory bulb and its central connections. The olfactory epithelium extends its surface area over only one or two turbinates (or conchae) and adjacent surfaces in humans (compared to three or more olfactory turbinates in many other mammals). Many mammals possess an “accessory olfactory system” that may be specialized to detect certain types of odorants (e.g., based on molecular weight). To this day, studies continue to debate the presence and functionality of this system in humans (Stoyanov et al., 2016). However, all thorough studies, which have histologically examined human nasal septal tissues, have demonstrated that humans bear only a nonfunctioning vestige of its peripheral element, the VNO (Smith et al., 1998; Trotier et al., 2000; Bhatnagar and Smith, 2001; Witt et al., 2002). Grossly, the human olfactory anatomy is relatively reduced compared to most mammals. Yet, the olfactory pathway (excluding the vomeronasal pathway) has a great degree of similarity at a microanatomical level to that of other mammals. The human olfactory system
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contains a full complement of functional elements, including a constantly regenerating population of olfactory sensory neurons. These send long ciliary processes that form a mucus-covered mat in which odorants are detected. The olfactory sensory neurons send their axons to the first synapse point of the olfactory system, within the olfactory bulb. Mitral and tufted cells then relay impulses from the bulb to other brain regions. It is likely that functional subclasses of mitral and tufted cells exist, with differing electrophysiological properties and distinct communications throughout the laminar organization of the olfactory bulb. Although the bulk of our knowledge on how olfactory cells function comes from studies of laboratory animals, the human olfactory system is very similar to that of most other mammals. For example, the human olfactory bulb has the same organization and cell types as occurs in rodents, including the different layers of the olfactory bulb (Bhatnagar et al., 1987). A fascinating literature has been amassed on sexual dimorphism of the mammalian olfactory systems. For example, it has been reported that female rodents have a greater number of neurons within the main olfactory bulb than male rodents (Samaulhaq et al., 2008). A recent study has suggested female-biased dimorphism in humans as well (Oliveira-Pinto et al., 2014), which could explain the ability of women to perform better than men on a range of olfactory tests (Doty and Cameron, 2009).
ACKNOWLEDGMENTS The authors are grateful for the editorial input of Dr. R.L. Doty. We also thank Dr. Sam Marquez for generously provided original images.
REFERENCES Bhatnagar KP, Kallen FC (1974). Cribriform plate of ethmoid, olfactory bulb and olfactory acuity in forty species of bats. J Morphol 142: 71–89. Bhatnagar KP, Kallen FC (1975). Quantitative observations on the nasal epithelia and olfactory innervation in bats. Acta Anat 91: 272–282. Bhatnagar KP, Smith TD (2001). The human vomeronasal organ—part III: postnatal development from infancy through the ninth decade. J Anat 199: 289–302. Bhatnagar KP, Smith TD (2006). The vomeronasal organ and its evolutionary loss in catarrhine primates. In: TM Preuss, JH Kaas (Eds.), The evolution of primate nervous systems, vol. 5. Elsevier, New York, pp. 141–148. Bhatnagar KP, Kennedy RC, Baron G et al. (1987). Number of mitral cells and the bulb volume in the aging human olfactory bulb: a quantitative morphological study. Anat Rec 218: 73–87.
Bird D, Amirkhanian A, Pang B et al. (2014). Quantifying the cribriform plate: influences of allometry, function, and phylogeny. Anat Rec 297: 2080–2092. Boehm N, Gasser B (1993). Sensory receptor-like cells in the human foetal vomeronasal organ. Neuroreport 4: 867–870. Brodal A (1963). General discussion of the terminology of the rhinencephalon. In: W Bargmann, JP Schade (Eds.), The rhinencephalon and related structures. Progress in brain research, vol. 3. Elsevier, pp. 237–244. Buck L, Axel R (1991). A novel multigene family may encode odorant receptors: a molecular basis for odor recognition. Cell 65: 175–187. Clerico DM, To WC, Lanza DC (2003). Anatomy of the human nasal passages. In: RL Doty (Ed.), Handbook of olfaction and gustation, second ed. Marcel Dekker, New York, pp. 1–16. Craven B, Neuberger T, Paterson EG et al. (2007). Reconstruction and morphometric analysis of the nasal airway of the dog (Canis familiaris) and implications regarding olfactory airflow. Anat Rec 290: 1325–1340. Craven BA, Paterson EG, Settles GS (2010). The fluid dynamics of canine olfaction: unique nasal airflow patterns as an explanation of macrosmia. J R Soc Interface 7: 933–943. Dennis JC, Aono S, Vodyanoy VJ et al. (2015). In: RL Doty (Ed.), Development, morphology and functional anatomy of the olfactory epithelium, third ed. John Wiley and Sons, Inc, Hoboken, NJ, pp. 93–107. Handbook of olfaction and gustation. Doty RL, Cameron EL (2009). Sex differences and reproductive hormone influences on human odor perception. Physiol Behav 97: 213–228. Doty RL, Kamath V (2014). The influences of age on olfaction: a review. Front Psychol 5: 20. Eiting TP, Smith TD, Perot JB et al. (2014). The role of the olfactory recess in olfactory airflow. J Exp Biol 217: 1799–1803. Fantana AL, Soucy ER, Meister M (2008). Rat olfactory bulb mitral cells receive sparse glomerular inputs. Neuron 59: 802–814. Farbman AI (1992). Cell biology of olfaction, Cambridge University Press, Cambridge. Forni PE, Taylor-Burdi C et al. (2011). Neural crest and ectodermal cells intermix in the nasal placode to give rise to the GnRH-1 neurons, sensory neurons, and olfactory ensheathing neurons. J Neurosci 31: 6915–6927. Harkema JR, Popper CG, Hyde DM et al. (1987). Nonolfactory surface epithelium of the nasal cavity of the bonnet monkey: a morphologic and morphometric study of the transitional and respiratory epithelium. Am J Anat 180: 266–279. Harkema JR, Carey SA, Wagner JG (2006). The nose revisited: a brief review of the comparative structure, function, and toxicologic pathology of the nasal epithelium. Toxicol Pathol 34: 253–269.
ANATOMY OF THE OLFACTORY SYSTEM Holbroook EH, Szumiwski KE, Schwob JE (1995). An immunochemical, ultrastructural and developmental characterization of the horizontal basal cells of rat olfactory epithelium. J Comp Neurol 363: 129–146. Kalmey JK, Thewissen JG, Dluzen DE (1998). Age-related size reduction of foramina in the cribriform plate. Anat Rec 251: 326–329. Katoh RH, Shibata R et al. (2011). The dual origin of the peripheral olfactory system: placode and neural crest. Mol Brain 4: 34. Kimbell JS, Godo MN, Gross EA et al. (1997). Computer simulation of inspiratory airflow in all regions of the F344 rat nasal passages. Toxicol Appl Pharmacol 145: 388–398. Laska M, Seibt A, Weber A (2000). Microsmatic’ primates revisited: olfactory sensitivity in the squirrel monkey. Chem Senses 25: 47–53. Lehmann A, D’Errico A, Vogel M et al. (2016). Spatiotemporal characteristics of inhibition mapped by optical stimulation in mouse olfactory bulb. Front Neural Circuits 10: 15. Leopold DA, Hummel T, Schwob JE et al. (2000). Anterior distribution of human olfactory epithelium. Laryngoscope 110: 417–421. Mackay-Sim A (2010). Stem cells and their niche in the adult olfactory mucosa. Arch Ital Biol 148 (2): 47–58. Mackay-Sim A, St John JA (2011). Olfactory ensheathing cells from the nose: clinical application in human spinal cord injuries. Exp Neurol 22: 9174–9180. Malz CR, Knabe W, Kuhn H-J (2000). Pattern of calretinin immunoreactivity in the main olfactory system and vomeronasal system of the tree shrew, Tupaia belangeri. J Comp Neurol 420: 428–436. Ma´rquez S, Pagano AS, Lawson W et al. (2015). Evolution of the human nasal respiratory tract: nose and paranasal sinuses. In: MP Fried, A Tabaee (Eds.), Sataloff’s comprehensive textbook of otolaryngology: head & neck surgery (rhinology/ allergy and immunology)—volume 2, JP Brothers Medical Publishers (P) Ltd, New Delhi, pp. 15–42. Menco BPM, Morrison EE (2003). Morphology of the mammalian olfactory epithelium: form, fine structure, function, and pathology. In: RL Doty (Ed.), Handbook of olfaction and gustation, second ed. Marcel Dekker, New York, pp. 17–49. Miyamichi K, Serizawa S, Kimura H et al. (2005). Continuous and overlapping expression domains of odorant receptor genes in the olfactory epithelium determine the dorsal/ ventral positioning of glomeruli in the olfactory bulb. J Neurosci 25: 3586–3592. Morrison EE, Constanzo RM (1990). Morphology of the human olfactory epithelium. J Comp Neurol 297: 1–14. Morrison EE, Constanzo RM (1992). Morphology of the olfactory epithelium in humans and other vertebrates. Microsc Res Tech 23: 49–61. Nagayama S, Everva A, Fletcher ML et al. (2010). Differential axonal projection of mitral and tufted cells in the mouse main olfactory system. Front Neural Circuits 4 (120). https://doi.org/10.3389/fncir.2010.00120.
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Oliveira-Pinto AV, Santos RM, Coutinho RA et al. (2014). Sexual dimorphism in the human olfactory bulb: females have more neurons and glial cells than males. PLoS One9 (11): e111733. https://doi.org/10.1371/journal.pone.0111733. Orona E, Rainer EC, Scott JW (1984). Dendritic and axonal organization of mitral and tufted cells in the rat olfactory bulb. J Comp Neurol 226: 346–356. Piesse GW (1857). The art of perfumery, Lindsay and Blakiston, Philadelphia. Read EA (1908). A contribution to the knowledge of the olfactory apparatus in dog, cat and man. Am J Anat 8: 17–47. Samaulhaq M, Tahir KP, Lone K (2008). Age and gender related differences in olfactory bulb glomeruli in humans. Biomedica 24: 12–17. Savic I (2005). Brain imaging studies of the functional organization of human olfaction. Chem Senses 30 (Suppl. 1): i222–i223. Schaal B, Porter RH (1991). “Microsmatic humans” revisited: the generation and perception of chemical signals. Adv Study Behav 20: 474–482. Schaeffer JP (1920). The nose, paranasal sinuses, nasolacrimal passageways and olfactory organs in man, Blakiston Sons and Co, Philadelphia. Schoenfeld TA, Cleland TA (2005). The anatomical logic of smell. Trends Neurosci 28: 620–627. Shepherd GM (1972). Synaptic organization of the mammalian olfactory bulb. Physiol Rev 52: 864–917. Smith TD, Bhatnagar KP (2004). Microsmatic primates: reconsidering how and when size matters. Anat Rec 279: 24–31. Smith TD, Bhatnagar KP (2017). Vomeronasal system evolution. In: R Dunbar (Ed.), Reference module in neuroscience and biobehavioral psychology, vol. 2017. pp. Elsevier, New York, pp. 1–10. Smith TD, Siegel MI, Burrows AM et al. (1998). Searching for the vomeronasal organ of adult humans: preliminary findings on location, structure, and size. Microsc Res Tech 41: 483–491. Smith TD, Eiting TP, Bonar CJ et al. (2014a). Nasal morphometry in marmosets: loss and redistribution of olfactory surface area. Anat Rec 297: 2093–2104. Smith TD, Laitman JT, Bhatnagar KP (2014b). The shrinking anthropoid nose, the human vomeronasal organ, and the language of anatomical reduction. Anat Rec 297: 2196–2204. Smith TD, Eiting TP, Bhatnagar KP (2015). Anatomy of the nasal passages in mammals. In: RL Doty (Ed.), Handbook of olfaction and gustation, third ed. John Wiley and Sons, Inc., Hoboken, NJ, pp. 37–62. Standring S (Ed.), (2005). Gray’s anatomy, 39th ed. Elsevier, New York. Stoyanov G, Moneva K, Sapundzhiev N et al. (2016). The vomeronasal organ—incidence in a Bulgarian population. J Laryngol Otol 130: 344–347. Takami S, Yukimatso M, Matsumura G et al. (2016). Morphological analysis for neuron-like cells in the vomeronasal organ of human fetuses at the middle of gestation. Anat Rec 299: 88–97.
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Trotier D, Eloit C, Wassef M et al. (2000). The vomeronasal cavity in adult humans. Chem Senses 25: 369–380. Truex RC, Carpenter MB (1969). Human neuroanatomy, sixth ed. Williams and Wilkins. Turner W (1891). The convolutions of the brain: a study in comparative anatomy. J Anat Physiol 25: 105–153. Valverde F (1965). Studies on the piriform lobe, vol. 138. Harvard University Press, Cambridge. Van Valkenburgh B, Curtis A, Samuels JX et al. (2011). Aquatic adaptations in the nose of carnivorans: evidence from the turbinates. J Anat 218: 298–310. Van Valkenburgh B, Craven BA, Smith TD (2014). The vertebrate nose: evolution, structure and function. Anat Rec 297: 1975–1984. Wellis DP, Scott JW, Harrison TA (1989). Discrimination among odorants by single neurons of the rat olfactory bulb. J Neurophysiol 61: 1161–1177.
Witt M, Georgiewa B, Knecht M et al. (2002). On the chemosensory nature of the vomeronasal epithelium in adult humans. Histochem Cell Biol 117: 493–509. Wyatt TD (2015). The search for human pheromones: the lost decades and the necessity of returning to first principles. Proc Biol Sci. 282 (1804): 20142994. https://doi. org/10.1098/rspb.2014.2994. Wysocki CJ, Kruczek M, Wysocki LM et al. (1991). Activation of reproduction in nulliparous and primiparous voles is blocked by vomeronasal organ removal. Biol Reprod 45: 611–616. Yang GC, Scherer PW, Zhao K et al. (2007). Numerical modeling of odorant uptake in the rat nasal cavity. Chem Senses 32: 273–284. Zhao K, Dalton P (2007). The way the wind blows: implications of modeling nasal airflow. Curr Allergy Asthma Rep 7: 117–125.
Chapter 2
Anatomy of the olfactory system TIMOTHY D. SMITH1* AND KUNWAR P. BHATNAGAR2 School of Physical Therapy, Slippery Rock University, Slippery Rock, PA, United States
1 2
Anatomical Sciences and Neurobiology, University of Louisville School of Medicine, Louisville, KY, United States
Abstract Of the principal sensory systems (vision, olfaction, taste, hearing, and balance), olfaction is one of the oldest. This ubiquitous system has both peripheral and central subdivisions. The peripheral subdivision is comprised of the olfactory epithelium and nerve fascicles, whereas the central subdivision is made up of the olfactory bulb and its central connections. Humans lack the “accessory olfactory system” of many other mammals, exhibiting only a nonfunctioning vestige of its peripheral element, the vomeronasal organ. Compared to most mammals, major elements of the human olfactory system are reduced; for example, humans have fewer turbinates than many mammals, and their olfactory epithelia are found only on one or two of these structures and their adjacent surfaces. Nonetheless, humans retain a full complement of functional cellular elements including a regenerating population of olfactory sensory neurons. These neurons extend long ciliary processes into the mucus that form a mat of cilia on which the odorant receptors are located. The olfactory sensory neurons send their axons directly to synapse within the olfactory bulb. Mitral and tufted cells then relay impulses from the bulb to other brain regions. This chapter describes the general anatomy and microanatomy of the olfactory system.
INTRODUCTION The human sense of smell is sometimes celebrated (Piesse, 1857) and, at other times, denigrated as a reduced, less useful version of the powerful tracking sense that some of our fellow mammals enjoy (Turner, 1891). To some degree this is due to the seemingly less elaborate anatomy of the human olfactory system. However, appearances deceive. While the gross anatomy relating to olfaction differs in humans from that of many other mammals, the human olfactory system is essentially equivalent at the microanatomical level. In this chapter, we review the major elements of the human olfactory pathway, including the olfactory epithelium, the bundles of olfactory receptor axons that project from the epithelium into the olfactory bulb, the olfactory bulb, and the connections to central brain structures that collectively make up the central olfactory structures, commonly referred to as the olfactory cortex. The latter structures and their connections are discussed in detail in other chapters of the volume.
STRUCTURES OF THE PERIPHERAL OLFACTORY SYSTEM Osseous support for the nasal cavity and associated soft tissues In skeletal anatomy, the human olfactory apparatus begins with structures constituting the boundaries of the nasal cavity (Fig. 2.1; Clerico et al., 2003; Márquez et al., 2015). The piriform aperture is the opening into the nasal cavity (Fig. 2.2). Within this cavity, two bony elements form a midline partition that divides it into right and left nasal fossae. This osseous nasal septum is composed of the perpendicular plate of the ethmoid and the vomer bone; these articulate with septal cartilage anteriorly. The lateral wall is the most complex boundary of the nasal fossa, being formed by parts of the maxillary, ethmoid, lacrimal, and palatine bones. This wall takes on added complexity in that it communicates with the paranasal cavities. The cribriform plate of the anterior cranial base is the roof of each nasal fossa, while the palate forms the floor (Fig. 2.2).
*Correspondence to: Timothy D. Smith, School of Physical Therapy, Slippery Rock University, Slippery Rock, PA, United States. Tel: +1-724-738-2885, Fax: +1-724-738-2113, E-mail: [email protected]
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T.D. SMITH AND K.P. BHATNAGAR Figs. 2.1 and 2.2). The middle and superior nasal turbinates are part of the lateral mass of the ethmoid bone. A fourth fold (the “supreme” nasal concha) is also present in some individuals (Schaeffer, 1920). The turbinates serve not only to filter, warm, and moisten inspired air but also provide the scaffold for the olfactory epithelium and airflow patterns that send odor-laden air to its receptor surface. In addition, they divide the nasal airways into three compartments, called meatuses. Each meatus is located lateral to its respective turbinate (inferior, middle, superior; Fig. 2.1).
Microanatomy of the nasal mucosa
Fig. 2.1. CT scan slice showing the human nasal cavity in a coronal plane. This is a relatively posterior level of the nasal cavity in which all three turbinates are visible. Olfactory epithelium is often restricted to the lateral surface of the superior nasal turbinate (SC), the adjacent level of the nasal septum, and the roof of the nasal cavity in the same region. IC, inferior nasal turbinate; IM, inferior meatus; MC, middle nasal turbinate; MS, maxillary sinus; NS, nasal septum; OF, olfactory fossa; Or, orbit. Image courtesy of Dr. Samuel Ma´rquez.
Soft tissue structures transform the osseous nasal cavity into bilateral spaces that lead to the pharynx posteriorly. These nasal passageways are the first (proximal) part of the upper respiratory tract. The piriform aperture is elaborated by cartilaginous support structures that house the downwardly oriented anterior nares. The anterior nares open to a dilated vestibule on each side. Separating the right and left vestibules is the columella, the fleshy anteriormost extension of the nasal septum (Fig. 2.2). The majority of the nasal septum, a mucosalined partition supported internally by bone and cartilage, is found within the main nasal chamber. The nasal passages end at the choanae (or posterior nares), paired apertures that transmit air from the nasal fossae into the nasopharynx (Fig. 2.2). The nasal choanae are divided by the posterior end of the nasal septum, which is supported internally by the vomer bone. The vestibule may be thought of as a simple outer chamber that communicates posteriorly into the nasal cavity proper; this occurs approximately at an internal ridge called the limen nasi, which corresponds roughly to the bony margin of the piriform aperture. In the nasal cavity proper, the air passages become more complex. The nasal turbinates (conchae, turbinals) are projections from the lateral walls that extend into the nasal passages. The inferior nasal turbinate articulates with the maxillary bone (a basis for its alternate name—maxilloturbinal;
In other mammals, nasal skeletal structure may be used to infer some physiologic specializations. For example, the maxilloturbinal (¼ inferior nasal turbinate) is highly branched or scrolled in various mammals, reaching great complexity in mammals with special homeothermic demands (e.g., aquatic species; Van Valkenburgh et al., 2011). The maxilloturbinal is often used as a quantifiable proxy for respiratory adaptations, although recent research has shown that other turbinals may have “mixed functions” for respiratory air-conditioning as well as olfaction. Thus, the actual functional anatomy is mostly dictated by the distribution of specific types of nasal mucosa. The nasal fossa is lined with four types of epithelia: stratified, respiratory, transitional, and olfactory (Harkema et al., 1987, 2006; Fig. 2.3). Stratified epithelium typically lines regions of the nasal fossa in which fluid transport is passive, such as the floor of the nasal cavity. Respiratory epithelium in the nasal fossa has numerous motile cilia, which propel mucus toward the pharynx. At least four other cell types are present within this epithelium, such as nonciliated, goblet (mucus-producing), secretory (small granule), and basal cells (Harkema et al., 1987). Transitional epithelium is a nonstratified type that occurs as a narrow strip between stratified squamous and respiratory epithelium (Harkema et al., 1987, 2006). Olfactory epithelium possesses nonmotile cilia involved in chemoreception, and is typically distributed dorsally in monkeys, the closest relatives of humans that have been quantitatively studied (Harkema et al., 1987; Smith et al., 2014a), as well as humans (see Clerico et al., 2003). The majority of the human nasal mucosa is nonolfactory. Regionally, the vestibule is simply lined with facial skin, including large hairs (vibrissae). After passing the limen nasi, nonkeratinized stratified squamous epithelium is found, followed by a narrow strip of transitional epithelium. Respiratory mucosa covers most of the remaining nasal fossa, although stratified squamous epithelium lines the floor. Olfactory mucosa covers the “roof” of the nasal cavity along surfaces that are ventral to the cribriform
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Fig. 2.2. Osseous and soft tissue structures that delimit the nasal cavity. (A) The anterior nares from below, also illustrating the nasal valve (Va) and columnella (C). (B) After passing through the anterior nares, air enters the vestibule (Ve) and then into the main chamber, where three pairs of turbinates are shown on lateral nasal wall. The superior turbinate (SC) consistently has olfactory mucosa. Air exits the main nasal chamber through the choana (arrow is passing through the choana on the right side). (C) A posterior view of the paired choanae (Ch). Note that turbinates can be seen inside the main nasal chamber. (D and E) Osseous boundaries of the nasal cavity are shown for lateral (D) and medial (E) walls. The maxilla (M), lacrimal, palatine (P), inferior nasal turbinates (IC), and ethmoid bone form the lateral wall. The superior and middle nasal turbinates (SC, MC) are projections of the ethmoid bone. The perpendicular plate of the ethmoid bone (PP) helps to form the medial boundary of each nasal cavity. It articulates inferiorly with the vomer bone (V) and anteriorly with the septal cartilage (SC). The location of the vestigial vomeronasal organ (VNO) is indicated. FS, frontal sinus; OB, olfactory bulb; OA, opening of auditory tube; OC, oral cavity; U, uvula. Modified after Smith, T.D., Eiting, T.P., Bhatnagar, K.P., 2015. Anatomy of the nasal passages in mammals. In: Doty, R.L. (Ed.), Handbook of olfaction and gustation, third ed. John Wiley and Sons, Inc., Hoboken, NJ, 37–62.
plate. It also generally covers the superior nasal turbinate and may extend over the middle nasal turbinate (Fig. 2.3). The septal mucosa adjacent to the superior turbinate also bears olfactory mucosa (Read, 1908; Leopold et al., 2000; see Clerico et al., 2003, regarding variations). A portion of the middle nasal turbinate may bear olfactory mucosa, though this varies (Fig. 2.3). The olfactory mucosa, comprising the surface neurosensory epithelium and supporting the lamina propria, has distinguishing characteristics throughout its depth. It is layered in the following order beginning from the nasal cavity: mucus layer surrounding the free processes (cilia) of olfactory sensory neurons (OSNs), microvillus of the supporting cells, openings of the mucus gland ducts, cell bodies within the olfactory epithelium, and the basal lamina. The thick lamina propria comprises connective tissue with glandular elements, olfactory nerve fascicles, blood and lymphatic vessels, and autonomic and terminal nerve elements. The olfactory epithelium is thicker than the adjacent respiratory epithelium by several orders of magnitude (Fig. 2.4)
and has more numerous rows of cells bodies (see following text). The lamina propria is also distinctive from that of the respiratory mucosa. Olfactory mucosa has distinctive glands and more numerous large bundles of axons. Bowman’s glands have a branched tubuloalveolar morphology and manufacture mucus secretions that are transmitted to the surface via a single duct (Fig. 2.4). The secretions protect the epithelium and its apical processes. It is further supposed that the secretions facilitate odorant access to receptor sites on olfactory cilia (Farbman, 1992). Nervous elements include collections of unmyelinated axons emanating from the base of the olfactory neuroepithelium. Together the characteristics of the epithelium and lamina propria make the olfactory mucosa distinctive by light microscopy (Fig. 2.4). The olfactory epithelium (Fig. 2.5) is composed of the bipolar OSNs, sustentacular (supporting) cells, microvillar cells, and basal cells, with olfactory gland ducts, autonomic nerves and blood vessels. The OSNs are large, oftentimes extending through the entire height of the epithelium (Figs. 2.4D and 2.5). The olfactory
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Fig. 2.3. Lateral nasal wall of adult human with olfactory (blue) and nonolfactory mucosa indicated. The location is based on a biopsy survey and literature review provided by Clerico et al. (2003). On the lateral nasal wall, part of the superior nasal turbinate (SC) and the superior extent of the middle nasal turbinate (MC) commonly bear olfactory mucosa. The extent to which it extends over the remainder of the superior turbinate varies (light blue). There is also olfactory mucosa on the roof of the nasal cavity, which is opposite the cribiform plate (CP), upon which the olfactory bulb (OB) rests, and finally the nasal septum (not shown) bears olfactory mucosa, approximately opposite to that shown here for the lateral wall. Na, anterior nares; Sph, sphenoid bone.
neuroepithelium is 60–80 mm thick (Morrison and Constanzo, 1992) and covers about 5 cm2 of the posterodorsal nasal fossa (Standring, 2005, p. 571). Electron microscopic studies offer a finer picture of human olfactory epithelium, which generally resembles that of other vertebrates (Morrison and Constanzo, 1990; Morrison and Constanzo, 1992; Menco and Morrison, 2003). The neuronal body is globose with a round nucleus with just a peripheral rim of cytoplasm. There are multiple cell types, most notably the OSNs. OSNs have unbranched dendrites that project to the apical surface of the epithelium, and axons that project to the basal aspect of the epithelium, where they project into the lamina propria (Fig. 2.5). Mitochondria are concentrated within the cytoplasm of the dendrites. The dendrites terminate as 1–2 mm thick swellings called olfactory knobs. These may extend beyond the apical border into the nasal cavity. From each olfactory knob, 10–25 olfactory cilia extend further, forming a dense tangle of cell processes coated with mucus. Although they contain some microtubules, these processes are nonmotile projections with transmembrane odorant receptors (Dennis et al., 2015). Each OSN expresses just one of the numerous odorant receptor proteins (Buck and Axel, 1991).
Fig. 2.4. Olfactory mucosa of an adult common marmoset stained with Gomori trichrome procedure (A, B), and periodic acidSchiff (C). The section in (D) is prepared to show immunoreactivity to neuron-specific beta tubulin (D). The olfactory mucosa is composed of the olfactory epithelium (OE) and the deeper connective tissue, the lamina propria. The lamina propria of the olfactory epithelium has bundles of OSN axons (ON, or fila olfactoria) and Bowman’s glands (BG). Nuclei of OSNs are indicated by arrowheads. Bowman’s glands produce neutral (see C, see magenta stain) or acidic mucins. Note that the beta tubulin immunohistochemistry emphasizes the olfactory epithelium indicated by presence of OSNs (arrowheads) and that the respiratory epithelium (RE) has short cilia (small arrows). Scale bars: A, 50 mm; B–D, 20 mm.
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Fig. 2.5. (A) Schematic diagram showing olfactory sensory neurons indicated in differing stages of maturity. Three of the cell types are represented, including supporting cells (SC), basal cells (BC), and sensory neurons. The mature sensory neurons (shaded light gray) span the entire distance of the epithelium from the base to the apex. At the apex, dendritic knobs protrude and emit elongated cilia that join other processes and secreted mucus in the mucociliary layer (MC). In this medium odorants may attach to olfactory receptor proteins found on the cilia. At the basal membrane are mature sensory neurons sending their axons into the lamina propria, where they join others in fascicles of the olfactory nerves (OF). Some sensory neurons located near the basal side are immature (dark gray). As yet, these cells lack dendrites that are long enough to reach the apex and are still undergoing axonogenesis. (B) Micrograph of olfactory epithelium in a nonhuman primate (Otolemur garnettii) prepared using immunohistochemistry to Growth-Associated Protein 43 (GAP43), a neuronal marker that identifies OSNs during axonogenesis. Note the GAP43 + OSNs (arrows) are close to the basal side of the epithelium. Scale bar, 20 mm. Redrawn after Farbman, A.I., 1992. Cell biology of olfaction. Cambridge University Press, Cambridge.
The receptor cell axons are unmyelinated and extremely thin (0.1–0.7 mm) in diameter. Within the epithelium, these axons intermingle as intraepithelial fascicles and, after they pass through the basement membrane, combine into larger bundles of axons (fila olfactoria). The fila olfactoria are readily observable by light microscopy within the lamina propria (Fig. 2.4). After piercing the basement membrane, the fila olfactoria are covered with olfactory ensheathing (glial) cells. These neural crest-derived cells likely promote axonogenesis (Forni et al., 2011; Katoh et al., 2011; Mackay-Sim and St John, 2011), presumably after stress/injury or for newly differentiating OSNs. After traversing the lamina propria the fila olfactoria enter the anterior cranial fossa through the cribriform plate of ethmoid spreading over the entire ventral surface of the olfactory bulb (Fig. 2.6; Bhatnagar and Kallen, 1974; Bhatnagar et al., 1987). Other columnar cells in the olfactory epithelium are supporting cells and microvillar cells. The supporting cells have narrow basal processes, intermingling with OSN axons. The oval nuclei form a prominent row closer to the apical border than those of other cells types. Short microvilli project from the broader apical end of the cell. The apical cytoplasm of supporting cells contains mitochondria, free ribosomes, Golgi apparatus, and vesicles, thus suggesting a secretory function, although they are not considered the source of the mucus coating at the olfactory surface. In a recent review, Dennis et al. (2015) suggested these cells offer structural support with
protective functions. The microvillar cells are neuronlike, but are as yet of unknown function (Morrison and Constanzo, 1990). Basal cells of the olfactory epithelium are morphologically differentiated into two types, horizontal and globose basal cells. The former are considered stem cells and divide slowly (Holbroook et al., 1995). The globose basal cells give rise to new OSNs which are in constant regeneration (Mackay-Sim, 2010). The stages of neuron regeneration and degeneration are seen in the basal regions of the olfactory epithelium. During differentiation, the axons develop first, sprouting through the basement membrane (Fig. 2.5). In most mammals, a specialized chemosensory epithelial organ is found near the base of the nasal septum anteriorly. This structure, the vomeronasal organ (VNO), is present bilaterally and often encapsulated by cartilage or bone. Most frequently, the VNO is an epithelial tube lined with two types of epithelia: chemosensory and receptor-free ciliated epithelia (Smith and Bhatnagar, 2017). Vomeronasal chemoreception in mammals has been most clearly linked to sociosexual communication as well as aggression (Wysocki et al., 1991). However, in humans and other catarrhine primates (Old World Monkeys and apes), a growing consensus has emerged that the organ is absent or vestigial in adults (Bhatnagar and Smith, 2001, 2006; Witt et al., 2002; Wyatt, 2015). During prenatal development, the human VNO has sensory neurons transiently (Takami et al., 2016). It may also be part of a migration route for luteinizing
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Fig. 2.6. Sagittal histological sections in two perinatal mammals, showing the fila olfactoria (FO) through openings (olfactory foramina) in the cribriform plate (CP). At the top is a mammal that possesses a complex nasal cavity (A, B: a flying lemur, Cynocephaus volans). Note the enlarged nasal airways (NA) that project toward the cranial base. In an anthropoid primate (C, D: marmoset, Callithrix jacchus), as in humans, the nasal airways are truncated posteriorly due to the convergent orbits. Note that the olfactory bulb of the macrosmatic flying lemur is larger with more numerous axon bundles penetrating the cribriform plate compared to the marmoset. Scale bars, A, 2 mm; B, D, 0.5 mm; C, 1 mm.
hormone-releasing hormone neurons that develop peripherally and migrate to the brain (Boehm and Gasser, 1993). Smith et al. (2014b) termed the human VNO a “chronological” vestige, because it fulfills this function during prenatal ontogeny and then becomes a vestigial epithelial tube.
Physiology of the nasal walls as relates to olfactory function The turbinates on the lateral nasal wall are significant to nasal air movements as well as to the cleansing and warming of the inspired air. The inferior nasal turbinate (or maxilloturbinal) resides in the ventral air space between the anterior nares and the choanal apertures posteriorly. Much inhaled air passes across this turbinate, which is covered only with nonolfactory epithelium. In particular, this turbinate and most or all of the middle nasal turbinate bears respiratory mucosa, a ciliated epithelium with a highly vascular lamina propria (Harkema et al., 1987). Air is humidified and warmed by this mucosa, prior to its passage to the lower respiratory airways. The surface area of respiratory mucosa can be quite complex in other mammals, especially in environments that put in place extreme thermoregulatory
demands (Van Valkenburgh et al., 2011). Olfactory mucosa is more restricted, as discussed earlier (Fig. 2.3). In a comparative perspective, many mammals have a cul-de-sac in the nasal cavity that is dedicated for olfactory function (Fig. 2.5). In this region, called the olfactory recess, air movements are slowed to enhance odorant access by the olfactory epithelium (Craven et al., 2007; Eiting et al., 2014; Smith et al., 2015). Humans and all other anthropoid primates lack this dedicated olfactory region (Fig. 2.6). This recess is one anatomical feature used to denote mammals with a nasal architecture that is optimized for olfaction, the “macrosmatic” mammals such as canids (see Craven et al., 2010). Although there is increasing evidence for the great importance of olfaction to the macrosmatic (“keen-scented”) mammals (see Van Valkenburgh et al., 2014, for review), this does not render olfaction insignificant in other mammals, including humans (Schaal and Porter, 1991; Laska et al., 2000; Smith and Bhatnagar, 2004). There is little doubt that the simpler anatomical arrangement (fewer turbinals, no olfactory recess) in humans and other anthropoids relates to markedly different airflow patterns in the two groups. Human nasal airflow, for instance, occurs in an arc flowing first
ANATOMY OF THE OLFACTORY SYSTEM dorsally and then turning ventrally toward the choanae. Dogs and many other mammals, in contrast, have separate streams of air dedicated mainly for air-conditioning or olfaction (Craven et al., 2007, 2010). In addition, most mammals can increase the percentage of air that reaches the olfactory region by forcefully inhaling air (sniffing) (Kimbell et al., 1997; Yang et al., 2007; Craven et al., 2010). Based on rodent models, the nature of receptivity of the olfactory epithelium may not be identical across the entire nasal airway. For example, the OSNs located along the more central ethmoturbinals project axons to spatially distinct portions of the main olfactory bulb compared to those along the frontoturbinals (Miyamichi et al., 2005). These regions may correspond to different odorant absorption properties, suggesting a structure– function relationship between the internal nasal anatomy and odorant absorption (Schoenfeld and Cleland, 2005; Zhao and Dalton, 2007). The bearing of this on the far simpler human nasal anatomy, if any, is as yet unexplored. Since entire regions of the nasal cavity have been lost or displaced during evolution of humans and their closest relatives (Smith et al., 2014a,b), it seems possible that sensitivity to a certain range of odorants may have also been lost.
THE CENTRAL OLFACTORY APPARATUS The communication of the cranial cavity with the nasal cavity and the central olfactory apparatus The anterior cranial fossa is a depression in the cranial base, which primarily supports the large frontal lobes of the cerebral hemispheres. However, in the center of the anterior cranial fossa are small depressions, the olfactory fossae, which support the olfactory bulbs (Figs. 2.1, 2.6, and 2.7). The bulbs represent the first part of the
Fig. 2.7. Schematic view of the human olfactory bulb receiving fila olfactoria (yellow). The bone is represented in black. CP, cribriform plate; OB, olfactory bulb; SNC, superior nasal concha.
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olfactory system within the brain itself. The floor of each olfactory fossa is the cribriform plate of the ethmoid bone, a thin lamina of bone that is perforated by numerous olfactory foramina, which transmit olfactory nerves. The sides of the olfactory fossa are the raised surfaces of the frontal bone, which are convex over the orbits. The olfactory foramina have generated considerable interest as osteological correlates in mammals relating to the olfactory axons that traverse the cribriform plate (Bhatnagar and Kallen, 1974, 1975; Bird et al., 2014). The axons of the fila olfactoria are unmyelinated. They pierce the cribriform plate to enter the olfactory bulb and form Layer I (the olfactory nerve fiber layer; Fig. 2.8). In humans, these foramina have a documented age-related decrease in number, that is, they apparently ossify as olfactory nerves decrease in number (Kalmey et al., 1998; Doty and Kamath, 2014). An age-related decrease in neuronal number has also been reported in the olfactory bulb (Samaulhaq et al., 2008).
THE OLFACTORY BULB Of the principal sensory systems (vision, olfaction, taste, hearing and balance), the sense of smell is one of the oldest. It has both peripheral and central subdivisions. Most mammals also possess an accessory olfactory bulb; it is absent in humans, Old World monkeys, fruit bats, and some other mammals (Bhatnagar et al., 1987; Bhatnagar and Smith, 2006). The accessory olfactory bulb is absent in humans even though vestiges of the peripheral receptor structure, the VNO, are present in the nasal cavity (see earlier text). The peripheral olfactory system comprises the olfactory epithelium and nerve fascicles. The central olfactory system can be described in one word, namely rhinencephalon. This term applies to the olfactory brain, especially to those structures that receive projections from the olfactory epithelium via the olfactory bulb (Brodal, 1963). The rhinencephalon, also known as the paleopallium or primitive olfactory lobe (Valverde, 1965), includes the olfactory bulb, tract, tubercle, lateral, oftentimes an intermediate, and medial striae, anterior olfactory nucleus, parts of the amygdaloid complex parts of the prepiriform cortex, the archipallium. The oldest cortical structures include the hippocampal formation, the dentate gyrus, the fasciolar gyrus, and the indusium griseum (supracallosal gyrus) (Standring, 2005). Most importantly, unlike other sensory systems, the afferent olfactory projections bypass the thalamus in reaching the olfactory cortex. The mammalian olfactory bulb is described as a six-layered structure (Shepherd, 1972; Farbman, 1992). It is stated that the olfactory fascicles/fila enter the tip of the olfactory bulb. Although this may be true of mammals with rostrally positioned olfactory foramina, this is not the case
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Fig. 2.8. (A) Schematic representation of the olfactory bulb showing the six layers. The same layers are also numbered in a micrograph of an adult human olfactory bulb (B). From the exterior, these are numbered layers I through VI. Layer I contains axons of olfactory sensory neurons (OSN) emanating from the olfactory epithelium. These axons synapse in layer II, within roughly spherical bodies containing OSN synapses with dendrites of mitral (MC) and tufted cells (TC). Glomeruli (Gl) are magnified in (C), where periglomerular cells are seen surrounding them (small arrows). Layer III is the external plexiform layer containing the main and secondary dendrites of mitral cells and tufted cells as well as the cell bodies of some tufted cells (TC). Layer IV is the mitral cell body. At higher magnification (C), one sees a row of large nuclei (M). Layer V is the internal plexiform layer containing mitral cell axons, recurrent and deep collaterals of mitral and tufted cells, and granule cells with their spine-covered processes. Layer VI contains granule cell columns. The cell bodies are in this layer, while their dendrites project to other layers. Panel (A): Modified after Smith, T.D., Bhatnagar, K.P., 2004. Microsmatic primates: Reconsidering how and when size matters. Anat Rec 279: 24–31. Scale bars: A, 250 mm; B, 100 mm.
in humans. The olfactory nerve fibers enter the ventral aspect of the olfactory bulb on the entire length of the nearly 11 mm long olfactory bulb (Bhatnagar et al., 1987). There are as many olfactory nerves (and not just one nerve) as there are perforations (a few let pass vascular branches) in the cribriform plate (Bhatnagar and Kallen, 1974; Bhatnagar et al., 1987). The perforations are not uniform. That results in the olfactory bulb tissue bulging toward the nasal fossa. The six layers (Fig. 2.8) are as follows: Layer I, the most superficial, is formed by the olfactory nerve fascicles (Cranial nerve I) after they pierce the cribriform plate to enter the olfactory bulb. This layer tends to be thickest ventrally where relatively more axonal bundles approach the bulb (Shepherd, 1972). Layer II, the glomerular layer, comprises of tiers of glomeruli, which are synaptic fields for the centrifugal and centripetal projections to the olfactory bulb. Glomeruli are round, ball-like structures that receive inputs from
the olfactory epithelium, mitral, and tufted cells of layers IV and III, and periglomerular cells. The periglomerular cells occur in a network that surrounds the entire glomeruli (Shepherd, 1972). Layer III, the external plexiform layer, carries the main and secondary dendrites of mitral cells and tufted cells. Displaced mitral and tufted cells, primarily the latter and their processes, are also seen in this layer (Bhatnagar et al., 1987). Extrinsic fibers from the telencephalon also enter this layer. Layer IV, the mitral cell layer, contain the cell bodies of mitral cells. Each mitral cell emits a principal dendrite to a glomerulus, secondary dendrites to the external plexiform layer, and an axon to the olfactory tract. A few granule cells are also found in layer IV. Mitral cells of the olfactory bulb are the largest neurons of the olfactory bulb, and for that reason they are able to send projections to distant cortical regions. This layer of cell bodies occurs in a narrow band, typically densely stained (Fig. 2.8).
ANATOMY OF THE OLFACTORY SYSTEM Layer V, the internal plexiform layer, contains mitral cell axons, recurrent and deep collaterals of mitral and tufted cells, and granule cells with their spine-covered processes. Granule cells are typical in not having an axon. Two principal spine-bearing dendrites of these cells ramify terminating in the external plexiform layer. Layer VI contains granule cell columns and projections of the olfactory bulbar axons described earlier. Granule cells are the most numerous of the cell types in the bulb (Farbman, 1992). The cell bodies are in this layer, while their dendrites project to other layers. Bulbar cells have functions related to transmission or to modulation. Mitral and tufted cells are those with projections to other brain regions. In mammals, including humans, each of these cells has a single primary dendrite that is associated with a particular glomerulus. Each mitral and tufted cell has secondary or collateral dendrites that project into the external plexiform layer. In rodents, two types of mitral cells have been identified (Orona et al., 1984), one of which is more responsive to OSN stimulation (Wellis et al., 1989). Tufted cells are more diverse than mitral cells and occur at multiple levels throughout the thickness of the external plexiform layer (Farbman, 1992). External and middle tufted cells have restricted projections within the cortex. The internal tufted cells, like mitral cells, project more extensively, including to deeper cortical levels (Nagayama et al., 2010). Malz et al. (2000) observed that diverse mitral and tufted cells communicate with one another and, thus, function distinctly within a laminar framework of the olfactory bulb. In other words, there are functional subclasses of these neurons. Periglomerular and granule cells modulate the transmission of olfactory signals. Periglomerular cells cover the glomerulus and have reciprocal dendrodendritic synapses with tufted and mitral cells. Granule cells lack axons, but have multiple branched dendrites. Many of the synapses with tufted cells and mitral cells occur in the external plexiform layer. Thus, the cells involved in signal projection have dendritic synapses in two parts of the bulb with these two cell types. These cells may refine olfactory reception. For example, lateral inhibition may take place in the glomerular layer with periglomerular cells of a particular glomerulus, inhibiting responses in neighboring glomeruli. It is thought that at high enough concentrations, a particular odorant might invoke this response. This may enhance odorant detection by reducing the signal to noise ratio (Farbman, 1992). In this model, a central glomerulus is a source of strong excitation and is transmitted by a mitral cell, and the surrounding field of glomeruli is inhibited (dense receptive field hypothesis). An alternative model has been discussed for olfaction (sparse receptive field hypothesis; Fantana et al., 2008), and this continues to
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be a point of discussion based on rodent models for olfactory function (Lehmann et al., 2016). Neurons of the olfactory cortex and anterior olfactory nucleus provide centrifugal projections to the olfactory bulb. Other inputs to this region come from neurons in the diagonal band of Broca, to the granule cell layer and the glomerular layer. The olfactory tract continues along the olfactory sulcus on the orbital surface of the frontal lobe. The neurons of the granule cell layer constitute the anterior olfactory nucleus continuing into the lateral, intermediate, and medial striae and olfactory trigone to the prepiriform cortex, the anterior perforated substance, and precommissural septal regions (see Truex and Carpenter’s Neuroanatomy, 1969, Fig. 3.15, p. 46). An intermediate stria sometimes forms leading into an olfactory tubercle. These striae are thinly covered with the gray matter of the corresponding olfactory gyri. The largest cortical olfactory area is the piriform cortex. Other areas such as the anterior olfactory nucleus, entorhinal and insular cortex and amygdale and areas of limbic lobe also receive direct inputs from the olfactory bulb, without any relay in the thalamus. The connectivity beyond the olfactory bulb is not so well understood, nor have such studies been undertaken in humans. Modern imaging techniques are beginning to provide some functional information of higher level communications (Savic, 2005).
SUMMARY The olfactory system has peripheral and central subdivisions. The peripheral olfactory system includes the olfactory epithelium and nerve fascicles, and the central olfactory system corresponds to the olfactory bulb and its central connections. The olfactory epithelium extends its surface area over only one or two turbinates (or conchae) and adjacent surfaces in humans (compared to three or more olfactory turbinates in many other mammals). Many mammals possess an “accessory olfactory system” that may be specialized to detect certain types of odorants (e.g., based on molecular weight). To this day, studies continue to debate the presence and functionality of this system in humans (Stoyanov et al., 2016). However, all thorough studies, which have histologically examined human nasal septal tissues, have demonstrated that humans bear only a nonfunctioning vestige of its peripheral element, the VNO (Smith et al., 1998; Trotier et al., 2000; Bhatnagar and Smith, 2001; Witt et al., 2002). Grossly, the human olfactory anatomy is relatively reduced compared to most mammals. Yet, the olfactory pathway (excluding the vomeronasal pathway) has a great degree of similarity at a microanatomical level to that of other mammals. The human olfactory system
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contains a full complement of functional elements, including a constantly regenerating population of olfactory sensory neurons. These send long ciliary processes that form a mucus-covered mat in which odorants are detected. The olfactory sensory neurons send their axons to the first synapse point of the olfactory system, within the olfactory bulb. Mitral and tufted cells then relay impulses from the bulb to other brain regions. It is likely that functional subclasses of mitral and tufted cells exist, with differing electrophysiological properties and distinct communications throughout the laminar organization of the olfactory bulb. Although the bulk of our knowledge on how olfactory cells function comes from studies of laboratory animals, the human olfactory system is very similar to that of most other mammals. For example, the human olfactory bulb has the same organization and cell types as occurs in rodents, including the different layers of the olfactory bulb (Bhatnagar et al., 1987). A fascinating literature has been amassed on sexual dimorphism of the mammalian olfactory systems. For example, it has been reported that female rodents have a greater number of neurons within the main olfactory bulb than male rodents (Samaulhaq et al., 2008). A recent study has suggested female-biased dimorphism in humans as well (Oliveira-Pinto et al., 2014), which could explain the ability of women to perform better than men on a range of olfactory tests (Doty and Cameron, 2009).
ACKNOWLEDGMENTS The authors are grateful for the editorial input of Dr. R.L. Doty. We also thank Dr. Sam Marquez for generously provided original images.
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