Special Senses

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Chapter 12 Special Senses

The special senses of olfaction, vision, taste, hearing and balance are conveyed to the brain in cranial nerves. In each case, highly specialized peripheral receptors respond to stimuli in the external environment or our relationship to it. The olfactory system has an ancient lineage, reflected by the fact that afferent olfactory pathways proceed directly to the cerebral cortex and bypass the thalamus. Its terminal fields are, likewise, primitive cortical areas in a phylogenetic sense and are considered to be parts of the limbic system. All other special senses have a thalamic representation that projects to specialized regions of the neocortex. The integrative functions related to the various special senses (e.g. control of ocular gaze) are also included here. Of particular importance is a detailed discussion of retinal functional anatomy.

Olfaction

Olfactory pathways subserving the sense of smell are described in this section. Details of the relationship between the olfactory pathways and the limbic system are shown in Figure 16.7.

The olfactory nerves arise from olfactory receptor neurones in the olfactory mucosa. The axons collect into approximately 20 bundles and enter the anterior cranial fossa by passing through the foramina in the cribriform plate. They attach to the inferior surface of the olfactory bulb, which is situated at the anterior end of the olfactory sulcus on the orbital surface of the frontal lobe, and terminate in the bulb. Apparently unique in the nervous system, olfactory receptor neurones are continually replaced throughout life by differentiation of stem cells in the olfactory mucosa. The olfactory bulb is continuous posteriorly with the olfactory tract, through which the output of the bulb passes directly to the olfactory cortex.

There is a clear laminar structure in the olfactory bulb (Fig. 12.1). From the surface inward, the laminae are the olfactory nerve layer, glomerular layer, external plexiform layer, mitral cell layer, internal plexiform layer and granule cell layer.

Fig. 12.1 Organization of the olfactory bulb. The radial organization of the bulb into ‘layers,’ with their principal neurone types, and an indication of their main connections are shown. Red, mitral and tufted neurones and their processes; light blue, internal granule neurones; dark blue, dopaminergic periglomerular neurones; black, olfactory receptor neurones and their processes. The olfactory tract consists of (1) centripetal axons of mitral and tufted cells, some of which synapse with neurones in the anterior olfactory nucleus, and (2) centrifugal axons (yellow), which terminate in the different zones indicated.

The olfactory nerve layer consists of unmyelinated axons of the olfactory neurones. The continuous turnover of receptor cells means that axons in this layer are at different stages of growth, maturity or degeneration. The glomerular layer consists of a thin sheet of glomeruli, where the incoming olfactory axons divide and synapse on terminal dendrites of secondary olfactory neurones—that is, mitral, tufted and periglomerular cells. The external plexiform layer contains the principal and secondary dendrites of mitral and tufted cells. The mitral cell layer is a thin sheet composed of the cell bodies of mitral cells, each of which sends a single principal dendrite to a glomerulus, secondary dendrites to the external plexiform layer and a single axon to the olfactory tract. It also contains a few granule cell bodies. The internal plexiform layer contains axons, recurrent and deep collaterals of mitral and tufted cells and granule cell bodies. The granule cell layer contains the majority of the granule cells and their superficial and deep processes, together with numerous centripetal and centrifugal nerve fibres that pass through the layer.

The principal neurones in the olfactory bulb are the mitral and tufted cells; their axons form its output via the olfactory tract. These cells are morphologically similar, and most use an excitatory amino acid, probably glutamate or aspartate, as their neurotransmitter. The mitral cell spans the layers of the bulb and receives the sensory input superficially at its glomerular tuft. The axons of mitral and tufted cells appear to be parallel output pathways from the olfactory bulb. It is not known whether they receive inputs from different olfactory sensory neurones.

The main types of intrinsic neurones in the olfactory bulb are periglomerular cells and granule cells. The majority of periglomerular cells are dopaminergic (cell group A15); some are GABAergic. Their axons are distributed laterally and terminate within extraglomerular regions. Granule cells are similar in size to periglomerular cells. Their most characteristic feature is the absence of an axon, hence their resemblance to amacrine cells in the retina. Granule cells have two principal spine-bearing dendrites that pass radially in the bulb to ramify and terminate in the external plexiform layer. They appear to be GABAergic. The granule cell is likely to be a powerful inhibitory influence on the output neurones of the olfactory bulb.

Centrifugal inputs to the olfactory bulb arise from a variety of central sites. Neurones of the anterior olfactory nucleus and collaterals of pyramidal neurones in the olfactory cortex project to the granule cells of the olfactory bulb. Cholinergic neurones in the horizontal limb nucleus of the diagonal band of Broca, part of the basal forebrain cholinergic system, project to the granule cell layer and also to the glomerular layer. Other afferents to the granule cell layer and the glomeruli arise from the pontine locus coeruleus and the mesencephalic raphe nucleus.

The olfactory tract leaves the posterior pole of the olfactory bulb to run along the olfactory sulcus on the orbital surface of the frontal lobe (see Fig. 16.7). The granule cell layer of the bulb is extended into the olfactory tract as scattered medium-sized multipolar neurones that constitute the anterior olfactory nucleus. They continue into the olfactory striae and trigone to the grey matter of the prepiriform cortex, anterior perforated substance and precommissural septal areas. Many centripetal axons from mitral and tufted cells relay in, or give collaterals to, the anterior olfactory nucleus; the axons from the nucleus continue with the remaining direct fibres from the bulb into the olfactory striae.

As the olfactory tract approaches the anterior perforated substance, it flattens and splays as the olfactory trigone. Fibres of the tract continue from the caudal angles of the trigone as diverging medial and lateral olfactory striae, which border the anterior perforated substance. An intermediate stria sometimes passes from the centre of the trigone to end in a small olfactory tubercle. The lateral olfactory stria follows the anterolateral margin of the anterior perforated substance to the limen insulae, where it bends posteromedially to merge with an elevated region, the gyrus semilunaris, at the rostral margin of the uncus in the temporal lobe (see Fig. 16.7). The lateral olfactory gyrus forms a tenuous grey layer covering the lateral olfactory stria; it merges laterally with the gyrus ambiens, part of the limen insulae. The lateral olfactory gyrus and gyrus ambiens form the prepiriform region of the cortex, passing caudally into the entorhinal area of the parahippocampal gyrus. The prepiriform and periamygdaloid regions and the entorhinal area (area 28) together make up the piriform cortex. The medial olfactory stria, covered thinly by the grey matter of the medial olfactory gyrus, passes medially along the rostral boundary of the anterior perforated substance toward the medial continuation of the diagonal band of Broca. Together, they curve up on the medial aspect of the hemisphere, anterior to the attachment of the lamina terminalis. The diagonal band enters the paraterminal gyrus. The medial stria becomes indistinct as it approaches the boundary zone, which includes the paraterminal gyrus, parolfactory gyrus and, between them, prehippocampal rudiment (see Fig. 16.7).

The olfactory cortex receives a direct input from the olfactory bulb, which arrives via the olfactory tract without relay in the thalamus. The largest cortical olfactory area is the piriform cortex. The anterior olfactory nucleus, olfactory tubercle, regions of the entorhinal and insular cortex and amygdala also receive direct projections from the olfactory bulb.

The entorhinal cortex (Brodmann’s area 28) is the most posterior part of the piriform cortex and is divided into medial and lateral areas (areas 28a and 28b). The lateral parts receive fibres mainly from the olfactory bulb and also from the piriform and periamygdaloid cortices.

Projections from the piriform olfactory cortex are widespread and include the neocortex (especially the orbitofrontal cortex), thalamus (especially the medial dorsal thalamic nucleus), hypothalamus, amygdala and hippocampal formation.

Vision

Eye

The eyeball, the peripheral organ of vision, is situated in the orbit, a skeletal cavity whose walls help protect the eye from injury (Fig. 12.2). The orbit also has a more fundamental role in the visual process itself: it provides rigid support and direction to the eye and forms the sites of attachment for its external muscles. This setting permits the accurate positioning of the visual axis under neuromuscular control and determines the spatial relationship between the two eyes—essential for binocular vision and conjugate eye movements.

Fig. 12.2 The organization of the eye, viewed from above. In this illustration, the left eye and part of the lower eyelid are depicted in horizontal section and also cut away to show internal structure.

The eyeball is embedded in orbital fat, separated from it by a thin fascial sheath. It is composed of the segments of two spheres of different radii. The anterior segment, part of the smaller sphere, is transparent and forms approximately 7% of the surface of the whole globe. It is more prominent than the posterior segment, which is part of the larger sphere and opaque and forms the remainder of the globe. The anterior segment is bounded by the cornea and the lens and is incompletely subdivided into anterior and posterior chambers by the iris. These chambers are continuous through the pupil. The anterior chamber is slightly overlapped by the sclera peripherally. The angle between the iris and cornea, therefore, forms an anulus of greater diameter than the limbus, the junction between the sclera and the cornea. The difference between these two varies from 1 to 2 mm, the angle being deeper above and below than at the sides of the eyeball. The posterior chamber lies between the posterior surface of the iris and the anterior aspect of the lens and its supporting ligament, the zonule, and is triangular in section. The apex of the triangle is the point where the iris touches the lens; the base, or zonular region, extends among the collagenous bundles of the zonule, sometimes even into a retrozonular space between the zonule and the vitreous humour in the posterior segment of the eyeball. The posterior segment consists of the parts of the eye posterior to the zonule and lens.

The anterior pole is the centre of the anterior (corneal) curvature, and the posterior pole is the centre of its posterior (scleral) curvature; a line joining these two points forms the optic axis. (By the same convention, the eye has an equator, equidistant between the poles: any circumferential line joining the poles is a meridian.) The optic axes of the two eyes, in their primary position, are parallel and do not correspond with the orbital axes, which diverge anterolaterally at a marked angle to each other. The optic nerves follow the orbital axes and are therefore not parallel; each enters its eye approximately 3 mm medial (nasal) to the posterior pole.

Visual Pathway

The visual pathway is illustrated in Figure 12.3. The first-order neurone of the visual system is a bipolar cell that is contained entirely within the retina. The second-order neurone is a ganglion cell whose axon enters the optic nerve.

Fig. 12.3 The visual pathway, showing the spatial arrangement of neurones and their fibres in relation to the quadrants of the retinae and visual fields. The proportions at various levels are not exactly to scale; in particular, the macula is exaggerated in size in the visual fields and retinae. In each quadrant of the visual field and the parts of the visual pathway subserving it, two shades of the respective colour are used—paler for the peripheral fields, and darker for the macular part of the quadrant. From the optic tract onward, these two shades are both more saturated to denote the intermixture of neurones from both retinae, with the palest shade reserved for parts of the visual pathway concerned with monocular vision. The pathway subserving the pupillary light reflex is also indicated.

The optic nerves pass posteromedially into the cranial cavity and meet in the midline, forming the optic chiasma, a flat mass of decussating fibres that lies at the junction of the anterior wall and floor of the third ventricle. The tuber cinereum and infundibulum lie posterior to the chiasma, and the third ventricle is dorsal to them. The termination of the internal carotid artery and the anterior perforated substance are lateral relations. The optic recess of the third ventricle passes over its superior surface to reach the lamina terminalis.

Optic nerve fibres arising from the nasal half of each retina, including half of the macula, cross in the chiasma to enter the contralateral optic tract. Fibres from the temporal hemiretinae continue into the ipsilateral optic tract. Decussating fibres loop a little backward into their ipsilateral optic nerve before crossing and then passing forward into the contralateral optic tract. Macular fibres, and those from an adjacent central area, occupy almost two-thirds of the central chiasma, dorsal to all peripheral decussating fibres. The most ventral axons are nasal fibres concerned with monocular fringes of the binocular field. They lie beneath fibres from the extramacular parts of both nasal hemiretinae, which occupy an intermediate position in the chiasma.

The optic chiasma is supplied with blood from a pial plexus that receives branches from the superior hypophysial, internal carotid, posterior communicating, anterior cerebral and anterior communicating arteries. The venous drainage of the chiasma is into the basal and anterior cerebral venous system.

Behind the optic chiasma, the optic tracts diverge dorsolaterally, each passing between the anterior perforated substance and tuber cinereum. The tract curves around the cerebral peduncle, to which it adheres. Optic tract fibres terminate primarily in the lateral geniculate nucleus of the thalamus, but also in the superior colliculus, pretectal area, suprachiasmatic nucleus of the hypothalamus and inferior pulvinar.

Axons from third-order visual neurones in the lateral geniculate nucleus run in the retrolenticular part of the internal capsule and form the optic radiation, which curves dorsomedially to the occipital cortex. Fibres representing the lower half of the visual field sweep superiorly to reach the visual cortex above the calcarine sulcus. Those representing the upper half of the visual field curve inferiorly into the temporal lobe (Meyer’s loop) before reaching the visual cortex below the calcarine sulcus.

Some neurones in the occipital cortex send descending axons to the superior colliculus, which therefore receives cortical and retinal afferents. From there, fibres travel by tectobulbar tracts to motor nuclei of the third, fourth, sixth and eleventh cranial nerves and the ventral horn of the spinal cord.

Retina

The retina is the sensory neural layer of the eyeball (Figs 12.4–12.6). It is a very complex structure and should be considered a special area of the brain, from which it is derived by outgrowth from the diencephalon (Ch. 15). It is dedicated to the detection and early analysis of visual information and is an integrated part of the much larger apparatus of visual analysis present in the thalamus, cortex and other areas of the central nervous system.

Fig. 12.4 Ophthalmoscopic photographs of the right human retina. A, Note the dichotomous branching of vessels. Arteries are brighter red and show a more pronounced ‘reflex’ to light, as a pale stria along their length. The veins are also larger in calibre; more of them cross arteries superficially than is usual. The optic disc, around the entry of the vessels, is a light pink, with a surrounding zone of heavier pigmentation. Compare with Figure 12.5A from the same Caucasian adult. B, Appearances in a heavily pigmented individual (an adult of African origin), with a paler optic disc than in A. Note accentuation of the edge of the disc by retinal and choroidal pigmentation. The arteries cross the veins superficially in this retina. C, Normal macula of a young Caucasian subject. The vessels radiate from the centrally placed fovea. The macular branches of the central retinal artery are approaching from the right. The macula is largely free of vessels of macroscopic size, but the capillaries here form a particularly close network, except at the fovea. D, The region of the optic disc in an eye with poorly developed pigmentation. Three cilioretinal arteries are curving around the edge of the disc (two on the left, one on the right). Between the two cilioretinal arteries, a single macular artery is apparent. Due to the depressed pigmentation, choroidal vessels are also visible, especially veins; on the left of the photograph, two large vorticose venous tributaries can be seen.

Fig. 12.5 Fluorescence angiograms of the retina. These are produced by photography with a fundus camera at known periods of time following the introduction of fluorescein into the circulation. A, Angiogram of the same retina shown in Figure 12.4A, taken in ‘mid-venous’ phase. The arteries display an even fluorescence, but the veins appear striped, owing to laminar flow. This appearance is the reverse of the arterial ‘reflex’ shown in Figure 12.4A and should not be compared with it. The background mottling is due to fluorescence from the choroidal vessels. B, Angiogram of the left optic disc, showing the major arteries and veins and also their smaller branches. Note particularly the radial pattern in the retinal capillaries. The laminar flow in the veins is less obvious than in Figure 12.4A. C, Angiogram showing the macular region of a right eye. The main macular vessels are approaching from the right. The subject is an elderly person with considerable macular pigmentation, which masks fluorescence from the choroidal circulation. D, Angiogram of the macula of a young subject (left eye) showing the macular capillaries in detail. Note the central avascular fovea. Compare with C.

Fig. 12.6 Section through the fovea centralis.

(By permission from Young, B., Heath, J.W., 2000. Wheater’s Functional Histology, Churchill Livingstone, Edinburgh.)

The retina lies between the choroid externally and the vitreous body internally. It is thin, being thickest (0.56 mm) near the optic disc; it diminishes to 0.1 mm anterior to the equator and continues at this thickness to the ora serrata. It also thins locally at the fovea of the macula. The retina is continuous with the optic nerve at the optic disc. Anteriorly, at the ora serrata, a thin, non-neural prolongation of the retina extends forward over the ciliary processes and iris as the ciliary and iridial parts of the retina, respectively; they consist of pigmented and columnar epithelial layers only. The optic part of the retina extends from the optic disc to the ora serrata. It is soft, translucent and purple in the fresh, unbleached state because of the presence of rhodopsin (visual purple), but it soon becomes opaque and bleached when exposed to light.

Near the centre of the retina, there is a region 5 to 6 mm in diameter that contains the macula lutea (see Fig. 12.5C, D), an elliptical yellowish area measuring approximately 2 mm horizontally and 1 mm vertically. Its colour is due to the presence of xanthophyll derivatives. The macula lutea contains a central depression, the fovea centralis or foveola, with a diameter of approximately 0.4 mm, where visual resolution is highest (see Fig. 12.6.) Here, all elements except pigment epithelium cone photoreceptors are displaced laterally. The minute size of the foveola is the reason why the visual axes must be directed with great accuracy to achieve the most discriminative vision.

About 3 mm medial (nasal) and 1 mm superior to the foveola, the optic nerve becomes continuous with the retina at the optic disc (‘blind spot’). It is approximately 1.5 mm in diameter. The name ‘optic papilla,’ which is often applied to the disc, is a misnomer because almost all of a normal disc is level with the retina. Centrally, it contains a shallow depression, where it is pierced by the central retinal vessels (see Figs 12.2, 12.4, 12.5A, B). The disc is devoid of photoreceptors and is therefore insensitive to light. By ophthalmoscopy, the disc is normally pink, but it is much paler than the retina and may be grey or almost white. In optic atrophy the capillary vessels disappear, and the disc is then white.

Microstructure

The retina is derived from the two layers of the invaginated optic vesicle; the outer layer becomes the layer of pigment cells, and the inner layer develops into a complex multilaminar structure of sensory and neural cells. Anteriorly, sensory and neural cells are absent from the retina as it approaches the ora serrata and merges into the ciliary body and iris.

The neural retina contains a variety of cell types, including photoreceptors (rod and cone cells), their first-order neurones (bipolar cells) and the somata and axons of the second-order neurones (ganglion cells); it also contains two major classes of interneurones, the horizontal and amacrine cells. In addition, the retina contains neuroglial elements and a rich vascular system, chiefly of capillaries. It is backed by specialized pigment epithelial cells.

Layers of the Retina

The retina is organized into layers or zones (Fig. 12.7), where distinctive components of its cells are clustered together or in register to form continuous strata. These layers extend uninterrupted throughout the photoreceptive retina except at the exit point of the optic nerve fibres at the optic disc, although certain layers are much reduced at the foveola where the photoreceptive elements predominate. The names given to the different layers reflect, in part, the components present within them, and also their position in the thickness of the retina. Conventionally, those structures farthest from the vitreous (i.e. toward the choroid) are designated as outer or external, and those toward the vitreous are inner or internal.

Fig. 12.7 Retina of a 60-year-old man. The section was made close to the optic nerve head, explaining the thickened nerve fibre layer. 1, pigment epithelial layer; 2, rod and cone layer; 3, external limiting membrane; 4, outer nuclear layer; 5, outer plexiform layer; 6, inner nuclear layer; 7, inner plexiform layer; 8, ganglion cell layer; 9, nerve fibre layer; 10, internal limiting membrane; 11, choroid; 12, sclera.

(Courtesy of the late Gordon L. Ruskell, Department of Optometry and Visual Science, The City University, London.)

Customarily, 10 retinal layers are distinguished (Fig. 12.8), beginning at the choroidal edge and passing toward the vitreous. These are the retinal pigment epithelium, layer of rods and cones (outer and inner segments), external limiting membrane, outer nuclear layer, outer plexiform layer, inner nuclear layer, inner plexiform layer, ganglion cell layer, nerve fibre layer, and internal limiting membrane. Some of these are subdivided into substrata, and an innermost plexiform layer between layers 8 and 9 has also been demonstrated.