Section 16: Special Senses

Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger, Jr., Ph.D.
Peer Review Status: Externally Peer Reviewed

Plates

The different sensations perceived by the human body are grouped into two major categories: those concerned with general sensations (touch, pressure, pain, and temperature) and those concerned with special sensations (olfaction, taste, vision, audition, and sense of position and movement). Illustrations of nerve endings concerned with general sensibility are found in Section 6, Nervous Tissue. This section is devoted to a consideration of the organs of special senses. Whereas nerve endings concerned with general sensibility are widely distributed, those concerned with special sensations are limited to specific areas of the body.

Olfaction

The olfactory organ is located in the mucous membrane lining the uppermost part of the roof of the nasal cavity. From the roof, the olfactory epithelium extends down both sides of the nasal cavity to cover most of the superior concha laterally and 1 cm of nasal septum medially. Man is a microsmatic animal in whom the surface area of olfactory mucous membrane in both nostrils is small (approximately 5 cm). The specialized nerve cells of the olfactory epithelium are highly sensitive to different odors. There are approximately 25 million nerve cells in each half of the nasal cavity. Olfactory neurons are continuously produced from basal cells of the olfactory epithelium and are continuously lost by the normal wear and tear process. The presence of these nerve cells at the surface exposes them unduly to damage; it is estimated that 1 per cent of the fibers of the olfactory nerves (axons of olfactory neurons) is lost each year of life because of injury to the perikarya. The sense of smell thus diminishes in the elderly as a result of the exposure of the olfactory epithelium to repeated infections and trauma in life. The presence of olfactory neurons at the surface represents the only exception to the evolutionary rule by which nerve cell bodies of afferent neurons migrate along their axons to take up more central and well-protected positions. The olfactory neurons (see Plate 298) are bipolar nerve cells with short peripheral processes (dendrites) reaching the surface of the epithelium and longer central processes (unmyelinated axons) that constitute the olfactory nerves. Olfactory nerve fibers enter the cranial cavity through foramina in the cribriform plate of the ethmoid bone and synapse on neurons in the olfactory bulb. The peripheral short processes end as bulbous enlargements (olfactory vesicles) bearing sensory receptor hairs. The surface of the epithelium is constantly moistened by secretions of Bowman's glands. The moistening of the epithelium helps dissolve the gaseous substances, facilitating stimulation of the olfactory epithelium. The continuous secretion prevents retention of dissolved odors.

It is believed that different basic odors stimulate different olfactory neurons that are not evenly distributed throughout the olfactory mucosa. Stimulation of different combinations of receptors for basic odors is believed to be the basis for man's ability to recognize all the varieties of odors to which he is exposed.

Taste

The gustatory (taste) sense organs in higher vertebrates are limited to the cavity of the mouth. The sensory organ of taste is the taste bud (see Plates 179 and 299), which is a pale, ovoid structure within the stratified squamous epithelium. It is estimated that one vallate papilla of the tongue contains 200 taste buds on its sides and about 50 buds in the wall of the trench opposite the papilla. This number decreases progressively with age. In addition to the vallate and fungiform papillae of the tongue, taste buds are found in the soft palate, oropharynx, and epiglottis. The taste bud contains neuroepithelial and supporting cells. There are approximately 4 to 20 receptor cells in the center of each taste bud. The apex of each receptor cell is modified into microvilli, which increase the receptor surface and project into an opening, the taste pore. Receptor cells decrease in number with age. The neuroepithelial cells are stimulated by substances in solution.

Although all taste buds look histologically alike, sensitivity to the four basic taste modalities is different in different regions of the tongue. Like olfaction, the sense of taste is a chemical sense. Although man can taste a large number of substances, only four primary taste sensations are identified: sour, salty, sweet, and bitter. Most taste receptors respond to all four primary taste modalities at varying thresholds but respond preferentially at a very low threshold to only one or two. Thus, taste buds at the tip of the tongue respond best to sweet and salty substances, and those at the lateral margins and posterior part of the tongue respond best to sour and bitter substances, respectively. The mechanism by which a substance is tasted is not well understood. Substances in solution enter the pore of the taste bud and come in contact with the surface of taste receptors. This will induce a change in the electrical potential of the membrane of the receptor cells (receptor or generator potential). The receptor potential will in turn generate an action potential in nerve terminals in opposition to the receptor cell surface.

Taste sensations from the anterior two thirds of the tongue are mediated to the central nervous system via the chorda tympani of the seventh (facial) cranial nerve, those from the posterior one third of the tongue via the ninth (glossopharyngeal) cranial nerve, and those from the epiglottis and lower pharynx via the tenth (vagus) nerve. These nerves contain the peripheral processes of pseudounipolar sensory nerve cells located in the geniculate ganglion (seventh nerve), petrous ganglion (ninth nerve), and nodose ganglion (tenth nerve). These peripheral processes enter the deep ends of the taste buds and establish intimate contact with the neuroepithelial cells of the buds. The central processes of these sensory neurons project to the nucleus of the tractus solitarius in the brain stem.

Vision

Vision is by far the most important of man's senses. Most of our perception of the environment around us comes through our eyes. Our visual system is capable of adapting to extreme changes in light intensity to allow us to see clearly; it is also capable of color discrimination and depth perception.

The organ of vision is the eye; accessory structures include the eyelids, lacrimal glands, and the extrinsic eye muscles. The eye has been compared to a camera. Whereas structurally the two are similar, the camera lacks the intricate automatic control mechanism involved in vision. As an optical instrument, the eye has four functional components: a protective coat, a nourishing lightproof coat, a dioptric system, and a receptive integrating layer. The protective coat is the tough, opaque sclera, which covers the posterior five sixths of the eyeball; it is continuous with the dura mater around the optic nerve. The anterior one sixth is covered by the transparent cornea, which belongs to the dioptric system. The nourishing coat is made up of the vascular choroid, which supplies nutrients to the retina and, because of its rich content of melanocytes, acts as a light-absorbing layer. it corresponds to the pia-arachnoid layer of the nervous system. Anteriorly, this coat becomes the ciliary body and iris. The iris ends at a circular opening, the pupil. The dioptric system includes the cornea, the lens, the aqueous humor within the anterior eye chamber, and the vitreous body. The dioptric system helps focus the image on the retina. The greatest refraction of incoming light takes place at the air-cornea interface. The lens is supported by the suspensory ligament from the ciliary body (see Plates 302 and 303), and changes in its shape permit change of focus. This is a function of the ciliary muscle, which is supplied by the parasympathetic nervous system. In late middle age, the lens loses its elastic properties and a condition known as presbyopia results, wherein accommodative power is diminished, especially to near vision. The amount of light entering the eye is regulated by the size of the pupil. Pupillary size is controlled by the action of the constrictor and dilator smooth muscles of the iris. The constrictor muscle is supplied by the parasympathetic nervous system, and the dilator by the sympathetic nervous system.

The receptive integrating layer is the retina, which is an extension of the brain, to which it is connected by the optic nerve. The rods and cones are the sensory retinal receptors (see Plates 306 to 308). The rods are about 20 times as numerous as the cones. The rods and cones differ in their distribution along the retina. in humans, a modified region of the retina, the fovea, contains only cones and is adapted for high visual acuity. At all other points along the retina, rods greatly outnumber cones. Rods function best for peripheral vision and during dim light vision; cones function for central vision, during bright light vision, and in color discrimination. The outer segments of rods and cones contain the visual pigments, rhodopsin and iodopsin (cone opsin), respectively. Light falling on these pigments results in a series of chemical changes leading to depolarization of the receptor cell membrane (receptor or generator potential) and the formation of an action potential, which is then conducted to the brain.

Audition

The organ of hearing (the organ of Corti) is located in the scala media (cochlear duct) of the inner ear and is separated from the underlying scala tympani by the basilar membrane (see Plates 311 and 312). Sound waves reaching the tympanic membrane will initiate vibrations that are transmitted through the bony ossicles of the middle ear to the oval window. Vibrations of the oval window are transmitted to the perilymph in the scala vestibuli and across the vestibular membrane to the endolymph of the cochlear duct (see Plate 312). Such induced pulsations in the endolymph will displace the basilar membrane on which the organ of Corti lies (see Plate 312) and alter the relationship of the tectorial membrane, which overlies the organ of Corti, to the hairs of the hair cells. Thus, bending or stretching of the hairs acts as a stimulus to the hair cells, causing release of a chemical neurotransmitter, generation of a receptor potential, and subsequent development of an action potential in the peripheral processes of bipolar neurons of the spiral ganglion (see Plate 105). The central processes of bipolar neurons constitute the auditory component of the eighth cranial nerve, which projects centrally to the cochlear nuclei. In man, the cochlea and the organ of Corti follow a spiral course of two and one half turns. The lower turns are wider than the apical turns. It is believed that the hair cells in the lower turns respond best to high frequency sounds, whereas those of the upper turns respond best to low-frequency sounds. Exposure to excessively loud sound as occurs in discos and around jet engines results in damage to hair cells in the lower turns of the cochlea (high-tone deafness).

Position and Movement (Vestibular Sensations)

The receptor organ of posture and equilibrium is a composite one located in the semicircular canals, the utricle, and the saccule of the inner ear. The utricle and saccule are located in the main cavity of the bony labyrinth, the vestibule; the semicircular canals, three in number, are extensions from the utricle. The dilated ends of the semicircular canals (ampullae) contain the cristae (see Plate 314), which constitute the neurosensory epithelium that responds to changes in rotational or angular acceleration. The apical processes of receptor cells are embedded in a dome-shaped, gelatinous protein- polysaccharide mass, the cupula (see Plate 314). The cupula swings from side to side in response to currents in the endolymph bathing it. Movement of the cupula bends or deforms the hairs of receptor cells embedded within it and thus modifies the rate of impulse discharge from these receptor cells. Each crista is stimulated by movements occurring in the plane of its semicircular canal. The neuroepithelial component of the utricle and saccule (macula; see Plate 315) provides information regarding static equilibrium and position of the head in space. The macula is similar in structure to the crista of the semicircular canals. The apical processes of receptor cells in the macula project into a gelatinous mass, the otolithic membrane. It is flat and contains numerous small crystalline bodies, the otoliths or otoconia, composed of calcium carbonate and protein. Gravitational pull acts on the otoconia on the surface of the macula, and the hair tufts of underlying neuroepithelial hair cells are thus stimulated. Stimuli from the vestibular sense organs travel by way of the peripheral processes of the bipolar neurons of the ganglion of Scarpa. The central processes form the vestibular component of the eighth nerve. Although we are normally not aware of the vestibular component of our sensory experience, this component is essential for the coordination of motor responses, eye movements, and posture.

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