Chapter summary

The special senses include smell, taste, vision, hearing, and equilibrium.

Olfaction: The sense of smell

Both smell and taste are chemical senses involving chemoreceptors. Olfaction involves the detection of volatile chemicals in solution.

Anatomy of olfactory receptors

1. The olfactory epithelium is located in the roof of the nasal cavity along the inferior surface of the cribriform plate of the ethmoid bone and extend­ing along the superior nasal concha and upper part of the middle nasal concha.

2. Olfactory receptors are bipolar neurons. At their apical end, the dendrite forms a knob from which several long cilia project. Axons from the olfac­tory receptors project into the olfactory bulb.

3. The supporting cells are columnar epithelial cells surrounding the olfactory receptors.

4. Basal stem cells are found between the support­ing cells. They continually undergo cell division, producing new olfactory receptors.

5. Olfactory (Bowman's) glands found in the con­nective tissue that supports the olfactory epi­thelium produce mucus that is carried to the surface of the olfactory epithelium and dissolves odorants.

6. The facial nerve (cranial nerve VII) innervates supporting cells and olfactory glands.

Physiology of olfaction

Odorants dissolve in the mucus membrane stimulat­ing G proteins, thus activating adenylate cyclase and producing cAMP, which causes Na⁺-channels to open. The influx of Na⁺ results in depolarization of the olfactory receptor.

Olfactory pathway

The axons of the olfactory receptors form the olfac­tory nerve (cranial nerve I), which terminates on neurons in the olfactory bulbs. Neurons from the olfactory bulb project to the lateral olfactory area in the temporal lobe. The olfactory neurons also project to the hypothalamus and other limbic areas, thus explaining how smell can evoke various memories and emotions.

Gustation: The sense of taste

Tongue

1. The tongue is involved in gripping, reposition­ing food, mixing food with saliva, and forming the compact mass of food called a bolus.

2. The surface of the tongue is covered with small bumps called papillae. Filiform papillae are thorn-shaped, giving the tongue roughness, thus aiding in licking and manipulating food. Fungi­form papillae are mushroom-shaped, have taste buds, and are thus mechanical and gustatory. Foliate papillae have leaf-shaped ridges, are located on the lateral borders of the tongue, and have a gustatory function. Vallate, or circumval­late, papillae are the largest and least numerous,

and resemble the fungiform papillae but are circled by a cleft containing taste buds.

Taste

1. Animals have innate preferences for flavors such as those that are sweet, while tending to reject unpleasant flavors such as bitter flavors. Each papilla has one to hundreds of taste buds.

2. Taste cells are electrically excitable and have voltage-gated Na⁺, K⁺, and Ca⁺⁺ channels on their surface.

3. While it was formerly generally believed that animals had four primary tastes, including salty, sour, sweet, and bitter, recently, a fifth category was added, called umami, meaning "delicious" in Japanese. It is stimulated by monosodium glutamate.

4. The binding of taste stimuli, called tastants, to extracellular receptors located on taste cells leads to depolarization of the taste cell, leading to the release of an unknown neurotransmitter, causing excitation in the sensory neuron.

Vision

Accessory structures of the eye

The accessory structures of the eye include the eyelids, eyelashes, eyebrows, lacrimal (tearing) apparatus, and extrinsic muscles of the eye.

Eyelids, eyelashes, and eyebrow

1. The upper and lower eyelids, or palpebrae, cover the eye during sleep, protect the eye from exces­sive light and foreign objects, and assist with lubricating the eye.

2. The gap between the two eyelids is the palpebral fissure. At either corner of the eyelid is the lateral and medial commissure, respectively. A small, reddish elevated area found in the medial com­missure is the lacrimal caruncle. It contains both sebaceous (oil) and sudoriferous (sweat) glands.

3. Domestic species have a nictitating membrane or third eyelid.

4. Located at the margin of the eyelids are the cilia (eyelashes). Located above each eyelid are the eyebrows. Located at the base of the hair follicles of the eyelashes are sebaceous ciliary glands that release a lubricating fluid. Infection of these glands is a sty.

Lacrimal apparatus

The lacrimal apparatus consists of structures that produce and drain tears (lacrimal fluid). Lacrimal glands secrete lacrimal fluid through excretory lacri­mal ducts. The fluid moves over the eye and enters two small openings called the lacrimal puncta. From there, the fluid enters the lacrimal canals, two ducts leading into the lacrimal sac. The nasolacrimal duct carries fluid from the lacrimal sac into the nasal cavity.

Extrinsic eye muscles

Movement of the eyeball is controlled by extraoc­ular muscles. The lateral and medial rectus muscles move the eye laterally and medially, respectively. The superior rectus and inferior rectus muscles elevate and depress the eye, respectively. The inferior oblique muscle elevates and turns the eye laterally while the superior oblique depresses and turns the eye laterally.

Anatomy of the eyeball

The eyeball consists of three layers: (1) fibrous tunic, (2) vascular tunic, and (3) retina.

Fibrous tunic

The fibrous tunic, or external coat of the eyeball, is avascular and consists of the anterior cornea and posterior opaque sclera. The cornea is a transparent layer covering the iris, the colored portion at the front of the eye.

Vascular tunic

1. The vascular tunic, or uvea, is the middle layer of the eyeball. It consists of three parts: choroid, ciliary body, and iris.

2. Many species of domestic animals, including cats, dogs, horses, and ruminants, have an addi­tional layer in the choroid called the tapetum lucidum. The tapetum Iucidum reflects light back toward the retina so that the animal can see in low light.

3. In the anterior, the choroid becomes the ciliary body. It consists of the ciliary processes and ciliary muscles. The ciliary muscles are a bundle of smooth muscles that alter the shape of the lens in order to allow for near or far vision.

4. The iris is the colored portion at the front of the eyeball that is shaped like a disk with a hole, the pupil, in the center.

Retina (the sensory tunic)

1. The innermost layer of the eye is the retina, which lines the posterior portion of the eyeball. The retina consists of two layers: an outer pig­mented layer and an inner neural layer.

2. The neural layer of the retina has three major layers—the photoreceptor layer, the bipolar cell layer, and the ganglion cell layer. The outer and inner synaptic layers separate these layers from each other. Before reaching the photoreceptor layer, light must first pass through the ganglion and bipolar cell layers. Interspersed among these cells are two other types of neurons—horizontal cells and amacrine cells.

3. Axons from the ganglion cells collectively form the optic nerve, which exits the eye at the optic disc. Since the optic disc lacks photoreceptors, it is also called the blind spot.

4. There are two types of photoreceptors, rods and cones. Rods outnumber cones 20:1, except in birds, which have more cones than rods. Rods are effective in dim light. Cones provide for color and high acuity vision.

5. The macula Iutea is found in the exact center of the posterior of the retina. It contains mostly cones. At its center is a small pit, the central fovea, which contains only cones and where the bipolar and ganglion cells are displaced to the sides.

6. The avian retina is avascular, and contains a unique structure called the pecten. This is a black pigmented structure extending from the ventral retina up to just above the area where the optic nerve exits.

Lens

The lens is a biconvex, transparent, avascular struc­ture that can change its shape to focus light on the retina. The lens is held in place by the suspensory ligament attaching it to the choroid process.

Chambers of the eye

1. The lens divides the eye into the anterior and posterior segments. The iris subdivides the ante­rior segment into the anterior chamber located between the cornea and iris, and the posterior chamber between the iris and lens.

2. The anterior segment is filled with aqueous humor. If the drainage of aqueous humor is blocked, intraocular pressure increases, causing compression of the retina and optic nerve, leading to glaucoma and blindness.

3. The posterior segment of the eye contains vitre­ous humor, a clear gel-like substance that pushes the retina against the pigmented layer of the choroid.

Physiology of vision

The eye can be likened to a camera. An image is focused on the retina by the lens, and the amount of light entering the eye is controlled by the pupil. The retina, lens, and pupil are analogous to the film, lens, and aperture of the camera, respectively.

Refraction

When light rays pass from one medium to another of a different density, the speed of light changes. As a result, the light rays are bent or refracted. Images are inverted, both upside down and backward, as they are focused on the retina. The brain reinter­prets this image so that objects are not perceived as inverted.

Accommodation

1. Increasing the refractive power of the lens is called accommodation. Therefore, as an object moves closer to the eye, the lens must become more convex, in order to focus the image on the retina.

2. Accommodation is accomplished by the actions of the ciliary muscle. When the ciliary muscle is relaxed, the fibers surrounding the lens pull on the lens, thus making it fatter or less convex.

3. Some species of birds possess a static mechanism allowing them to keep objects in focus regardless of their distance. This is accomplished from asymmetries in the eye allowing it to be emme­tropic (i.e., light rays focus directly on the retina) in its upper portions while becoming increas­ingly myopic (i.e., nearsighted) toward its lower portions.

4. Horses have a limited accommodating ability due to weak ciliary muscles. To compensate, horses have a ramp retina that allows them to use a form of static accommodation in which they move their heads to focus an object at dif­ferent locations on the retina.

5. The far point of vision is that distance beyond which no accommodation (no change in lens shape) is necessary for focusing. The near point of vision is the closest point at which the animal can focus clearly.

Refraction problems

1. A normal eye is said to be emmetropic. As animals age, the lens loses its elasticity and its ability to accommodate, a condition called presbyopia.

2. Myopia, or nearsightedness, is when an animal can see close objects, but distant objects are blurred. In hypermyopia, also called hyperopia or farsightedness, the animal can see distant objects but is unable to focus near objects because the eyeball is too short. An irregular curvature in either the lens or cornea results in astigmatism.

Pupil diameter

The amount of light that can enter the eye is con­trolled by the diameter of the pupil. Circular muscle fibers control the pupil diameter.

Field of vision

1. The field of vision is the spatial area that can be seen by a single eye. The location of the eyes within the head has an impact on the field of vision. The field of vision of the two eyes gen­erally overlaps, providing an area of binocular vision.

2. The eye location varies between species and within breeds of species. The wider the set of the eyes, the greater the panoramic field of vision. Herbivores tend to have their eyes set wide, thus providing them with a panoramic field of vision.

Photoreception

1. Photoreception involves conversion of light energy to an electrical signal carried by the optic nerve.

2. Rods and cones consist of an outer segment involved in photoreception and an inner segment containing the cell nucleus, Golgi complex, and mitochondria.

3. Within the outer segments are stacks of membra­nous discs in which the visual pigments, or phot­opigments, are embedded.

4. There is one type of rod and three (four in birds) types of cones, distinguished by different visual pigments.

Chemistry of visual pigments

1. The light-absorbing photopigment in rods is rho­dopsin. It consists of opsin and a vitamin A derivative called retinal. The opsins found in the each of the three types of cones permit them to absorb primarily either blue, green, or yellow- orange wavelengths of light.

2. In the dark, retinal is found as 11-czs-retinal, which binds strongly with opsin. In rods, this forms rhodopsin. When exposed to light, cis- retinal is isomerized to all-fraπs-retinal, which no longer binds to retinal. Therefore, fraπs-retinal and opsin dissociate, thereby forming a colorless photopigment.

Light transduction by photoreceptor

1. In the dark, cyclic GMP (cGMP) binds to sodium channels located on the plasma membrane and keeps them open. The influx of sodium depo­larizes the membrane, allowing continuous release of the neurotransmitter glutamate, which induces inhibitory postsynaptic potentials in bipolar cells.

2. The presence of light converts 11-czs-retinal to all-fra/zs-retinal, causing opsin to dissociate from the photopigment. Opsin then interacts with a G-protein subunit called transducin an enzyme that breaks down cGMP to GMP The break­down of cGMP causes the sodium channels on the plasma membrane to close, thus hyperpolar­izing the membrane potential. This decreases the release of glutamate.

Retinal processing of visual information

Ganglion cells produce action potentials while the amacrine, horizontal, and bipolar cells produce graded potentials. Ganglion cells have circular recep­tive fields consisting of a circle within a circle. The circular zone at the centeris called the on-center area, while that on the periphery is called the off-center, or surround, area. The on-center ganglion cells are excited when light illuminates rods or cones in the central area, while the off-center area is inhibited.

Light or dark adaptation

1. The eye adapts to a sudden increase in light intensity by decreasing its sensitivity, a process called light adaptation. Light adaptation in­volves contraction of the pupil and bleaching of

photopigments. In addition, bright light causes the cGMP-gated channels to close.

2. Dark adaptation is essentially the reverse of light adaptation.

Visual pathway

1. The axons of the ganglion cells converge to form the optic nerve. At the optic chiasma, fibers from the medial portion of the eye cross to the oppo­site side and continue via the optic tracts. Each optic tract contains fibers from the temporal (lateral) aspect of the eye on the ipsilateral side and fibers from the nasal (medial) aspect of the contralateral eye.

2. Most of these axons then travel to the lateral geniculate body of the thalamus, where they synapse on neurons that travel through the inter­nal capsule forming the optic radiation. These fibers project to the primary visual cortex in the occipital lobes.

2. The vestibule is the central region of the bony labyrinth. Within the vestibule are the utricle and saccule. Projecting from the vestibule are three semicircular canals called the anterior, posterior, and lateral semicircular canals.

3. The cochlea, a bony spiral canal resembling a snail shell, lies anterior to the vestibule. The cochlea consists of three channels. The upper channel is the scala vestibule, the lower channel is the scala tympani. The third channel, lying between the other two, is the cochlear duct or scala media. The cochlear duct is separated from the scala vestibuli by the vestibular membrane and from the scala tympani by the basilar membrane.

4. The spiral organ, or organ of Corti, sits on the basilar membrane. It consists of epithelial cells, supporting cells, and hair cells, the receptors for hearing. On the apical (top) surface of each hair cell is a hair bundle consisting of many stereo­cilia and one long kinocilium, which are imbed­ded in the tectorial membrane.

Hearing and balance

Anatomy of the ear

The ear consists of three regions: the outer, middle, and inner ear.

Outer (external) ear

The outer ear consists of the pinna, or auricle, and the external acoustic meatus.

Middle ear

1. Separated from the outer ear by the tympanic membrane, the middle ear is an air-filled cavity within the temporal bone. It is separated from the inner ear by two openings: the superiorly located oval window and the round window.

2. Extending from the tympanic membrane to the oval window are three auditory ossicles called the malleus, incus, and stapes, commonly called the hammer, anvil, and stirrup, respectively.

Inner ear

1. The inner ear consists of two main sections: an outer bony labyrinth enclosing an inner mem­branous labyrinth. The bony labyrinth lies in the temporal bone and consists of (1) the semicircu­lar canals, (2) the vestibule, and (3) the cochlea. The first two contain receptors for equilibrium and the cochlea contains receptors for hearing.

Sound

The pitch is related to the frequency of the sound wave while the intensity is related to the amplitude of the sound wave.

Physiology of hearing

1. The stapes make contact with the oval window so that the sound wave is thus transferred to the perilymph in the scala vestibuli. This sound wave moves through the scala vestibuli, and into the scala tympani, which finally causes the round window to vibrate. This sound wave causes the basilar membrane to move up and down.

2. As the basilar membrane oscillates, it causes the cilia and kinocilia on the hair cells to shear.

3. If the hair cells shear toward the kinocilia, then the hair cell is depolarized.

Physiology of equilibrium

The ear is involved in the sense of balance. Receptors in the semicircular canals and vestibule collectively make up the vestibular apparatus, the part of the ear associated with equilibrium.

Otolithic organs within the utricle and saccule

1. On the walls of both the utricle and saccule is the macula. They are perpendicular to one another, and are mostly involved in static equilibrium, but also have a role in dynamic equilibrium.

2. Each macula consists of hair cells embedded in an otolithic membrane. The otolithic membrane is a jellylike mass on which sit crystals of calcium carbonate called otoliths.

3. As the head moves in a linear direction, the oto­liths cause the otolithic membrane to shear in the opposite direction. When the hair cells bend toward the single kinocilium, they depolarize, while bending in the opposite direction causes hyperpolarization. This causes a change in the impulse rate in the vestibular nerve.

The crista amputaris and dynamic equilibrium

1. The crista ampularis, located in the ampulla of each semicircular canal, is the receptor for dynamic equilibrium. It consists of supporting cells and hair cells. The hair cells have Sterocilia and one kinocilium that project into a gelatinous mass called the capula.

2. The crista ampularis responds to changes in velocity of rotation of the head. The inertia exerted by the endolymph in the semicircular canals causes the hair cells to bend in the oppo­site direction of movement, causing depolariza­tion or hyperpolarization of the hair cells.

Review questions and answers are available 3 online.

References

Constantinescu, G.M. 2001. Guide to Regional Ruminant Anatomy Based on the Dissection of the Goat. Iowa State Press, Ames, Iowa.

Constantinescu, G.M. 2002. Clinical Anatomy for Small Animal Practitioners. Iowa State Press, Ames, Iowa.

Getty, R. 1964. Atlas for Applied Veterinary Anatomy. Iowa State Press, Ames, Iowa.

Kandel, E.R., J.H. Schwartz, and T.M. Jessel. 2000. Principles of Neural Science. McGraw Hill, New York.