THE SPECIAL SENSES (1) THE CHEMICAL SENSES: TASTE (GUSTATION) AND SMELL (OLFACTION)

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1 THE SPECIAL SENSES Senses allow the body to maintain homeostasis by constantly receiving information regarding internal and external environmental changes. There are many ways we sense things, but there are four traditional senses that are called the special senses: smell, taste, sight and hearing. Receptors for a fifth special sense known as equilibrium are located within the ear, along with the organs for hearing. In a previous chapter, we learned about the general senses. Please recall that they are scattered throughout the body and are classified according to the nature of the stimulus that excites them. They are: (1) nociceptors, (2) thermoreceptors, (3) mechanoreceptors and (4) chemoreceptors. The special senses are not just dendrites; rather they are distinctive receptor cells, which are confined to the cephalic (head) region of the body. The area monitored by a receptor cell is known as its receptive field. (1) THE CHEMICAL SENSES: TASTE (GUSTATION) AND SMELL (OLFACTION) Gustation (taste) Taste buds are the sensory structures for taste and they are located primarily in the oral cavity. We have 10,000 or more taste buds and most of them are located on our tongue, a few on the inner surface of the cheeks and a couple on the soft palate, pharynx and epiglottis. The basic taste sensations are: (1) sweet, (2) salty, (3) sour, (4) bitter and (5) umami. Taste buds are often found in stumpy-structures called papillae. Each bud consists of epithelial cells of three types: (1) supporting cells, (2) basal cells and (3) receptor cells. There are three major types of papillae: (1) filiform, (2) fungiform and (3) circumvallate. The seventh (facial), ninth (glossopharyngeal) and tenth (vagus) cranial nerves monitor the taste buds. There are distinct individual differences in taste sensitivity, all of which have a genetic basis. PTC paper (phenylthiourea a/k/a phenylthiocarbamide) for example, can be tasted by 70% of the Caucasian population. Ability to taste declines with age. We start off with around 10,000 taste buds, but as we age the number declines. At 50 the decline becomes rapid. That is probably why you often hear older people complaining that the food just doesn t taste as good as it used to! Olfaction (smell) Paired olfactory organs are located on either side of the nasal septum in the upper portion of the nasal cavity. These cells are specialized endings of the fibers that make up the olfactory nerve. They are surrounded by specialized epithelial cells that each end in a tuft of cilia. Chemicals will bind to the cilia and stimulate the transmission of nerve impulses that are transmitted directly to the olfactory bulb of the cerebral cortex. The senses of taste and smell complement each other. They are jointly interpreted by the cerebral cortex. Hence, when you have a cold food seems to lose its taste. (2) THE VISUAL SENSES Vision is our dominant sense. Over 75% of our sensory receptors are visual receptors. Nearly half of the cerebral cortex is involved in the processing of visual stimuli. Photoreceptors are located within the eyes, which are in the orbits formed by the union of seven of the bones of the skull (frontal, lacrimal, ethmoid, zygomatic, maxilla, sphenoid and palatine bones). 47

2 Each orbit has muscles called extrinsic muscles, which anchor the eye in place within the orbit. Three cranial nerves provide regulation for these muscles: the oculomotor, abducens and trochlear. The muscles are: (1) superior rectus (rolls eyes upward; innervated by oculomotor nerve), (2) inferior rectus (rolls eyes downward; innervated by oculomotor nerve), (3) lateral rectus (turns eyes out from midline; innervated by the abducens nerve), (4) medial rectus (turns eye towards midline; innervated by oculomotor nerve), (5) superior oblique (rotates eye counterclockwise; innervated by the trochlear nerve) and (6) inferior oblique (rotates eyes clockwise; innervated by the oculomotor nerve). Extrinsic muscles affect two types of movement: (1) saccades (jerky movements that allow the whole visual field to be seen in a short period of time) and (2) scanning ( tracking ) movements (follow an object and keep focused on it as we track it through our visual field). Eyelids protect the eye and are operated by the orbicularis oculi muscles, which close the lids and the levator palpebrae superioris, which raise the lids. Sebaceous glands produce an oily secretion that lubricates the eyelid. These glands are called tarsal or Meibomian glands. Modified sweat glands known as ciliary glands lie between the hair follicles of the eyelashes. Eyebrows shade the eye from the sun. Contraction of the orbicularis oculi muscle depresses the eyebrow and contraction of the corrugator muscle raises the eyebrow. The lacrimal apparatus consists of a lacrimal gland (which is located over eye in the orbit) and nasolacrimal sac with its ducts. Tears produced by the gland pass through two ducts into the sac before they drain into the nose via the nasolacrimal duct. Lacrimal fluid contains mucus, antibodies and lysozyme: an enzyme that destroys bacteria. Human tears contain endorphins. Therefore, crying may be a function of trying to decrease human stress. Anatomy of the eye: (1) Lens biconvex, transparent and flexible. Changes shape to focus light. Held in place by the suspensory ligament. (2) Iris - part of vascular tunic and is the colored part of the eye. Has the shape of a flat doughnut and lies between cornea and lens. It is continuous with the ciliary body at the iris posterior edge. (3) Pupil - round central opening of the iris. Allows for the passage of light into the visual apparatus. Acts as a diaphragm due to the smooth muscles of (circular and radial) of the iris. (4) Choroid - part of vascular tunic (uvea). Highly vascular. Dark brown membrane. The choroid has numerous blood vessels to provide nutrition to whole eye. Brown pigment comes from melanocytes. (5) Sclera - part of fibrous tunic of the eye. Forms posterior portion and bulk of the eye. Is white and opaque. Seen on anterior side as the whites of the eyes. This layer is continuous with the dura mater at the posterior end of the eye. (6) Cornea -anterior sixth of the fibrous tunic. Transparent. Forms a window to let in light and also refracts light to assist in visual process. (7) Vitreous humor - posterior segment of the eye. Located behind the lens and filled with a clear gel. Transmits light, supports posterior surface of the lens, holds retina in place and helps maintain intraocular pressure. (8) Aqueous humor - fills part of eye called anterior segment (area in front of lens). Forms and drains continuously. Not a stable gel like the vitreous humor. Watery like blood plasma. Drains via the canal of Schlemm, a/k/a/the sclera venous sinus. 48

3 (9) Ciliary body - part of vascular tunic (uvea). Anterior end of the choroid membrane. Thick ring of tissue that encircles the lens. Made of interlacing smooth muscles called ciliary muscles. (10) Ciliary muscles - the interlacing smooth muscles associated with the structure of the ciliary body of the choroid coat; help control shape of the lens. (11) Retina - constitutes the sensory tunic. This is the innermost of the three tunics (fibrous, vascular and sensory). The outer layer of retina is a pigmented layer, which attaches to the choroid, covers the ciliary body and also covers the posterior face of the iris. The cells of the pigmented layer absorb light, are phagocytes and store vitamin A. The inner layer is called the neural layer and it extends anteriorly to the posterior margin of the ciliary body. This point of union is called the ora serrata retinae. The retina is actually an out-pocketing of the brain; it contains millions of photoreceptors. It contains three major types of neurons: (1) photoreceptors (receive light stimulus - the two kinds are rods and cones), (2) bipolar cells (accept light stimulus and transfer it to ganglion cells) and (3) ganglion cells (accept light stimulus and generate action potentials). (12) Rods - dim-light and peripheral vision receptors. Do not provide sharp images or color vision but are more sensitive to light than cones are. (13) Cones - provide accurate color vision. (14) Optic disc - point where optic nerve exits the eye. It is a weak spot in the structure of the eye because it is not reinforced by attachment to the sclera. It is called the blind spot because it lacks photoreceptors. (15) Macula lutea - lateral to blind spot of each eye. A/k/a yellow spot, it has a tiny opening at its center called fovea centralis. The macula consists of mostly cones. (16) Fovea centralis - tiny opening in macula lutea that allows the light to pass into the photoreceptors, especially the cones. The fovea is almost totally cones. (3) THE AUDITORY SENSES The organization of the ear provides for these responsibilities: hearing and equilibrium. Receptors that accomplish functions for both of these tasks are located within the inner ear. The outer ear includes the (1) Pinna (a/k/a auricle - outer ear. Shell-shaped. Directs sound waves into auditory canal; (2) Auditory canal - external. Goes from pinna to eardrum and is lined with epithelium that has hairs, sebaceous glands and modified sweat glands that produce wax, which are called ceruminous glands; and the (3) Tympanic membrane - eardrum. Boundary between outer and middle ears. A thin CT membrane covered by skin on its external surface and mucosa internally. Shaped like a flattened cone. Its apex protrudes into the middle ear and it transfers sound waves to the bones of the inner ear. 49

4 The middle ear includes the (1) Oval window - one of two openings in the bony wall that borders the middle ear cavity (a/k/a tympanic cavity) on the side opposite the eardrum. Superior to the round window and deep to the stapes; (2) Round window - inferior to the oval window. One of two openings in the body wall that borders the middle ear cavity on the side opposite the eardrum; (3) Ossicles - tiny bones. The three smallest bones in the body. They are named for their shape. Malleus - called the hammer. Secured by the attachment of handle of the hammer to the eardrum. Incus - called the anvil. Positioned between the malleus and the stapes. Stapes - called the stirrup. Secured to the oval window by the annular ligament. Secured to the bony wall by the stapedius muscle. Eustachian tubes (4) Eustachian tubes a/k/a/ pharyngotympanic (auditory) tube. Links the middle ear with the nasopharynx. Used to equalize pressure between middle ear and external environment. The inner ear includes (1) Osseous labyrinth - located within temporal bone behind the eye socket. A system of twisted tubes within the bone. Contains the vestibule, cochlea and semicircular canals. The bony labyrinth is filled with perilymph, which is a fluid similar to CSF. (2) Membranous labyrinth - continuous series of sacs within the bony labyrinth. Contains a fluid called endolymph. Together with perilymph, these fluids conduct sound waves. NOTE: these fluids are not related to fluids of the lymphatic system! (3) Semicircular canals - part of the osseous labyrinth. Three canals exist and each is oriented in one of the three planes of space (anterior, posterior and lateral). The canals contain ducts that have ampullae. (4) Ampullae - enlarged swelling of a semicircular duct. Contains equilibrium receptors known as cristae ampullaris. (5) Vestibule - central, egg-shaped cavity of the osseous labyrinth. Contains two sacs: the utricle and the saccule. These structures work to maintain control of positional changes of the head. (6) Utricle - located in the vestibule. Contains equilibrium receptors known as maculae that respond to gravity and the position of the head. Connects to saccule. (7) Saccule - located in the vestibule. Contains equilibrium receptors known as maculae that respond to gravity and the position of the head. Connects to utricle. (8) Otoliths - calcium carbonate crystals located in the maculae. Increase the weight of the membrane and its inertia and assist in maintenance of equilibrium. 50

5 (9) Cochlea - snail-shaped. Spiral, conical, bony chamber. Contains the organ of Corti, which is a receptor for hearing. Cochlea is divided into three chambers: (1) scala vestibuli, (2) scala media and (3) scala tympani. (10) Organ of Corti - receptor for hearing. Supported by the basilar membrane. (11) Cochlear canal - the basilar membrane forms the lower wall of this canal. This lower wall contains hair cells with cilia that contact a membrane called the tectorial membrane. When the basilar membrane vibrates, the hairs touch the membrane, bend and nerve impulses begin in the cochlear nerve and are transmitted to the brain. The hair cells plus the tectorial membrane are called the organ of Corti. Hearing occurs when sound waves enter the auditory canal. These waves travel by the successive vibration of molecules. When numbers of these molecules strike the eardrum, they cause it to vibrate slightly. Ossicles receive the vibrations from the eardrum and transmit them to the oval window, having amplified them approximately twenty times. When the oval window vibrates it causes pressure waves in form in the fluid of the cochlea. The cochlea contains three canals: vestibular, tympanic and cochlear. The cochlear canal is where the actual hearing mechanism is located. 51

6 DISORDERS/DISEASES Cataract Color Blindness Conduction Deafness Conjunctivitis (Pinkeye) Glaucoma Hyperopia Macular Degeneration Meniere s Syndrome Myopia Night Blindness Otitis Media Otosclerosis Retinal Detachment Sty Tinnitus Clouding of the lens. Some are congenital and others caused by agerelated hardening and thickening of the lens. Can result from diabetes mellitus. Sex-linked (X-linked) disease, therefore more common in males. Cannot perceive certain colors. Most frequent pattern is red-green color blindness. Lack of genetic codes for the production of certain types of cones. Results from interference with the conduction of sound vibration through the fluids of the inner ear. Inflammation of the conjunctiva. It is called pinkeye only when the inflammation is due to infection by bacteria or viruses. Blockage of ducts used to drain aqueous humor results in an increase in pressure and damage to retina and optic nerve. Can see things far away but not near. Point of focus falls behind the retina. The fovea centralis of the macula lutea contains a lot of cones. In this disorder the visual field becomes distorted because the field of cones degenerates. Blurring or a blind spot results. The disorder has a genetic basis and light-eyed people are more often affected. Most frequent cause of legal blindness in the USA. A labyrinth disorder that affects the semicircular canals and the cochlea. Results in tinnitus, vertigo, nausea and vomiting. Can see near objects, but not distant ones. Caused by focal point falling in front of the retina. A/k/a nyctalopia, a serious impairment of rod functioning. Most common cause is prolonged vitamin A deficiency. Inflammation of the middle ear. Can result from infection or food allergies. Hardening of the ossicles. Age-related. Leads to conduction deafness. Staples foot is fused to base of oval window by overgrowth of bony tissue. The pigmented and nerve layers of the retina separate, allowing the vitreous humor to seep out. This can cause permanent blindness. Infection of any small gland associated with the eye. Known as a chalazion if the infection is in the larger tarsal gland. Constant white noise in the ears. May result from neural damage or from drug use. 52