CHAPTER 17: THE SPECIAL SENSES:



THE EYE AND VISIONAccessory structures of the eye: Eyelids: The eyelids function to protect the eyes from excess light, perspiration, and foreign objects. They, also, spread lacrimal fluid over the eyeballs to help lubricate the surface of the eyes.Eyelashes: The eyelashes function to protect the eyes from excess light, perspiration, and foreign objects. Eyebrows: The eyebrows function to protect the eyes from excess light, perspiration, and foreign objects. 13970008255000Lacrimal apparatus: The lacrimal apparatus is a group of structures the produces and drains lacrimal fluid.Lacrimal glands: These glands are located in the orbits superior to the eyeballs, and they function to produce lacrimal fluid and secrete it onto the surface of the upper eyelids.Lacrimal fluid (tears): This is a watery fluid composed of salts, mucus, and lysozyme (an antibacterial enzyme), and it functions to protect, clean, lubricate, and moisten the eyeballs.Excretory lacrimal ducts: These ducts extend from the lacrimal glands and function to drain lacrimal fluid onto the surface of the upper eyelids.Extrinsic eye muscles: The extrinsic eye muscles a group of six skeletal muscles that function to provide smooth, precise, and rapid movements of the eyes. They allow lateral, medial, superior, and inferior movements of the eyes.Superior rectusInferior rectusLateral rectusMedial rectusSuperior obliqueInferior obliqueAnatomy of the eyeball: The eyeball is about 1 inch in diameter and about 1/6 of it is exposed. It’s protected by the orbits of the skull.Fibrous tunic (fibrous layer): The fibrous tunic is the superficial, avascular layer of the eyeball, and it includes the cornea and sclera.Cornea: The cornea is a curved, transparent structure composed of an external covering of stratified squamous epithelium, an internal layer of simple squamous epithelium, and an abundance of nerve endings, especially nociceptors (pain receptors). It functions to focus light rays onto the central fovea of the retina to facilitate the formation of clear images.Sclera: The sclera is the superficial white structure of the eye composed of dense irregular connective tissue. It covers the entire the eyeball, except the cornea, and it functions to protect the inner structures of the eye and to provide shape for the eyeball.Scleral venous sinus (canal of Schlemm): The scleral venous sinus is an opening located at the junction of the cornea and sclera, and it functions to allow aqueous humor to drain into the venous blood from the anterior chamber of the eyeball.Vascular tunic (vascular layer): The vascular tunic is the middle, vascularized layer of the eyeball, and it includes the choroid, ciliary body, and iris.Choroid: The choroid is a highly-vascularized, darkly-pigmented membrane located in the posterior region of the vascular tunic; it lines the inner portion of the sclera. The capillaries of the choroid function to nourish the posterior surface of the retina, and the melanocytes that provide its pigmentation function to absorb stray light rays, which prevents the scattering of light within the eyeball.Ciliary body: The ciliary body is located in the anterior region of the vascular tunic.Ciliary processes: The ciliary processes are folds on the surface of the ciliary body that contain capillaries and function to secrete aqueous humor.Zonular fibers (suspensory ligaments): The zonular fibers are thin structures that extend from the ciliary processes and attach to the lens. They function to hold the lens in place.Ciliary muscles: The ciliary muscles are a circular band of smooth muscle fibers that function to allow the lens to change shape for near or far vision by tightening and loosening the zonular fibers.Iris: The iris is the colored portion of the eyeball located between the cornea and lens. It’s attached to the ciliary processes via the zonular fibers, and it’s composed of melanocytes and smooth muscle fibers. It functions to regulate the amount of light entering the eyeball through the pupil via contraction and relaxation of the circular and radial muscles.Pupil: The pupil is the hole in the center of the iris. It appears black due to the dark pigmentation of the choroid, and its diameter is regulated by the autonomic nervous system.Circular muscles (sphincter pupillae): The circular muscles are smooth muscle fibers of the iris, which contract in near vision and when bright light stimulates the eye. Contraction of these muscles causes constriction of the pupil.Radial muscles (dilator pupillae): The radial muscles are smooth muscle fibers of the iris, which contract in far vision and when dim light stimulates the eye. Contraction of these muscles causes dilation of the pupil.Retina: The retina is the highly-vascularized, inner layer of the eyeball, which is located superficial to the choroid. It’s composed of two layers: a pigmented layer and a neural layer.Pigmented layer: The pigmented layer is a thin, dark structure, which consists of a single layer of melanocytes (epithelial cells). This layer functions to absorb stray light rays that enter the eye, act as phagocytes to remove dead/damaged photoreceptors, and store vitamin A for photoreceptors.Neural layer: The neural layer is a transparent structure located deep to the pigmented layer, and it consists of photoreceptors, bipolar cells, ganglion cells, horizontal cells, and amacrine cells. The photoreceptors detect light rays and relay this information to the bipoloar cells and, finally, the ganglion cells. The ganglion cells function to generate action potentials in response to light/visual stimulus. The horizontal cells and amacrine cells function to facilitate visual processing at the level of the retina when a visual stimulus is detected.Photoreceptors: The photoreceptors are specialized receptors cells located in the neural layer of the retina, and they function to transduce visual stimuli into electrochemical (nerve) impulses that the brain can analyze and interpret.Rods: The rods are photoreceptors that become activated in response to dim light, and there are approximately 120 million of these cells located in the periphery of the retina (no rods located in the central fovea). They provide no color vision, only shades of gray, and they are more sensitive than the cones.Cones: The cones are photoreceptors that become activated in response to bright light, and there are approximately 6 million of these cells located in the central fovea of the macula lutea (no cones in the periphery of the retina). They provide color vision due to the stimulation of three types of cones: red, blue, and green.Optic disc (blind spot): The optic disc is an area of the retina, where the optic nerve exits the eye. It’s known as the blind spot because it contains no photoreceptors, so images cannot be formed on this area.Macula lutea: The macula lutea is the exact center of the retina and includes the central fovea.Central fovea: The central fovea is a small depression in the center of the macula lutea, which contains cones. Because of the high density of cones in this area, it provides the highest visual acuity (sharpest vision) on the retina when all light rays focus here.Lens: The lens is a transparent, biconvex, elastic structure composed of lens epithelium and crystallins. It’s located posterior to the pupil and iris and held in place by the zonular fibers. It functions to focus light rays onto the central fovea of the retina to facilitate the formation of clear images.Crystallins: The crystallins are transparent proteins located in the fibrous cells of the lens epithelium.Interior chambers: The interior chambers of the eyeball are divided by the lens, and they include the anterior cavity and vitreous chamber.Anterior cavity: The anterior cavity is the space located anterior to the lens, and it includes the anterior chamber and posterior chamber. This cavity contains aqueous humor.Anterior chamber: The anterior chamber is the space located between the cornea and the iris.Posterior chamber: The posterior chamber is the space located between the iris and the lens and zonular fibers.Aqueous humor: Aqueous humor is a watery fluid that’s filtered and secreted from the capillaries of the ciliary processes, and it functions to nourish the cells of the lens and the cornea.Vitreous chamber: The vitreous chamber is a large space in the eyeball located between the lens and the retina, and it contains the vitreous body.Vitreous body: The vitreous body is a jelly-like substance that functions to push the retina against the choroid to provide a smooth, even retinal surface to facilitate the formation of clear images. It, also, contains phagocytes to catabolize cellular debris in order to keep the vitreous chamber clear and unobstructed.Intraocular pressure: Intraocular pressure is the pressure within the eye, which is produced by the aqueous humor and vitreous body, and it functions to maintain the shape of the eyeball.Physiology of eyes and vision: Image formation: The objective of image formation is to focus light rays onto the central fovea and to provide clear images on the retina.STEP 1 – Refraction of light rays: The cornea refracts (bends) light rays as they enter the eye, and the lens further refracts the light, so the light rays come into exact focus on the central fovea of the macula lutea, located on the retina.STEP 2 – Accommodation of the lens: As light rays enter the eye, the curvature of the lens will increase in order to allow greater refraction of light rays. This helps improve the focusing power of the lens for near vision.STEP 3 – Constriction of pupil: The pupils constrict due to contraction of the circular muscles of the iris. This action narrows the diameter of the pupil to regulate the amount of light entering the eye, and it helps focus light rays on the retina by preventing stray light rays from entering the eye through the periphery of the lens.Refraction abnormalities: Myopia, hyperopia, and astigmatism are conditions that affect image formation.Emmetropic eye: This is the normal eye, which is diagnosed with 20/20 vision or better.Myopic eye (myopia): This condition is also known as nearsightedness, and it occurs because the eyeball is too long of the lens is thicker than normal. This abnormality causes the light rays to focus in front of the retina.Hyperopic eye (hyperopia): This condition is also known as farsightedness, and it occurs because the eyeball is too short or the lens is thinner than normal. This abnormality causes the light rays to focus behind the retina.Astigmatism: Astigmatism occurs due to irregular curvature of the cornea and/or lens, and this causes light rays to refract preventing them from focusing on the central fovea of the retina. It results in blurred, distorted vision.Convergence: Convergence is the medial movement of the eyeballs in order to track stimuli, and it occurs due to coordination of the extrinsic eye muscles.THE EAR AND HEARINGAnatomy of the ear: The ear includes distinct regions and structures, which function in hearing and equilibrium.9537704699000External (outer) ear: The outer ear includes the auricle (pinna), external auditory canal, and tympanic membrane.Auricle (pinna): The auricle (pinna) is composed of elastic cartilage covered by skin, and it’s attached to the head via ligaments and muscles. It functions to direct sound waves into the external auditory canal.External auditory canal: The external auditory canal is a curved tube that lies in the temporal bone, and it leads to the tympanic membrane. It functions to allow sound waves to enter the ear.Ceruminous glands: The ceruminous glands are modified sudoriferous (sweat) glands located in the epithelium of the external auditory canal. A high density of these glands is located near the opening of the external auditory canal, and they function to produce and secrete cerumen (earwax) to prevent dust particles and other foreign objects from entering the ear.Tympanic membrane (eardrum): The tympanic membrane is a thin, semi-transparent partition between the outer ear and middle ear. It’s covered by stratified squamous epithelium and is composed of epithelial and connective tissues. It functions to vibrate when stimulated by sound waves, causing the auditory ossicles to vibrate, thereby allowing sound waves to stimulate middle ear structures.Middle ear: The middle ear is a small, air-filled cavity in the temporal bone, which is lined with epithelium, and it contains the auditory ossicles, skeletal muscles, and the Eustachian tube. This area is located between the tympanic membrane and the oval and round windows (membrane-covered openings).Auditory ossicles: The auditory ossicles are the smallest bones in the body, and they articulate together via synovial jointsMalleus (hammer): The malleus is the distal bone of the series, and it attaches to the internal surface of the tympanic membrane and articulates with the incus.Incus (anvil): The incus is the middle bone of the series, and it articulates with the malleus and the stapes.Stapes (stirrup): The stapes is the proximal bone of the series, and it articulates with the incus and covers the oval window leading to the inner ear.Skeletal muscles: The skeletal muscles in the middle ear attach to the auditory ossicles.Tensor tympani muscle: The tensor tympani muscle functions to limit movement and increase tension on the tympanic membrane, which helps prevent damage to the inner ear from loud noises. Stapedius muscle: The stapedius muscle is the smallest skeletal muscle in the body, and it functions to protect the oval window from loud noises by limiting the movement of the stapes as it pushes on the oval window.Eustachian (auditory) tube: The Eustachian tube connects the middle ear to the nasopharynx (superior portion of the pharynx located posterior to the nasal cavity), and it’s composed of bone and hyaline cartilage. It opens during yawning and swallowing to allow air to enter and leave the middle ear, which equalizes air pressure in the middle ear with air pressure in the atmosphere (external environment). If air-pressure equalization does not occur, pair, hearing impairment, ringing in the ear, and vertigo can result. It’s, also, a route for pathogens to travel from the nasal cavity and pharynx to enter the middle ear, which can cause common ear infections (i.e. otitis media).Internal (inner) ear: The inner ear is also known as the labyrinth because it’s composed of a complicated series of canals.Bony labyrinth: The bony labyrinth is the bony covering of inner ear structures. It’s a series of cavities in the temporal bone lined with periosteum, and it contains perilymph (fluid) that surrounds the membranous labyrinth.Membranous labyrinth: The membranous labyrinth is a series of sacs and tubes located inside the bony labyrinth which is lined with epithelium, and it contains endolymph, a fluid that facilitates the generation of action potentials in response to vibrations and movement of the head.Vestibule: The vestibule is part of the bony labyrinth, and it’s composed of the utricle and saccule, which are part of the membranous labyrinth. It functions in static equilibrium.Semicircular canals: The semicircular canals are located posterior and superior to the vestibule and are part of the bony labyrinth. They are three canals at right angles to each other, and they function in dynamic equilibrium.Semicircular ducts: The semicircular ducts are located inside the semicircular canals and are part of the membranous labyrinth. They connect to the utricle and saccule and contain endolymph.Ampullae: The ampullae are the swollen ends of the semicircular canals that contain endolymph and cristae with hair cells. The hair cells are specialized receptor cells that detect movement of the endolymph as the head moves, and they transduce mechanical stimuli into electrochemical (nerve) impulses that the brain can analyze and interpret.Cochlea: The cochlea is a bony spiral canal composed of bony and membranous labyrinths. It’s located anterior to the vestibule, and its canals are divided into three chambers: scala vestibuli (contains perilymph), scala media (contains endolymph), and scala tympani (contains perilymph). It functions in an of Corti (spiral organ): The organ of Corti is a structure located in the cochlear duct (scala media) superficial to the basilar membrane, and it’s composed of inner hair cells, outer hair cells, supporting cells, and the tectorial membrane. The inner and outer hair cells are specialized receptors cells, which function to detect movement of the tectorial membrane as the endolymph moves in response to sound waves and vibrations. The hair cells transduce auditory stimuli into electrochemical (nerve) impulses that the brain can analyze and interpret.Physiology of ear and hearing:STEP 1 – The auricle/pinna directs sound waves into the external auditory canal.STEP 2 – The sound waves strike the tympanic membrane, which causes it to vibrate (slow vibration in response to low-frequency sounds; rapid vibration in response to high-frequency sounds).STEP 3 – Vibration of the tympanic membrane causes the auditory ossicles to vibrate.STEP 4 – Vibration of the stapes vigorously pushes on the membrane of the oval window causing vibration of it.STEP 5 – Movement of the membrane of the oval window causes fluid-pressure waves in the perilymph of the cochlea.STEP 6 – The fluid-pressure waves are transmitted throughout the three channels of the cochlea and cause the walls of the cochlea to deform.STEP 7 – The movement of the perilymph causes fluid-pressure waves to develop in the endolymph of the cochlea.STEP 8 – The fluid-pressure waves in the endolymph cause the tectorial membrane of the organ of Corti to vibrate, which move the hair cells, and the hair cells respond by generating action potentials.STEP 9 – The electrical impulses are sent to the brain (temporal lobe).Types of equilibrium: Equilibrium refers to maintaining balance of the body.Static equilibrium: Static equilibrium involves maintaining balance of the body relative to gravity, and the otolithic organs (utricle and saccule) function to maintain this type of equilibrium.Dynamic equilibrium: Dynamic equilibrium involves maintaining balance of the body in response to sudden movements (i.e. acceleration, deceleration, rotation), and the semicircular canals function to maintain this type of equilibrium.Vestibular apparatus: The vestibular apparatus includes all of the structures involved in maintaining static and dynamic equilibrium.Utricle and Saccule (otolithic organs): The utricle and saccule comprise the membranous labyrinth of the vestibule and are located in the inner ear. Both contain small, thickened regions called the maculae, which contain hair cells, supporting cells, otoliths, and the otolithic membrane.Hair cells: The hair cells are specialized receptor cells, located in the maculae of the utricle and saccule, that function to detect movement of the otolithic membrane as the head moves.Supporting cells: The supporting cells, located among the hair cells in the maculae of the utricle and saccule, function to secrete a thick gelatinous glycoprotein layer called the otolithic membrane.Otolithic membrane: The otolithic membrane is secreted by the supporting cells of the maculae and is composed of otoliths. The stereocilia (“hairs”) of the hair cells are embedded in the membrane, and it moves when the head moves stimulating the stereocilia of the hair cells.Otoliths: Otoliths are calcium carbonate crystals that form a dense superficial layer in the otolithic membrane. These crystals move the in the membrane as the head moves in response to gravity.Semicircular ducts: The three semicircular ducts comprise the membranous labyrinth of the semicircular canals, and they are located at right angles to each other. They contain endolymph and function to permit detection of acceleration, deceleration, and rotation of the body. The swollen ends of the semicircular ducts are called ampullae, which contain cristae and cupulae. The cristae contain hair cells and supporting cells, and the cupula is a mass of gelatinous material superficial to the cristae (stereocilia of hair cells are embedded in the cupula). When the head moves in response to velocity changes or rotation, the endolymph (within the semicircular ducts) and ampullae move, which stimulates the stereosilia (“hairs”) of the hair cells in the cristae.Physiology of ear and equilibrium:STEP 1 – When you move your head, the otoliths and otolithic membrane of the maculae or the endolymph inside the ampullae move.STEP 2 – The movement of the otolithic membrane or movement of the cupula by the endolymph stimulate the stereocilia of the hair cells.STEP 3 – The hair cells respond by generating action potentials.STEP 4 – The action potentials are sent to the brain (medulla, pons, and cerebellum) in order to maintain posture and balance.OLFACTION: THE SENSE OF SMELLAnatomy of olfactory receptors: The nose contains 10-100 million chemoreceptors for olfaction.Olfactory epithelium: The olfactory epithelium is a membrane located in the superior region of the nasal cavity, which contains olfactory receptors, supporting cells, basal cells, and olfactory glands.Olfactory receptors: Olfactory receptors are bipolar neurons with knob-like dendrites located in the olfactory epithelium. They function to detect odorants that enter the nasal cavity and transduce the chemical stimuli into nerve impulses. Olfactory hairs: Olfactory hairs are cilia that project from the dendrites of olfactory receptors, and they function to increase the surface area of the olfactory epithelium in order to detect odorants that enter the nasal cavity.Odorants: Odorants are molecules that stimulate the olfactory receptors.Supporting cells: Supporting cells are columnar-shaped epithelial cells located among the olfactory receptors in the olfactory epithelium. They function to provide physical support, nourishment, and electrical insulation for the olfactory receptors, and they, also, aid in the detoxification of odorants that contact the olfactory epithelium.Basal cells: Basal cells are stem cells located at the bases of the supporting cells, and they function to undergo somatic cell division in order to produce new olfactory receptors (olfactory receptors are replaced every 30-60 days!).Olfactory (Bowman’s) glands: Olfactory (Bowman’s) glands are located in the connective tissue superficial to the olfactory epithelium, and they function to produce and secrete mucus that moistens the surface of the olfactory epithelium and dissolves odorants so the olfactory hairs can quickly detect chemical stimuli.Physiology of olfactory epithelium and olfaction:STEP 1 – The odorants (chemical stimuli) enter the nasal cavity.STEP 2 – The olfactory hairs detect the odorant molecules.STEP 3 – The odorant molecules stimulate the olfactory hairs, which activate the olfactory receptors. The olfactory receptors transduce the chemical stimuli into action potentials.STEP 4 – The action potentials are sent to the brain (temporal lobe) to be perceived and interpreted.Threshold of olfaction: Only a few molecules of an odorant are necessary to stimulate olfactory receptors, which indicate a low-stimulus threshold.Odor adaptation: Olfactory receptors adapt to odorants in one minute or less, which indicates rapid adaptation to odorants.GUSTATION: THE SENSE OF TASTEAnatomy of taste buds and papillae: The taste buds can only detect five primary tastes: sour, sweet, salty, bitter, and umami.Taste bud: Taste buds are located in the papillae on the tongue, on the soft palate, on the pharynx (throat), and on the epiglottis. They are composed of gustatory receptor cells, supporting cells, and basal cells.Gustatory receptor cells: Gustatory receptor cells have gustatory hairs that project from them, and they function to detect tastants and transduce the chemical stimuli into nerve impulses. These cells emain alive for about 10 days in the taste buds.Tastants: Tastants are molecules that stimulate gustatory receptor cells.Supporting cells: Supporting cells are located among the gustatory receptor cells, and they function to develop into the gustatory receptor cells.Basal cells: Basal cells are stem cells located in the periphery of the taste buds, and they function to undergo somatic cell division and develop into the supporting cells.Papillae: Papillae are the elevations on the dorsal surface of the tongue, and taste buds are located in the papillaeCircumvallate papillae: These large, circular papillae form an inverted V-shape at the back of the tongue, and each of these papilla contain about 100-300 taste buds.Fungiform papillae: These mushroom-shaped papillae are scattered over the dorsal surface of the tongue, and each of these papilla contain about 5 taste buds.Foliate papillae: These papillae are located on the lateral margins of the tongue, and most of these papillae degenerate in childhood.Filiform papillae: These pointed, threadlike structures cover the dorsal surface of the tongue and do not contain any taste buds. Instead, they contain tactile receptors, which detect the presence of food in the oral cavity and increase friction between the tongue and food to help move food in the oral cavity.Physiology of taste buds and gustation:STEP 1 – The tastants (chemical stimuli) enter the oral cavity and dissolve in the saliva.STEP 2 – The molecules of the tastants in the saliva contact the gustatory hairs of the gustatory receptor cells.STEP 3 – The gustatory hairs become stimulated by the tastant molecules, which activate the gustatory receptor cells. The gustatory receptor cells transduce the chemical stimuli into action potentials.STEP 4 – The action potentials are sent to the brain (temporal lobe) to be perceived and interpreted.Threshold of gustation: The stimulus threshold for the five primary tastes varies.Taste adaptation: Gustatory receptor cells adapt to tastants in one to five minutes with constant exposure of the receptors to the chemical stimuli. ................
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