Special Senses Lecture PDF

SPECIAL SENSES JENINA V. TOLENTINO,PHD. Lecturer  The special senses respond to stimuli involved in vision, hearing, balance, smell, and taste. A variety of receptors, housed in special sense organs such as the eye, ear, and nose, help detect sti...

SPECIAL SENSES JENINA V. TOLENTINO,PHD. Lecturer  The special senses respond to stimuli involved in vision, hearing, balance, smell, and taste. A variety of receptors, housed in special sense organs such as the eye, ear, and nose, help detect stimuli in your surroundings. The other four “traditional” senses—smell, taste, sight, and hearing—are called special senses. Receptors for a fifth special sense, equilibrium, are housed in the ear, along with the organ of hearing. special sense receptors are either large, complex sensory organs (eyes and ears) or localized clusters of receptors (taste buds and olfactory epithelium).  THE EYE AND VISION Nearly 70 percent of all sensory receptors in the body are in the eyes. The optic tracts that carry information from the eyes to the brain are massive bundles, containing over a million nerve fibers. Anatomy of the Eye The adult eye is a sphere that measures about 1 inch (2.5 cm) in diameter. The accessory structures of the eye include the extrinsic eye muscles, eyelids, conjunctiva, and lacrimal apparatus. Anteriorly the eyes are protected by the eyelids, which meet at the medial and lateral corners of the eye, the medial commissure (canthus) and lateral commissure (canthus), space between the eyelids in an open eye is called the palpebral fissure. Projecting from the border of each eyelid are the eyelashes. Modified sebaceous glands associated with the eyelid edges are the tarsal glands. These glands produce an oily secretion that lubricates the eye Ciliary glands, which are modified sweat glands, lie between the eyelashes (cilium = eyelash), and their ducts open at the eyelash follicles. On the medial aspect of each eye is the lacrimal caruncle A delicate membrane, the conjunctiva ,lines the eyelids and covers part of the outer surface of the eyeball. It ends at the edge of the transparent cornea by fusing with the corneal epithelium. The conjunctiva secretes mucus, which helps to lubricate the eye- ball and keep it moist. The lacrimal apparatus consists of the lacrimal gland and a number of ducts that drain lacrimal secretions into the nasal cavity. The lacrimal glands are located above the lateral end of each eye. They continually release a dilute salt solution (tears) onto the anterior surface of the eyeball through several small ducts. The tears flush across the eyeball into the lacrimal canaliculi medially, then into the lacrimal sac, and finally into the nasolacrimal duct, which empties into the inferior meatus of the nasal cavity Tears also contain mucus, antibodies, and lysozyme ,an enzyme that destroys bacteria. Thus, they cleanse and protect the eye surface as they moisten and lubricate it. When lacrimal secretion increases substantially, tears spill over the eyelids and fill the nasal cavities, causing congestion and the “sniffles.”  Internal Structures: The Eyeball The eye itself, called the eyeball, is a hollow sphere. Its wall is composed of three tunics, or layers, and its interior is filled with fluids called humors that help to maintain its shape. The lens, the main focusing apparatus of the eye, is supported upright within the eye cavity, dividing it into two chambers. Layers Forming the Wall of the Eyeball Fibrous Layer The outermost layer, called the fibrous layer, consists of the protective sclera and the transparent cornea. The sclera (thick white connective tissue) is seen anteriorly as the “white of the eye.” The cornea is the most exposed part of the eye, and it is very vulnerable to damage. Vascular Layer The middle, or vascular layer, of the eyeball, has three distinguishable regions. Most posterior is the choroid ,a blood- rich nutritive tunic that contains a dark pigment. The pigment prevents light from scattering inside the eye. Moving anteriorly, the choroid is modified to form two smooth muscle structures, the ciliary body, which is attached to the lens by a suspensory ligament called the ciliary zonule, and the iris. The pigmented iris has a rounded opening, the pupil, through which light passes. Circularly and radially arranged smooth muscle fibers form the iris, which acts like the diaphragm of a camera. In distant vision and dim light, the radial fibers contract to enlarge (dilate) the pupil, which allows more light to enter the eye. Cranial nerve III (oculomotor) controls the muscles of the iris. Sensory Layer The innermost sensory layer of the eye is the delicate two-layered retina, which extends anteriorly only to the ciliary body. The outer pigmented layer of the retina is composed of pigmented cells that, like those of the choroid, absorb light and prevent light from scattering inside the eye. The transparent inner neural layer of the retina contains millions of receptor cells, the rods and cones, which are called photoreceptors because they respond to light. Electrical signals pass from the photoreceptors via a two-neuron chain—bipolar cells and ganglion cells—before leaving the retina via the optic nerve and being transmitted to, and interpreted by, the optic cortex. The result is vision. The photoreceptor cells are distributed over the entire retina, except where the optic nerve leaves the eyeball; this site is called the optic disc. Since there are no photoreceptors at the optic disc, it results in a blind spot in our vision. The rods allow us to see in gray tones in dim light, and they provide our peripheral vision Lens Light entering the eye is focused on the retina by the lens, a flexible biconvex crystal- like structure. Recall the lens is held upright in the eye by the ciliary zonule and attached to the ciliary body The lens divides the eye into two segments, or chambers. The anterior (aqueous) segment, anterior to the lens, contains a clear watery fluid called aqueous humor. The posterior segment, posterior to the lens, is filled with a gel-like substance called vitreous humor. Aqueous humor is reabsorbed into the venous blood through the scleral venous sinus, or canal of Schlemm, which is located at the junction of the sclera and cornea. Anatomy of the Ear Anatomically, the ear is divided into three major areas: the external, or outer, ear; the middle ear; and the internal, or inner, ear. External (Outer) Ear The external ear, or outer ear, is composed of the auricle and the external acoustic meatus. The auricle or pinna ,is what most people call the “ear”—the shell-shaped structure surrounding the auditory canal opening. The external acoustic meatus (or auditory canal) is a short, narrow chamber (about 1 inch long by 1⁄4 inch wide) carved into the temporal bone of the skull. In its skin-lined walls are the ceruminous glands, which secrete waxy yellow cerumen, or earwax, which provides a sticky trap for foreign bodies and repels insects. Sound waves entering the auditory canal eventually hit the tympanic (tympanum = drum) membrane, or eardrum, and cause it to vibrate.  Middle Ear The middle ear cavity, or tympanic cavity, is a small, air-filled, mucosa-lined cavity within the temporal bone. It is flanked laterally by the eardrum and medially by a bony wall with two openings, the oval window and the inferior, membrane- covered round window. The pharyngotympanic (think throat-eardrum: pharynx-tympanic) tube, or auditory tube, runs obliquely downward to link the middle ear cavity with the throat, and the mucosae lining the two regions are continuous. Internal (Inner) Ear The internal ear is a maze of bony chambers called the bony labyrinth, or osseous labyrinth; “maze”), located deep within the temporal bone behind the eye socket. The three subdivisions of the bony labyrinth are the spiraling, pea-sized cochlea (“snail”), the vestibule and the semicircular canals. The vestibule is situated between the semicircular canals and the cochlea. The bony labyrinth is filled with a plasma like fluid called perilymph. Suspended in the perilymph is a membranous labyrinth, a system of membrane sacs that more or less follows the shape of the bony labyrinth. The membranous labyrinth itself contains a thicker fluid called endo- lymph  Hearing Within the cochlear duct, the endolymph-containing membranous labyrinth of the cochlea is the spiral organ of Corti , which contains the hearing receptors, or hair cells The chambers (scalae) above and below the cochlear duct contain perilymph. Sound waves that reach the cochlea through vibrations of the eardrum, ossicles, and oval window set the cochlear fluids into motion , fluids of the inner ear into motion, and these pressure waves set up vibrations in the basilar membrane. The “hairs” of the receptor cells are embedded in the stationary tectorial membrane such that when the basilar membrane vibrates against it, the “hairs” bend Once stimulated, the hair cells transmit impulses along the cochlear nerve (a division of cranial nerve VIII—the vestibulocochlear nerve) to the auditory cortex in the temporal lobe, where interpretation of the sound, or hearing, occurs. hearing is the last sense to leave our awareness when we fall asleep or receive anesthesia and is the first to return as we awaken.  SMELL AND TASTE The receptors for taste and olfaction are classified as chemoreceptors ,because they respond to chemicals in solution. Five types of taste receptors have been identified, but the olfactory receptors (for smell) are believed to be sensitive to a much wider range of chemicals. Olfactory Receptors and the Sense of Smell The thousands of olfactory receptors, receptors for the sense of smell, occupy a postage stamp–sized area in the roof of each nasal cavity. Air entering the nasal cavities must make a 90° turn to enter the respiratory passage- way below, so sniffing, which causes more air to flow superiorly across the olfactory receptors, intensifies the sense of smell. The olfactory receptor cells are neurons equipped with olfactory hairs, long cilia that protrude from the nasal epithelium and are continuously bathed by a layer of mucus secreted by underlying glands. The olfactory nerve conducts the impulses to the olfactory cortex of the brain. There the odor is interpreted, and an “odor snapshot” is made.  Taste Buds and the Sense of Taste The taste buds, or receptors for the sense of taste, are widely scattered in the oral cavity. Of the 10,000 or so taste buds that we have, most are on the tongue. The dorsal tongue surface is covered with small peg like projections, or papillae The taste buds are found on the sides of the large round vallate papillae, or circumvallate) papillae, on the tops of the more numerous fungiform papillae and in the foliate papillae on the sides of the tongue The specific receptor cells that respond to chemicals dissolved in the saliva are epithelial cells called gustatory cells. Their long microvilli—the gustatory hairs—protrude through the taste pore; and when they are stimulated, they depolarize, and impulses are transmitted to the brain. Three cranial nerves—VII, IX, and X— carry taste impulses from the various taste buds to the gustatory cortex. The facial nerve (VII) serves the anterior part of the tongue. The other two cranial nerves—the glossopharyngeal nerve and vagus nerve—serve the other taste bud–containing areas.  Five major types of taste buds. The sweet receptors respond to substances such as sugars, saccharine, some amino acids, and some lead salts (such as those found in lead paint). Sour receptors respond to hydrogen ions (H+), or the acidity of the solution; bitter receptors to alkaloids; and salty receptors to metal ions in solution. Umami ( “delicious”), a taste discovered by the Japanese, is elicited by the amino acid glutamate, which appears to be responsible for the “beef taste” of steak and the flavor of monosodium glutamate, a food additive. QUIZ