Special Senses PDF

SPECIAL SENSES Prepared by: MARIEL ANNE R. BUENO Intended Learning Outcomes Upon completion of the course, the student should be able to: Identify the eye structures and the functions of each Describe image formation on the retina Trace the pathway of light through the eye to the reti...

SPECIAL SENSES Prepared by: MARIEL ANNE R. BUENO Intended Learning Outcomes Upon completion of the course, the student should be able to: Identify the eye structures and the functions of each Describe image formation on the retina Trace the pathway of light through the eye to the retina Interpret result of visual acuity test using the Snellen’s chart Identify the structures of the ears and the functions of each Devise a diagram showing the pathway of sound conduction Explain the function of the organ of Corti in hearing Describe how the equilibrium organs help maintain balance Describe the location, structure and function of the olfactory and taste receptors Describe changes that occur with age in the special sense organs Predict physiological consequences of any alteration in the structure and function of eyes and ears Sense- the ability to perceive stimuli Sensation- the input obtained by sensory receptors, and then sent to the brain Perception- conscious awareness of the stimuli once the action potentials reach the cerebral cortex Sensory receptors- generate action potential as a response to stimuli General senses- have receptors distributed over a large part of the body Special senses- more specialized in structure and are localized to specific parts of the body Special Senses smell hearing taste equilibrium sight Sensory Receptors mechanoreceptors- respond to mechanical stimuli such as bending or stretching of receptors, touch, vibration, pressure, and sound chemoreceptors- respond to chemicals such as taste and odor molecules photoreceptors- respond to light thermoreceptors- respond to temperature changes nociceptors- respond to stimuli that result in pain THE EYE AND VISION 70 percent of all sensory receptors in the body are in the eyes. Only the anterior one-sixth of the eye’s surface is normally seen. Anatomy of the Eye eyelids- fold of skin protecting the eyes anteriorly; meet at the medial and lateral commissure (canthus); bordered by eyelashes palpebral fissure- space between the eyelids in an open eye  tarsal glands (Meibomian glands)- modified sebaceous glands associated with the eyelid edges, which produce an oily secretion that lubricates the eye ciliary glands- modified sweat glands that lie on the margin of the eyelid next to the base of the eyelashes conjunctiva- delicate membrane that lines the eyelids (palpebral conjunctiva) and covers part of the outer surface of the eyeball (bulbar conjunctiva), fusing with the corneal epithelium; secretes mucus lacrimal apparatus- consists of the lacrimal gland and a number of ducts that drain lacrimal secretions into the nasal cavity  lacrimal glands- located at the superior lateral orbit that continually release tears lacrimal canaliculi- small ducts where excess tears are collected lacrimal punctum- small opening that lead to the canaliculi lacrimal sac- dilated superior part of the nasolacrimal duct that drains tears nasolacrimal duct- carries tears from the lacrimal sac of the eye into the nasal cavity Tears- contain mucus, antibodies, and lysozyme (an enzyme that destroys bacteria) Soucre: https://www.aafp.org/pubs/afp/issues/2024/0800/conjunctivitis.html 6 extrinsic eye muscles (external eye muscles)- produce gross eye movements Internal Structures of the Eyeball Tunics / Layers of Eyeball Fibrous layer- outside layer Vascular layer- middle layer Sensory layer- inside layer Humors- are fluids that fill the interior of the eyeball Layers Forming the Wall of the Eyeball a. fibrous tunic consists of the sclera and cornea sclera- anterior thick white connective tissue; “white of the eye”; helps maintain the shape of the eye; provides attachment site to extrinsic eye muscles cornea- the most exposed convex, transparent part of the eye supplied with nerve endings; permits light to enter b. vascular tunic (uvea)- has 3 regions choroid- most posterior, blood-rich nutritive tunic that contains a dark pigment that prevents light from scattering inside the eye; modified anteriorly to form two smooth muscle structures  ciliary body- consists of vascular network, attached to the lens by a suspensory ligament called the ciliary zonule; change the shape of the lens when eyes focus on a near object; synthesizes aqueous humor iris- colored part of the eye which is attached to the anterior margin of the ciliary body, anterior to the lens; regulates the amount of light that enters the eye; consists of smooth muscles that surround the pupil *pupil- rounded opening of the iris through which light passes Cranial nerve III (oculomotor)- controls the muscles of the iris  Lens- flexible, biconvex crystal-like structure which is the main focusing apparatus of the eye that divides the eye into anterior and posterior chambers -held in place by a suspensory ligament attached to the ciliary body c. sensory/ nervous tunic- consists of the delicate two- layered retina pigmented layer- outer layer composed of pigmented cells that absorb light and prevent it from scattering; acts as phagocytes to remove dead or damaged receptor cells and store vitamin A needed for vision  neural layer- inner layer that contains millions of photoreceptors that respond to light *rods- are densest at the periphery, or edge, of the retina; help give us good vision in low light in gray tones, and provide our peripheral vision *cones- densest in the center of the retina; requires much more light and provide color vision -has three varieties of cones: blue, green, red optic disc- the only spot on the retina that has no photoreceptors, making it a "blind spot”; site where the optic nerve leaves the eyeball  macula- area in the center of the retina responsible for central vision fovea centralis- pit inside the macula that contains high density of cones, making it the area of greatest visual acuity (point of sharpest vision) Electrical signals pass from the photoreceptors via a two neuron chain—bipolar cells and then ganglion cells— before leaving the retina via the optic nerve and being transmitted to, and interpreted by, the optic visual cortex. Chambers of the eye anterior (aqueous) segment- anterior to the lens, contains a clear watery fluid called aqueous humor posterior (vitreous) segment- posterior to the lens, filled with a gel-like substance Aqueous humor- similar to blood plasma and is continually secreted by the ciliary body -helps maintain intraocular pressure (IOP) -provides nutrients for the avascular lens and cornea -keeps the cornea moist and maintains the shape of an eyeball -reabsorbed into the venous blood through the scleral venous sinus, or canal of Schlemm, located at the junction of the sclera and cornea Vitreous humor- helps maintain eye pressure -holds the lens and retina in place -does not circulate ophthalmoscope- medical instrument used to examine the internal structures of the eye, allowing the retina, optic disc, and internal blood vessels at the fundus (posterior wall of the eye) to be viewed and examined  PHYSIOLOGY OF VISION accommodation- ability of the eye to focus specifically for close objects The eye is set for distant vision (over 20 feet away). The greater the lens convexity, or bulge (which makes it thicker), the more it bends the light. The flatter (thinner) the lens, the less it bends the light. Light is refracted by the cornea, aqueous humor, lens, and vitreous humor. Image formed on the retina is a real image: reversed from left to right, upside down, smaller than the actual object  Light first passes through the cornea, then through the aqueous humor which fills small chambers behind the cornea. Light passes through the pupil, with the iris adjusting the size of the pupil to control the amount of light that enters the eye. Light penetrates the lens while the attached muscles contract or relax in order to change its shape to produce clarity of images Light passes through the vitreous humor at the posterior eye. Light finally reaches the retina, where rod and cone cells are stimulated to release split-second chemical reactions converting the light to electrical impulses. Visual information from the retina is relayed through the thalamus to the primary visual cortex Source: https://www.cornea.org/Learning-Center/What-is-a-Cornea.aspx  Visual Fields and Visual Pathways to the Brain optic nerve- CN II; a bundle of nerve fibers that transmits sensory information for vision in the form of electrical impulses from the retina to the brain optic chiasma- part of the brain where the optic nerves cross over to the opposite side of the brain optic tracts- contains fibers from the lateral side of the eye on the same side and the medial side of the opposite eye; synapse with neurons in the thalamus, whose axons form the optic radiation, which runs to the occipital lobe of the brain for visual interpretation.  visual field- the total area in which objects can be seen in the peripheral vision while focusing the eyes on a central point -Each eye “sees” a slightly different view but that their visual fields overlap quite a bit. Binocular vision- provides for depth perception, also called “three-dimensional” vision, as the visual cortex fuses the two slightly different images delivered by the two eyes into one “picture” Eye Conditions emmetropia- 20/20 sight; eye focuses images correctly on the retina myopia- nearsightedness; close objects look clear but far objects look blurry; occurs when the parallel light rays from distant objects are focused in front of the retina Correction: concave corrective lenses hyperopia- farsightedness; objects up close appear out of focus; occurs when the parallel light rays from distant objects are focused behind the retina Correction: convex corrective lenses astigmatism- unequal curvatures in different parts of the cornea or lens; blurry images occur because points of light are focused not as points on the retina but as lines Correction: cylindrical lenses EYE REFLEXES photopupillary reflex- pupils immediately constrict when suddenly exposed to bright light; prevents excessively bright light from damaging the delicate photoreceptors accommodation pupillary reflex- pupils constrict reflexively when viewing close objects  THE EAR: HEARING AND BALANCE mechanoreceptors- involved in hearing, detection of equilibrium, and detect stimuli such as touch, pressure, and vibration ANATOMY OF THE EAR 1. external ear 2. middle ear 3. inner ear EXTERNAL EAR pinna / auricle- visible part of the ear surrounding the auditory canal opening external acoustic meatus / auditory canal- a short, narrow chamber (about 1 inch long by ¼ inch wide) carved into the temporal bone of the skull ceruminous glands- secrete waxy yellow cerumen (earwax) that provides a sticky trap for foreign bodies and repels insects tympanic membrane / eardrum- thin layer of tissue that separates the external from the middle ear; vibrates as the sound waves hit it MIDDLE EAR middle ear cavity / tympanic cavity- a small, air-filled, mucosa-lined cavity within the temporal bone oval window & round window- two openings on the medial side of the middle ear covered by connective tissue membrane pharyngotympanic tube / auditory tube / eustachian tube- links middle ear cavity with the back of the nose and throat; flattened and closed, but swallowing or yawning can open it briefly to equalize ear pressure between the outside air and the middle ear cavity, to allow the eardrum to vibrate  ossicles- smallest bones in the body that transmit and amplify the vibratory motion of the eardrum to the fluids of the inner ear a. Malleus (hammer) b. Incus (anvil) c. Stapes (stirrup) Pathway of sound vibrations: malleus → incus → stapes → oval window of inner ear INNER EAR bony/osseous labyrinth- bony chambers within the temporal bone behind the eye socket perilymph- plasmalike fluid that fills the space within the bony labyrinth membranous labyrinth- smaller set of membranous tunnels and chambers located inside the bony labyrinth endolymph- thick fluid in the membranous labyrinth Regions of the bony labyrinth a. cochlea- fluid filled, spiral-shaped, pea-sized cavity that plays a vital role in the sense of hearing as it participates in the process of auditory transduction b. vestibule- central part of the bony labyrinth that provides the sense of balance and the information about body position; separated from the middle ear by the oval window c. semicircular canals- three tiny, fluid-filled tubes that help keep our balance and sense head position Subdivisions of the membranous labyrinth a. Vestibular labyrinth- contains otolith organs that detect linear movements that are related to gravity Utricle- detects horizontal motion Saccule- detects vertical movement b. Cochlear labyrinth- contains the receptors for hearing; the auditory part of the inner ear HEARING spiral organ / Organ of Corti- located within the cochlear duct, containing hairlike microvilli hearing receptors, called hair cells (stimulated by the vibrating movement of the basilar membrane) tectorial membrane- rigid gel-like membrane that lies over the receptor cells, that when the basilar membrane vibrates against it, the “hairs” bend. cochlear nerve (CN VIII)- the sensory nerve that transfers auditory information from the cochlea to the auditory cortex in the temporal lobe  vibrations create sound waves, collected by the auricle sound waves travel through the external auditory canal toward the tympanic membrane causing it to vibrate eardrum vibration causes vibration of the ossicles, amplifying the force of vibration vibrations are transferred to the oval window, producing waves in the perilymph of the cochlea which pushes against the round window  waves in the perilymph pass through the vestibular membrane that causes vibration of the endolymph waves within the cochlear duct cause displacement of the basilar membrane hair cells move the microvilli of the hair cells bend because their other ends are embedded in the nonmoving tectorial membrane  the bending of the microvilli induces action potentials in the cochlear nerves sends the signals to the auditory cortex to be interpreted as sounds basilar membrane: narrower and denser near the oval window; wider and less dense near the tip of the cochlea High-pitched sounds disturb the short, stiff fibers of the basilar membrane near the oval window. Low-pitched sounds disturb the long, floppy fibers near the tip of the cochlea ORGAN OF CORTI EQUILIBRIUM -sense of balance vestibular apparatus- equilibrium receptors of the inner ear Branches: 1. static equilibrium- associated with the vestibule; involved in evaluating the position of the head relative to the gravity 2. dynamic equilibrium- associated with the semicircular canals; involved in evaluating changes in the direction and rate of head movements Static Equlibrium maculae- receptors in the vestibule that report on changes in the position of the head in space with respect to the pull of gravity when the body is not moving; help us keep our head erect otolithic membrane- gelatinous mass where the cilia/hairs of the maculae are embedded otoliths- tiny stones in the membrane, made of protein and calcium salts Movements cause otoliths to roll and bend hair cells, sending impulses along the vestibular nerve (CN VIII) to the cerebellum of the brain, informing it of the position of the head in space. Dynamic Equilibrium -receptors respond to angular or rotational movements of the head rather than to straight-line movements crista ampullaris- receptor regions within the ampulae consisting of a tuft of hair cells covered with a gelatinous cap called the cupula The cupula bends with the body’s motion dragging against the stationary endolymph. >> stimulates the hair cells, and impulses are transmitted up the vestibular nerve to the cerebellum Hearing and Equilibrium Deficits Deafness- any degree of hearing loss Conduction deafness- results when something interferes with the conduction of sound vibrations through the external and middle ear Sensorineural deafness- occurs when there is degeneration or damage to the receptor cells in the nervous system structures Ménière’s syndrome- inner ear problem that causes progressive deafness, ringing in the ears, and sensation of spinning  Chemical Senses: SMELL AND TASTE chemoreceptors- receptors for taste and olfaction Sense of Smell olfactory receptors- receptors for the sense of smell found in the roof of each nasal cavity that transmit impulses along the olfactory filaments, which are bundled axons of olfactory neurons that collectively make up the olfactory nerve (CN I) olfactory receptor cells- neurons equipped with olfactory hairs, long cilia that protrude from the nasal epithelium and are continuously bathed by a layer of mucus where chemicals are dissolved olfactory nerve- conducts the impulses to the olfactory cortex of the brain where odor is interpreted anosmia- partial or full loss of smell Sense of Taste taste buds- receptors for the sense of taste Locations of taste buds: tongue- majority soft palate superior part of the pharynx cheeks papillae- projections covering the tongue surface that contain the taste buds vallate papillae / circumvallate papillae- on backside of tongue fungiform papillae- anterior tongue foliate papillae- sides of tongue gustatory cells- receptor cells that respond to chemicals dissolved in the saliva gustatory hairs- long microvilli of the gustatory cells CN VII, IX, and X- carry taste impulses from the various taste buds to the gustatory cortex Basic taste sensations: sweet receptors- respond to substances such as sugars, saccharine, alcohols, some amino acids sour receptors- respond to hydrogen ions (H+) bitter receptors- respond to alkaloids salty receptors- respond to metal ions in solution umami- savory; respond to amino acid glutamate and other meaty flavor DEVELOPMENTAL ASPECTS OF THE SPECIAL SENSES All of the special senses are functional, to a greater or lesser degree, at birth. VISION Vision is the only special sense that is not fully functional at birth.(all babies are hyperopic / farsighted) >> eyeballs continue to enlarge until the age of 8 or 9, but the lens grows throughout life. Lacrimal glands are not fully developed until about 2 weeks after birth.  At 5 y.o., color vision is well developed, visual acuity has improved to about 20/30, and depth perception is present, providing a readiness to begin reading. At 40 y.o., presbyopia (farsightedness) begins to set in as the result of decreasing lens elasticity. Lacrimal glands become less active as we age, resulting in dry eyes which make eyes more vulnerable to bacterial infection and irritation. Lens loses its crystal clarity and becomes discolored.  The dilator muscles of the iris become less efficient; thus, the pupils are always somewhat constricted. >> decrease the amount of light reaching the retina, and visual acuity is dramatically lower HEARING Maternal infections can have a devastating effect on ear development, and maternal rubella during the early weeks of pregnancy results in sensorineural deafness. Newborn can hear sounds, but initial responses are reflexive.  3-4mos.: the infant is able to localize sounds and will turn to the voices of family members Toddler listens and begin to imitate sounds. otitis- ear inflammation secondary to bacterial infections or allergy presbycusis- sensorineural hearing loss secondary to gradual deterioration and atrophy of the spiral organ of Corti, leading to a loss in the ability to hear high tones and speech sounds otosclerosis- fusion of ossicles as a result of abnormal bone remodeling in the middle ear TASTE & SMELL Taste and smell are sharp at birth. In the mid-40s, ability to taste and smell diminishes, which reflects the gradual decrease in the number of the receptor cells. References Marieb, E. N., Keller, S. M. (2022). Essentials of Human Anatomy & Physiology, Global Edition. Pearson Education Limited VanPutte, C. et.al (2023). Seeley's Anatomy and Physiology, 13th Edition. McGraw-Hill Education