CH 21 The Sensory System

The Eye and Eyeball

'Ophthalmo', the organ of vision -Lies in ball shaped cavity of the skull, the orbit/ -Contains more than 65% of body's sensory receptors. Has a 200 degree viewing angle, can see 2.7 million different colors and can see candle flicker 14 miles away -Eyelids, 'palpebrae', are retractable covers for the eyes anterior surface. Prefix: 'blepharo' -Oval opening b/w upper and lower eyelids called the 'palpebral fissure'. -Several types of glands are in the eye secreting sebum and sweat -The conjunctiva, is thin transparent mucous membrane that covers and protects the anterior eye, beneath and lining the eyelids (the cornea and sclera), supplied with blood vessels and nerve endings. -Lacrimal glands produce tears (lacrimal fluid-1ml/day) to moisten and lubricate eyes surface. Lacrimal glands located at outer corner (lateral canthus) of each eye. Tears drain from small opening ,the 'punctum', into the 'nasolacrimal duct' located in inner corner of eye (medial canthus). Tears contain enzyme that help protect eye from bacterial infection.

Effects of aging on the sensory system

**Vision/Eye Changes** -Lens accommodation decreases; lens loses elasticity Results in Presbyopia (age related farsightedness) NI: Corrective lenses helpful, especially for reading (bifocals or trifocals) Advise a vision check for corrective lenses. -Depth perception decreases. Results in Difficulty judging the height of curbs and steps Falls common NI: Encourage use of hand rails, canes, and walkers. Advise to avoid fast moves or turns. Make client aware of dangers. -Peripheral vision decreases. Results in Driving may be dangerous NI: Encourage a defensive driving class. Avoid standing at client's side. -Pupil size decreases; Ability to react to darkness and to bright light decreases; night vision decreases. Results in Takes longer for eyes to adjust when entering a dark room or bright sunlight, may require additional light for reading NI: Advise use of a night light. Advise person to avoid night driving if possible. Reading glasses may be needed, Wear sunglasses (especially amber, orange, or brown lenses) when outside -Color perception decreases, Depth perception decreases, Clouding of lenses. Results in Difficulty discerning hues of blue, green, and violet and distances. Cataract (may occur at any age) NI: Use yellow, red, and black for signs. Surgical removal and replacement with artificial lens -Grayish white ring (arcus senilis) forms around the iris due to deposits of calcium and cholesterol salts. Vitreous gel liquefies, Results in May lower self-esteem and body image (does not affect vision). Posterior visual detachment (PVD) causing 'floaters; in eyeball. NI: Enhance client's self-esteem. Usually not treated -Tear formation decreases. Results in Dry, itchy eyes, more susceptible to infections NI: Advise about medication ("artificial tears"). Advise against rubbing eyes -Fluid circulation in eye decreases. Results in Increased risk for glaucoma NI: Encourage regular visual examinations, including intraocular pressure measurement.

Balance and Equilibrium Taste Smell Touch/Tactile Sense

*Balance and Equilibrium* Sense of static balance (person at rest) is centered in the utricle and saccule of the inner ear. -Balance with movement (active balance) is associated with semicircular canals. -CN VIII (Vestibulocochlear nerve) plays important role in both balance and hearing. Transmits impulses to other cranial nerves responsible for control of head and neck movement. Also sends and receives information (about body's static and active positions) from inner ear processes, including the semicircular canals so the body can maintain positional balance and equilibrium. *Taste* -Sense of taste (gustation)is based on perceptions of sweet, salty, sour, bitter,metallic, and fatty or meaty (umami) favors and their combinations. Chemoreceptors (gustatory cells) in taste buds detect chemicals in solution in the mouth. -Two cranial nerves carry the sensation of taste to the brain; the facial nerve (CN VII) and the glossopharyngeal nerve (CN IX). The sense of taste is interpreted in the parietal and temporal lobes of cerebral cortex. *Smell* Nerve of smell, olfactory, is the olfactory nerve (CN I) -Odor molecules enter the nose and rise in the upper nasal cavity to the sensory area (olfactory epithelial tissue). Interpretation occurs in the olfactory center of the temporal lobe. In order for odors to be detected, chemicals must be dissolved in the watery mucus lining in the nasal cavity. *Touch* Developed in utero, first sense to appear. -Tactile receptors or corpuscles found mostly in skin. Sensory receptors constantly receiving nerve impulses, transmitting pain, vibration, pressure, or pleasures of warmth and softness. Brains parietal lobe interprets most tactile stimuli. -Some sensory signals are relayed immediately to the spinal cord and brain stem. Temp, pain, pressure, and proprioception are internal tactile stimuli **Thermoreceptors stop functioning at about 40F (4.4C). At this point, skin feels numb. Temp of more than 113F (45C), pain receptors take over

The Ear - Middle Ear

*Middle Ear* On other side of tympanic membrane, small air-filled cavity in temporal bone which is line with mucosa. Three small bones within this cavity: -The malleus (hammer) has a long process, the manubrium (handle), attached to movable section of the eardrum. -The incus (anvil) is the bridge b/w the malleus and stapes. -The stapes (stirrup) is the smallest bone in the body that transmit sound vibrations to the fluid-filled inner ear of the oval window, which separates the middle ear from the inner ear. These three bones, (malleus, incus, stapes) collectively called ossicles, are so small that sound waves can set them in motion. -Two small muscles attach to the stapes and malleus, that reflexively contract at sudden, loud noises. This reflex stops the ossicles vibrations protecting vital internal organs of the middle and inner ear from injury. (Hearing in external & middle ear accomplished by air conduction. Inner ear utilizes bone conduction) Extending from middle ear are two openings, one leads into the mastoid cells behind it and the other leads down into the eustachian tube, or auditory tube which communicates with the nasopharynx. -To function properly, middle ear pressure must equal to external atmospheric pressure. Eustachian tube opens during swallowing or yawning to equalize pressure in middle ear with atmospheric pressure, allowing eardrum to vibrate freely, otherwise hearing is impaired. Unequal pressures will cause pain, can be severe and may cause eardrum to rupture. -Tube also helps drain the middle ear. The middle ear, eustachian tube, nasopharynx, and the passage to the mastoid cells are lined with continuos coating of mucous membrane. Clenching jaw inhibits eardrum movement and impairs hearing.

Light Transmission through the Eyeball

Cornea (refraction) Anterior chamber and aqueous humor (refraction) Pupil (constriction and dilation) Lens (accommodation) Vitreous chamber and vitreous humor (refraction) Retina--rods and cones (receiving images of black/white and color) Central fovea (light rays for sharpest vision) Optic disk (nerve fibers converge, called the blind spot) Optic Nerve (CN II) Optic Chiasm (cranial nerve cross) Thalamus Cerebral Cortex (interprets impulses)

Nerves and Muscles of the Eye Table 21-2

Cranial Nerve II (optic): Carries visual images to the brain Cranial Nerve III (oculomotor): Constricts and dilates pupil, elevates eyelid, innervates superior, inferior, and medial rectus, and inferior oblique muscles. Cranial Nerve IV (trochlear): voluntary eye movement Cranial Nerve V (trigeminal {ophthalmic branch}): carriers sensations of eye pain and temp Cranial Nerve VI (abducens): innervates the lateral rectus muscle Cranial Nerve VII (facial): controls blinking reflex Superior rectus: Controls upward movement Inferior rectus: Controls downward movement Lateral rectus: Controls outward movement Medial rectus: Controls inward movement Superior Oblique: Controls upward and outward movement Inferior Oblique: Controls downward and inward movement

Which cranial nerves are responsible for the blink reflex, pupillary changes, visualization, and pain sensation in the eye?

Cranial Nerve II (optic): Carries visual images to the brain Cranial Nerve III (oculomotor): Constricts and dilates pupil, elevates eyelid, innervates superior, inferior, and medial rectus, and inferior oblique muscles. Cranial Nerve V (trigeminal {ophthalmic branch}): carriers sensations of eye pain and temp Cranial Nerve VII (facial): controls blinking reflex

Vision

Depends on four factors -Size of object -Brightness (luminance); intensity of light and amount of reflection -Contrast b/w object and background -Speed or time allowed to see object (more difficult to see a fast moving object) The visual areas of the cerebral cortex fuse images into a single image with a 3-dimensional effect, binocular vision, responsible for depth perception and possible because the coordinated muscles of both eyes move the eyeballs in tandem. **Vestibuloocular reflex (VDR) stabilizes images on the retina when the head moves. -Smooth-pursuit movement (SPM): phenomenon that allows the eyes to follow a moving object, usually requiring conscious effort. -Vergence movement: causes the eyes to converge (move toward each other) when viewing close object or diverge for far away objects. Closely related to accomodation

Effects of aging on the sensory system cont.

Hearing/Ear Changes Numerous functional and structural changes occur in ear components. resulting Taste changes: Taste sensations decreases; may be cause by lack of sensation transmission to brain; decreased # and function of taste buds; may be related to decreased function of smell. Results in decrease appetite; less enjoyment of food, may try to compensate by increasing salt and sugar intake, aggravating condition such as hypertension and diabetes; risk for consuming spoiled foods. Smell Changes: Smell Perception decreases. Results in may not smell smoke or poisonous substances. Affects sense of smell. NI: Teach client to install smoke and CO detectors and preventive safety measures. Teach safe dietary modifications. Tactile Changes Efficiency and the # of sensory nerve endings (all sensations affected) decrease. Results in stronger stimuli needed for person to perceive sensations; pain may not be perceive. NI: Monitor clients overall condition, do not ignore complaints, observe body language signs of pain

Differentiate between myopia, hyperopia, and astigmatism

If eyeball is too short, the light rays land behind the retina causing farsightedness (hyperopia or hypermetropia). The refracted light rays do not come together directly on the retina, but are absorbed by it therefore person cannot see close objects clearly. In nearsightedness (myopia), light rays are focused in front of the retina because the lens muscles contract too tightly, not allowing enough light to enter the eye or because the eyeball is too long. Person see close objects clearly, but distant objects are blurry. Astigmatism is caused by irregularities in the curvature of the cornea and lens. The eye cannot bring horizontal and vertical lines into focus at the same time, causing blurry vision.The retina's receptors send visual impulses through the nerve fibers to the optic nerve (CN II), which meets the retina at the optic disk ('blind spot' not light sensitive). Optic nerve carries the information to the cerebral cortex of the brains occipital lobe for interpretation. KEY CONCEPT: Farsightedness, hyperopia, is caused when light rays focus behind the retina. Nearsightedness, myopia, occurs when light rays focus in front of the retina.

Define Presbyopia Ptosis Presbycusis

Presbyopia: far-sightedness, difficulty seeing close objects or reading Ptosis: Upper lids droop; lower lids droop (ectropion) Presbycusis: progressive hearing loss (highest pitches lost first) certain consonants difficult to hear K, T, S, P

Functions and Placements of Rods and Cones

Rods -Placement; Widespread over retina -Function:Color; Receive black and white and shapes -Function:Vision; Scotopic (night) vision Cones -Placement; Center of retina -Function:Color; receive color -Function:Vision; Photopic (bright light) vision

The Sensory System

Sensory perceptions are those of seeing, hearing, tasting, smelling, and touching. Also receive impressions of warmth, softness, pressure, vibration, and pain through sensory system. In order to be aware of information from surroundings, person must posses: >Receptors to receive a stimulus >Nerve routines to carry stimulus to brain >Centers in brain to interpret stimulus General sense organs include: microscopic sensors widely distributed in muscles, tendons, joints, and internal organs. Sensory organs are: eyes, ears, tongue, nose, and skin. Functions: -Visual sense receives images (light) -Hearing receptors process sound waves (auditory sense) -Through prioprioceptors and the inner ear, the system helps to maintain a sense a balance, equilibrium, and position in space -Chemoreceptors in mouth obtain information about tastes(gustatory sense) -Chemoreceptors in nose receive sensations of odors (olfactory sense) - Touch receptors receive information about surrounding world - Internal organs receive sensations of pain, pressure, fullness, vibration - The brain interprets most of sensations

Nerves and Pathway of sound

Sound is conducted via the pinna (external ear) and middle ear to the inner ear's auditory nerves. -The nerve receptors for hearing are within the cochlear in the organ of Corti. These hair-like receptors connect to the cochlear nerve, a division of the acoustic or auditory nerve (CN VIII). -Sound is conducted to the center for hearing, in the temporal lobe of the brains cerebral cortex. -The acoustic nerve has 2 divisions; the cochlear nerve (for transmission of sound) and vestibular nerve (for transmission relating balance and position) . Thus CN VIII is sometimes called the vestibulocochlear nerve

Hearing

Sound is energy that moves in waves of pressure; sound waves are perceived by the brain via firing of auditory nerve cells. -Perception of sound is audition, hearing measured by audiometer -Sound waves enter the ear's external auditory canal and strike the tympanic membrane (eardrum) which vibrates at various speeds in response to various pitches of sounds. -The ossicles within the middle ear contact each other and act as a movable bridge to transmit vibrations to the oval window. -Sound waves are concentrated because the oval window is much smaller than the tympanic membrane. This amplifies them (hydraulic principle) -Base of the stapes fit into oval window, when stimulated the stapes vibrate against this membrane, setting cochlear fluid (in inner ear) in motion. This wave like action transmits vibrations onto tiny hair like nerve endings (receptors) in the organ of Corti. The protein filaments are mechanoreceptors which release a chemical neurotransmitters when stimulated. (The sound waves bend the filaments causing them to fire). The stimuli from the organ of Corti are sent to the vestibulocochlear nerve ( a portion of CN VIII) and then to the temporal lobe in cerebral cortex, where sounds are interpreted. External Environment--> external auditory canal--> tympanic membrane, where vibrations begin. Vibrations travel through the middle ear-->oval window into the inner ear, where they travel through the cochlear fluid (perilymph and endolymph) --> receptors (hair cells) of organ of Corti. From here, the vibrations are transmitted to auditory nerve fibers--> vestibulocochlear nerve--> cerebral cortex, where they are interpreted

Sound Amplification

Sound waves are amplified in 3 ways: -Ear canal is open and resonance there approx. doubles sound waves -The ossicles (hammer, anvil, and stirrups) act as levers. This mechanical advantage amplifies sound apporx. 3-fold (This, when multiplied by amplification in the ear canal, now equals a total of a 6-fold amplification) -The relative sizes between the eardrum and oval window amplify or increase sound waves approx. 30x more Hearing can be damaged by extremely loud noises or music (noise trauma) by certain drugs (amino-glycoside--streptomycin, loop diuretics--furosemide, or aspirin--ototoxicity, blast injuries, foreign objects, or specific illnesses. Can be temporary or permanent.

The Ear - Inner Ear

The division between the middle ear and inner ear is the oval window (fenestra ovalis). The inner ear is embedded in the temporal bone, the densest bone in the body. Here, the bony labyrinth contains the membranous labyrinth. Both labyrinths are filled with fluid similar to CSF. (Fluid in the bony labyrinth [perilymph] is thinner, that in the membranous labyrinth is thicker [endolymph]. Thes labyrinths are important in sound wave transmission and in determination of body balance and positional changes. The 3 sections of the inner ear are the cochlea, vestibule, and semicircular canals. *Cochlea* -Shaped like a hollow snail shell. -Inside the bony cochlea lies the cochlear duct, containing the organ of Corti, a very small but very intricate structure. Referred to as 'true organ of hearing' because transmission of nerve stimuli related to sound begins here. *Vestibule* -B/w the cochlea and semicircular canals, contains 2 membranous sacs, the utricle and saccule, suspended in perilymph. -Utricle and saccule have specialized areas of hair cells, maculae, which send sensory information to the cerebellum and midbrain. These areas of the brain relay changes in body position, primarily when at rest )static balance) *Semicircular Canals* -Shaped like horseshoes , lie behind the cochlea, and are primarily concerned with balance when the body is moving (acceleration/deceleration or head movements) -At the end (ampulla) of each semicircular canal is a receptor, the crista (crista ampullaris), which contains sensory hairs. Movement activates endolymph within the canals, which sets in motion the tiny hairs of crista, transmitting information about the body's position to the brain. Each canal is on different planes so endolymph flows in various directions and senses more types of motion. -Sensory receptors for equilibrium in the moving body are on nerve endings in semicircular canals (active balance). Sensations transmitted to the brain via the vestibular division of CN VIII(8)

The Eyeball -Sclera and Cornea

The eyeball is a hollow sphere with only the anterior surface being visible. Consists of 3 layers of tissue, the tunics: the sclera and cornea, choroid layer, and retina. -The sclera is the tough, fibrous, protective outer layer of eyeball. Helps maintain shape, its continuous with the transparent, yet tough, section over front of the eyeball, the cornea, which permits light rays to enter the eye. Junction b/w cornea and sclera is the limbus. -The cornea is covered with a protective coating of stratified squamous epithelium, influences visual acuity by refracting light rays. Very sensitive to touch and pain, often removed after death for corneal transplantation. (avascular) The eye is protected by surrounding bony eye socket, also includes a cushioning fat layer. Eyebrows/lids/lashes provide further protection. Eye lid also contains sebaceous glands that provide lubrication. Blink reflex protects eye from foreign objects or blows.

The Eyeball -Lens and Retina

The lens, an opening controlled by ciliary body muscles (including the iris and 3 auxiliary muscles). -Located immediately behind pupil and iris and has a major role in focusing light rays on retina. -Space behind lens, posterior chamber, filled with transparent gelatin like material called vitreous humor which helps maintain the eyeballs shape and contributes to intraocular pressure. *Loss of vitreous humor results in blindness. The retina is the eyeballs inner layer. -Its an incredible light-sensitive membrane. Pigmented layer composed of simple cuboidal epithelium and contains the optic nerve receptor. -Optic disk, "blind spot" is an oval area on the posterior retina that allows optic nerve to enter a the nerve head. 'Physiologic cup' is a depression within optic disk. -Retina also contains specialized neurons, rods, and cones (photoreceptor cells). >Rods receive sensations of black and white and can register shapes, but not colors, useful in night vision (scotopic vision). >Color vision depends on the cones, which are concentrated in the retina center and function in daylight and bright light (photopic vision). Majority of cones are concentrated around the 'fovea centralis' (about 5% of the retina) the fovea lutea. The cones of of 3 individual classes; each receives red, blue, or green light waves, combined to forms color. Cones also add to visual acuity (visual sharpness) but require a significant amount of light, thus you see shades of gray, rather than color in dim light, because only the roads are receiving stimuli. **Color Blindness is an error in production of photopigments in the cones. It causes a distinguishing between colors, particularity red and green. -The macula is the center of the retina, lateral to the optic disk, and surrounding the fovea centralis. Since this area contains the most cones, its a major receptor for vision and color. The macula is considered the region of greatest visual acuity. A disruption, macular degeneration, results in visual difficulties, can lead to blindness.

The Eyeball -Choroid Layer

The middle tunic, the 'choroid', contains the iris and ciliary body. This vascular layer brings o2 + nutrients to all layers (tunics) of the eye. -The choroid contains dark pigment, to prevent scattering of light rays. -The colored iris controls amount of light entering. -Ciliary body contains muscles that adjust shape and thickness of lens and also secretes aqueous humor, which flows through anterior chamber of eye in the space b/w cornea and lens. Aqueous humor maintains intraocular pressure (24mmHg) and provides nutrients and 02 to avascular lens and cornea. *Anterior chamber:space from front of cornea to back of lens and iris , produces aqueous humor. Posterior chamber begins behind the iris, contains vitreous humor, a thicker clear gel. -Over the front of the eyeball, the choroid develops into pigments section called, the iris, which gives eye its specific color. Amount of pigment determines eye color. Smooth and radial muscles in the iris control the size of the pupillary opening. -Pupil is the black center opening within the eye that allows light to enter. In strong light, iris muscles constrict the pupil to allow less light to enter. Reverse occurs under low light situations (purkinje effect) KEY CONCEPT: The adaptation to darkness (Purkinje effect) depends on good blood flow to the eye. Blood flow is inhibited by vasoconstrictors, including tobacco and alcohol.

The Ear-External Ear

The organ of hearing (auditory sense) and equilibrium. Otology is the study of ear disorders. Auditory refers to hearing and measurement of hearing acuity. *External Ear* -Also called Pinna or auricle, only readily visible part of ear. -Composed mostly of cartilage and is funnel-shaped to gather and guide sound waves into its small opening, which extends into the auditory canal. This opening is the 'external auditory meatus'. Its covered with tiny hairs and contains ceruminous glands which secrete cerumen. Hairs and wax protects ear from foreign objects. -Auditory canal is very short and extends to a thin membrane, the eardrum (tympanic membrane), that contains fibrous tissue and mucous membrane, separates external from middle ear. External auditory meatus angled different in a child.

Trace the path of light rays as they enter the eye and focus on the retina; describe transmission to the brain

When you look at a distant object, the ciliary muscles around the lenses relax and lens flattens. Looking at a nearby object causes the muscles to contract, pulling the choroid coat forward and causing the lens to bulge. Light rays enter the eye through the cornea and pass through the aqueous humor, pupil opening, lens, and vitreous humor before focusing on the retina. -Refraction (bending of light rays) occurs within these components KEY CONCEPT: When light rays converge on the retina, the image is upside-down. In the visual cortex of the brain, the image is corrected and perceived as upright. -Within the lens, parallel light rays are refracted (bent) so they focus directly on the retina making the image clear. -Ciliary muscles also play a role in clear image formation. As the eye moves, the exposure is adjusted by the iris and also as a result of a chemical reaction. The formed image is reflected on the retina, the nerve center of the eye. -The central fovea ( fovea centralis) is responsible for the sharpest vision. Goal is to have light rays fall precisely on this central fovea. -Lens play a major role in this process by adjusting light rays, adjustment cause a clear image, accommodation, is controlled by the autonomic nervous system. Field of vision: the entire area visible when the eyes are fixed in one position. Peripheral vision (side vision): the eyes see on the side when looking straight ahead; all visible outside the central area of focus. 95% of retina is concerned with peripheral vision