Special Senses Chapter 8

Special Senses Smell, taste, sight, and hearing Equilibrium Special sense receptors: large, complex sensory organs (eyes, ears) localized clusters of receptors (taste buds and olfactory epithelium) Senses blend to give us our sensations

The Eye & Vision

External Anatomy of the Eye Only anterior 1/6 of eye’s surface can normally be seen Accessory structures: Extrinsic eye muscles Eyelids Conjunctiva Lacrimal apparatus

Accessory Structures Extrinsic eye muscles Six muscles are attached to outer surface of each eye Produce gross eye movements

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Accessory Structures Eyelids 4 layers: skin, muscle, connective tissue, conjunctiva (mucous membrane) Moved by orbicularis oculi muscles (close) and levatator palpebrae superioris (open)

Accessory Structures Lacrimal apparatus Consists of lacrimal gland & ducts that drain lacrimal secretions into nasal cavity Lacrimal glands: continuously secrete dilute salt solution (tears) onto anterior surface of eyeball Secretion contains mucus, antibodies, and lysozyme which is an enzyme that destroys bacteria Cleans and protects eye surface as it moistens & lubricates it

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Internal Structures Eyeball is a hollow sphere Wall composed of three layers: Fibrous layer Sclera Cornea Vascular layer Choroid Ciliary body Iris Sensory layer Retina Pigmented layer (prevent light from scattering) Neural layer (photoreceptors)

Fibrous Layer Sclera: thick, white connective tissue (“white of the eye”) Opaque due to large, disorganized collagenous and elastic fibers For protection & attachment Cornea: clear “window” through which light enters eye anterior portion of eye many nerve endings (pain fibers), easily repairs itself avascular, so only tissue in the body that can be transplanted without fear of rejection helps focus entering light rays Connective tissue with thin layer of epithelium; unusually regular fiber pattern

Fibrous Layer: Sclera & Cornea

LASIK link

Vascular Layer link Choroid: blood-rich nutritive tunic that contains a dark pigment Prevents light from scattering inside the eye Modified anteriorly to form ciliary body & iris Produces melanin to absorb light Contains blood vessels Ciliary body: two smooth muscle structures to which the lens is attached with the ciliary zonule (ligament) Forms internal ring around front of eye (muscle fibers & ligaments) Iris: pigmented, has an opening called the pupil through which light passes Thin diaphragm composed of muscle tissue and connective tissue Circular and radial muscle fibers control size of pupil (stimulated by photons of light) Regulates the amount of light which enters the eye

Choroid

Ciliary Body

Sensory Layer: Retina Retina – two layers which extends anteriorly only to the ciliary body Pigmented layer composed of pigment cells that absorb light and prevent light from scattering inside the light act as phagocytes to remove dead or damaged receptor cells store vitamin A needed for vision

Sensory Layer: Retina Neural layer – contains millions of receptor cells Photoreceptors (rods & cones): respond to light, bipolar neurons 70% of sensory receptors are in the eyes Rods: more sensitive in low light, gives general outline, seen as black and white Cones: less sensitive in low light, sharp picture, color Electrical impulse leaves the retina via the optic nerve & nerve impulses are transmitted to the optic cortex which results in vision Fovea: all cones, sharpest vision (visual acuity) Optic disc (“blind spot”) – where optic nerve leaves eyeball

Retina

Sensory Layer: Retina link Night blindness: fewer working rods (lack of vitamin A) Day blindness: lack of working cones

Age-related Macular Degeneration

Retinitis Pigmentosa Genetic disorder FDA approval Genetic disorder involves abnormalities in photoreceptors or retinal tissue that leads to progressive vision loss Bionic Eye (Argus II Retinal Prosthesis System) Mostly restores black & white vision Did restore color vision in some

Sensory Layer: Retina Read pg. 286 Rods: rhodopsin breaks down into opsin & retinal when struck by photons; initiates chemical reaction (action potential) which is sent to visual cortex (occipital lobe) In bright light, nearly all rhodopsin I broken down, reducing rod sensitivity Cones: three different light sensitive proteins connected to retinal – each most sensitive to a particular wavelength of visible light Depending on which is stimulated, brain interprets that color Erythrolabe: red Chlorolabe: green Cyanolabe: blue Mixing & interpretation of color occurs in the brain, not the retina! (i.e. red light in one light & green light in another eye will be seen as yellow) Read pg. 286

Visual Pigments If all are stimulated at once, see white!

Color Blindness Total color blindness: lack of all three cones Partial color blindness: lack of one cone type (lack of red or green receptor is most common) Sex-linked condition (carried on X chromosome)

Color Blindness 1. Normal Color Vision: A: 29, B: 45, C: --, D: 26 4 Sex-Linked Traits:  1. Normal Color Vision:  A: 29,  B: 45,  C: --,  D: 26  2. Red-Green Color-Blind:  A: 70,  B: --,  C: 5,  D: --  3. Red Color-blind:  A: 70,  B: --,  C: 5,  D: 6  4. Green Color-Blind:  A: 70,  B: --,  C: 5,  D: 2

Internal Structures: Lens Lies directly behind iris & pupil Focuses light entering the eye on the retina (changes shape to focus) Held upright in the eye by a suspensory ligament (ciliary zonule) attached to ciliary body Epithelial cells (cytoplasm is transparent part)

Cataracts With age, lens becomes increasingly hard and opaque Cataracts result from this process and cause vision to become hazy and distorted Can eventually cause blindness in affected eye Treatment: surgical removal of lens and replacement with lens implant or special cataract glasses

Internal Structures: Humors Aqueous humor Anterior to the lens, clear watery fluid Fills space between cornea and lens Nourishes, helps hold shape Vitreous humor Reinforces eyeball internally Posterior to lens

Glaucoma aqueous humor made more quickly than can be removed or drainage is blocked pressure builds, damage results from compression of retina and optic nerve

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Physiology of Vision Light rays are bent (refracted) as light encounters the cornea, aqueous humor, lens, and vitreous humor Refractory power of the lens can be changed by changing its shape (ciliary body controls shape of lens) Accommodation: ability of eye to focus for close objects (less than 20 ft. away) Image formed on retina is a real image (inverted)

Vision

Astigmatism

Visual Pathways Axons carrying impulses from retina are bundled together at posterior aspect of eyeball and issue from back of eye as optic nerve Optic chiasma: fibers from medial side of eye cross over to opposite side of brain Fiber tracts that result are optic tracts Each optic tract contains fibers from the lateral side of the eye on the same side and the medial side of the opposite eye Optic tract fibers synapse with neurons in the thalamus, whose axons form the optic radiation which runs to the occipital lobe of brain (visual interpretation occurs) Each side of brain receives visual input from both eyes Visual fields overlap to give humans binocular vision

The Ear: Hearing & Balance

Anatomy of the Ear Divided into three major areas External (outer) ear Middle ear Internal (inner) ear

External (Outer) Ear Auricle (pinna) – “ear” Collects and directs sound waves into the auditory canal (function largely lost in humans) External acoustic meatus (auditory canal) Short, narrow chamber carved into temporal bone of skull Ceruminous glands – secrete cerumen (earwax) Tympanic membrane (eardrum) – vibrate when sound waves strike it; separates external & middle ear

Swimmer’s Ear Otitis Externus

Middle Ear Small, air-filled, mucosa-lined cavity within temporal bone Transfers vibrations via the ossicles (smallest bones in body) malleus (hammer), incus (anvil), and stapes (stirrup) Stapes passes vibration to the oval window of the inner ear Pharyngotympanic tube (auditory tube): pressure needs to be equalized to enable eardrum to vibrate

Middle Ear

Otitis Media Inflammation of the middle ear Pharyngotympanic tubes run more horizontally in children Ear tubes – implanted in ear drum to allow pus to drain into external ear canal Infants with bottles “propped” or fed lying flat can get fluid in their ears through the pharyngotympanic tube

Internal (Inner) Ear Bony (osseous) cavity located behind the eye socket Filled with perilymph (fluid) Membranous labyrinth suspended in perilymph, contains endolymph Three subdivisions Cochlea Vestibule Semicircular canals Lined with hair cells (mechanoreceptors)

Equilibrium Vestibular apparatus: equilibrium receptors of ear link Vestibular apparatus: equilibrium receptors of ear Static equilibrium: maculae receptors Report on changes in position of head when body not moving (keep head erect) Otoliths: tiny stones that roll in response to changes in pull of gravity Dynamic equilibrium: bending of cupula indicates rotation (gelatinous cap) Report on changes when body moving (i.e. spinning) Receptors stimulate hair cells, which send impulses via the vestibular nerve to the cerebellum Work together with proprioceptors for control & balance

Hearing link Spiral organ of corti – contains hair cells (hearing receptors) Vibrations set cochlear fluids in motion, pressure waves cause vibrations impulses are sent via cochlear nerve to temporal lobe (auditory cortex)

Loud Sounds and Hair Cell Damage

Cochlear implant link

Chemical Senses: Smell & Taste

Olfaction Olfactory receptors (chemoreceptors): receptors for sense of smell, occupy a postage stamp-size area in roof of each nasal cavity 10-100,000,000 receptors in nose Olfactory filaments (axons) make up olfactory nerve (cranial nerve I) which conducts impulses to olfactory cortex of brain Olfactory lobes of brain (gray matter) – situated over nose (bottom of frontal lobe) Olfactory impulses closely tied to limbic system Sensitive receptors, just a few molecule can activate them (thousands of smells) Adapt rather quickly when exposed to unchanging stimulus

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Gustation: Taste Taste buds: specific receptors for taste widely scattered in the oral cavity Live 7-10 days! protection (low threshold – bitter) 10,000 taste buds (mostly on tongue) Five tastes: sweet, salty, sour, bitter, and umami (savory) Flavors: combination of 5 tastes and olfactory and touch sensations 1st order neuron  medulla  hypothalamus or thalamus (limbic)  parietal lobe (conscious perception of taste)

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