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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
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
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
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
Vascular Layer• 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
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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
• 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
Sensory Layer: Retina
Retina
Sensory Layer: Retina
• Night blindness: fewer working rods (lack of vitamin A)
• Day blindness: lack of working cones
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Retinitis Pigmentosa• 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
FDA approval
Sensory Layer: Retina• 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 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
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
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 EarOtitis 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
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
– 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
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Hearing• 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)
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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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