Special Senses Flashcards
Eye + vision
70% of all sensory receptors are in the eye
- half of cerebral cortex is involved in processing visual info
- most of eye protected by fat cushion + bony orbit
Eye accessory structures
Protects + aid eye function
- eyebrows: shade from sun
- eyelids (palpebrae): protection/glands
- conjunctiva: transparent membrane
(cunjunctivitis - pinkeye - infection of conjunctive)
- lacrimal apparatus: tears –> lubricate eye, washes debris, nourish cornea + conjunctiva
- extrinsic eye muscles (CN 3, 4, 6)
Eyeball structure
Wall of eyeball contains 3 layers:
1. Fibrous: sclera + cornea
- avascular, outermost CT
2. Vascular: iris, ciliary body, choroid
- middle pigmented layer
3. Sensory: retina
- delicate 2 layered membrane
- internal cavity filled w/ fluids called humors
- lens separates internal cavity into anterior + posterior cavities
Fibrous layer
- Sclera (white of your eye)
- protects + shapes eyeball
- anchors extrinsic eye muscles
- continuous w/ dura mater of brain posteriorly - Cornea
- transparent anterior portion of fibrous layer
- bends light as it enters eye
- numerous pain receptors contribute to blinking/tearing reflexes
Vascular Layer (uvea - middle pigmented layer)
- Choroid region: supplies blood to all layers of the eyeball
- Ciliary body: smooth muscle bundles (ciliary muscles) control lens shape (accommodation)
- Iris: colored part of eye
- pupil: central opening regulating amt of light entering eye
Iris muscles
Parasympathetic
- sphincter pupillae: muscle contraction DECREASES pupil size
- dilator pupillae: muscle contraction INCREASES pupil size
Sensory layer (retina)
- Pigmented Layer (outer layer)
- absorbs light + prevents scattering - neural layer (inner layer)
- photoreceptor: transduce light energy
- photoreceptors (rods/cones) –> bipolar cells –> ganglion cells where AP generated
Retina
Ganglion cell axons
- run along inner surface of retina
- leave eye as optic nerve CN2
Optic disc (blind spot)
- site where optic nerve leaves eye
- lacks photoreceptors
- blind spot
Photoreceptors
- Rods
- numerous at peripheral region
- DIM lights
- indistinct, fuzzy, NON color peripheral vision - Cones
- macula lutea: concentrated at fovea centralis
- BRIGHT light
- high-acuity COLOR vision
Macular degeneration: loss of vision in center of eye due to retina damage
Fundus of eye
place where small vessels can be observed directly
Internal chambers + fluids
Posterior segment contain vitreous humor
- transmit light
- holds neural retina firmly against pigmented layer
- contributes to intraocular pressure
Anterior segment contains aqueous humor
- 2 chambers:
- anterior chamber: bw cornea + iris
- posterior chamber: bw iris + lens
Glaucoma: compression of retina + optic nerve if drainage of aqueous humor is blocked
- second leading cause of blindness in US
Lens
- biconvex, transparent, flexible, elastic, and avascular
- precise focusing of light on retina
- lens = denser, convex, less elastic w/ age
cataracts: clouding of lens, conseq/ of aging, diabetes, mellitus, smoking, exposure to intense sunlight
Focusing light on Retina
Pathway of light entering eye: cornea, aqueous humor, lens, vitreous humor, neural layer of retina, photoreceptors
- light refracted (bending of light ray) 3x:
1. at cornea
2. entering lens
3. leaving lens - change in lens curvature allows for fine focusing of an image
Focusing for close vision
Parasympathetic input contracts ciliary muscle, causing lens to bulge + focus on near objects
Rods
Best suited for NIGHT VISION (DIM LIGHT) and peripheral vision
- perceived input is in GRAY tones only
- fuzzy/indistinct images
- rod pigment: rhodopsin, doesn’t detect color, 120-130 million rods, located in periphery
- Night blindness: condition is which rod function is impaired, most common cause is deficit in vit A
Cones
Functional characteristics
- need BRIGHT LIGHT for activation
- cone pigment: 3 types, sensitive to primary colors (red/green/blue), located in macula
- detailed, HIGH RES vision
Light adaptation
Occurs when moving from darkness –> bright light
- large amt of pigments broken down, producing glare
- pupils constrict
- cones + neurons rapidly adapt
- visual acuity improves over 5-10 mins
Dark adaptation
occurs when moving from bright –> darkness
- reverse of light adaptation
- cones stop functioning in low intense light
- pupils DILATE
- rhodopsin accumulates in dark + retinal sensitivity increases 20-30 mins
- longer to adapt to dark than light
Visual pathways
- axons of ganglion cells form optic nerve
- some fibers of optic nerve cross at optic chiasma
- most fibers of optic tracts continue to the thalamus (way station)
- optic fibers connect to primary visual cortex in occipital lobes
Depth perception
Both eyes view same image from slightly different angles
- depth perception (3D vision) results from cortical fusion of slightly different images
Ear: hearing/balance
3 parts
1. External (outer) ear
- hearing
2. Middle (tympanic cavity) - air filled
- hearing
3. Internal (inner) - fluid filled
- both hearing + equilibrium (balance)
-receptros (mechanoreceptors) for hearing + balance - respond to separate stimuli
Tympanic + middle ear
Boundary bw external/middle ears
- CT membranes vibrates to sound
- transfer sound energy to bones of middle ear
Cochlea
cavity of cochlea is divided into 3 chambers
- spiral bony chamber
- scala vestibuli
- scala media (cochlear duct)
- scala tympani
Hearing physiology
- sound waves (vibrations in air) cause eardum to vibrate
- eardrum vibration causes movement of auditory ossicles (middle ear)
- ossicle movement presses fluid of inner ear + waves formed
- tiny hair cells excited + stimulate local neurons of CN VIII (vestibulocochlear N)
