chapter 15 Flashcards
The eye and vision
- > 70% of all sensory receptors are in the eye
- nearly half of the cerebral cortex is involved in processing visual info
- most of the eye is protected by a cushion of fat and bony orbit
- eye is like a camera
accessory structures of the eye
- protect the eye and aid eye function:
- eyebrows: shade from sun
- eyelids (palpebrae): protection/glands
- conjunctiva: transparent membrane
- lacrimal apparatus: tears
- extrinsic eye muscles (CN 3,4,6)
eye muscles controlled by cranial nerves 3,4,6
yes
structure of the eyeball
wall of eyeball contains 3 layers:
- fibrous-sclera and cornea
* avascular, outermost CT - vascular: iris, ciliary body, choroid. *middle pigmented area
- sensory- retina
* delicate 2 layered membrane
- internal cavity is filled with fluids called humors
- the lens separates the internal cavity into anterior and posterior segments (cavities)
fibrous layer
- sclera (white of your eye):
- protects and shapes eyeball
- anchors extrinsic eye muscles
- continous w/ dura mater of brain posteriorly - cornea:
- transparent anterior portion of fibrous layer
- bends light as it enters the eye
- numerous pain receptors contribute to blinking and 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:
- the colored part of the eye
- pupil: central opening that regulates the amount of light entering the eye
what 2 muscles does the iris have and how does it affect the parasympathetic and sympathetic NS system?
Iris has 2 muscles:
- sphincter pupillae
- dilator pupillae
Parasympathetic division:
sphincter pupillae muscle contraction decreases pupil size
sympathetic division:
dilator pupillae muscle contraction increases pupil size. (think when you in a bad situation, say, running in a forest while being chased, you need to see everything so eyes dilate. fight or flight
sensory layer: retina
- pigmented layer (outer layer):
- absorbs light and prevents its scattering - Neural layer (inner layer):
- photoreceptor- transduce light energy
- photoreceptors (rods/cones)->bipolar cells-> ganglion cells where action potential is generated
the retina
ganglion cell axons:
- run along the inner surface of the retina
- leave the eye as the optic nerve (CN 2)
Optic disc (blind spot):
- site where the optic nerve leaves the eye
- lacks photoreceptors
Photoreceptors
- rods:
- more numerous at peripheral region
- operate in dim light
- provide indistinct, fuzzy, non color peripheral vision - cones:
- found in macula lutea; concentrated in the fovea centralis
- operates in bright light
- provide high-acuity color vision
macular degeneration-loss of vision in the center of eye due to damage to the retina
fundus of eye
only place where small vessel can be observed directly
internal chambers and fluids
- > Posterior segment contains VITREOUS humor that:
- transmits light
- holds the neural retina firmly against the pigmented layer
- contributes to intraocular pressure
- > Anterior segment contains aqueous humor:
- -is composed of 2 chambers-
- anterior chamber- between the cornea and the iris
*posterior chamber-between the iris and the lens
Glaucoma: compression of the retina and optic nerve if drainage of aqueous humor is blocked. 2nd leading cause of blindness in US
lens
- biconvex, transparent, flexible, elastic, and avascular
- allows precise focusing of light on the retina
- lens become denser, more convex, and less elastic with age
Cataracts (clouding of lens) occur as a consequence of aging, diabetes mellitus, heavy smoking, and frequent exposure to intense sunlight
focusing light on the retina
pathway of light entering the eye: cornea, aqueous humor, lens, vitreous humor, neural layer of retina, photoreceptors
- light is refracted (bending of light ray) 3X:
1. at the cornea
2. entering the lens
3. leaving the lens
-change in lens curvature allows for fine focusing of an image
parasympathetic input contracts the ciliary muscle, allowing the lens to bulge and focus on near objects
yes
rods
- best suited for night vision (dim light) and peripheral vision
- perceived input is in gray tones only
- fuzzy and indistinct images
- rod pigment: RHODOPSIN, do not detect color. 120 million to 130 million rods, located in periphery
- night blindness-condition in which rod function is impaired, most common cause is deficit in vitamin A
cones
functional characteristics:
-need bright light for activation
- cone pigment: 3 types of pigments, sensitive to primary colors (red, green, blue), 6-7 million cones, located in macula
- detailed, high-resolution vision
light adaptation
- occurs when moving from darkness into bright light
- large amounts of pigments are broken down instantaneously, producing glare
- pupils constrict
- cones and neurons rapidly adapt
- visual acuity improves over 5-10 min
dark adaptation
occurs when moving from bright light into darkness
- cones stop functioning in low-intensity light
- pupils dilate
- Rhodopsin accumulates in the dark and retinal sensitivity increases within 20-30 min
takes longer to adapt to dark than it does to light
visual pathway
- axons of ganglion cells form the optic nerve
- some fibers of the optic nerve cross at the optic chiasma
- most fibers of the optic tracts continue to the thalamus
- the optic fibers connect to the primary visual cortex in the occipital lobes
depth perception
both eyes view the same image from slightly different angles
-depth perception (3-dimensional vision) results from cortical fusion of the slightly different images
Focusing an object on the _______ provides the highest visual acuity
fovea centralis
The ear: hearing and balance
3 parts of the ear:
- external (outer) ear
- middle ear (tympanic cavity)- air filled
- internal (inner) ear- fluid filled
The ear: hearing and balance
- external ear and middle ear are involved with hearing
- internal ear functions in both hearing and balance
- receptors (mechanoreceptors) for hearing and balance
- respond to separate stimuli
The cochlea-spiral bony chamber
The cavity of the cochlea is divided into 3 chambers:
- scala vestibuli
- scala media (cochlear duct)
- scala tympani
physiology of hearing
- sound waves (vibrations in air) cause ear drum to vibrate
- eardrum vibration causes movement of auditory ossicles (middle ear)
- ossicle movement presses fluid of inner ear and waves are formed
- tiny hair cells are excited and stimulate local neurons of Cranial Nerve VIII (8)(vestibulocochlear N)
auditory processing
- impulses from specific hair cells are interpreted as specific pitches
- loudness is detected by increased numbers of action potentials that result when the hair cells experience larger deflections
- localization of sound depends on relative intensity and relative timing of sound waves reaching both ears
equilibrium and orientation
-vestibular apparatus consists of the equilibrium receptors utricle/saccule and semicircular canals
A.) Utricle and saccule receptors (in vestibule):
*effects of gravity, linear acceleration (ex: driving in a car)
B.) semicircular canal receptors monitor dynamic equilibrium
*angular movements (ex: spinning)
equilibrium pathway to the brain
- pathways are complex and poorly traced
- impulses travel to the vestibular nuclei in the brain stem then to cerebellum (balance center)
- 3 modes of input for balance and orientation
- vestibular receptors
- visual receptors
- somatic receptors
