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BIOL 245
Vision
I. Neural visual apparatus
A. Rods
1. most common
2. very light-sensitive
3. no color vision
4. rhodopsin
a. opsin + retinal
b. light causes conformational change
c. all rods sensitive to similar wavelength of light
d. AP generated as a result of change in rod cell membrane permeability
5. dark adaptation
B. Retinal cell potentials
1. photoreceptors, bipolar cells, horizontal cells
2. ganglion cells
3. dark current - constant stream of Na+ into photoreceptor cell
a. light --> rhodopsin bleaching, blocks dark current
b. light stimulation may be breaking down cGMP which causes Na+
channels to close ---> stops glutamic acid secretion by rod cell
c. glutamic acid stimulates or inhibits bipolar cells (2 types)
C. Cones
1. require more light
2. color vision
blue, green, red cones (wavelength specific)
3. retinal not associated with same opsin
D. Bipolar neurons
1. modify output of rods and cones
2. processes visual info
E. Horizontal cells - synapse with several photoreceptors
F. Amacrine cells - synapse with several ganglion cells
G. Ganglion cells
1. largest retinal neurons
2. input from multiple bipolar neurons (usually)
3. axons form optic nerve fibers
H. Macula lutea (yellow spot)
1. on retina; directly posterior to lens
2. fovea centralis in center
a. focus point of light for direct vision
b. densely packed with cones
c. clear path for light rays
3. each cone in fovea has direct input to ganglion cell
4. vision in dark best with peripheral vision
I. Optic disc (blind spot)
1. medial to macula lutea
2. no photoreceptors
3. blood vessels enter; optic nerve exits
II. Visual Neural Pathways
A. Optic nerve---> optic chiasm
Hemidecussation:
1. medial retinal signals cross
2. lateral retinal input to ipsilateral brain
3. visual field goes to opposite side of brain
B. Optic tracts
1. terminate in thalamus
a. optic radiations: thalamus---> visual cortex
2. signals for visual reflexes--> sup. colliculi & pretectal nuclei instead
leads to motor pathway---> eye and body movements
a. smooth pursuit movements
b. pupillary and accommodation reflexes
III. Strabismus - wandering eye
IV. Blindness
A. Cataracts
1. clouding of lens
2. usually remove lens to correct
B. Glaucoma
1. excessive pressure in anterior compartment (aqueous humor)
2. circulation defect
3. pressure can constrict blood vessels
C. Diabetes mellitus
1. nerve degeneration
2. retinal detachment
3. result of circulatory problems
D. Retinal detachment
separation of sensory from pigmented retina