L7: Vision II Flashcards
layers of the retina
photoreceptors horizontal cells bipolar cells amacrine cells ganglion cells
rods and cones interact w/ ______ cells via NT ?
bipolar cells
glutamate
amacrine cell NTs
8 or more kinds
including:
GABA, glycine, dopamine, ACH, indolamine
**note all are inhibitory
retinal layer - photoreceptors
rods and cones
transmit signals to outer plexiform layer
synapse w/ bipolar cells and horizontal cells
plexiform layer = ?
layer of synaptic connections
horizontal cells
- transmit signals from rods/cones to bipolar cells
- transmit signals to outer plexiform layer
- output is always inhibitory — lateral inhibition
bipolar cells
- transmit signals from rods/cones and horizontal cells
- transmit signals to inner plexiform layer
- synapse w/ amacrine and ganglion cells
amacrine cell signals
transmit signals to :
–directly from bipolar to ganglion cells
—w/in inner plexiform layer from axons of bipolar cells to dendrites of ganglion cells or to other amacrine cells
ganglion cells
transmit signals from retina to brain**
axons make up optic nerves**
axons of the optic nerve
only retina cells that transmit action potentials
interplexiform cells
transmit from inner plexiform to outer layer (retrograde signals)
inhibitory signals – lateral inhibition
foveal region
cone vision
3 neurons in direct pathway
3-neuron cone vision
cones
bipolar cells
ganglion cells
4-neuron rod vision
rods
bipolar cells
amacrine cells
ganglion cells
describe amacrine cells
- -30 kinds
- -dif type does dif things
ex. rod conduction, continuing visual signals, offset of visual signals, dark or light vision, mvt
because of their various functions, amacrine cells are ?
interneurons
help analyze visual signals before they leave retina
describe ganglion cells
~60 rods and 2 cones per ganglion cell to optic nerve fiber
approaching the fovea, ____ rods and cones converge on each optic fibers, rods and cones also become more ______ .
fewer
slender
central fovea – only cones ~35k
what is the result of rods and cones sliming down
increases visual acuity in central retina
describe peripheral retina
–more sensitive to low light
up to 200 rods converge on a single optic fiber in periphery
types of ganglion cells
3 types
W, X, Y
W ganglion cells
40% of ganglion cells
small
transmit 8 m/sec
most excitation due to rods by way of small bipolar and amacrine cells
W ganglion cell in periphery
broad fields in peripheral retina
because dendrites spread widely in inner plexiform layer
X ganglion cells
55% of ganglion cells
medium size
transmit 14 m/sec
X ganglion cell signals
small fields – discrete retinal locations
every x ganglion receives input from at least one cone cell
cells probably responsible for all color vision
x ganglion cells
Y ganglion cells
5%
large
transmit 50 m/sec and up
respond to rapid changes in visual image
Y ganglion to CNS
transmit almost instantly
when a new visual event occurs anywhere in the field
but w/o great accuracy w/ respect to location
dorsal lateral geniculate nucleus
receives input from optic nerve (in thalamus)
relays info from optic tract to visual cortex by optic radiation
= geniculocalcarine tract
decussation in optic chiasm
50%
layers of lateral geniculate nucleus
6 layers
2,3,5 = receive signals from lateral half of ipsilateral retina
1,4,6 = from medial half of opposite retina
layers 1 and 2
magnocellular layer
large neurons
input almost entirely from Y ganglion
provide rapid conduction
only black and white
layers 3 and 4
parvocellular layers
small/medium neurons
input from X ganglion
moderate conduction
color
describe transmission gating
the lateral geniculate nucleus controls how much of the signal is allowed to pass to the cortex
source of gating control
corticofugal fibers from primary visual cortex
reticular areas of mesencephalon
describe sources of gating control
both sources are inhibitory
and
help highlight visual info that is allowed to pass
the primary visual cortex is also called ?
striate cortex
characteristics of primary visual cortex
in occipital lobe
6 distinct layers
geniculocalcarine fibers terminate in layer 5
where do signals from macular area terminate in visual cortex?
signals from macular area terminate near occipital pole
where do peripheral retina signals terminate in visual cortex?
term in concentric half circles anterior to the pole but still along the calcarine fissures
layer 4 of primary visual cortex
organized into subdivisions
4calpha = signals from Y ganglion
4cbeta and 4a = signals from x ganglion
the primary visual cortex is organized into several million vertical ______ of ______.
columns of neurons
30 - 50 micrometer diameter
~1000 neurons per column
describe color blobs
special column-like areas
–located among the columns of 2ndary visual areas
receive lateral signals from adjacent visual columns
are activated specifically by color signals
signals from the 2 separate eyes enter _______ stripes of columns in layer ___.
alternating stripes
layer 4
what does the cortical area do?
deciphers whether the respective areas of the 2 visual images from each eye register w/ each other
accommodation in children
refractive power of the lens can be voluntarily increased from 20 to 34 diopters
accommodation of 14
accommodation in young person – relaxed state
no tension on capsule
lens assumes almost spherical shape
accommodation in young person – normal eye conditions
suspensory lig. attached radially around lens
create tension
causes lens to remain flat
meridional fibers
part of ciliary muscle
contract and release tension on lens
circular fibers
part of ciliary muscle
decrease tension on lens
_______ controls both sets of ciliary muscles
CN III
optic nerve
accommodation in older person
lens is larger and thicker w/ age
less elastic
accommodation power decreased by 2 diopters by 50
decreased to 0 by 70
define presbyopia
farsightedness
due to loss of elasticity of the lens
occurs typically in middle to old age
**need for reading glasses
