physiology of the visual field Flashcards
refraction
the fact or phenomenon of light, radio waves, etc being deflected in passing obliquely through the interface between one medium and another or through a medium of varying density
first sight of refraction
cornea
- not variable
- 2/3 of light bending
second sight of refraction
lens
- variable
- depends on curvature of lens under physiologic control
- rounder = more refraction
- flatter = less refraction
what is required to change the curvature of the lens?
- ciliary muscle
- suspensory ligaments
- normal lens
increase curvature of lens
- ciliary m contracts
- allows suspensory ligaments to loosen
- lens more rounded shape by natural recoil
- near vision
decrease curvature
- ciliary m relaxes
- suspensory ligaments tighten
- lens pulled tight flattening it
- used for far vision
presbyopia
lens becomes stiffer in aging, loss of elasticity
near response
- contraction of ciliary ms
- convergence of eyes to the point of focus
- constriction of pupil
- reduces opening for light to enter
- eliminates diverging light rays
- allows better focus
path of light from when it enters at cornea to activating photoreceptor
cornea, lens, hummor, vitreous, GCL, IP, INL, OPL, ONL
5 neuron types in the retina
vertically oriented
- receptor cells (rod and cone)
- bipolar cells
- ganglion cells - MG cells
horizontally oriented
- horizontal cells
- amacrine cells
photoreceptors
rods and cones
rod system
- convergence: many rods + many bipolars –> 1 ganglion cell
- allows to see in dim light
- sacrifices acuity to gain sensitivity
- off center
cone system
- less convergence: 1 receptor –> 1 bipolar cell –> 1 ganglion cell
- maximizes acuity
- bright light
- center
what do rods and cones constantly release?
glutamate
when is glutamate release highest?
dark
when is glutamate release lowest?
light
-stimulation by photons –> hyperpolarize –> less glutamate release
activation of bipolar cell by cone
1 photon stimulates photoreceptor
2 photoreceptor hyperpolarizes
3 glutamate release onto the bipolar cell DECREASES
ON center
CENTER: causes depolarization
PERIPHERY: causes hyperpolarization
increase discharge rate to luminance increments in the receptive field center
OFF center
CENTER: hyperpolarizes
PERIPHERY: depolarizes
increase discharge rate to luminance decrements in the receptive field center
ON center bipolar cell in darkness
- glutamate would activate Gi GPCR metabotropic receptor on the ON-center bipolar cell
- results in a decrease in cation influx into the bipolar cell
- hyperpolaries cell
ON center bipolar cell in brightness
- light photons decrease the presence of glutamate
- less glutamate around
- less activation of metabotropic receptor on the ON center bipolar cell
- less Gi signaling
- results in an INCREASE in cation influx into the bipolar ell
- depolarizes the cell
OFF center bipolar cell in darkness
- glutamate would activate AMPA receptor on the OFF center bipolar cell
- results in an increase in cation influx into the bipolar cell
- depolarizes cell
OFF center bipolar cell in brightness
- light photons decrease presence of glutamate
- less glutamate around
- less activation of AMPA receptor on the OFF center bipolar cell
- results in decrease in cation influx into the bipolar cell
- hyperpolarizes the cell
Do ganglion cells have ON center and OFF center varieties?
yes –> activated/deactivated by glutamate released when bipolar cells depolarize
- ganglion cell axons become fibers of optic nerve
- in cortex, ganglion cells will release glutamate
activation of bipolar cell by rod photoreceptors
- many rods converge on one ON center bipolar cell
- connects to a “rod-bipolar cell” and “rod amacrine cell” which both function as interneurons inhibiting the cones with glycine or GABA
- connects to a “cone-bipolar cell”
- connects to ganglion cell
direct targets of the retina
- LGB
- superior colliculus
- pretectum
- hypothalamus
- accessory optic nuclei
LGB functions
- control the motions of the eyes to converge on a point of interest
- control the focus of the eyes based on distance
- determine relative position of objects to map them in space
- detect movement relative to an object
optic radiations
axons of LGB relay cells to visual cortex on same side
-maintains retinotopic organization
anatomical areas of visual processing
17 primary visual cortex
18 parastriate cortex
19 peristriate cortex
functional areas of visual processing
V1 primary visual cortex (17) V2 greater part of 18 V3 narrow strip of 18 V4 area 19 V5 middle temporal (MT) area of 19
primary visual cortex layers
I, II, III - networking with IV
IV receives input from LGB
V and VI main output layers: LGB, thalamus, subcortical regions
What layers of the cortex do columns span?
all 6 layers
ocular dominance columns
a slab of cells that preferentially respond to input from one eye or the other
orientation columns
organized region of neurons that are excited by visual line stimuli of varying angles spanning 6 layers of cortex oriented perpendicular to cortical surface
blobs in primary visual corex
collections of 6 layers of the cortex organized region of neurons that are sensitive to color assemble into cylindrical shapes.
-3 color coding cones required for color detection
S-cone
blue 437 nm
rod
498 nm
M-cone
green 533 nm
L-cone
red 564 nm
stripes
ocular dominance
swirls
orientation columns
cytochrome oxidase stain
blobs
V1 major job
edges and contours of objects
V2 major job
depth
V3 major job
ID of motion
V4 major job
color processing
dorsal pathway
from primary visual cortex and goes tot the parietal/frontal cortex
- primary path associating vision with movement
- completes motor acts based on visual input
- passes through V3
ventral pathway
from primary visual cortex to the inferior temporal cortex
- primarily involved in interpreting images (recognizing or copying shapes, forms, faces) and complex patterns
- copying/naming objects are separate functions in temporal lobe. damage to one area is possible without damaging the other
- facial recognition is a specialized area
what are MG cells?
a subset of ganglion cells detects light directly via the blue-sensitive photopigment melanopsin
What does melanopsin do?
light, via melanopsin causes changes in calcium levels in MG cells
-non-image-forming light -responsive system –> project to hypothalamus (suprachiasmatic nucleus) (circaidian rhythms)
