eye/visual pathway Flashcards
3 functions of pigment epithelium
- takes up old pigment disks (12 days)
- replace photopigment molecules
- provide nourishment for photoreceptors
structure of rods and cons
have an inner segment and outer segment
- outer segment is the part that actually senses the light
- the other end contains the synaptic terminal
visual pigments
light-sensing molecules in photoreceptor cells
- rhodopsin - visual pigment in rods
rods: pigment molecules are embedded in the bilipid membrane of the outer membrane which makes up the disks
cones: pigment molecules are embedded amongst the teeth of the comb (outer segment)
how rods become hyperpolarized
photoreceptors have both cation-conductive channels (which conduct primarily Na+) and K+ channels. In the dark, many of the cation channels are open, allowing Na+ to flow into the cell - dark current - serves to keep photoreceptors in a depolarized state
via a biochemical cascade, light triggers closing of the cation-conductive channels. the K+ channels which have remained open throughout, now dominate, dragging the membrane potential toward the K+ equilibrium potential
hence light hyperpolarized photoreceptors
structure of visual pigments
rods: rhodopsin - consists of an opsin molecule and a retinal molecule. the retinal is the actual light sensor
cones: opsin molecule affects the light wavelength sensitivity of the neuron
molecular mechanism of light transduction in rods
rhodopsin works just like a G-protein coupled receptor but instead of being triggered by ligand binding, it responds to light
the transducin molecule activates PDE which breaks down cGMP, reducing its conc
reduced cGMP leads to Na+ channel closing and cell hyperpolarization
transducin
the g-protein used as a signaling molecule in rods
light allows replacement of a bound GDP with GTP, activating transducin
once activated the alpha subunit starts the signal cascade
steps of light transduction in rods
- light stimulation of rhodopsin leads to activation of a G-protein, transducin
- activated G-protein activates cGMP PDE
- PDE hydrolyzes cGMP, reducing its conc
- this leads to closure of Na+ channels
steps in phototransduction
light -> photoisomerization -> reduced cGMP -> closing of Na+ channels -> hyperpolarization
steps in recycling of retinol isomers
trans-retinal is transported into pigment epithelium, which converts it to the cis isomer and sends it back into the outer segment
luminance sensitivity of rods
rods are used to detect changes in overall luminance (black and white vision)
highly sensitive to light but saturate in bright light and are no longer useful
luminance sensitivity of cones
cones enable color vision
require more light to respond but enable accurate color vision in bright light
non-functional in very dim light
distribution of rods and cones on the retina
fovea is covered with cones with virtually no rods
rods are more prevalent in the periphery
cones in the fovea are smaller and more densely packed, this leads to increased acuity
color constancy
two objects returning different spectra to the eye can appear to be the same color
receptive field
the area of the retina from which the activity of a neuron can be influenced by light
on-center receptive field
a spot of light at the center of the receptive field leads to a strong response
off-center receptive field
cells are inhibited by a light spot in the center of their receptive field and excited by light in the surround
photoreceptors have only
graded potentials
ganglion cells are excited by
glutamate and fire action potentials
two flavors of bipolar cells
- d bipolar or “on-center” - expresses a metabotropic glutamate receptor which is inhibitory. these cells depolarize when light is received by their photoreceptor
- H bipolar or “off-center” - these cells have a normal excitatory synapse with the photoreceptor. light decreases glutamate release and causes them to hyperpolarize
neural responses leading up to the on-center ganglion cell response
- light causes reduction in glutamate release from photoreceptor
- D bipolar cell is released from inhibition and depolarizes
- bipolar cell releases glutamate which activates ganglion cell
neural responses leading up to the off-center ganglion cell response
- light causes reduction in glutamate release from photoreceptor
- H bipolar cell is normally depolarized by glutamate release from photoreceptor. now, it has reduced input and hyperpolarizes
- bipolar cell reduces glutamate which inhibits ganglion cell
horizontal cells
release GABA at synapses with rods and cones and are inhibitory
they extend dendrites over large areas of the retina and are thought to mediate surround inhibition
also receive an excitatory input from rods and cones
have graded potentials
when light shines in the surround of a particular on-center ganglion cell
- the surround cone hyperpolarizes, reducing its glutamate release
- the horizontal cell becomes hyperpolarized and reduces its GABAergic suppression of the center cone
- response of the center cone to light is reduced (it reverts to the resting, depolarized state). it releases more glutamate
- a D bipolar cell is suppressed (hyperpolarized) by increased glutamate. this cell reduces its glutamate released onto the ganglion cell
- the response of the ganglion cell is suppressed
hyperpolarization of the surround causes
depolarization of the center
a center surround receptive field causes
suppression on one side of a dark-light boundary and excitation on the other side
this amplifies the effect of the luminance edge and enhances edge detection
- the brain cares about differences
retinogeniculostriate
mediates seeing
retina -> LGN -> V1
striate cortex
primary visual cortex (V1)
macula
highly pigmented part of the retia that includes the fovea
has the highest density of receptors and best spatial acuity
3 types of neurons in the LGN
Magnocellular
parvocellular
koniocellular
magnocellular
- large cell bodies; large receptive fields
- fast conduction velocities
- short processing time, but with little detail
- input from all cone types (S, M, L) so they are not color selective
- fast motion detection/processing
parvocellular
- small cell bodies; smaller receptive fields
- center and surround have input from different cone types, so they are color selective
- slow conducting velocities
- long processing time, but carries lots of info
- picture/scene analysis
koniocellular
- very small cell bodies
- located in between m and p layers
- function is unclear, linked with integrating visual info with other sensory info and color perception
foveal
great color and detail
peripheral
good luminance contrast and temporal frequency sensitivity
pinwheels
neighboring neurons have similar orientation selectivity
ocular dominance columns
bands of neurons that receive input from one eye
found in LGN and V1
they alternate, and are a way for the brain to integrate binocular input
ocular dominance columns
injecting an anterograde tracer into the retina of one eye produces strips in the primary visual cortex
hypercolumn
the slab of material which satisfies all the criteria for ocular dominance in V1
V1 is organized so that for every retinotopic location, there is a complete set of cells to represent ocular dominance, all orientations, and both blob and interblob regions
stereopsis
depth perceptions
disparities between the images falling on the two retinas is the basis of stereopsis
extrastriate
regions around V1
two general pathways for visual info
dorsal (where and how)
ventral (what)
ventral pathway
what pathway
concerned with object identity and has neurons sensitive to color and form
dorsal pathway
where pathway
concerned with movement and where things are in space
respond selectively to motion
area MT
specialized for motion processing
motion maps form pinwheels
akinetopsia
focal damage to area MT
an inability to perceive motion
patient perceives the world as a series of still images, as if viewed via a strobe light
achromatopsia
damage to certain extrastriate areas can induce a selective deficit in color vision
color opponent cells
color sensitive ganglion cells and LGN cells often have centers and surrounds that respond to opposing colors
these are the first step in coding color in the visual system
descending inputs
connections are glutamatergic but contact both primary LGN relay cells and inhibitory interneurons
cortex can shape its inputs which in turn changes cortical activity and so forth in a feedback loop
illusory contours
visual illusion
visual cues yield the perception of an object boundary where no physical boundary cues actually exist
neurons that respond to illusory contours
some neurons in V2 and V4 but not V1
top-down processing
face-responsive neurons
neurons in the temporal cortex that respond selectively to intact faces
these neurons are near the top of the visual cortex processing hierarchy
fusiform gyrus
region of the human cortex which responds selectively to faces
prosopagnosia
a selective deficit in perceiving faces
produced from lesions in the fusiform gyrus
