Visual System Physiology Flashcards
What are photoreceptor cells connected to?
Bipolar cells —> connect to ganglion cells —> connect to the brain
Describe the rod system
Convergence: many rods + many bipolar cells: one ganglion cell
Allows the rod system to operate in dim light
Sacrifices acuity to gain sensitivity
Density of rods peak at an eccentricity of about 20 degrees (peripheral vision)
Describe the cone system
Less convergence - receptor may send info to one bipolar cell that contacts one ganglion cell
Maximizes acuity
Density of cones peak at the fovea
Describe the events that occur in conditions of darkness
Photoreceptor not stimulated by light -> photoreceptor depolarizes -> graded potential is generated -> NT (glutamate) is released and acts as an inhibitory signal -> bipolar cell does not depolarize (no impulse sent to the brain) -> you don’t see anything bc its dark
Describe the events that occur during conditions of light (brightness)
Photoreceptor stimulated by light (photons) -> photoreceptor hyperpolarizes -> graded potential does not occur -> no inhibitory glutamate is released -> bipolar cell depolarizes -> visual impulse is sent to the brain
When is glutamate release the highest?
When it is dark (no stimulation by photons)
When is glutamate release from rods and cones the lowest?
When there is light (stimulated by photons)
The cells hyperpolarize under these conditions
True or false: Rods and cones constantly release glutamate
True
What glutamate receptor is expressed in bipolar cells?
mGluR6 which is a GPCR that closes cGMP gated Na channels (similar to light transduction in photoreceptors) which allows glutamate to be inhibitory
Closure of Na channels causes the bipolar cell to not become depolarized
When the bipolar cell is activated/depolarized what occurs?
It release glutamate to depolarize and therefore excite the ganglion cell (here glutamate will bind to NMDA or non-NMDA receptor) —> AP generated —> release of glutamate in visual cortex
What are direct targets of the retina?
LGN, superior colliculus, pretectum, hypothalamus and accessory optic nuclei (AON)
Describe the lateral geniculate body (LBG/LGN)
Major target of the retina
Maintains retinotopic organization
Has a lamellar structure (6 layers - half from nasal and half from temporal fields)
Signals from the two eyes are kept apart
Regulates the flow of info to the primary visual cortex
What are the functions of the lateral geniculate body?
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
Describe the superior colliculus
Connects with tectospinal tract to send projections to cervical anterior horn cells
Creates a map of visual space to activate appropriate motor responses required to move the eyes into their intended position within the orbits
Specifies movement intention rather than fixation of movement upon a target
Describe the pretectum
Important in the pupillary light reflex
Sends projections to Edinger Westphal nucleus then to the ciliary ganglion
Describe the hypothalamus
A small number of fibers branch off the optic tract, forming the retinohypothalamic tract, and terminate in the supraoptic, suprachiasmatic and pareventricular nuclei of the hypothalamus
Visual input to the hypothalamus drives the light dark entrainment of neuroendocrine function and other circadian rhythms
Describe the accessory optic nuclei (AON)
Advanced visual processing, optokinetic nystagmus in response to prolonged large field motion
Describe optic radiations
Axons of LBG relay cells to visual cortex on same side
Maintains retinotopic organization
Which 3 cortical areas make up the visual cortices?
Primary visual cortex, parastriate cortex and extrastriate cortex
Describe the primary visual cortex
V1
Major job is to identify the edges and contours of objects
Also decodes visual input to re-direct info in subsequent steps of analyses to other visual areas
What is the function of area V2 of extrastriate cortex?
Important in depth perception which is detected by analyzing the disparities between the two eyes
What is the function of V3a?
Major function is to identify motion
Is motion happening? Yes or no?
What is the function of V4?
Complete processing of color inputs
Lesions here are known to cause achromatopsia (problem seeing all colors)
Unilateral lesions may go unnoticed by pts
What is the function of MT/V5?
Tracks motion across a scene of directionality and context of the background and foreground
Does so by containing neurons that selectively respond to the direction of a moving edge
Ex. Watching a ball move through the air
