Visual System Physiology

Question 1

What are photoreceptor cells connected to?

Answer 1

Bipolar cells —> connect to ganglion cells —> connect to the brain

Question 2

Describe the rod system

Answer 2

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)

Question 3

Describe the cone system

Answer 3

Less convergence - receptor may send info to one bipolar cell that contacts one ganglion cell
Maximizes acuity
Density of cones peak at the fovea

Question 4

Describe the events that occur in conditions of darkness

Answer 4

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

Question 5

Describe the events that occur during conditions of light (brightness)

Answer 5

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

Question 6

When is glutamate release the highest?

Answer 6

When it is dark (no stimulation by photons)

Question 7

When is glutamate release from rods and cones the lowest?

Answer 7

When there is light (stimulated by photons)

The cells hyperpolarize under these conditions

Question 8

True or false: Rods and cones constantly release glutamate

Answer 8

True

Question 9

What glutamate receptor is expressed in bipolar cells?

Answer 9

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

Question 10

When the bipolar cell is activated/depolarized what occurs?

Answer 10

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

Question 11

What are direct targets of the retina?

Answer 11

LGN, superior colliculus, pretectum, hypothalamus and accessory optic nuclei (AON)

Question 12

Describe the lateral geniculate body (LBG/LGN)

Answer 12

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

Question 13

What are the functions of the lateral geniculate body?

Answer 13

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

Question 14

Describe the superior colliculus

Answer 14

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

Question 15

Describe the pretectum

Answer 15

Important in the pupillary light reflex

Sends projections to Edinger Westphal nucleus then to the ciliary ganglion

Question 16

Describe the hypothalamus

Answer 16

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

Question 17

Describe the accessory optic nuclei (AON)

Answer 17

Advanced visual processing, optokinetic nystagmus in response to prolonged large field motion

Question 18

Describe optic radiations

Answer 18

Axons of LBG relay cells to visual cortex on same side

Maintains retinotopic organization

Question 19

Which 3 cortical areas make up the visual cortices?

Answer 19

Primary visual cortex, parastriate cortex and extrastriate cortex

Question 20

Describe the primary visual cortex

Answer 20

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

Question 21

What is the function of area V2 of extrastriate cortex?

Answer 21

Important in depth perception which is detected by analyzing the disparities between the two eyes

Question 22

What is the function of V3a?

Answer 22

Major function is to identify motion

Is motion happening? Yes or no?

Question 23

What is the function of V4?

Answer 23

Complete processing of color inputs
Lesions here are known to cause achromatopsia (problem seeing all colors)
Unilateral lesions may go unnoticed by pts

Question 24

What is the function of MT/V5?

Answer 24

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

Question 25

What are the three types of columns in the visual cortex?

Answer 25

Ocular dominance columns, orientation columns, blobs

Question 26

Describe ocular dominance columns

Answer 26

Columns that span all 6 layers of the cortex
How: a slab of cells that preferentially respond to input from one eye or the other (ocular dominance)
Mapped into stripes

Question 27

Describe orientation columns

Answer 27

Columns that span all 6 layers of the cortex
How: organized region of neurons that are excited by visual line stimuli (horizontal/vertical) of varying angles
Where: orientated perpendicular to the cortical surface
Mapped into swirls

Question 28

Describe blobs

Answer 28

Span all layers of the cortex
Organized region of neurons that are sensitive to color assemble into cylindrical shapes
All 3 color coding cones are required for accurate color detection
Can be visualized with cytochrome oxidase stains

Question 29

Describe melanopsin ganglion (MG) cells

Answer 29

1-3% of ganglion cells can directly sense light
Due to expression of melanopsin they are sensitive to blue light wavelengths of light
Provides light sensing info for regulation of circadian rhythm
This is considered a non-image forming light responsive system

Question 30

MG cells project directly to what?

Answer 30

Hypothalamic suprachiasmatic nucleus, the neuroendocrine effector in the pineal gland which produces melatonin in a rhythmic pattern

Question 31

Which two pathways are important for processing what you see around you?

Answer 31

Dorsal (where) pathway

Ventral (what) pathway

Question 32

Describe the dorsal pathway

Answer 32

From primary visual cortex and goes to the parietal/frontal cortex
Primary path associating vision with movement
Completes motor acts based on visual input
Arises from V3
V1 -> V2/3 -> V5/MT -> parietal lobe

Question 33

Describe the ventral pathway

Answer 33

From primary visual cortex to the inferior temporal cortex
Primarily involved in interpreting images (recognizing or copying shapes, forms, faces) and complex patterns
V1 -> V2 -> V4 -> temporal lobe