Retina stuff

Major retinal neuron types

Photoreceptors, Bipolar Cells, Ganglion Cells, Horizontal Cells, Amacrine Cells

Rods/location/end/synapse type

Most numerous photoreceptor, most in peripheral retina, ends in spherule, invaginating synapses

Fovea

Contains only cones, foveola= only red and green cones

Percentage of cones that are S cones

7–10%

Why doesn’t red and green cone variability affect color vision

Color depends on relative activation of cone populations rather than absolute cone numbers

Cone terminal name

Pedicle

Triad

Structure formed by rod bipolar and horizontal cells in rod spherules

Cone ON bipolar synapse type

Invaginating

Cone OFF bipolar synapse type

Flat

Signal generated by bipolar cells

Graded potentials

Ribbon Synapses

Continuously release neurotransmitter vesicles, reason bipolar cells can continuously signal, present on ON BP cells

ON bipolar cell response to photoreceptor hyperpolarization

Depolarization

ON bipolar cell receptor type

Metabotropic glutamate receptor

OFF bipolar cell response to photoreceptor hyperpolarization

Hyperpolarization

OFF bipolar cell receptor type

Ionotropic glutamate receptor

Definition of receptive field

Region of retina where stimulation affects a neuron

Function of receptive fields

Enhance edge detection and reduce responses to uniform illumination

Primary source of lateral inhibition

Horizontal cells

ON-center OFF-surround receptive field

Light in center excites light in surround inhibits

OFF-center ON-surround receptive field

Light in center inhibits light in surround excites

Primary role of amacrine cells

Inhibitory interneurons involved in retinal processing

Main neurotransmitters used by amacrine cells

GABA and glycine

Amacrine cells that use acetylcholine

Starburst amacrine cells

Amacrine cells involved in rod-cone switching

AII amacrine cells

Typical ganglion cell receptive field

Center-surround

Percentage of ganglion cells that are midgets

About 80%

Four types of midget ganglion cells

Red-ON Red-OFF Green-ON Green-OFF

Importance of midget ganglion cells

High visual acuity and color vision

Three mechanisms of light adaptation

Pupil size change, photoreceptor adaptation (slowest), network adaptation (fastest, switching from rods to cones using AII cones)

Purkinje shift

Visual sensitivity shifts toward longer wavelengths in light and shorter wavelengths in dark

Reason rods are insensitive to red light

Rod sensitivity peaks at shorter wavelengths

Primary visual pathway

Retina → LGN → Cortex

Percentage of ganglion cell axons terminating in LGN

About 90%

Major retinal recipient nuclei

LGN (vision)>> Superior Colliculus (saccades) > Suprachiasmatic Nucleus (circadian), Pretectum (pupillary reflex), Accessory Optic System (steadying gaze)

Blindsight

Limited visually guided behavior (reflexes) without conscious vision after damage to retina-LGN-cortex pathway

Effect of destroying geniculocortical pathway

Functional blindness with some reflexes preserved

Function of accessory optic system

Eye stabilization

Retinal slip

Motion of most of the visual scene across the retina due to unstable eye position

SCN

Hypothalamus above the optic chiasm, circadian rhythm regulation (uses light to “set” biological clock)

Pretectum

Pupillary light reflex (pupil constriction in olivary nuclei)

Major functions of superior colliculus

Saccades and orienting reflexes

Sensory maps in superior colliculus

Visual auditory, somatosensory maps, motor map

Effect of stimulating superior colliculus map location

Saccade toward corresponding location in space

Definition of LGN

Visual relay nucleus of the dorsal thalamus (part of diencephalon)

Definition of retinotopy

Neighboring retinal locations remain neighboring throughout visual pathways, allows lesions to produce predictable visual field defects

Visual field defect rule after optic chiasm

Homonymous

LGN contralateral eye input

Layers 1 4 and 6

LGN ipsilateral eye input

Layers 2 3 and 5

Magnocellular LGN layers

Layers 1 and 2, parasol cells

Magno cells

Receive input from parasol ganglion cells, are “color blind”, respond well to low-contrast stimuli, low spatial acuity, very fast, sometimes called Y (homologous)

Parvocellular LGN layers

Layers 3, 4, 5, and 6, midget cells

Parvo cells

Receive input from midget ganglion cells, has color vision, fine detail, doesn’t respond to low contrast stimuli, slow, highest spatial acuity, sometimes called X (homologous)

Functions of magnocellular pathway

Motion detection, low contrast sensitivity, rapid temporal responses

Functions of parvocellular pathway

Color vision and fine spatial detail

Major LGN neuron types

Relay neurons (project to cortex) and interneurons (only local connections)

Major computational strategy of retina

Information compression and feature extraction

Purpose of center-surround organization

Edge detection and contrast enhancement

Theme underlying magno and parvo pathways

Parallel processing

First location where information from both eyes converges

Cortex

LGN layers are monocular or binocular

Monocular

Superior colliculus

Recieves cortical input from frontal eye fields, part of midbrain, generates saccadic eye movements and reflexes, superficial layers= retina and cortex, deep layers= somatic and auditory inputs, head and neck orienting movements and multisensory integration

LGN lesions

*macular sparing is not obligate for visual cortex lesions

Ganglion cells

Only cells producing action potentials in the retina

First site of binocular integration

Cerebral cortex

Major theme of visual system

Specialized parallel pathways process different aspects of vision simultaneously