VS131 Visual Neuroscience

Retinal Recipient Nuclei

Which responds faster to a change in light, ON or OFF bipolar cells?

20%

20%

20%

20%

20% 1. ON Bipolar Cells

2. OFF Bipolar Cells

3. ON and OFF bipolars are the same speed

4. This is a wrong answer

5. This is also a wrong answer

The major targets of the retinal ganglion cells:

RETINA LGN CORTEX (“vision’)

Superior Colliculus (saccades)

Suprachiasmatic Nucleus (circadian)

Pretectum (pupilary light reflex)

Accessory Optic System (‘steadicam’)

Only about 10% of retinal ganglion cells do not terminate in the Lateral Geniculate Nucleus (LGN).

‘BLINDSIGHT’: preservation of very limited ability to perform visually guided tasks after destruction of the retina to LGN to cortex pathway, in the apparent absence of conscious perception.

Bottom line: take out the LGN-cortical system and you are for all practical purposes completely blind. Some reflexes and other limited abilities remain, however – but this does not mean that the LGN and cortex can compensate if one of the ‘lesser’ systems is damaged!!!

Relevant sections of Oyster that you should read:

Superior Colliculus: pg.150, 204-205

Suprachiasmatic Nucleus: pg.205-207

Accessory Optic System: pg.205

Pretectum (& Edinger-Westphal): pg.204-205, 224, 233-235, 433-434.

Accessory Optic System: stabilize the eyes using information from the visual system (as opposed to from the vestibular system).

-> Eye stabilization is critical for vision

-> RGC axons go to three small nuclei in the brain stem: dorsal, lateral, and medial terminal nuclei.

-> Only a few thousand axons.

-> These nuclei have cells that respond to large areas of motion, they are selective for both direction and velocity.

-> Medial, Lateral, and Dorsal Terminal Nuclei (MTN, LTN, DTN)

-> Definition of retinal slip.

Suprachiasmatic Nucleus (SCN): A small cluster of cells in the hypothalamus above (duh) the optic chiasm.

-> Has cells fire in direct proportion to the total amount of light (presumably not center-surround!).

-> In animals lesioning this area causes disturbances of diurnal rhythms.

-> Visual inputs entrain the natural rhythm in the SCN.

-> Only other known clock in the human body is in the retina itself, but that one only seems to be involved with specifically retinal adaptation and maintenance. The SCN does jet lag etc.

Pretectum: Part of the Midbrain.

-> Nucleus of the optic tract: eye movement control and stabilization.

-> Pretectal Olivary Nuclei: pupillary light reflexes.

Superior Colliculus: also part of the midbrain.

-> Has a variety of functions, including generating saccadic eye movements and visual orienting reflexes.

-> Relatively bigger and more important in other species, such as reptiles (“Gamera”) where it has turned into the optic tectum, a homologous structure to our superior colliculus.

Superior colliculus (SC) has seven layers.

-> Three sensory maps:

visual map

body map

sound map

-> Also a motor map

-> Maps are aligned.

Superficial layers: direct input from retina and also cortex

Deeper layers: somatic and auditory inputs, also motor outputs.

-> If you stimulate the SC in a particular place in the map, you will generate a rapid saccadic eye movement to that place in space represented by that location on the map.

-> SC also gets inputs from the frontal eye fields in cortex, direct volitional as well as reflex control of saccades.

-> SC projects to regions of brainstem that are involved in oculomotor control, and to other regions as well.

-> Also involved in reflex control of head and neck movement.

-> Eye movements are incredibly fast and precise. It takes a lot of ‘machinery’ to give you that capability. The SC is just one (very important!!) part of this system.