Neuro 6: Photoreceptors and Central Vision Pathways

Convert light energy to electrical signals

Primary function of photoreceptors

Photopigments, rhodopsin, cone opsins

What is inside of the discs of photoreceptors?

Cone opsins

Have peak sensitivities for specific colors like red, green, and blue

Red, green, blue

All other colors are represented by different combinations of absorbance by these three pigments

Wavelengths

Different opsins are tuned to different ___ of light.

Rhodopsin, opsin, glutamate

When light hits ___, retinal absorbs the light and changes shape, causing the ___ to activate and initiate a cascade of events in the disk.

This cascade will modulate ___ release.

Glutamate

Main neurotransmitter used by photoreceptors to communicate with bipolar cells, which then relay the signal to ganglion cells and ultimately to the brain

Phototransduction

What is this process called?:

1. In the dark: influx of + ions: depolarized state and release of glutamate

2. Retinal absorbs light, changes configuration, and prompts opsin to activate transducing ptn. Leading to break down cGMP and ion channels close, becoming hyperpolarized due to potassium ions, and glutamate release decreases

3. This decrease in neurotransmitter release acts as a signal that a light stimulus is present

+, glutamate, transducing PTN, potassium

Phototransduction:

1. In the dark: influx of _ [+/-] ions: depolarized state and release of ___

2. Retinal absorbs light, changes configuration, and prompts opsin to activate ___. Leading to break down cGMP and ion channels close, becoming hyperpolarized due to ___ ions, and glutamate release decreases

3. This decrease in neurotransmitter release acts as a signal that a light stimulus is present

L-cones, M-cones, S-cones

Types of cones

L-cones

Long wavelength (red)

M-cones

Medium wavelength (green)

S-cones

Short wavelength (blue)

Dichromatic

Most domestic mammals are ___ and thus are red-green colorblind.

Opsins

Cones have ___ tuned to different wavelengths of light.

S-cones, M-cones

Rods have a wavelength between the wavelengths of ___ and ___.

429, 555

Two color absorbance peaks for dogs

Bes

Which species?:

Trichromatic but do not see red

Tropical fish and birds

Which species?:

Can have 4 or 5 cones and see a vast range of color

Reptiles and amphibians

Which species?:

4 or 5 cone types

Photoreceptors (rods and cones), bipolar cells, retinal ganglion cells, CN2, optic chasm, optic tract or lateral geniculate nucleus of thalamus

Conscious visual pathway

Contralateral, ipsilateral

Conscious visual pathway:

Axons from the medial (nasal) aspect of the retina cross over to the ___ side.

Axons from the lateral (temporal) aspect of the retina remain ___ in their course.

Nasal retina, temporal retina

Each optic tract contains fibers from the ___ of the contralateral eye and ___ of the ipsilateral eye.

Lateral geniculate nucleus, thalamus

The ___ in the ___ serves a strategic role in the gating of information flow to the cortex.

Retinal ganglion cell

___ axons leave via the optic disc, then bundle together to become the optic n.

Diencephalon

Optic n. travels to the optic chiasm at the base of the diencephalon.

Ganglion axons

Past the optic chiasm, the ___ form the optic tract.

Diencephalon, lateral geniculate nucleus (LGN), thalamus

Some fibers in the optic n. target the ___ and help regulate circadian rhythms.

However, the majority of fibers are targeting the ___ in the ___.

Contralateral

Deficits in one eye may represent a lesion in the ___ pathway.