PS231: Photoreceptors and phototransduction

Visual information pathway in the retina

Photoreceptor cells (rods and cones) to bipolar cells, and then to ganglion cells. The axons of these ganglion cells form the optic nerve, which transmits signals to the brain

Phototransduction

turning light into nerve impulses (shared by rods and cones)

Outer segment

contains photoreceptor dissk packed with light-sensitive photopigment (opsins)

Rods contain more disks than cones

makes them have greater sensitivity to light (not active in daylight)

Rods are very sensitive

makes them good for low-light vision, but this comes at the expense of acuity (mostly located in peripheral retina, achromatic → no color vision)

Cones are much less sensitive to light

have more acuity (high resolution) and have different opsins that allow us to see colors (different wavelengths of light)

How many cones are there?

6m

How many rods are there?

125m

Cones are present

at a low density throughout the retina with a sharp peak in the fovea

Rods are present

at high density throughout most of the retina, with the exception of the fovea

Blind spot

lack of photoreceptors at the optic disk where RGC axons gather to leave the retina, forming the optic nerve

Where does phototransduction occur?

the photoreceptor outer segment

Light hyperpolarizes receptors

The absorption of photons of light ultimately leads to a reduction in neurotransmitter (glutamate) release by rods and cones—photoreceptors are therefore inhibited/hyperpolarized by light

Photons are absorbed by opsin proteins

Absorption of light causes a conformational change in the retinal molecule (vitamin A derivative) within the opsin protein, initiating an intracellular signaling cascade

Phototransduction pathway in the light

1. A photon of light is absorbed by retinal within the opsin protein (“planar”/ active form)

2. This stimulates transducin (G-protein)

3. This activates PDE, which works to close sodium channels

4. Neurotransmitter (glutamate) release decreases due to this hyperpolarization

Photoreceptors are depolarized in the dark

1. Retinal within the opsin protein is not activated (“bent”/ inactive form)

2. Thus, transducin (G-protein) is not activated

3. Phosphodiesterase (PDE) is therefore not activated

4. Without PDE activity, Na+ channels are open, allowing Na+ influx, which causes the photoreceptor to depolarize

Photoreceptors only release glutamate in the dark

Photoreceptors are more active in the dark, shown by depolarization and glutamate release in darkness (“dark current”)

On pathway

cells are depolarized by light due to mGluR6 (GPCR) activation (“sign inverting”)— report information about “light” to the brain

Off pathway

cells are hyperpolarized by light due to ionotropic AMPARs (“sign conserving”)—report information about “darkness” to the brain

Light pathway

ganglion cells → bipolar cells → photoreceptors