The Visual System

Into. to Neurobiology

2010

The Retina

PHOTORECEPTORS BIPOLAR CELLS GANGION CELLS

Light

© Stephen E. Palmer, 2002

The Retina

• Photoreceptors and bipolar cells produce graded potentials rather than action potentials.• Ganglion cells generate action potentials. The ganglion cells’ axons join together to form the optic nerve, which exits through the back of the eye at the optic disk.

Photoreceptors

rod

cone

The Photoreceptors: RODS

• ~120 million rods are distributed over most of the retina.

• There are no rods at the very center of your visual field (the fovea); we can’t see detail in poor illumination.

• Rods are connected in groups (send less optic nerves than the cones).

• Rod vision detects edges and motion very well.

• Rod pigment, Rhodopsin (=visual purple), is bleached by light. Therefore rods are less effective in bright light.

• Rods take about 20 - 30 minutes of ‘dark adaptation’ before they are most efficient.

The Photoreceptors: CONES

• There are 5 million cones in the retina.

• Concentrated in fovea, in a region of about 1.5 mm in diameter. Most acute vision limited to foveola, covering ~0.4 mm.

• Color vision is provided by 3 types of cone with different colored light absorptions: red, green, and blue cones.

RodsConesOuter segment is rod

(stick) shapedOuter segment is cone

shaped

108-109 cells per eye, distributed throughout the retina, used for peripheral

vision .

106 cells per eye, found mainly in the fovea, so can only detect images in the

center of the retina .

Good sensitivity to low light levels (scotopic vision)

Poor sensitivity, active at higher light levels (photopic

vision)

Only 1 type monochromatic vision

3 types (R, G & B) colour vision

Many rods connected to one bipolar cell poor acuity = poor resolution

Each cone is connected to one bipolar cell good acuity = good resolution

Bleaching = transformation of Opsin

Light (μs)

Rhodopsin

• Rhodopsin is a pigment of the photoreceptor cells in the retina.

• It is responsible for the first events in the perception of light.

• It is comprised of Opsin (a protein with 7 transmembrane helices) and Retinal (photoreactive chromophore)

11 - cis- retinal all- trans - retinal

In the Dark…

• Na channels are open, causing depolarization.

• Rod cells release the neurotransmitter Glutamate in the dark.

• Their synapse with bipolar cells is inhibitory, thus it does not generate an action potential.

In the Light…

• As cis retinal is converted to trans retinal, the Na channels begin to close.

• Less neurotransmitter is released.

• If the threshold is reached, the bipolar cell will be depolarised and will fire an AP which is then passed to the ganglion cells.

The Role of cGMP in Vision

• cGMP amlifies the signal from a single photon via a cascade, causing 300 channels to close in rod cells.

• In the dark, cGMP keeps cGMP-gated cationic channels open. Thus sodium and calcium ions enter the photoreceptor cell. The Na current (called the "dark current") depolarizes the cell.

• Exposure to light causes the activation of the enzyme cGMP PDE which hydrolyzes cGMP. This closes Na and Ca channels and stops the "dark current" = hyperpolarizes the photoreceptor cell, causing a nerve impulse. • In order for vision to continue, the concentration of cGMP must be replenished. This is accomplished by inactivation of PDE.

Photoreceptor Adaptation

• Dark Adaptation (~ 20 min)

- Starts with pupil dilation.

- Shift from cone to rod vision.

- Resynthesis of rhodopsin, which was bleached in the light.

• Light Adaptation (~ 5 min):

- Starts with pupil constriction.

- Sensitivity of the retina decreases dramatically.

- Shift from rod to cone function.

http://webvision.med.utah.edu/movies/trasduc.mov

Processing in the Retinal Ganglion Cell

• Receptive field - the area of visual space to which a cell responds

• Ganglion cell receptive fields are circular.

• On-center cells: excited by light falling on the center of the receptive field and inhibited by an illuminated surround.

• Off-center cell: excited by light in the surround and inhibited by an illuminated center.

Each ganglion cell compares the degree of illumination between the center and the surround of its receptive field

Receptive fields in Bipolar cells and Lateral Inhibition

D (“on”-center)H (“off”-center)

Receptive fields in Bipolar cells and Lateral Inhibition

H (“off”-center)

H D

Visual Illusions and Lateral Inhibition

Mach bands

Summary Animation:

http://www.sumanasinc.com/webcontent/animations/content/receptivefields.html