BSCI450: Phototransduction (Dark vs. Light)

Phototransduction

The process by which photoreceptors convert light energy into electrical signals that can be interpreted by the brain.

Main Goal of Phototransduction

To convert light into action potentials that travel through ganglion cells to the brain.

Phototransduction in the Dark (rods)

In the absence of light, photoreceptors remain depolarized and continuously release glutamate.

Na+ channels open → depolarize cell

Ca2+ (calcium) influx which releases inhibitory neurotransmitters to not activate bipolar cell to not stimulate ganglion cell

Step 1 (Dark)

No light is present, so opsin remains inactive.

Transducin is inactive because opsin has not been activated and phosphodieserase

Step 2 (Dark)

cGMP concentration remains high inside the photoreceptor.

High cGMP keeps cyclic nucleotide-gated sodium (Na⁺) and calcium (Ca²⁺) channels open.

Step 3 (Dark)

Na⁺ and Ca²⁺ continuously enter the photoreceptor (the "dark current").

The influx of Na⁺ and Ca²⁺ is greater than the loss of K⁺, keeping the photoreceptor depolarized and more positive

Step 4 (Dark)

Glutamate inhibits bipolar cells.

Step 5 (Dark)

Because bipolar cells are inhibited, ganglion cells do not reach threshold and do not generate action potentials.

Result in Darkness

No visual signal is sent to the brain because ganglion cells remain inactive.

Phototransduction in Light

Light causes photoreceptors to hyperpolarize, reducing glutamate release and allowing ganglion cells to generate action potentials when excitatory neurotransmitters go through.

Step 1 (Light)

Light strikes the photoreceptor.

Transducin activates phosphodiesterase (PDE).

Step 2 (Light)

PDE converts cGMP into GMP.

Low cGMP causes Na⁺ and Ca²⁺ channels to close.

Step 3 (Light)

Potassium channels remain open, so K⁺ continues leaving the cell.

The continued loss of K⁺ without Na⁺ and Ca²⁺ entering causes the photoreceptor to hyperpolarize → hyperpolarization greatly decreases glutamate release.

Step 4 (Light)

bipolar cells are no longer inhibited.

Bipolar cells depolarize and release excitatory neurotransmitter → ganglion cells depolarize and gets stimulated which becomes activated

Result in Light

A visual signal reaches the brain, allowing vision.

Phosphodiesterase (PDE)

Enzyme that converts cGMP into GMP.

cGMP

Molecule that keeps Na⁺ and Ca²⁺ channels open in darkness.

What can cGMP be used for when light enters? What does it do to Na+ and K+ channels?

cGMP gets converted to GMP by phosphodiesterase → inactivates and closes Na+ channel and K+ channels open to hyperpolarize bipolar cell → releases excitatory neurotransmitters to turn on ganglion cell

Glutamate

Neurotransmitter continuously released in darkness that inhibits bipolar cells → from Ca2+ influx and release action potential