Phototrasduction

PHOTOTRANSDUCTION

Visual or phototransduction is the process by which light energy is converted into receptor potential in visual receptors. Resting membrane potential in other sensory receptor cells is usually between –70 and –90 mV. However, in the visual receptors during darkness, negativity is reduced and resting membrane potential is about –40 mV. It is because of influx of sodium ions. Normally in dark, sodium ions are pumped out of inner segments of rod cell to ECF. However, these sodium ions leak back into the rod cells through membrane of outer segment and reduce the electronegativity inside rod cell. Thus, sodium influx maintains a decreased negative potential up to –40 mV. This potential is constant and it is also called dark current. Influx of sodium ions into outer segment of rod cell occurs mainly because of cyclic guanosine monophosphate (cGMP) present in the cytoplasm of cell. The cGMP always keeps the sodium channels opened. Closure of sodium channels occurs due to reduction in cGMP. Concentration of sodium ions inside the rod cell is regulated by sodium potassium pump. When light falls on retina, rhodopsin is excited leading to development of receptor potential in the rod cells.

Phototransduction Cascade of Receptor Potential

Following is the phototransduction cascade of receptor

potential :

1. When a photon (the minimum quantum of light energy) is absorbed by rhodopsin, the 11-cis

retinal is decomposed into metarhodopsin through few reactions mentioned earlier. Metarhodopsin

II is considered as the active form of rhodopsin. It plays an important role in the development of

receptor potential.

2. Metarhodopsin II activates a G protein called transducin that is present in rod disks Activated transducin activates the enzyme called cyclic guanosine monophosphate phospho diesterase

(cGMP phosphodiesterase), which is also present in rod disks

4. Activated cGMP phosphodiesterase hydrolyzes cGMP to 5’-GMP

5. Now, the concentration of cGMP is reduced in rod cell

6. Reduction in concentration of cGMP immediately causes closure of sodium channels in the membrane

of visual receptors

7. Sudden closure of sodium channels prevents entry of sodium ions leading to hyperpolarization. The

potential reaches –70 to –80 mV. It is because of sodium-potassium pump.

Thus, the process of receptor potential in visual receptors is unique in nature. When other sensory

receptors are excited, the electrical response is in the form of depolarization (receptor potential). But, in visual receptors, the response is in the form of hyperpolarization.

Significance of Hyperpolarization

Hyperpolarization in visual receptor cells reduces the release of synaptic transmitter glutamate. It leads to

development of response in bipolar cells and ganglionic cells so that, the action potentials are transmitted to cerebral cortex via optic pathway.

PHOTOSENSITIVE PIGMENTS IN CONES

Photosensitive pigment in cone cells is of three types, namely porphyropsin, iodopsin and cyanopsin. Only one of these pigments is present in each cone. Photopigment in cone cell also is a conjugated protein made up of a protein and chromophore. Protein in cone pigment is called photopsin, which is different from scotopsin, the protein part of rhodopsin. However, chromophore of cone pigment is the retinal that is present in rhodopsin. Each type of cone pigment is sensitive to a particular light and

the maximum response is shown at a particular light and wave-length. Various processes involved in phototransduction in cone cells are similar to those in rod cells.