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.
0 Comments