In vertebrate eyes, the light beams enter through the pupil and these light beams are focused on highly organized collection of light sensitive neurons known as retina. Retina is composed of two types of light sensing neurons which are discussed below:
(a) Rod cells: These cells sense low level of light but can not differentiate colors.
(b) Cone cells: These cells are less sensitive to light but can discrimate colors.
Structure of Rod and Cone
Both cells are long, narrow, photo sensory neuron having two distinct cellular compartments ; the outer segment consists of number of membranous disk containing membrane protein rhodopsin and the inner segment is made up of nucleus and many mitochondria required for the production of ATP which is utilized during photo transduction. The inner segment also contains Na+K+ ATPase which creates transmembrane electrical potential. This membrane potential is reduced by flow of Na+ or Ca2+ through an ions channel present in outer segment. This ion channel is gated by cGMP. In dark, rod cells consist of certain level of cGMP which keep this channel open. The membrane potential value depends upon the net difference between Na+ and K+ concentration which is created by Na+ K+ ATPase present in inner segment of cells and the influx of Na+ or Ca2+ through the ion channels of the outer segment.
Molecular Mechanism involved in vision
The signal transduction starts when light falls on rhodopsin. Rhodopsin is the integral membrane protein with seven membranes spanning Î± helices. Following events happen during the process of signal transduction in vision.
Step 1: Rhodopsin is composed of light absorbing pigment 11-cis retinal and covalently attached protein which is known as opsin. When photon is absorbed by rhodopsin, the energy of photon causes conformation change in rhodopsin by converting 11-cis retinal into all-trans retinal.
Step 2: Excited rhodopsin interact with second protein transducin belongs to GTP binding proteins containing three subunits namely TÎ±,TÎ² and TÎ³. Transducin can bind either with GTP or GDP. In dark when no signal is obtained, GDP is bound and all three protein subunits remain bound. When rhodopsin is excited by photon, the rhodopsin interacts with transducin catalyzing the replacement of bound GDP by GTP from cytosol. TÎ± subunit of transducin dissociate from TÎ²Î³ subunit.
Step 3: The next step in signal transduction pathway is activation of cGMP phosphodiesterase. Phosphodiesterase is enzyme which converts cGMP into 5' GMP. The activation of this enzyme results into lowering of cGMP concentration in outer segment. Lower cGMP level blocks cGMP gated ion channels inhibiting the reentry of Na+ and Ca2+ into outer segment of photosensory cells causing hyperpolarization of the membrane of photosensory cells. This causes the change the change in membrane potential of cell membrane. This signal passes to the visual cortex of brain and produces signals.
Step 4: Continued efflux of Ca2+ through the Na+ Ca2+ exchange reduces cytosolic Ca2+. This reduction of Ca2+ concentration activates guanylyl cyclase which inhibit phosphodiesterase enzyme. The inhibition of this enzyme cause increase in cGMP level leads to the reopening of cation. In this way the membrane potential returns to its prestimulus potential.
Step 5: The conformational change which is caused due to photon absorption results into exposure of several Thr and Ser residues. The residues are quickly phosphorylation by rhodopsin kinase. A Ca2+ binding protein recoverin act as an inhibitor of rhodopsin kinase. The phosphorylated rhodopsin is bound by protein arrestin1. This arrestin1 protein prevents interaction between rhodopsin and transducin. In due course of time, all trans-retinal of an excited rhodopsin molecule is removed and replaced by 11-cis retinal.
Amplification involved in this Signal transduction
Each excited rhodopsin molecule activates at least five hundred molecules of transducin and each transducin molecule in turn activate phoshodiesterase enzyme. This phosphodiesterase enzyme hydrolyses four thousand two hundred molecule of cGMP per second as phosphodiesterase enzyme has high turn over number.
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