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Eyes of animals are very sensitive to light. Light detection by eyes is accomplished by sensory neurons that are specialized to use signal transduction mechanisms similar to those that detect hormones, neurotransmitters and growth hormones. The initial signal is amplified by mechanisms involved in gated ion channels, intracellular second messenger. The input signal from different several receptors is integrated before the final signal reaching the brain.
Step 1: Rods and cones of the vertebrate eye are lightly hyperpolarizable
The light-sensing cells are of two types; one is rod cell and the other is cone cell. Rod cells sense the low level of the light but can’t discriminate the colors while cone cells are less sensitive to light but can discriminate colors. These types of cells are long narrow specialized sensory neurons. The outer segment contains rhodopsin protein and the inner segment contains a nucleus, mitochondria.
Like other neurons, rods and cones have a transmembrane potential produced by the electrogenic pumping of the Na+/Ka+ ATPase in the plasma membrane of the inner segment. A cGMP-gated ion channel in the outer segment also contributes to the membrane potential by allowing the passage of Na+ or Ca++ ions.
In the dark, rod cells contain enough cGMP to keep the Ca++ or Na+ channel open in the outer segment. The mechanism of signaling in the retinal rod or cone cell is a light-induced and decrease in the concentration of cGMP which causes to close the cGMP-gated channel.
Step 2: Change in the conformation of the rhodopsin receptor is triggered by light
When light falls on the rhodopsin present in the disc of the outer segment of rods and cones then visual transduction begins. Rhodopsin is an integral membrane protein having seven transmembrane domains (serpentine transmembrane domains) of alpha-helices with chromophore 11-Cis retinal covalently attached to opsin which is the proteinous part of the rhodopsin.
Rhodopsin belongs to the G-protein-linked cell-surface receptor family. When a retinal component of rhodopsin absorbs a photon, the photochemical changes occur due to the absorbed energy that converts 11 cis-retinal to all-trans-retinal thereby results in the activation of rhodopsin by changing its conformation.
Step 3: Action of excited rhodopsin is through the G protein transducin, to lower the concentration of cGMP
The activated rhodopsin interacts with transducin (T), a type of GTP binding heterotrimeric protein. The transducin contains a bound-GDP α-subunit, β, and a γ-subunit all of which are attached together. When activated rhodopsin interacts with the bound GDP of the transducin, bound GDP is replaced by GTP from the cytosol. The α-subunit now bound with GTP gets dissociated and moves toward cGMP phosphodiesterase (PDE), an integral protein.
GTP-bound α-subunit activates cGMP phosphodiesterase (PDE) by replacing its inhibitory by itself. The activated PDE degrades cGMP to 5‘GMP reducing the concentration of cGMP in the outer segment so that cGMP-gated channels be closed. Closed cation channels prevent the influx of Na+, & Ca++ and membrane are hyperpolarized sending signals to the brain.
Continuous efflux of Ca through Na+, Ca++ exchanger reduces the concentration of Ca in the cytosol.
Step 4: Visual signals are amplified in rod and cone cells
There are at least 500 molecules of transducin each of which can activate a molecule of PDE. All transducin molecules are activated initially by a single excited molecule of rhodopsin and each of the activated molecules of PDE hydrolyze 4200 molecules of cGMP per second. This amplification of the signal is highly sensitive to light and, therefore, a single photon can close 1000 or more ion channels.
Step 5: The visual signal is terminated quickly
Termination of illumination of rod and cone cell causes the photosensory system to shuts off. The α-subunit of transducin has intrinsic GTPase activity. With the decrease in light intensity, GTP is hydrolyzed to GDP by intrinsic GTPase activity and α-subunit is reassociated with other subunits of transducin.
The inhibitory subunit of PDE attached to the GTP-bound transducin is then released and reassociates with PDE inhibiting it. Then the guanylyl cyclase enzyme converts GTP to cGMP increasing the concentration of cGMP. All these events are due to the fall in Ca++ ion in the cytosol
Step 6: Desensitization of rhodopsin by phosphorylation
With the prolonged illumination, Thr & Ser residues of the rhodopsin are phosphorylated by rhodopsin kinase, an enzyme functionally and structurally similar to that of the β-adrenergic kinase. Thereby desensitizing the β-adrenergic receptor.
These residues are located near to the c terminal domain and the phosphorylation reaction is stimulated by the low concentration of Ca++ and recoverin, a regulatory calcium-binding protein.
Furthermore, arrestin binds to phosphorylated c terminus inactivating rhodopsin. Slowly arrestin, a regulatory protein, dissociates, rhodopsin is dephosphorylated and all-trans-retinal is replaced by 11-cis-retinal. Then rhodopsin is then ready to take over another phototransduction cycle.