Transducin is required for –
Correct Answer: Vision
Description: Ans. is 'd' i.e., Vision o Transducin is a G protein which is involved in transmission of visual signals.o Both the rods and the cones release glutamate at their synapses with the bipolar cells.o The mechanism of signal tranduction, i.e., the process by which light is converted into electrical signal (nerve impulse), is almost similar in both rods and conesQo But both photoreceptors have differences with other sensory receptors.o The photoreceptors do not behave like other receptors when exposed to their stimulus : they are hyperpolarized instead of depolarized.o The photoreceptor cell has a membrane potential of - 40 mV.o The photoreceptors cell are in depolarized state in the dark.o It is due to Na+ channels in outer segment, which are open in dark state which allow influx of Na+ and depolarization,o Therefore, a steady state of neurotransmitter (glutamateQ) is released from photoreceptors.o In light photoactivation of the opsin is accompanied by a conformational change, which in turn brings about a similar change in a membrane protein, transducinQ, in the plasma membrane of outer segment,o Transducin is a member of G-proteins which activates phosphodiesterase and decreases intracellular cGMP due to degradation by cGMP.o Decrease in cGMP causes closure of Na+ channels (cGMP keeps Na+ channels open in outer segment) and hyperpolarization.o Phototransduction is thus a unique example of sensory transduction that is associated with hyperpolarizationQ and a reduction in neurotransmitter release from the sensory receptor (all other sensory receptors develop depolarization/hypopolarization in response to stimulus),o Unlike other receptors, photoreceptors (and bipolar) cells do not produce an action potential but only a graded changes of the membrane potential (hyperpolarizing local graded potential).o Because the distance is very short from the outer segment-where the membrane potential changes are produced-to the synapses between the photoreceptors and the bipolar cells, even small fluctuations of the membrane potential cause alterations of the transmitter release from the photoreceptors,o There is thus no need for the production of action potentials, which are important when signals are to be propagated over long distances.o In contrast to the photoreceptors and the bipolars, the ganglion cells produce action potentials (conducted in the optic nerve to the higher visual centres).
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