Wald’s Visual Cycle – Phototransduction Nerve Impulse Generation

Updated : Jan 11, 2020 in Articles

Wald’s Visual Cycle – Phototransduction Nerve Impulse Generation


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in this video we will be studying the walds visual cycle
before discussing the actual cycle let us have an overview about what happens in this
cycle as you all know the human eye contains two
main types of photoreptors first is the rods and second are the cones.
basic structure of rods is as follows these are the axons and this is the nucleus
. now it contains the discs which have the pigment
rhodopsin the basic step in the walds visual cycle is
that when rhodopsin is exposed to light it forms an intermediate compound known as metarhodopsin
2 which further breaks down into opsin and 11 cis retinal
now this meta rhodopsin 2 which is formed through generation of various enzymes it triggers
the nerve impulse now let us look at the details of the walds visual cycle
so the first step in the visual cycle is the exposure of rhodpsin to light
whcih causes its breakdown into luminorhodopsin this luminorhodopsin spontaneously converts
into metarhodopsin one which further converts into metrhodopsin 2 . now this metrhodopsin
2 gives 2 compounds the first is the opsin and second is the all
trans retinal now this all trans retinal has to be converted
back into 11 cis retinal so that it can combine with the opsin and lead to the reformation
of rhodopsin now this conversion occurs through two main
pathways the first of which operates in the eye
the all trans retinal is converted into 11 cis retinal
by the enzyme retinal isomerase and this 11 cis retinal combines with opsin
and forms rhodopsin second pathway which operates is in the liver
here the enzyme is the alcohol dehydrogenase the first the all trans retinal is converted
into all trans retinol and this is conveted by the enzyme isomerase
to 11 cis retinol which is acted upon again by the alcohol dehydrogenase and converted
into 11 cis retinal now the next important thing to know is how
the nerve impulse is generated through this cycle
now the question which arises here is how does metarhodopsin leads to the generation
of nerve impulse for understanding this let us understand how the various ion channels
are distributed on a rod cell htis is the axon and these are the terminal
knobs and this is the nucleus the first major type of the ion channels are
located in the rod cells are the sodium potassium atpase pump.
the main function of this pump is to pump out the sodium ions and pump in the potassium
ions now this creates a state of electronegativity inside the rod cells
another major type of the ion channel are the ungated potassium leak channels through
this the potassium ions constantly leak out and create a state of electronegativity
another ion channel are the gated sodium channels through which there is a constant entry of
the sodium inside the rod cell this entry creates a state of electronegativity
of minus 40 millivolts which is much less as compared to minus 80 millivolts of other
excitatory cells now here are the calcium channels through which there is constant entry
of calcium inside the axon adn through the terminal knobs there is the
release of glutamate now this metarhodopsin 2 causes activation
of regulatory protien know as transducin this transducn cause activation of the enzyme
phosphodiesterase this phosphodiesterase enzyme cause the conversion of cyclic gmp to 5 dash
gmp and the concentration of cyclic gmp decreases
due to this step now since the cyclic gmp is essential for
the opening of the gated sodium channels due to the decrease in cyclic gmp concentration
the sodium channels close and this leads to the decrease entery of sodium
in the rod cells and their concentration decreases since there is a constant efflux of potassium
ions through the leak channels there is excess negativity inside the cells
and this leads to the hyperpolarisation of the rod cell
this hyperpolarisation causes the closure of the gated calcium channels since this calcium
is essential for the release of the glutamte through the terminal knobs
the release of the glutamte stops and due to the decrease in the release of the glutamate
there is the generation of electic potential of the bipolar cells and further conductance
follows.

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