Voltage-gated ion channel

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A voltage-gated ion channel is a membrane residing protein[1], the function of which being to control the diffusion of specific ions down their concentration gradient. The main ions are namely K+, Na+, Ca2+ and Cl-. They connect the exterior to the cytosol with narrow passages that facilitate rapid movement due to the opening and closing of gates which are dependant on changes of transmembrane voltage. Voltage-gated ion proteins have absolute specificity in the sense that they only allow passage of one ion. These ion channels have a crucial job in maintenance of homeostasis and the initiation and proliferation of nerve impulses[2].

Nerve impulses

A nerve impulse is initiated because of the initiation of a change in the membrane potential, caused by the change in membrane potential from the opening and closing of sodium and potassium voltage-gated ion channels. When a graded potential is large enough to reach the threshold voltage, an action potential is initiated. When the membrane potential reaches -55mV, enough Na+ voltage-gated ion channels have been activated by the activation of the activation gate to facilitate the diffusion of Na+ ions into the axon. The movement of positive ions inwards causes K+ ion channels to open and the inactivation gates of sodium channels to cause sodium channels to begin to close. As potassium channels open, sodium channels close resulting in hyperpolarisation of the cell membrane to a voltage of around -90 mV. At this membrane potential, the potassium ions then begin to be inactivated. When both channels are inactivated there is a refractory period meaning that the ion channel cannot be reactivated, allowing for the directional depolarisation down the axon.

References

  1. Alberts; Johnson; Lewis; Raff; Roberts; Walter; Molecular biology of the cell, Fifth edition, Garland science, 2008.
  2. http://ieeexplore.ieee.org/abstract/document/1402408/?reload=true
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