Question

ACTION POTENTIAL (AP) ON A NEURON

1.     For each of the following channels, indicate what specifically opens it, what specifically closes it, and what goes through (& in what direction) when it is open.

Voltage-gated Na+ channel

Voltage-gated K+ channel

2.     What are leak channels? What purpose do they serve in neurons?

Answer 1

Resting membrane potential is due to the distribution of more cations outside the cell membrane and more anions inside the cell membrane. at this point the sodium is more in the extracellular fluid but it cannot enter the cell due to impermeability of the membrane.

Depolarization phase:- when threshold stimulus is applied to the cell membrane, at the point of stimulation the permeability of the membrane for Sodium ions increases. At first the rise of permeability for Sodium is slow till it reaches the firing level. When the membrane depolarizes, sodium channel start opening up. The opening of the sodium channels (m gate) depolarizes the membrane further, leading to the opening of a greater number of sodium channels. as the concentration gradient and electrical gradient of this Ion are directed inward, there occurs a rapid influx of sodium ions into the cell. This rapid entry of sodium is sufficient to overwhelm the repolarization forces.

Repolarization phase:- it occurs due to decrease in further sodium influx and potassium is flux through the voltage gated K channels which open later than sodium channels but remain activated for prolonged period.

After depolarization:- after the rapid initial repolarization, the further repolarization occurs slowly. this is due to the fact that the rate of potassium efflux slows down, as electrical gradient responsible for initial rapid diffusion declines.

After hyperpolarization:- the slow efflux of potassium continues even after the resting membrane potential is reached, resulting in a prolonged phase of hyperpolarization during which the membrane potential falls upto -72 mV. However little after, the voltage gated potassium channels also close.

Leak channels are the simplest type of ion channels. These are the small pores inside the cell membrane which allow diffusion of some ions. Most importantly leak channels are useful for K and Cl. Some leak channels are also present for the Na ions.

Significance:- These leak channels primarily, K and Cl channels, are responsible for the ionic distribution to be brought back to the resting level after hyperpolarization along with Na-K ATPase pump.