Question

in autorthythmic cells, influx of these two ions bring the cell threahold?

A) sodium

B) calcium

C) chloride

D) potassium

Answer 1

Autorhythmic cells are the pacemaker cells and conduction fibers that make conduction system of heart. They are present in SA node and AV node. They are different because they lack resting membrane potential. Thus they generate spontaneous action potentials. The two ions that generate these potentials due to their influx are sodium and calcium.

The pacemaker cells have 3 phases for generation of action potential. In phase 4, there is spontaneous depolarization. Initially there is depolarization caused by funny sodium channels. This decreases the membrane potential from -60 mV to -50 mV. T type calcium channel then open at -50 mV, allowing calcium ions into the cell. This causes depolarization to -40 mV, when L-type calcium channels open. This leads to further depolarization. There is closure of K+ channels and this leads to decreased potassium currents. The entry of calcium ion causes depolarization in phase 0. Depolarization closes funny sodium channels and T-type calcium channels. In phase 3, K+ channels open to cause repolarization.

Chloride channels are not involved in pacemaker cells.

Right choice (if select all that apply): Two ions that are involved are-

A) Sodium

B) calcium

[If only one choice has to be selected, the right choice will be B) calcium, as calcium is responsible for the greater depolarization]