Physiology

Question 1

Explain the phases of the myocardial action potential (non-pacemaker cells)

Answer 1

Cells are polarized meaning that there is an electrical voltage across the cell membrane. In a resting cell the resting membrane potential is usually negative (non-pacemaker cell -90mV).
1. Resting membrane potential - more Na+ and Ca2+ outside the cell and more K+ inside the cell.
2. Depolarization (rapid change in the membrane voltage) due to a large influx of Na+ into the cell, rapidly increasing the voltage to +50 mV.
3. Phase 1 - rapid closure of NA+ channels and brief opening of the K+ channels allow for brief K+ entrance into the cell.
4. Phase 2 - “plateau phase” K+ leaves slowly the cell and Ca2+ slowly enters the cell, near balance of charge moving in and out of the cell.
5. Phase 3 “rapid depolarisation” - Ca2+ channels close and K+ gradually enters the cell

Question 2

Explain the action potential of pacemaker cells.

Question 3

Explain the two cardiac cell refractory periods and correlate them with the cell action potential phases.

Answer 3

Cardiac cells have two refractory periods, the first from the beginning of phase 0 until part way through phase 3; this is known as the absolute refractory period during which it is impossible for the cell to produce another action potential. This is immediately followed, until the end of phase 3, by a relative refractory period, during which a stronger-than-usual stimulus is required to produce another action potential