Question

The pre-potential (resting potential) of pacemaker cells is produced by the opening of ________ at the end of an action potential.

A. fast Ca2+ channels

B. slow Na+ channels

C. slow Ca2+ channels

D. K+ channels

Answer 1

Pacemaker (or autorhythmic) cells account for a small minority of cells in the myocardium. The rest (99%) are contractile cells that are responsible for pumping the blood.  Pacemaker cells are concentrated at two ‘nodes’: the sinoatrial node (SA node), located in the right atrium near the superior vena cava; and the atrioventricular node (AV node), located at the base of the right atrium near the junction of the four heart chambers. They are unique because they can spontaneously produce action potentials in rhythm to control the heart beat. They do this because they exhibit a specialized membrane potential- the pacemaker potential.

The membrane potential in cardiac cells is different from the rest of the body due to 3 reasons:

1. An increased influx of sodium (Na+)
   1. This is the main factor driving the pacemaker potential towards threshold. Cardiac cells possess unique Na+ ion channels (‘funny channels’) that open in response to hyperpolarisation. Therefore after an action potential, these open and allow sodium to flow into the cell, pushing the membrane potential towards the resting potential for sodium (60mV)
2. A decreased efflux of potassium (K+)
   1. On top of the effect of sodium, potassium channels in the heart begin to close during hyper polarisation (of the action potential). This limits the amount of potassium that can leave the cell, pushing the membrane potential up further.
3. Calcium flow is much more important.
4. The pacemaker potential is the main determinant of the heart rate. Because the pacemaker potential represents the non-contracting time between heart beats (diastole), it is also called the diastolic depolarization. The amount of net inward current required to move the cell membrane potential during the pacemaker phase is extremely small, in the order of few pAs, but this net flux arises from time to time changing contribution of several currents that flow with different voltage and time dependence. Evidence in support of the active presence of K⁺, Ca²⁺, Na⁺ channels and Na⁺/K⁺ exchanger during the pacemaker phase have been variously reported in the literature, but several indications point to the “funny”(I_f) current as one of the most important.^[4](see funny current). There is now substantial evidence that also sarcoplasmic reticulum (SR) Ca²⁺-transients participate to the generation of the diastolic depolarization via a process involving the Na–Ca exchanger.
5. The threshold potential is the potential an excitable cell membrane, such as a myocyte, must reach in order to induce an action potential. This depolarization is caused by very small net inward currents of calcium ions across the cell membrane, which gives rise to the action potential.