Question

The rising phase of a cardiac pacemaker action potential is caused by a __________ current.

a) Sodium
b) Potassium
c) Calcium
d) Chloride

Answer 1

The rising phase of a cardiac pacemaker action potential is caused by a sodium current.

Step-by-step explanation:

The rising phase of a cardiac pacemaker action potential is caused by a sodium current.

The action potential of a cardiac pacemaker cell refers to the sequence of electrical events that occur within these specialized cells in the heart. Pacemaker cells are primarily found in the sinoatrial (SA) node, which is the natural pacemaker of the heart. They play a crucial role in generating rhythmic electrical impulses that initiate each heartbeat. Here's an overview of the cardiac pacemaker action potential:

Resting Membrane Potential (Phase 4): The pacemaker cells have a unique property called automaticity, which means they can spontaneously depolarize without external stimulation. During the resting state, the membrane potential gradually becomes more positive (depolarizes) due to a slow influx of sodium ions (Na+) through funny channels (If channels).

Threshold (Phase 0): As the membrane potential reaches a certain threshold, voltage-gated calcium channels open, allowing an influx of calcium ions (Ca2+). This rapid increase in calcium ions leads to a more pronounced depolarization and the initiation of an action potential.

Depolarization (Phase 0): The influx of calcium ions causes a rapid depolarization of the membrane potential. This phase is responsible for the rising phase of the action potential.

Repolarization (Phase 3): After depolarization, potassium ions (K+) channels open, allowing the efflux of potassium ions. This results in repolarization, bringing the membrane potential back to its resting state.

Resting State (Phase 4): The cell returns to its resting state, and the cycle repeats. The gradual depolarization during the resting phase is what distinguishes pacemaker cells from other cardiac cells.

The unique property of automaticity in pacemaker cells allows them to spontaneously generate action potentials, initiating the electrical impulses that travel through the heart, leading to the contraction of the atria and ventricles.

Answer 2

The rising phase of a cardiac pacemaker action potential is primarily initiated by a Calcium current. In pacemaker cells, the influx of Calcium ions during the rising phase triggers the depolarization necessary for the action potential. Thus the correct option is c.

Step-by-step explanation:

The action potential in cardiac pacemaker cells is distinctive due to the involvement of various ion channels. During the rising phase of the action potential, the influx of Calcium ions primarily drives depolarization. This influx of Calcium ions occurs through voltage-gated Calcium channels in the pacemaker cells.

Unlike the action potentials in other excitable cells, such as neurons or cardiac myocytes, the rising phase in pacemaker cells is primarily governed by Calcium influx rather than Sodium. This Calcium current initiates the gradual depolarization that leads to the threshold potential, triggering the action potential and the subsequent contraction of the heart muscle.

Therefore, the correct answer to the question about the current responsible for the rising phase of a cardiac pacemaker action potential is Calcium.

Therefore, the correct option is c.