what is the unique feature of the action portenial of a cardiac non contractile cell
The correct answer and explanation is:
The unique feature of the action potential of a cardiac non-contractile cell is the presence of a “pacemaker potential” or “pre-potential,” which gradually depolarizes until it reaches a threshold and triggers an action potential. These cells, also known as pacemaker cells, are found in structures like the sinoatrial (SA) node and the atrioventricular (AV) node of the heart.
Unlike contractile cardiac cells, which require a rapid depolarization to initiate their action potential, pacemaker cells do not have a stable resting membrane potential. Instead, their membrane potential gradually depolarizes due to the slow influx of ions, mainly sodium (Na+) and calcium (Ca2+), and a reduced efflux of potassium (K+). This gradual depolarization is called the pacemaker potential. Once the membrane potential reaches a critical threshold, an action potential is initiated.
The pacemaker potential is primarily caused by the movement of ions through specific ion channels. The “funny current” (I_f), a mixed sodium and potassium current, is responsible for the initial slow depolarization. This is followed by the opening of calcium channels, which further depolarizes the cell until the threshold is reached. Once the threshold is surpassed, voltage-gated calcium channels open, causing a rapid depolarization phase of the action potential. This is followed by repolarization, which is primarily due to the efflux of potassium ions.
This pacemaker potential allows the heart to generate rhythmic electrical impulses automatically, which is essential for maintaining a regular heartbeat. Unlike contractile cells, which depend on external stimulation from a nerve or neighboring cell, pacemaker cells have the inherent ability to generate spontaneous action potentials and regulate heart rate. The automaticity and rhythmicity of these cells are vital for the coordinated contraction of the heart and its ability to pump blood efficiently.