Action Potential - Voltage-Gated Calcium Channels

Cardiac Pacemaker Cells

• Pacemaker cells are mostly found in the sinoatrial node (SAN), which is situated in the upper part of the wall of the right atrium.
• These cells have natural automaticity, meaning they can generate their own action potentials.
• The atrioventricular node (AVN) and the Purkinje fibres also have latent pacemaker cells, but they are normally overridden by the SAN.

Action Potential in Sinoatrial Node

• The action potential in the SA node occurs in three phases: pacemaker potential, depolarisation, and repolarisation.
• Phase 4 (Pacemaker potential): slow depolarisation of pacemaker cells towards the membrane potential threshold, achieved by activation of hyperpolarisation activated cyclic nucleotide gated channels (HCN channels).
• HCN channels allow Na+ entry into the cells, enabling slow depolarisation, and are activated when the membrane potential is lower than -50mV.
• Phase 0 (Depolarisation): voltage-gated calcium channels open, allowing an influx of Ca2+, which produces a faster rate of depolarisation to reach a positive membrane potential.
• HCN channels inactivate, and K+ channels open at the peak of the action potential.
• Phase 3 (Repolarisation): efflux of K+ ions out of the cells, resulting in the repolarisation of the membrane.

Control by the Autonomic Nervous System

• The autonomic nervous system (ANS) alters the slope of the pacemaker potential to alter heart rate.
• Parasympathetic activity lengthens the interval between pacemaker potentials, slowing heart rate, via acetylcholine acting on M2 muscarinic receptors at the SAN.
• Sympathetic activity shortens the interval between impulses, increasing heart rate, via noradrenaline acting on B1 adrenoceptors.
• If all autonomic inputs are blocked, the intrinsic heart rate is about 100 beats per minute.

Learn about the role of voltage-gated calcium channels in generating action potentials in neurons. Understand how these channels interact with HCN channels to produce a rapid depolarization.