University:
Camosun College
Course:
BIOL 252 | Pathophysiology for Nursing 1
Academic year:

2024
Views:

93

Pages:

4
Author:

Aysha A.

Cardiac Myocyte AP (Action Potential) (For a Contractile Myocyte): Phase 0 is the Depolarization Phase: Influx of cations (Na⁺). This phase marks the rapid upstroke of the action potential. Phases 1, 2, 3, 4 are the Repolarization Phases: Efflux of cations (K⁺). Phase 2 is also known as the Plateau Phase: The duration of the AP is approximately 200-250 milliseconds. At Rest: The RMP (Resting Membrane Potential) is -90 mV. In depolarization by Purkinje fibers, cation influx occurs mainly through gap junctions. The influx of Na⁺ through voltage-gated Na⁺ channels occurs, raising the potential to +40 mV. Phase 1: This phase involves Initial Repolarization. K⁺ efflux through leaky K⁺ channels occurs in Phase 1 and Phase 2. Plateau Phase: The potential remains the same because K⁺ efflux and Ca²⁺ influx occur simultaneously through L-type voltage-gated Ca²⁺ channels. Phase 3: Begins with Ca²⁺ influx and ends with Ca²⁺ efflux, leading to further repolarization. Absolute Refractory Period: Also known as the Effective Refractory Period. During this time, myocytes cannot undergo contraction or relaxation, ensuring the cell doesn’t undergo tetanus (continued contraction). Characteristics of Different Layers: Epicardial Cells: Shortest AP duration (~200 ms). Mid-Myocardial Cells: Longest AP duration (~250 ms). Endocardial Cells: AP duration (~210-220 ms). AP of Auto-Excitable Cells: Phases: Phase 0 (Depolarization): Ca²⁺ influx. Phase 3 (Repolarization): K⁺ efflux. Phase 4 (Hyperpolarization): Resting membrane potential fluctuations between -60 mV and -50 mV, influenced by leaky K⁺ channels. Phase 4 (Pacemaker Potential): Hyperpolarization-induced cyclic nucleotide channels open spontaneously when the membrane potential is less than -60 mV, allowing Na⁺ and K⁺ to enter the system. This leads to a gradual depolarization, eventually triggering Ca²⁺ channels to open and begin the next action potential. Key Concepts: Depolarization: L-type Ca²⁺ channels allow Ca²⁺ into the cell, raising the membrane potential and reaching the threshold for the next AP. The depolarizing current is counteracted by the repolarizing current. Repolarization: Driven by K⁺ efflux. Effective Refractory Period: The period during which cardiac cells cannot be excited again, ensuring proper heart function and avoiding tetany.