Cardiac Ion Channels II

Cardiac Ion Channels: Action Potentials
Overview

  • We'll look at two main types of electrical signals (called 'action potentials') in the heart: 'fast response' found in the main pumping chambers (ventricles) and 'slow response' found in the heart's natural electrical wiring system (cardiac conduction system).

  • We'll also learn how these electrical signals start and create the heart's rhythm, focusing on two special areas called the SA node and AV node, which are like the heart's natural pacemakers.

Types of Action Potentials

Fast Response Action Potential

  • Location: These are mainly found in the muscle cells of the heart's upper chambers (atria) and lower, main pumping chambers (ventricles).

  • Phases: Five distinct phases:

    • Phase 0 (Depolarization):

    • Starts when the heart muscle cell is 'at rest' (not actively pumping), with an electrical charge of about −90mV-90mV.

    • This phase begins with a very quick rush of sodium ions (Na+Na^+) into the cell through special 'doors' (channels) that open when the voltage changes.

    • This rush of positive sodium makes the cell's inside much more positive very quickly, causing a sharp upward spike on the electrical graph.

    • Phase 1 (Early Repolarization):

    • The sodium 'doors' close, and new potassium 'doors' (K+K^+) open.

    • This causes a small dip in the cell's electrical charge, but it doesn't go back to negative yet.

    • Phase 2 (Plateau Phase):

    • Special calcium 'doors' (Ca2+Ca^{2+}) open, letting calcium into the cell. At the same time, potassium leaves the cell, and these two movements of ions largely balance each other out.

    • This balance keeps the cell's electrical charge fairly stable for a short period, like a 'plateau' on a graph.

    • Phase 3 (Rapid Repolarization):

    • The calcium 'doors' close, and even more potassium (K+K^+) rapidly leaves the cell through other potassium 'doors'.

    • This outward flow of positive potassium makes the cell's inside rapidly become negative again, bringing it back to about −60mV-60mV, starting its return to the resting state.

    • Phase 4 (Resting Membrane Potential):

    • The cell fully resets its electrical charge back to its most negative resting state, about −90mV-90mV.

    • This resting state is maintained mostly by specific potassium 'doors' (K+K^+) that allow some potassium to slowly move.

    • The cell is now ready to fire another electrical signal.

Slow Response Action Potential

  • Location: These are found in the heart's natural pacemakers, specifically in the SA node (the main pacemaker) and AV node (a secondary pacemaker).

  • Phases: Three phases present:

    • Phase 0 (Depolarization):

    • Unlike fast response cells, these cells start depolarizing from a less negative (more positive) electrical charge, around −40mV-40mV.

    • The main electrical change here is caused by calcium ions (Ca2+Ca^{2+}) rushing into the cell through L-type calcium 'doors'.

    • This means these pacemaker cells never truly 'rest' at a stable negative charge; they are always slowly preparing to fire again.

    • Phase 3 (Repolarization):

    • The calcium 'doors' close, and potassium 'doors' (K+K^+) open, letting potassium leave the cell.

    • This makes the cell's electrical charge become more negative, moving towards about −60mV-60mV.

    • Phase 4 (Pacemaker Potential):

    • This is the special part: the cell slowly becomes more positive (depolarizes) on its own. This happens because special 'funny channels' let sodium (Na+Na^+) slowly leak in, and at the same time, less potassium (K+K^+) is leaving the cell.

    • This slow, automatic change in charge is what allows these pacemaker cells to constantly trigger new heartbeats on their own, setting the heart's rhythm.

Comparison of Action Potentials

  • Fast Response Action Potential:

    • Have five distinct stages (0, 1, 2, 3, and 4).

    • Phase 0 is mainly driven by a huge rush of sodium into the cell, creating a very sharp upward spike.

    • They get very positive very quickly (strong depolarization) and then return to negative relatively fast (quick repolarization).

  • Slow Response Action Potential:

    • Have only three main stages (0, 3, and 4).

    • Their electrical changes are mostly caused by calcium entering the cell, not much by sodium.

    • Their electrical charge is always changing, which shows they can generate their own rhythm without needing an external trigger (this is called 'automaticity').

Physiological Importance

  • These electrical signals are super important for how the heart works. They make sure all the heart muscle cells contract (squeeze) and relax in a perfectly timed way.

  • The SA node is like the heart's main conductor, starting all the electrical signals. It can do this all by itself, without any commands from the brain or nerves, thanks to its 'automaticity'.

  • How quickly these electrical signals spread throughout the heart ensures that blood is pumped efficiently through the body (systemic circulation) and to the lungs (pulmonary circulation).

Summary

  • Knowing the differences between these 'fast' and 'slow' electrical signals is key to understanding how the heart truly functions.

  • The way different electrically charged particles (ions) move in and out of the cells during these signals explains how the heart works and keeps its rhythm, especially highlighting the crucial jobs of the SA and AV nodes in sending these electrical messages through the heart.

  • Outside influences, like signals from your nervous system (think 'fight or flight' (sympathetic) or 'rest and digest' (parasympathetic)), also help control how these electrical signals behave.