RhythmicalExcitation

of the Heart

RhythmicalExcitation

of the Heart

Chapter 10Chapter 10

1. Internodal pathways and transmission through atria.

2. A-V node with delay of impulse from atria to ventricles.

3. Rapid transmission in ventricular Purkinje system.

Transmission of the cardiac impulse through the heart.Figure 10-4, Guyton.

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1. Internodal pathways and transmission through atria.

2. A-V node with delay of impulse from atria to ventricles.

3. Rapid transmission in ventricular Purkinje system.

Transmission of the cardiac impulse through the heart.Figure 10-4, Guyton.

Phase 0

Na

Phase 0 - depolarization

> Voltage gated Na channels in membrane: in resting state (Phase 4), the h-gate (inactivation gate) is open, but m-gate (activation gate) is closed.

> Depolarization - when cell is stimulated, m-gate opens and Na ions rush into cell making inside of cell more positive via “fast sodium channels”.

Phase 1

As membrane potential peaks at +20 mV during phase 0, a negative feedback system inactivates the inward Na channels = responsible for partial repolarization of phase 1.

H-gates (inactivation gates) close and remain closed = can’t generate another action potential.

Phase 1 – Transient repolarization

Phase 2

Phase 2 – Plateau of Action Potential

Slow Ca influx (L-type channels) and K outflux are balanced = responsible for prolonged plateau. K permeability is reduced from normal nearly 5 fold at this time

Long plateau phase leaves time for Ca ions to diffuse into the cell for contraction.

Phase 3 - RepolarizationPhase 3

Phase 4K+

Voltage-gated K channels open that repolarize the cell (Ca sensitive K channels are relatively slow in activating).

Phase 4 – Return to resting state

As membrane potential returns to resting state, the voltage-gated K channels close; outward flow of K ions through leak channels is now sufficient to hold Km close to 90 mV

Pumps restore intracellular composition to normal,