Cardiac conduction

Question 1

What is an action potential?

What are the factors effecting resting membrane potential (RMP)?

Answer 1

An action potential describes sequential changes in membrane potential resulting in the propagation of an electrical impulse. These changes occur due to alterations in the membrane permeability and movement of ions through ion channels.

Resting membrane potential (RMP) is the transmembrane voltage that exists when an excitable cell is not producing an action potential and is negative inside with respect to the outside of the cell.

The factors which contribute to RMP are:

• Na/K ATPase pump; 3 Na+ ions are pumped OUT and 2 K+ in (there is a net loss of one +ve charge from within the cell per cycle of the pump)
• Differential permeability of membrane to K+ and Na+
• ‘Held’ negatively charged molecules inside the cell (proteins & phosphate)- Donnan effect

The Nernst equation calculates the membrane potential for an individual ion at equilibrium.

The Goldman equation examines the contribution of multiple ions across membrane.

Question 2

Pacemaker cells

What do cardiac pacemaker cells demonstrate?

What is the rate of discharge of:

• SA node
• AV node
• Ventricle

Detail the ranges of action potentials and duration of an action potential

Answer 2

Cardiac pacemaker cells exhibit automaticity. This means they lack stable membrane potential and spontaneously decay towards threshold. This is known as pre-potential.

The rate of discharge varies with the conduction system:

SA: 70-80/min

AV node 60/min

Ventricular 40/min

Maximal negative -60mV

Threshold: -40mV

Peak Potential +20mV

Duration 150ms

Question 3

Draw a graph to show cardiac pacemaker cell action cycles and detail what is happening at each point

Answer 3

Phase 4: ‘Pre-potential’

Pacemaker cells do not maintain a stable RMP. There is a slow decrease in membrane permeability to K+, meaning +ve charge builds up in the cells.

Phase 0: Depolarization

Influx of Na+ ions and Ca2+ to a smaller extent. Slow response action potential.

Phase 3: Repolarization

Due to inactivation of slow Ca2+ channels and increase K+ outflow

Stimulation of either the sympathetic or parasympathetic nervous systems will modulate the pre-potential phase of cardiac pacemaker cells and hence alter heart rate.

Question 4

Ventricular muscle cells

Detail the values for action potential in their cycle and draw a labeled diagram to explain the cycle of action potentials

Answer 4

Max negative potential= -90mV

Threshold -70mV

Peak +20mV

Duration 200ms

Phase 0: Rapid depolarization

Fast Na+ channels open at threshold of -70mV and Na+ pores into cell down conc and electrical gradient

Phase 1: Spike

Onset of depolarisation

Phase 2: Plateau

Ca2+ current flow into cell through slow-L type calcium channels. Open as AP passes -35mV. Balances efflux current of K+ so remain +ve. ABSOLUTE REFRACTOTY PERIOD. Prevents tetanic contraction

Phase 3: Repolarization

Closure of Ca2+ channels and large efflux of K+ restores RMP

Phase 4: RMP

Stable diastolic potential. Maintained by differential permeability of membrane to K+ vs Na+ (100:1) and 3Na:2K ATP pump.

Question 5

Describe excitation-constraction coupling

Answer 5