2002A02 Draw a labelled diagramme of cardiac muscle action potential
highlighting the sequence of changes in ionic conductances.
Explain the terms “threshold”, “excitability”, “irritability” with the aid of a diagramme.

Action potential

·   Allows rapid, co-ordinated signalling

·   Rapid depolarisation followed by repolarization

·   Ion channels cycle through resting -> activated -> inactivated states

Determinants of ion flux

·   Channel permeability: voltage dependent

·   Electrochemical gradients

o Na⁺ and Ca²⁺ into cell, K⁺ out of cell

o Mainly due to Na⁺K⁺ATPase

Threshold

·   Membrane potential at which a self-propagating action potential is triggered

Excitability

·   % of Na⁺ channels returned from inactivated to resting state

·   ∝ rate of rise phase 0

Irritability

·   Closeness to threshold