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.

· Intro

· Graph and conductances

· Definitions

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