Depolarisation
Depolarisation is the stimulation of the cardiac cell. A change in the cell membrane permeability results in electrolyte concentration changes within the cell. This causes the generation of an electrical current, which spreads to neighbouring cells causing these in turn todepolarise. Depolarisation is represented on the ECG as P waves and QRS complexes.
Repolarisation
Repolarisation is the process by which the cardiac cell returns to its original resting state. Ventricular repolarsiation is represented on the ECG as T waves (atrial repolarisation is not visible on the ECG as it is masked by the QRS complex).
Automaticity
Automaticity is the ability of specialised cardiac cells (automatic or pacemaker cells) to initiate electrical impulses without any external stimulation. The sinus node normally has the fastest firing rate and therefore assumes the role of pacemaker for the heart. If another focus in the heart has a faster firing rate, it will then take over as pacemaker.
Cardiac action potential
Cardiac action potentialis the term used to describe the entire sequence of changes in the cell membrane potential, from the beginning of depolarisation to the end of repolarisation, i.e. from the beginning of the P wave to the end of the T wave.Resting cardiac cells have high potassium and low sodium concentrations (140mmol/l and 10mmol/l, respectively). This contrasts sharply with extracellular concentrations (4mmol/l and 140mmol/l, respectively)
The cell is polarised and has a membrane potential of -90mV.
Cardiac action potential results from a series of changes in cell permeability to sodium, calcium and potassium ions. Following electrical activation of the cell, a sudden increase in sodium permeability causes a rapid influx of sodium ions into the cell. This is followed by a sustained influx of calcium ions. The membrane potential is now +20mV. This is referred to as phase 0 of the action potential. The polarity of the membrane is now slightly positive.
Phases of the cardiac action potential
Phase Action
0 Upstroke or spike due to rapid depolarisation
1 Early rapid depolarisation
2 The plateau
3 Rapid repolarisation
4 Resting membrane potential and diastolic depolarisation
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