Properties of Cardiac Muscle

Key points

• • Cardiac muscle is striated like skeletal muscle, but its cells are branched and joined end to end by intercalated discs, which contain gap junctions.
• • The gap junctions let the impulse pass directly from cell to cell, so the heart behaves as a single unit (a functional syncytium) and contracts together.
• • Automaticity (rhythmicity) means the heart contains specialised pacemaker cells that depolarise on their own, and the sinuatrial node fires fastest and so sets the pace as the pacemaker.
• • The cardiac action potential has a long plateau caused by calcium entry, which gives a long refractory period, so the heart cannot be tetanised and always relaxes enough to refill.
• • The force of contraction rises with how much the muscle is stretched before it contracts, within physiological limits, a property described by the Frank-Starling law of the heart.
• • Cardiac muscle depends almost entirely on aerobic metabolism, so it is packed with mitochondria and is very sensitive to a lack of oxygen.