Striped and branching cells with a single central nucleus, joined end to end by dark transverse bands
Cardiac muscle tissue — the intercalated discs.
Anatomy · Cells & tissues
Cardiac muscle tissue
The involuntary striped muscle of the heart wall
Muscle tissue · myocardium
The muscle tissue of the heart wall, or myocardium. Its cells (cardiomyocytes) are short, striped and branching, usually with a single central nucleus, and are joined end to end by intercalated discs that contain strong anchoring junctions and gap junctions. The gap junctions let the electrical wave pass so quickly from cell to cell that the heart contracts as one unit, and some of the cells are self-exciting pacemakers. It is involuntary, and is packed with mitochondria so it can work without rest.
How to spot them
- ▸Striped like skeletal muscle, but the cells are short and branch like the letter Y
- ▸Usually one, sometimes two, pale oval nuclei in the centre of each cell
- ▸Dark transverse lines that cross the cells at intervals: the intercalated discs
- ▸Cells interlocked into a woven network rather than parallel unbranched fibres
- ▸Abundant capillaries running between the cells and a great many mitochondria
What to know
Special junctions let each cell electrically excite its neighbours, so the whole wall beats as a functional syncytium
Cardiac muscle tissue — gap junctions within intercalated discs.
Some of its cells spontaneously generate the impulse that sets the heart's rhythm
Cardiac muscle tissue — pacemaker cells of the sinoatrial node show autorhythmicity.
It has a long refractory period, so it cannot be tetanised as other muscle can
Cardiac muscle tissue — the long plateau of the action potential prevents sustained contraction.
It has almost no ability to regenerate after damage, so a heart attack leaves a scar
Cardiac muscle tissue — dead cardiomyocytes are replaced by fibrous tissue.
Roughly a third of the volume of its cells is mitochondria, and it is supplied by the coronary arteries
Cardiac muscle tissue — it uses aerobic metabolism almost exclusively.
Its involuntary contractions are speeded or slowed by the autonomic nerves but need no nerve to begin
Cardiac muscle tissue — the nerves modulate an intrinsic rhythm.
Key ideas
- An action potential starts in the sinoatrial node, spreads through the atrial and ventricular cells via the intercalated discs, and triggers calcium entry that releases more calcium from the sarcoplasmic reticulum, causing each beat.
- The heart's tissue behaves as two functional syncytia, atrial and ventricular, separated by the fibrous skeleton of the heart and joined only through the atrioventricular node.
Quick-fire facts
- •Cells are roughly 100 micrometres long and 10–25 micrometres wide, much shorter than skeletal fibres
- •Most cells have one central nucleus; a minority have two
- •Intercalated discs contain desmosomes, fascia adherens junctions and gap junctions
- •Mitochondria make up a large share of cell volume — about a third in some estimates
- •The heart beats roughly 100,000 times a day, or about 2.5–3 billion times in a lifetime
- •Renewal is very slow: measurements using carbon-14 from nuclear tests suggest that about 1% or fewer of cardiomyocytes are replaced each year in adults
- •Purkinje fibres are specialised conducting cells at the inner ventricular wall
- •The heart wall's three layers are the epicardium, myocardium (this tissue) and endocardium
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