Muscle: The Three Types of Muscle

Contractility is a fundamental property of cells and the majority of them contain essentially the same contractile machinery as that found in muscle cells. In muscle cells, however, a larger proportion of the cells' resources are given over to this function than in other cell types.

These are the three types of muscle:

Skeletal Muscle

Contractions move part of the skeleton. Also called 'voluntary' because usually its contractions are under your control.

It has a stripy appearance, because of the repeating structure of the muscle: there are many myofibrils (fibers), each one of which is made up of repeating units called muscle sarcomeres. Each sarcomere is 2.5 mm long. Can you work out how many sarcomeres are there (placed end to end) in your biceps muscle, which approximately 25cm long?

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Cardiac Muscle

photo of cardiac muscle

Cardiac muscle makes up the muscular walls of the heart (myocardium). It is 'involuntary' because its contractions are not under your control. However, it has a similar ultrastructural organisation to skeletal muscle. So, it too has a stripy appearance because of the repeating units called muscle sarcomeres.

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Smooth Muscle

photo of smooth muscle

Found in the walls of most blood vessels and tubular organs such as the intestine. It is also 'involuntary'. However, it does NOT have a stripy appearance, because it does not have repeating sarcomeres. The contractile proteins, myosin and actin are much more randomly arranged than in skeletal or cardiac muscle

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Skeletal Muscle and Cardiac Muscle are also called 'striated muscle', because they have dark and light bands running across the muscle width when they are looked at under the microscope.

Confusingly the prefixes myo- and sarco- (respectively from the Latin and Greek, both meaning muscle) are often used when naming structures and organelles associated with muscle.

Thus the plasma membrane of muscle cells is sometimes called the sarcolemma and their cytoplasm sarcoplasm.

Their endoplasmic reticulum is called sarcoplasmic reticulum and their mitochondria are sometimes called sarcosomes.

The contractile fibres that lie in the sarcoplasm are known as myofibrils and the embryonic precursors of skeletal muscle cells are called myoblasts.