Cardiac Muscle and Smooth Muscle

The muscles of the heart and internal organs, while generally similar to skeletal muscles, also have a number of distinctive features. They contract much slower than skeletal muscles: if skeletal muscle fibers contract and relax in 0.1 seconds, then the cardiac muscle requires from 1 to 5 seconds, and smooth muscle from 3 to 180 seconds. The duration of all contraction phases has been increased.

Smooth muscles show great differences in tone; it may be almost relaxed or strongly contracted. In addition, it appears to be able to maintain a state of tonic shortening without expending energy, perhaps due to the reorganization of the protein chains that form the fibers.

Each heartbeat is a single contraction.

The heart muscle has a long refractory period, that is, the period following stimulation, during which it cannot respond to any other stimulation.

Therefore, the heart muscle is not capable of tetanus, since one single contraction cannot follow another quickly enough to maintain it in a state of contraction.