Muscle Contraction Physiology: Skeletal, Cardiac, Smooth, and Rigor Mortis

Muscle Contraction Physiology and Rigor Mortis

The Mechanism of Rigor Mortis

Rigor mortis occurs when muscles become rigid a few hours after death. This rigidity results because muscle cells run out of ATP and can no longer pump calcium back into the sarcoplasmic reticulum.

Calcium ions, upon leaving the reticulum, bind to accessory proteins, allowing the formation of cross-bridges. Without ATP, contraction cannot occur, but calcium remains present, and cross-bridges persist, preventing the muscles from stretching. Rigor mortis typically disappears 15 to 20 hours later, when muscle cells begin to break down.

Skeletal Muscle Contraction Control

The nervous system controls the contraction of skeletal muscle. Motor neurons activate specialized synapses called neuromuscular junctions in striated muscles.

The nervous system controls the strength and degree of muscle contraction by controlling two factors:

• The number of muscle fibers that are stimulated.
• The frequency of action potentials in each fiber.

A single action potential does not cause a muscle cell to contract fully; this requires many action potentials in rapid succession.

Motor Units and Innervation

Almost all motor neurons form synapses over a group of muscle fibers. A neuron and the fibers it innervates form a structure called a motor unit. The number of muscle fibers within a motor unit depends on the specific muscle.

Specialized Muscle Tissues

Cardiac Muscle Structure and Function

Heart muscle is only found in the heart. It is spontaneously active and has a striated appearance under the microscope.

Like skeletal muscle, cardiac muscle contraction is induced when an action potential reaches the cell through T-tubules and causes the release of calcium from the sarcoplasmic reticulum. However, calcium also enters from the extracellular fluid.

This type of muscle can initiate its own contractions (autorhythmicity). This quality is found in specialized fibers in the sinoatrial node, which acts as the heart's pacemaker. Action potentials spread rapidly through the heart by intercalated discs (or gap junctions), which allow the potential to travel directly from one cell to another and synchronize contractions.

Characteristics of Smooth Muscle

Smooth muscle lacks the striated appearance of the other muscle types. Lining the walls of the digestive tract, large blood vessels, among others, it produces slow and sustained contractions, which are involuntary.

Smooth muscle lacks sarcomeres, and, like heart muscle cells, its cells are almost all directly connected to each other by intercalated links (gap junctions), which allows synchronized contractions.

Smooth muscle also lacks a well-developed sarcoplasmic reticulum; the calcium necessary for contraction enters from the extracellular fluid during the action potential. The contraction of smooth muscle may be initiated by a stretch, by hormones, nerve signals, or some combination of such stimuli.