Muscles are multicellular contractile units. They are divided into three types:

As you read about each type of muscle, think about the similarities and differences between them in terms of structure and function.

Skeletal muscle is mainly responsible for the movement of the skeleton, but is also found in organs such as the globe of the eye and the tongue. It is a voluntary muscle, and therefore under conscious control. Skeletal muscle is specialized for rapid and forceful contraction of short duration.

Skeletal muscle cells contain similar components and structures as other cells but different terms are used to describe those components and structure in skeletal muscle cells. The plasma membrane of skeletal muscle is called the sarcolemma; its cytoplasm is known as sarcoplasm; the endoplasmic reticulum is called the sarcoplasmic reticulum.

Each muscle cell is defined by a sarcolemma and contains many nuclei along its length. The nuclei are displaced peripherally within a cross section of the sarcoplasm while a large number of longitudinal myofibrils, groups of arranged contractile proteins, occupy most of the center space. The myofibril contains several important histological landmarks:

Each myofibril can be understood as a series of contractile units called sarcomeres that contains two types of filaments: thick filaments, composed of myosin, and thin filaments, composed of actin. The individual filaments do not change in length during muscle contraction; rather the thin filaments slide over the thick filaments to shorten the sarcomere. The nature of these filaments can be understood in the context of the histological landmarks of the myofibril.

Skeletal muscles are divided into two muscle fiber types:

Most muscles contain a mixture of these extreme fiber types. In humans, the fiber types cannot be distinguished based on gross examination, but require specific stains or treatments to differentiate the fibers.

Skeletal muscle cells are innervated by motor neurons. A motor unit is defined as the neuron and the fibers it supplies. Some motor neurons innervate one or a few muscle cells whereas other motor neurons can innervate hundreds of muscle cells. Muscles that require fine control have motor neurons that innervate fewer muscle cells; muscles that participate in less controlled movements may have many fibers innervated by each neuron. Motor axons terminate in a neuromuscular junction on the surface of skeletal muscle fibers. The neuromuscular junction is composed of a pre-synaptic nerve terminal and a post-synaptic muscle fiber. Upon depolarization, the pre-synaptic vesicles containing the neurotransmitter acetylcholine fuse with the membrane, releasing acetylcholine into the cleft. Acetylcholine binds to receptors on the post-synaptic membrane and causes depolarization of the muscle fiber, which leads to its contraction. Typically, one action potential in the neuron releases enough neurotransmitter to cause one contraction in the muscle fiber.

In muscle cells, the sarcolemma or plasma membrane extends transversely into the sarcoplasm to surround each myofibril, establishing the T-tubule system. These T-tubules allow for the synchronous contraction of all sarcomeres in the myofibril. The T-tubules are found at the junction of the A- and I- bands and their lumina are continuous with the extracellular space. At such junctions, the T-tubules are in close contact with the sarcoplasmic reticulum, which forms a network surrounding each myofibril. The part of the sarcoplasmic reticulum associated with the T-tubules is termed the terminal cisternae because of its flattened cisternal arrangement. When an excitation signal arrives at the neuromuscular junction, the depolarization of the sarcolemma quickly travels through the T-tubule system and comes in contact with the sarcoplasmic reticulum, causing the release of calcium and resulting in muscle contraction.

Smooth muscle forms the contractile portion of the wall of the digestive tract from the middle portion of the esophagus to the internal sphincter of the anus. It is found in the walls of the ducts in the glands associated with the alimentary tract, in the walls of the respiratory passages from the trachea to the alveolar ducts, and in the urinary and genital ducts. The walls of the arteries, veins, and large lymph vessels contain smooth muscle as well.

Smooth muscle is specialized for slow and sustained contractions of low force. Instead of having motor units, all cells within a whole smooth muscle mass contract together. Smooth muscle has inherent contractility, and the autonomic nervous system, hormones and local metabolites can influence its contraction. Since it is not under conscious control, smooth muscle is involuntary muscle.

Smooth muscle fibers are elongated spindle-shaped cells with a single nucleus. In general, they are much shorter than skeletal muscle cells. The nucleus is located centrally and the sarcoplasm is filled with fibrils. The thick (myosin) and thin (actin) filaments are scattered throughout the sarcoplasm and are attached to adhesion densities on the cell membrane and focal densities within the cytoplasm. Since the contractile proteins of these cells are not arranged into myofibrils like those of skeletal and cardiac muscle, they appear smooth rather than striated.

Smooth muscle fibers are bound together in irregular branching fasciculi that vary in arrangement from organ to organ. These fasciculi are the functional contractile units. There is also a network of supporting collagenous tissues between the fibers and the fasciculi.

Cardiac muscle shares important characteristics with both skeletal and smooth muscle. Functionally, cardiac muscle produces strong contractions like skeletal muscle. However, it has inherent mechanisms to initiate continuous contraction like smooth muscle. The rate and force of contraction is not subject to voluntary control, but is influenced by the autonomic nervous system and hormones.

Histologically, cardiac muscle appears striated like the skeletal muscle due to arrangement of contractile proteins. It also has several unique structural characteristics:

Collagenous tissues are found surrounding individual cardiac muscle fibers. There is abundance vascularization within this supporting tissue, which is required to meet the high metabolic demands of cardiac muscle.

The cardiac muscle fibers are joined end to end by specialized junctional regions called the intercalated discs. The intercalated discs provide anchorage for myofibrils and allow rapid spread of contractile stimuli between cells. Such rapid spread of contraction allows the cardiac muscles to act as a functional syncytium. The intercalated discs contain three types of membrane-to-membrane contact:

In addition to the contractile cells, there is a specialized system made up of modified muscle cells whose function is to generate the stimulus for heartbeat and conduct the impulse to various parts of the myocardium. This system consists of sinoatrial node, atrioventricular node, bundle of His, and Purkinje fibers.