Muscle Physiology

The smallest contractile unit in muscle tissue is the sarcomere. Myofibrils are made up of many sarcomeres attached end-to-end at a series of dark lines (hence the term "striated") called Z lines. Each sarcomere contains actin and myosin filaments, which pull together during contraction to shorten the sarcomere.

Cardiac muscle cells are striated, branching cells that are linked by structures called intercalated discs. These discs are composed of gap junctions, which allow the free passage of electrical signaling between heart cells, and very strong attachment points called desmosomes.

During a muscle cell action potential, calcium enters the cell via t-tubules, which are specialized invaginations of the sarcoplasmic reticulum. Calcium binds with troponin, which pulls the tropomyosin strand away from actin binding sites and allows myosin heads to bind. Neither sodium nor potassium bind to troponin, and ATP and ADP both bind to myosin, rather than troponin.

The A band in the sarcomere is created by the bipolar myosin filaments, joined at the M band. During contraction, the heads of the myosin filaments bind with the actin filament and pull it toward the M band at the center of the sarcomere. The myosin filaments do not themselves change length, and because of that the width of the A band does not change during contraction.

The individual contractile unit of a muscle fibril is referred to as the sarcomere. These units are made of actin and myosin filaments and joined by Z-lines. The sliding filament theory refers to the idea that muscle contraction is the result of myosin strands within the fibril pulling themselves along actin strands similar to pulling on a rope, which shortens the whole sarcomere. The sarcolemma is the specialized cell membrane around the muscle fibril, and the sarcoplasmic reticulum is the smooth endoplasmic reticulum found within the muscle cell.

According to the sliding filament theory of muscle contraction, ATP binds to the myosin head and is hydrolyzed to ADP and inorganic phosphate. The energy released during this change draws the myosin head back into a high energy state, from which it is able to bind with actin and execute its "power stroke," leading to muscle contraction. ADP and inorganic phosphate then are released from the myosin head and replaced by a new molecule of ATP, which allows the myosin head to detach from the actin binding site.

The only type of muscle that is not striated is smooth muscle. Smooth muscle does not use sarcomeres for contraction - rather, each muscle cell is a spindle that is covered in a mesh of contractile fibrils. These fibrils contract in unison when calcium enters the cell.

The muscle body is encased in a fibrous elastic sheath called the epimysium (epi meaning on or above and mys meaning muscle). It is composed of dense irregular connective tissue and is continuous with tendon fibers. The perimysium surrounds muscle fascicles, while the endomysium surrounds muscle fibers.

The sliding filament theory of muscle contraction involves the myosin head attaching to actin and then pulling during the power stroke. Troponin is a protein attached to tropomyosin, a thin strand wrapping around the actin filament. When calcium enters the cell, troponin moves toward it, pulling the tropomyosin strand away from actin binding sites and allowing the myosin head to bind.

Muscle cells have a specialized endoplasmic reticulum called the sarcoplasmic reticulum. The sarcoplasmic reticulum regulates the calcium ion concentration in the cytoplasm of striated muscle cells, and so plays a significant role in muscle contraction and relaxation. The T-tubule is a specialized invagination of the sarcoplasmic reticulum, and the sarcomere is the single contractile unit of a muscle fibril. There is no muscle structure called the mycoplasmic reticulum.