Skeletal muscle histology

Terminology

Special terms are used to describe structures associated with skeletal muscle tissue. Muscle tissue terms often begin with myo-, mys-, or sarco-. The cytoplasm of a muscle cells is referred to as sarcoplasm. The plasma membrane is called the sarcolemma and the endoplasmic reticulum is called the sarcoplasmic reticulum. A muscle fiber may also be referred to as a myofiber.

Individually, skeletal muscles cells are referred to as muscle fibers. The length of a skeletal muscle fiber varies by location. In the anterior thigh, a muscle fiber may be a meter long. In contrast, muscle fibers making up the stapedius, a small muscle of the inner ear, are only a few millimeters in length. Myofibrils are rod shaped subunits of muscle cells. The actin and myosin filaments making up the myofibrils are organized into sarcomeres. Under a microscope, sarcomeres give skeletal muscle a striated appearance.

Skeletal muscle tissue develops through the fusion of individual myoblasts, or early muscle cells. This fusion results in a characteristic multinucleated structure. Because the cells are fused and multinucleated, they form a structural syncytium. Nuclei of skeletal muscle tissue are oval-shaped and located at the periphery of the cell. They are accompanied by satellite cells between the external lamina and sarcolemma. Satellite cells are precursors to skeletal muscle cells and are responsible for the ability of muscle tissue to regenerate. They have a small amount of cytoplasm and because of their locations can sometimes be mistaken for skeletal muscle cell nuclei.

Skeletal muscle fibers

Sarcomeres

The sarcomere is the functional unit of a skeletal muscle cell. Each sarcomere is about 2.5 micrometers in length. It is made up of multiple myosin and actin filaments oriented in parallel. The actin and myosin filaments overlap in certain places creating several bands and zones. A Z disc forms the boundary of the sarcomere on either side. Thin actin filaments project in either direction off of a Z disc but do not cross the entire length of the sarcomere. They are almost 8 nm in diameter and have tightly bound regulatory proteins called troponin and tropomyosin.

The center of the sarcomere lacks actin filaments and is referred to as the H zone. An M line runs down the middle of the H zone perpendicular to the filaments. Thick myosin filaments are found between the actin filaments. Their diameter is approximately 15 nm with a globular head region consisting of heavy and light chain. This head has an ATPase activity and ability to bind and move along actin filament. They are not connected to the Z discs but do traverse the H zone.

The sarcomere is broken up into three bands. The A band is in the middle and corresponds to the myosin filaments together with the thin filaments overlapping on both ends. There are two I bands on either side of the A band and represent the area in which only actin filaments are present.

Sarcoplasmic structures

• Sarcolemma - cell plasma membrane of a muscle cell
• Sarcoplasmic reticulum - modified endoplasmic reticulum. The sarcoplasmic reticulum resembles a piece of lace that surrounds the myofibril. It is made up of a network of tubules with a reservoir at either end. This reservoir is referred to as the terminal cisterna.
• Terminal cisterna - enlarged regions on either end of the sarcoplasmic reticulum. The terminal cisternae sequester calcium to be used in the contraction cycle. There is a terminal cisterna on either side of a t-tubule. All together they are called a triad.
• T-tubules - also called transverse tubules. The t-tubules are pits along the surface of the muscle cells. Their walls are continuous with the sarcolemma, meaning the internal surface of the t-tubule is exposed to the extracellular matrix. The t-tubules are responsible for moving action potentials to the inner region of the muscle cell.

Accessory proteins

The thick and thin filaments in the myofibrils are supported by accessory proteins. These proteins maintain the speed and alignment of filaments during the contraction cycle. Some of them are described below.

• Titin is a large, elastic protein the anchors thick filaments to Z lines to prevent excessive stretching of the myofibril
• Tropomodulin acts like an actin cap. It attaches to the free end of an actin filament to maintain its length.
• α-Actinin is a short, rod-shaped protein that arranges thin filaments into parallel bundles and anchors them to the Z line
• Desmin is an intermediate filament that forms a lattice surrounding the sarcomere near the Z lines to attach them to each other and the plasma membrane.
• Nebulin is a thin, elongated protein running parallel to thin filaments. It assists α-Actinin in binding the thin filaments to the Z line and is thought to be important during the development of muscle tissue.
• Dystrophin is thought to link actin filaments to the external lamina of the muscle cell
• Myomesin anchors thick filaments to the M line.

Neuromuscular junction

A neuromuscular junction, abbreviated NMJ,  is the location at which a motor nerve ending makes a synapse with a muscle cell. Motor neurons release acetylcholine into the synaptic cleft of the neuromuscular junction. The acetylcholine molecules traverse the synaptic cleft and bind to receptors on the muscle fiber membrane. This results in depolarization and subsequent contraction of the muscle cell.

Contraction

The mechanism for muscle contraction is referred to as the sliding filament theory. In this process, actin and myosin filaments slide past each other shortening the sarcomere. The sliding filament mechanism requires the hydrolysis of adenosine triphosphate (ATP) molecules and the binding and release of myosin heads on actin filaments.

• Following excitation by a motor nerve ending, ATP binds to myosin, decreasing the affinity of actin to myosin. Myosin releases actin.
• Myosin hydrolyses ATP to adenosine diphosphate (ADP) and a phosphate molecule. This provides energy needed for the myosin head to rotate  to a 90° angle and reattach to  a new actin.
• Power stroke - When the muscle is not contracting, the myosin binding site is partially blocked by a troponin molecule. During contraction calcium binds to troponin, decreasing its affinity to the myosin binding site and uncovering it so that myosin binds more tightly and releases a phosphate. The release of phosphate causes the myosin head to swing toward the M line and the attached actin filament slides with it.
• At the end of the power stroke, myosin releases the ADP molecule. The head returns to its original position and is ready to begin the cycle again.

Control of the skeletal muscle contraction

Two main proprioceptors influence the strength and duration of muscle contraction: Golgi tendon organs and muscle spindle fibers. Golgi tendon organs relay information about the force of contraction. Muscle spindles detect changes in muscle length.