Functional anatomy of the skeletal muscle and muscle fibers

When we think of muscles, we visualize each muscle as a whole, that is, as a single unit. This is natural because a skeletal muscle seems to act as a single entity. But skeletal muscles are far more complex than that.

If a person were to dissect a muscle, he or she would first cut through the outer connective tissue covering. This is the epimysium. It surrounds the entire muscle, holding it together. Once one had cut through the epimysium, one would see small bundles of fibers wrapped in a connective tissue sheath. These bundles are called fasciculi. The connective tissue sheath surrounding each fasciculus is the perymysium.

Finally, by cutting through the perimysium and using a microscope, one would see the muscle fibers, which are the individual muscle cells. A sheath of connective tissue, called the endomysium, also covers each muscle fiber. It is generally thought that muscle fibers extend from one end of the muscle to the other; but under the microscope, muscle bellies often divide into compartments or more transverse fibrous bands(inscriptions). Because of this compartmentalization, the longest human muscle fibers are about 12cm, which corresponds to about 500,000 sarcomeres, the basic functional unit of the myofibril. The number of fibers in different muscles ranges from several hundred(e.g. tensor tympani, attached to the eardrum) to more than a million(e.g. medial gastrocnemius muscle).

Muscle Fiber

Muscle fibers range in diameter from 10 to 120 µm, so they are nearly invisible to the naked eye. The following sections describe the structure of the individual muscle fiber.

Plasmalemma

If one looked closely at an individual muscle fiber, one would see that it is surrounded by a plasma membrane, called the plasmalemma. The plasmalemma is part of a larger unit reffered to as the sarcolemma. The sarcolemma is composed of the plasmalemma and the basement membrane. Some textbooks use the term sarcolemma to describe just the plasmalemma. At the end of each muscle fiber, its plasmalemma fuses with the tendon, which inserts into the bone. Tendons are made of fibrous cords of connective tissue that transmit the force generated by muscle fibers to the bones, thereby creating motion. So typically, individual muscle fibers are ultimately attached to bone via the tendon.

The plasmalemma has several unique features that are important to muscle fiber function. It appears as a series of shallow folds along the surface of the fiber when the fiber is contracted or in a resting state, but these folds disappear when the fiber is stretched. This folding allows stretching of the muscle fiber without disrupting the plasmalemma. The plasmalemma also has junctional folds in the innervation zone at the motor end plate, which assists in the transmission of the action potential from the motor neuron to the muscle fiber as discussed later in this chapter. Finally, the plasmalemma helps to maintain acid-base balance and transports metabolites from the capillary blood into the muscle fiber.

Satellite cells are located between the plasmalemma and the basement membrane. These cells are involved in the growth and development of skeletal muscle and in muscle’s adaptation to injury, immobilization, and training.

Sarcoplasm

Inside the plasmalemma, a muscle fiber contains successively smaller subunits, as shown in the figure. The largest of these are myofibrils, which we discuss separately. For now, consider myofibrils to be rodlike structures running the length of the muscle fibers. A gelatin-like substance fills the spaces within and between the myofibrils. This is the sarcoplasm. It is the fluid part of the muscle fiber – its cytoplasm. The sarcoplasm contains mainly dissolved proteins, minerals, glycogen, fats and the necessary organelles. It differs from the cytoplasm of most cells because it contains a large quantity of stored glycogen as well as the oxygen-binding compound myoglobin, which is quite similar to hemoglobin.

The Transverse Tubules

The sarcoplasm also houses an extensive network of transverse tubules(T-tubules), which are extensions of the plasmalemma that pass laterally through the muscle fiber. These tubules are interconnected as they pass among the myofibrils, allowing nerve impulses received by the plasmalemma to be transmitted rapidly to individual myofibrils. The tubules also provide pathways from outside the fiber to its interior, enabling substances to enter the cell and waste products to leave the fibers.

The Sarcoplasmic Reticulum

A longitudinal network of tubules, known as the sarcoplasmic reticulum(SR), is also found within the muscle fiber. These membranous channels parallel the myofibrils and loop around them. The SR serves as a storage site for calcium, which is essential for muscle contraction. Figure shown depicts the T-tubules and the SR.

“Physiology of sport and exercise”, fourth edition; Jack H. Wilmore, David L. Costill, W. Larry Kenney