Biochemical Foundations of Human Vital Activity - Volkov, N. I., Nesen, E. N. 2000

Biochemistry of Sports
Biochemistry of Muscle and Muscle Contraction
Structural Organization of Muscle Fibers

The MORPHOLOGICAL Structure OF a Muscle Cell is largely similar to that of Cells in other Tissues (see Chapter 2). However, owing to its specific contractile function, it possesses several distinct features.

A muscle cell is enclosed by an electrically excitable surface membrane known as the sarcolemma. The sarcolemma features specialized contact sites with motor nerve endings called synapses (neuromuscular junctions, see Fig. 113). It exhibits selective permeability to various substances and contains transport systems that maintain concentration gradients of Na⁺, K⁺, and Cl^- ions between the intracellular and extracellular fluid, thereby generating a Membrane Potential across its surface. The generation of a membrane Action Potential triggered by a Nerve Impulse is a prerequisite for muscle fiber excitation.

The internal space of the muscle fiber is filled with an intracellular fluid called sarcoplasm. Approximately 80% of the fiber volume is occupied by long contractile threads known as myofibrils.

Myofibrils are the contractile elements of the muscle fiber, and their number can reach several thousand. Under a Microscope, myofibrils display cross-striations consisting of repeating dark and light bands, or discs (Fig. 115). The dark bands, or A-discs, feature a lighter H-zone in their center, which is bisected by a dark M-line. The light bands, or I-discs, are intersected at their center by a narrow Z-line (Z-membrane).

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Fig. 115 Structure of a myofibril and sarcomere

The segment of a myofibril between two Z-membranes is called a sarcomere. It is the smallest contractile unit of a muscle. Sarcomeres are arranged sequentially along the myofibril, repeating at intervals of 1500–2300 nm. A single myofibril may contain several hundred sarcomeres. Their length and quantity within the myofibril determine the velocity and force of Muscle contraction. The striated appearance of skeletal Muscles visible under a Light Microscope results from a high degree of Organization, whereby most muscle cells align such that their sarcomeres lie parallel to one another. Electron Microscopy of myofibril cross-sections reveals that each myofibril consists of numerous parallel thick and thin muscle threads, or filaments, which impart a longitudinal striation to the muscle.

Thick filaments are located within the A-discs and are composed of the protein Myosin. Thin filaments reside in the I-discs and contain the Proteins Actin, Tropomyosin, and troponin. These thin filaments are arranged around the thick (myosin) filaments at the corners of a hexagon in such a way that each thin filament occupies a symmetrical position equidistant from three thick filaments, while each thick filament is symmetrically surrounded by six thin filaments (see Fig. 115). Thick and thin myofibrillar filaments interact via cross-bridges distributed along the length of the thick myosin filament.

During muscle contraction, the lengths of the thick and thin filaments remain constant, while the distance between the Z-membranes within the sarcomeres decreases. Consequently, changes in muscle length result from the sliding of thick and thin filaments past one another, accompanied by an alteration in the degree of their overlap. The tension developed during muscle contraction is proportional to the degree of filament overlap as well as the number of cross-bridges formed. Upon maximum contraction, sarcomeres shorten by 20–50%, whereas during passive stretching, they can elongate up to 120% of their resting length.

Muscle fibers are characterized by a network of transverse tubular invaginations of the sarcolemma—known as T-systems—which course between the myofibrils and the sarcoplasmic reticulum, ensuring the rapid propagation of nerve impulses deep into the fiber toward the muscle's contractile elements (Fig. 116).

The sarcoplasmic reticulum (SR) is an intracellular membrane system of interconnected flattened vesicles and tubules (cisternae) that envelops the myofibrillar sarcomeres (see Fig. 116). Its inner membrane contains proteins capable of binding Calcium Ions. The primary function of the SR is to regulate the concentration of calcium ions in the space between Actin and myosin, a level that fluctuates between 10^-5 and 10^-7 mol ⋅ L^-1 during the contraction-relaxation cycle. In response to a nerve impulse, the SR releases Ca²⁺ ions, and upon cessation of the stimulus, it reabsorbs them. The SR membrane also bears Ribosomes, which are sites of METABOLISM/35.html">Protein Biosynthesis.

Mitochondria represent one of the most vital Organelles of the muscle fiber. They lie parallel to the myofibrils in close contact with the reticulum. Mitochondria function as the "power plants" of the muscle fiber, as they generate ATP—the energy currency for muscle contraction. The number of mitochondria increases in trained muscles compared to untrained ones.

Fig. 116 Diagram of the arrangement of T-systems and the sarcoplasmic reticulum in a muscle fiber