Muscle Fiber Anatomy

To understand the muscle fiber anatomy, we must first grasp what a muscle fiber is. It is a highly specialized cell which constitutes the structural and functional unit of the muscular tissue. In other words, the apparatus of muscular contraction revolves around it, or is based on it. Billions of muscle fibers make up a skeletal muscle, which is anchored to bone through tendons. When these cells contract, a body part moves, such as an arm or leg, or when we turn the head.

Histological Description

The muscle fiber anatomy is the histological description of the fundamental unit of the muscular tissue. This cell, called myocyte, measures between 10 and 12 cm in length, and 70 and 80 μ (micron) in diameter. Each muscle fiber is covered by a membrane called sarcolemma, which surrounds and holds the sarcoplasm and many nuclei. The sarcoplasm is the muscle fiber cytoplasm, which contains the organelles of the myocyte. Some of these organelles have specific function. The main organelle is the myofibril (myofibrillae), which contains myofilaments.

Like any other eukaryotic cell cytoplasm, the sarcoplasm also has smooth endoplasmic reticulum, which is called sarcoplasmi reticulum. The sarcoplasmic reticulum is completely involved in regulating the calcium concentration around myofibrils. It forms an anastomosing network of interconnected cisternae (flat sac-like structures) which communicate with dilated terminal cisternae.

The myofibril's myofilaments are minutest thread-like structures, which are made of a special protein that render them able to contract. These myofilaments overlap to form sarcomeres, which are composed of myosin and actin (thick and thin filament, respectively). The physico-optical properties of the myofibrils differ along the length of the muscle fiber, making it striated due to the presence of alternating bands. Under polarized light, some of them glisten due to the presence of anisotropic discs that double-refract light, while others fail to refract light; these are dark isotropic discs.

Each muscle fiber and small group of fibers are surrounded by a connective tissue membrane called endomysium. Large group of muscle fibers form muscle bands, or fasciculi. These fasciculi, together with the whole muscle, are enclosed in a connective tissue membrane, which is known as the perimysium. Blood vessels and nerves reach the muscle fibers in the layers of connective tissue.

Each muscle has a developed network of blood vessels. The contraction of the muscle promotes the fast flow of blood as if it were a peculiar pump which forces the blood forward. Under conditions of reduced motor activity (hypokenesia), this function of the skeletal muscle is excluded, as a result of which the blood flows slower and metabolic processes are reduced and the consumption of glucose or ketone bodies drops. In contrast, under conditions of intense motor activity, the reserve capillaries open, new capillaries are formed, and nutrition of skeletal muscles improves.

Below, a portion of a muscle fiber showing striations and myofibrils.