Skeletal Muscle

The main bulk of muscular tissue of our body is of skeletal type. In males, the skeletal muscles organize about 40% of the body weight. There are about 656 numbers of voluntary muscles in the human body. The skeletal muscles are attached to bones by means of tendons which are found at the ends of the muscles. A tendon is a cord-like white fibrous connective tissue but it is non-elastic in nature. Some tendons are expanded and flat, forming membranous sheet-like structures known as aponeuroses.
Each anatomical muscle is composed of a large number of elongated cells which remain parallel to the long axis of the muscle and grouped in several bundles which are called fasciculi. The whole muscle, the individual fasciculi and the individual muscle fibers remain participated by connective tissue coverings. They are known as epimysium, perimysium and endomysium respectively. These coverings are interconnected. They are forming a continuous framework which serves to bind the individual fibers together and to integrate their action.

Structure of skeletal muscle :

Skeletal muscle is made up of numerous muscle fibers. Every muscle fiber is a single elongated cell. They are cylindrical in shape. The fibers are un-branched, having no syncytial bridges in between them. The cell membrane of a muscle fiber is named sarcolemma. Under ordinary microscope, the skeletal muscle fibers appear to be diagonally striated due to the presence of alternate dark and light diagonal bands along their length. For this reason, the skeletal muscles are also called striated muscles. Each skeletal muscle fiber is multinucleated. The nuclei are flattened and oval or elongated in shape and they lie peripherally in the cell i-e., just beneath the sarcolemma of the muscle fiber. The cytoplasm of the muscle fiber is divisible into three parts. Those are : -
[1J Sarcoplasm,
[2] Myofibrij
[3] Sarcotubular system.

Sarcoplasm :

Sarcoplasm is the fluid part of cytoplasm of muscle cells similar to the cytoplasm of other cells,containing several mitochondria, small Golgi bodies, endoplasmic reticulum etc.The smooth endoplasmic reticulum of muscle cells is highly developed and is called sarcoplasmic reticulum. Unlike the cytoplasm of other cells, the sarcoplasm of muscle cells stores glycogen which is a polysaccharide and myoglobin which is a hemoglobin-like pigment.

Myofibril :

Skeletal Muscle

Skeletal Muscle

In the sarcoplasm of muscle fibers, many special type of protein made fibrils remain arranged longitudinally and parallel. These are called myofibrils. The collection of myofibrils is called the contractile system of the muscle cell because they are responsible for muscle contraction. Each myofibril shows the presence of alternate dark and light transverse bands along its length and it appears to be striated. Jn a skeletal muscle fiber, the dark and light bands of the adjacent, parallel arranged myofibrils remain more or less on the same line so that the entire muscle fiber appears to be striated.
Under a polarized microscope, the dark band appears to be highly refractile and birefringent (i.e., doubly refractive) and it is also called A-band (A stands for anisotropic) whereas the light band being less refractile and monorefringent is called I-band (I stands for isotropic). At the middle of the A-band, there is a less dark band called H-band whereas at the middle of the I-band, there is a dark line called Z-line. In a Myofibril, the region between the two adjacent Z-lines is termed sarcomere. Thus, each sarcomere consists of a complete A-band at the middle, on either side of which there is a half I-band. A sarcomere is designated as the contractile (or functional) unit of a muscle because contraction of the whole muscle is achieved by contraction of its individual sarcomeres.

In a sarcomere, the region between myosin filament terminals and Z-line of each side is the half I-band where only actin filaments are present. The region covering throughout the length of myosin filaments is the A-band (where actin filaments are also present surrounding the myosin) and the central part of the myosin filaments (which is not covered by actin filaments) establishes the H-band.
In the myofibrils, in addition to the two chief muscle proteins, actin and myosin, two other proteins called tropomyosin and troponin are also found that remain attached to the actin filaments.

Sarcotubular system :

The sarcoplasm of muscle cells possess a specially developed system of tubules known as sarcotubular system. It is also designated as the conducting system of the skeletal muscle cells because it helps in transmission of impulse all over the muscle cells. The sarcotubular system consists of two kinds of tubules— transverse tubules and longitudinal tubules.
Transverse tubules-The sarcolemma of muscle fibres invaginates transversely at intervals to form some tubular structures ; these are called transverse tubules. In mammalian skeletal muscle fibers the transverse tubules pass through the junction of A and I bands, on the other hand in amphibian skeletal muscles Transverse-tubules pass through the Z-lines.

Longitudinal tubules-

The smooth endoplasmic reticulum of muscle cells are specially developed and modified into some tubules that remain arranged longitudinally between the adjacent transverse tubules. These are called longitudinal tubules or in short L-tubules. The L-tubules arborize to form a reticulum (or network) at the center of the sarcomere, hence they are collectively called sarcoplasmic reticulum. The peripheral ends of the L-tubules expand to form terminal cisternae that remain in contact with the transverse-tubules.