Skeletal Tissue

Skeletal muscle is one of the important types of muscles in the human body that remains attached to the bones by means of tendons. They are also known as voluntary muscles since the nervous system of our body controls them. Skeletal muscles are multinucleated, cylindrical, and striated- striations due to the differential arrangement of proteins within the cells. Each skeletal muscle consists of numerous muscle fibres, parallelly oriented and bundled together by connective tissue.

Structure of Skeletal Tissue

In a skeletal muscle, a rich network of nerves and blood vessels nourishes and controls each muscle cell. These muscle fibres are wrapped and then bound together by several layers of fibrous connective tissue.

Structural Plan

A single skeletal muscle cell, also known as a muscle fibre, develops when multiple embryonic myocytes fuse into one long, elongated, and multinucleated cell.
These muscle fibres are bound together into fascicles that are supplied with a rich network of blood vessels and nerves. The fascicles bundle together to form the muscle.

Connective Tissue Layers

The epimysium (Gr. epi, above; mys, muscle) is a dense fibrous connective tissue layer surrounding the entire muscle. Each skeletal muscle is formed from several fascicles of skeletal muscle fibres.
Each fascicle is surrounded by the layer of perimysium (Gr. peri, around).
Each single muscle cell is individually covered with a fine layer of reticular connective tissue (with some areolar fibres) around each muscle fibre called endomysium (Gr.endo, within).
All the connective tissue layers are continuous with each other and extend beyond muscle fibres, forming the tendons to anchor the muscle to the bone.

Myofibril

Each skeletal muscle cell is surrounded by the cell membrane (sarcolemma).
The cytoplasm (sarcoplasm) contains a large amount of glycogen and the protein myoglobin.
In the intracellular space, cylindrical intracellular specialised contractile elements called myofibrils are present. Each muscle fibre contains numerous myofibrils that extend throughout the length of a muscle fibre.
The myofibrils are densely packed and push the nuclei towards the periphery right under the sarcolemma.
Each myofibril consists of an arrangement of highly organised cytoskeletal elements- the thick and the thin filaments. The thick filaments are special assemblies of myosin protein, whereas the thin filaments are primarily made of actin protein.

Ultrastructure of Myofibril

Under electron microscopic view, a myofibril displays alternating dark bands (the A bands) and light bands (the I bands) due to alternately stacked thick and thin filaments. These bands of all the myofibrils, lined up parallel to one another, collectively produce the striated appearance of a skeletal muscle fibre.

An A band is made up of a set of thick filaments along with the regions of overlapping the thin filaments on both ends of the thick filaments. The thick filaments lie only within the A band and extend its entire width.
The H zone is a lighter area within the middle of the A band, where the thin filaments do not reach, and the central portions of the thick filaments are present.
M line is a system of supporting proteins that vertically hold the thick filaments together within each stack, which extends down the middle of the A band.
An I band consists of the remaining portion of only the thin filaments that do not overlap into the A band.
The Z line is a flat, cytoskeletal disc that appears as a dense vertical line in the middle of each I band; it connects the thin filaments of two adjoining sarcomeres.
The area between two Z lines is called the sarcomere, which is the functional unit of a skeletal muscle.