The skeletal muscle is surrounded by a connective tissue covering called Epimysium (Epi-on and mys- muscle). The individual muscles are surrounded by this dense layer of collagen fibres which holds them together.
The fibres of epimysium at one end of the muscle are continuous with those of an attached tendon. These tendon fibres are in turn interwoven with the fibres of the periosteum of the bone.
That means if the muscle contracts, it will exert a pull on the tendon and finally the bone to which it is attached. The blood vessels and nerves pass between the adjacent muscles or over the surface of the epimysium.
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These blood vessels and nerves, branch into their smaller branches and penetrate the epimysium to reach the perimysium (peri- around).
Just inside the epimysium, the connective tissue sheath called perimysium divides the internal structure of the muscle into series of compartment.
This compartment contains small bundles of muscle fibres called the muscle bundle or fasciculi. Thus, the perimysium is the connective tissue sheath which covers each fasciculus.
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The fibres of the perimysium and epimysium are again interwoven and continuous outside the muscle. Thus, the perimyseal fibres to extend into the attached tendon.
The endomyseal and the perimyseal fibres are again interwoven. Thus, a single muscle cells contraction will exert a pull on the tendon. But normally, the contraction of the muscle cells takes place in groups, which is under the neural control. The neurons control the fasciculi, after branching within the endomysium.
Sarcolemma:
Just beneath the endomysium, the fibres are covered by the muscle cell membrane called the sarcolemma. This membrane consists of a true cell membrane, the plasma membrane with an outer coat consisting of a thin layer of poly- sacchrahide material containing several thin collagen fibres.
At one end of the muscle fibre, the layer of sarcolemma fuses with the tendon fibres and these tendon fibres inserts to the bone. Thus, every individual muscle fibre is finally attached to the bone by the tendon.
Sarcoplasm:
Just inside the sarcolemma, the muscle fibre contains the fluid part. It is a gelatin like substance which fills the internal structure of the muscle fibre. This is called sarcoplasm.
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The sarcoplasm mainly contains dissolved proteins, minerals, glycogen, fats, K, Mg, P, enzymes and other cellular organelles like nucleus, mitochondria, sarcoplasmic reticulum, T- tubules and myofibrils.
Sarcoplasmic Reticulum and the Transverse Tubules:
It is an endoplasmic reticulum which is made of network of tubules and vesicles. The tubules as they run parallel to the myofibril are called longitudinal tubules. On the either end these tubules terminate to form vesicles or cisterns which are also called as outer vesicles or cisterns.
The two outer vesicles and the T-tubules separating them are called triad. The T-tubules helps in transmission of nervous impulses from the sarcolemma to the deep portions of the fibre. Ca++ is stored in the outer vesicles.
When the nervous impulses are transmitted over the T-tubules and between the outer vesicles, the Ca++ is released. The sarcoplasmic reticulum, serves to bind the calcium ions responsible for initiating contraction with ATP. Further, it also assists in relaxation of the muscle.
Myofibrils:
Myofibrils are the protein’ thread like strands which are embedded in the sarcoplasm. Each myofibril is about 1 to 2 mm in diameter and is continuous through the entire length of the muscle fibre. They constitute about 80% of the fibre volume. There are several hundred to several thousands of myofibrils in each muscle fibre.
The actin and the myosin filament lie parallel and play an important role in muscle contraction. The myofibrils are further designed into a series of repeating light and dark patterns, called the sarcomere. The sarcomere is also called the functional unit of myofibrils.
A typical myofibril consists of 10,000 or more sarcomere which is held together. The average length of a sarcomere is 2.6 mm.
Each sarcomere consists of two sets of actin filaments and a set of myosin filament. As the actin and myosin filaments partially interdigitate, it results in the formation of light and dark bands in the myofibrils.