The structure of skeletal muscle
The muscle belly is completely surrounded by a thick connective tissue called epimysium (epi- means ‘upon’ or ‘on’, and myo means ‘muscle’). This is continuous and eventually forms tendons which join the muscle onto bones. Each part of the muscle is covered by connective tissue to help provide shape and add strength.
The muscle belly is composed of many bundles of fibres known as fasiculi, and these are covered with perimysium (peri- means ‘around’). The epimysium, perimysium and endomysium are all composed of irregular collagen fibres.
Each fibre within a single fasciculus is surrounded by a membrane called the sarcolemma and contains many smaller fibres called myofibrils which provide the contractile unit of the muscle. These myofibrils are surrounded by endomysium, an irregular collagen sheath (endo- means ‘within’) and have characteristic light and dark bands (striations) which represent a sarcomere.
Other organelles of the cell, for example, the mitochondria are found between the myofibrils. The myofibrils are embedded in the cell’s sarcoplasm. Surrounding the myofibrils within the sarcoplasm is the sarcoplasmic reticulum, a series of channels that store and secrete calcium (essential for muscle contraction).
Also in the sarcoplasm are transverse tubules/T vesicles that transmit the nerve stimulus from the sarcolemma into the cell, causing the sarcoplasmic reticulum to release calcium.