Skeletal muscles are those which we have direct control over and whose main function is to generate forces and produce movement.
These muscles vary in size and shape, with some being extremely small such as those in the middle ear, to other very large, strong muscles such as those in the Quadriceps group. Shape and fibre orientation also differ between muscle groups.
However, the main structure of a skeletal muscle remains the same. If you were to take one whole muscle and cut through it, you would find the muscle is covered in a layer of connective tissue known as the Epimysium. The Epimysium protects the muscle from friction against other muscles and bones.
It also continues at the end of the muscle to form (along with other connective tissues) the muscles tendon. Looking at the cross section of the muscle you can see bundles of fibres, known as Fasciculi, which are surrounded by another connective tissue, called the Perimysium. Each Fasciculi contains anywhere between 10 and 100 muscle fibres, depending on the muscle in question.
A large strong muscle, such as thoses forming your Quadriceps would have a large number of fibres within each bundle. A smaller muscle used for precision movement, such as those in the hand would contain far fewer fibres per Fasciculi.
Looking at each muscle fibre in detail, you can see they too are covered in a fibrous connective tissue, known as Endomysium which insulates each muscle fibre. Muscle fibres can range from 10 to 80 micrometers in diameter and may be up to 35cm long.
Beneath the Endomysium and surrounding the muscle fibre is the Sarcolemma which is the fibres cell membrane and beneath this is the Sarcoplasm, which is the cells cytoplasm, a gelatinous fluid which fills most cells.
This contains Glycogen and Fats for energy and also Mitochondria which are the cells powerhouses, inside which the cells energy is produced.
Each muscle fibre itself contains cylindrical organelles known as Myofibrils. Each muscle fibre contains hundreds to thousands of Myofibrils. These are bundles of Actin and Myosin proteins which run the length of the muscle fibre and are important in muscle contraction.
Surrounding the Myofibril there is a network of tubules and channels called the Sarcoplasmic Reticulum in which Calcium is stored which is important in muscle contraction.
Transverse tubules pass inwards from the Sacrolemma throughout the Myofibril, through which nerve impulses travel.
Each Myofibril can then be broken down into functional repeating segments called Sarcomeres which are separated by sheets of protein known as Z lines. Each Sarcomere demonstrates a striated appearance, with different bands called the I band, A band and H band visible, due to the overlapping of Actin and Myosin filaments.
Actin is a thin filament and Myosin is thicker and so the I band which is light in appearance contains only Actin filaments, the A band is dark, where both filaments overlap and the H band in the centre is where you can only see Myosin filaments. The movement and overlapping of the Actin and Myosin filaments, causing a decrease in Sarcomere size is thought to be the way in which muscles contract and shorten.
