Anatomical and cellular organisation of striated muscle

Primary organisation

Seen in cross section, skeletal striated muscles show a three-level organisation, each level being separated from the next by supporting connective tissue: the muscle as a whole is bounded by the epimysium; it is made up of fascicles bounded by the perimysium, which is itself made up of muscle fibres separated from one another by the endomysium. This primary organisation was observed under light microscopy using histological techniques, which are now routinely used in the diagnosis of neuromuscular diseases.

Muscle types and fibre types

Histological techniques have made it possible to refine the concept of muscle types introduced by Ciaccio in 1898, building on the description given by Stefano Lorenzini as early as 1678. In 1898, Ciaccio distinguished red muscles from white muscles. Ranvier (1874) was the first to suggest a physiological difference between red fibres and white fibres in the rabbit, associating them respectively with fast contraction and slow contraction. Grützner (1884) then confirmed these findings in the frog and, after extending the studies to other species, concluded that this typing was universal. With the development of histochemical techniques (from 1935 onwards, by Semenoff), fibre typing was made easier by the technique for detecting myosin ATPase activity (Engel, 1962; Brooke and Kaiser, 1970). Slow-twitch fibres (known as type I) work aerobically and show oxidative metabolic activity. These fibres are able to sustain prolonged tonic activity. Fast-twitch fibres (type II) work anaerobically and show oxidative-glycolytic (type IIA fibres) or glycolytic (type IIB fibres) metabolic activity. These muscle fibres fatigue rapidly and are suited to phasic activity.

Innervation

Each skeletal striated muscle fibre receives a single innervation, from an alpha motor neuron (motor nerve fibres). Their cell bodies are located in the brainstem or the spinal cord (anterior horn). Their axons are myelinated. When the motor axon reaches the vicinity of a muscle, it divides into numerous branches. Each of these branches forms a single junction with one muscle fibre. Together, the motor neuron and the fibres it innervates make up the motor unit. The muscle fibres of a given motor unit are randomly scattered throughout the muscle. The electrical activity of a motor neuron therefore controls the contractile activity of all the fibres in the motor unit. Close to the muscle fibre, a branch of a motor neuron ends in a fine arborisation that is embedded in the membrane of the muscle fibre, forming the neuromuscular junction. This junction is therefore the synapse between the axon terminals of the motor neuron and the striated muscle fibre.

Blood supply

As described above, muscle fibres are surrounded by a capillary network with a rectangular mesh. On average, four to six capillaries surround each fibre. The density of these capillaries per mm², however, varies with fibre type and therefore with the size and metabolism of the fibre. It is high for small fibres with an aerobic metabolism, that is, for slow-twitch fibres. It decreases to reach its lowest level in the large fibres with an anaerobic metabolism, namely the fast-twitch fibres.

Myotendinous junction

Striated muscle fibres attach to bone through the tendons; this attachment is called the myotendinous junction. It is here that the forces generated by the contraction of the myofibrils are transmitted to the tendon. At this point, the sarcoplasm and the basal lamina of the muscle fibres form numerous folds that increase the contact surface between the tendon and the muscle fibres by a factor of 10 to 50. The collagen microfibrils are in close contact with the sarcoplasm, inserting themselves into the folds of the sarcoplasm.