Types of Skeletal Muscle

Types of Skeletal Muscle

Skeletal muscle cells respond to stimulation with a brief maximal contraction - they are of the twitch type. Individual muscles fibres cannot maintain their contraction over longer periods. The sustained contraction of a muscle depends on the "averaged" activity of often many muscles fibres, which individually only contract for a brief period of time.
The force generated by the muscle fibre does depend on its state of contraction at the time of excitation. Excitation frequency and the mechanical summation of the force generated is one way to graduate the force generated by the entire muscle. Another way is the regulation of the number of muscle fibres which contract in the muscle. Additional motor units, i.e. groups of muscle fibres innervated by one motor neurone and its branches, are recruited if their force is required. The functional properties of the muscle can be "fine-tuned" further to the tasks the muscle performs by blending functionally different types of muscle fibres:

Type I fibres (red fibres)

Red muscles contain predominantly (but not exclusively) red muscle cells. Red muscle fibres are comparatively thin and contain large amounts of myoglobin and mitochondria. Red fibres contain an isoform of myosin with low ATPase activity, i.e. the speed with which myosin is able to use up ATP. Contraction is therefore slow. Red muscles are used when sustained production of force is necessary, e.g. in the control of posture.

Type II fibres White muscle cells, which are predominantly found in white muscles, are thicker and contain less myoglobin. ATPase activity of the myosin isoform in white fibres is high, and contraction is fast. Type IIA fibres (red) contain many mitochondria and are available for both sustained activity and short-lasting, intense contractions. Type IIB/IIX fibres (white) contain only few mitochondria. They are recruited in the case of rapid accelerations and short lasting maximal contraction. Type IIB/IIX fibres rely on anaerobic glycolysis to generate the ATP needed for contraction.

Skeletal muscle fibres do not contract spontaneously. Skeletal muscle fibres are notinterconnected via GAP junctions but depend on nervous stimulation for contraction. All muscle fibres of a motor unit are of the same type.

Fibre type is determined by the pattern of stimulation of the fibre, which, in turn, is determined by the type of neuron which innervates the muscle. If the stimulation pattern is changed experimentally, fibre type will change accordingly. This is of some clinical / pathological importance. Nerve fibres have the capacity to form new branches, i.e. to "sprout", and to re-innervate muscle fibres, which may have lost their innervation as a consequence of an acute lesion to the nerve or a neurodegenerative disorder. The type of the muscle fibre will change if the type of stimulation provided by the sprouting nerve fibre does not match with the type of muscle. The process of reinnervation and type adjustment may result in fibre type grouping within the muscle, i.e. large areas of the muscle are populated by muscle fibres of one type.