Skeletal muscle fibres are multinucleate cells (Fig. 9.1) formed by the fusion of numbers of elongated uninucleate cells called myoblasts. Mature fibres may be as long as the muscle of which they form part, and 10 to 100 |xm in diameter. The nuclei are arranged around the edge of the fibre. Most of the interior of the fibre consists of the protein filaments which constitute the contractile apparatus, grouped together in bundles called myofibrils. The myofibrils are surrounded by cytoplasm (or sarcoplasm), which also contains mitochondria, the internal membrane systems of the sarcoplasmic reticulum and the T system, and a fuel store in the form of glycogen granules and sometimes a few fat



Fig. 9.1. Diagram to show the arrangement of fibres in a vertebrate striated muscle. The cross-striations on the myofibrils can be seen with light microscopy; their ultrastructural basis is shown in Fig. 10.6. After Schmidt-Nielsen (1990).

droplets. We shall examine the structure of the contractile apparatus and the internal membrane systems in more detail in the following chapter.

The muscle fibre is bounded by its cell membrane, sometimes called the sarcolemma, to which a thin layer of connective tissue (the endomysium) is attached. Bundles of muscle fibres are surrounded by a further sheet of connective tissue (the perimysium) and the whole muscle is contained within an outer sheet of tough connective tissue, the epimysium. These connective tissue sheets are continuous with the insertions and tendons which serve to attach the muscles to the skeleton.

Muscles have an excellent blood supply, with blood capillaries forming a network between the individual fibres. Sensory and motor nerve fibres enter the muscle in one or two nerve branches. The sensory nerve endings include those on the muscle spindles (sensitive to length), in the Golgi tendon organs (sensitive to tension), and a variety of free nerve endings in the muscle tissue, some of which are involved in sensations of pain.

In mammals the gamma motoneurons provide a separate motor nerve supply for the muscle fibres of the muscle spindles, while the bulk of the muscle fibres are supplied by the alpha motoneurons. Each alpha motoneuron innervates a number of muscle fibres, from less than ten in the extraocular muscles (those which move the eyeball in its socket) to over a thousand in a large limb muscle. The complex of one motoneuron plus the muscle fibres which it innervates is called a motor unit. Since they are all activated by the same nerve cell, all the muscle fibres in a single motor unit contract at the same time. Muscle fibres belonging to different motor units may well contract at different times, however.

Most mammalian muscle fibres are contacted by a single nerve terminal, although sometimes there may be two terminals originating from the same nerve axon. Muscle fibres of this type are known as twitch fibres, since they respond to nervous stimulation with a rapid twitch. In the frog and other lower vertebrates another type of muscle fibre is commonly found, in which there are a large number of nerve terminals on each muscle fibre. These are known as tonic fibres, since their contractions are slow and maintained. There are some tonic fibres in the extraocular muscles of mammals, and also in the muscles of the larynx and the middle ear.

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