The function of muscle cells is to contract: to shorten and develop tension. This means that the end-product of cellular activity can be measured with considerable precision, by mechanical measurement of the change in length or tension or both. Such activity must obviously involve the consumption of energy, some of which may appear as heat.

In this chapter and the next two we shall be concerned mainly with the properties of rapidly contracting vertebrate skeletal muscles, such as frog sartorius and the rabbit psoas. Skeletal muscles are activated by motoneurons, as we have seen in previous chapters. Their cells are elongate and multinuclear and the contractile material within them shows cross-striations; hence skeletal muscle is a form of striated muscle. Some of the special properties of other muscles are examined in chapter 21.

The normal stimulus for the contraction of a muscle fibre in a living animal is an impulse in the motor nerve by which it is innervated. The sequence of events following the nerve impulse is shown schematically in fig. 18.1. We have examined stages 1 to 4 of this sequence (the excitation processes) in previous chapters. This chapter is concerned with some of the overall consequences of contraction (stage 6); details of the cellular mechanisms involved in stages 5 and 6 are considered in the following chapters.

Anatomy

Skeletal muscle fibres (fig. 18.2) are multinucleate cells formed by the fusion of numbers of elongated uninucleate cells called myoblasts.