Till now we have considered nervous activity as that initiated by stimuli acting on or in the body and producing responses which are either reflex actions or which are experienced as sensations. Many of our activities are, however, not due to impulses set up by receptors, but arise in the conscious part of the brain itself. Such activities are of the voluntary type. Those muscles which are controlled at will are therefore termed voluntary muscles, having striped or striated fibres forming most of that part of an animal termed the ’meat ’as distinct from ’offal. ’ We have no voluntary control over plain or unstriped muscle like that forming the walls of the gut and blood vessels which carry out their activities in a purely reflex fashion, e.g. we cannot by deciding to do so make the heart beat more quickly, stop ourselves from blushing, alter the size of the pupil of the eye or of artery walls. The muscles involved in such actions are therefore termed involuntary muscles. The converse, however, is not true—our voluntary muscles do not only carry out voluntary actions but involuntary ones as well—the example of riding a bicycle given above should make this abundantly clear.
Involuntary muscle is composed of narrow spindle-shaped cells but voluntary muscle is made up of long fibres which are multicellular 412 in origin. In each fibre are many longitudinal fibrils with cross-striping formed from the cytoplasm of the cells forming the fibre. The nuclei of these cells are visible just beneath the sheath binding the fibrils together.