Involuntary muscle

Involuntary muscle makes up the muscular tissue of the bladder, gut walls, stomach and womb, and also forms a layer around the blood vessels. It is therefore involved in functions such as peristalsis and in maintaining the tone of blood vessels to control the blood pressure and distribution of blood to the various organs of the body.

The activity of involuntary muscle is controlled by two different sets of nerves, the parasympathetic and sympathetic nervous systems (the two parts of the autonomic nervous system). Some muscles are innervated by both parts, in which cases the nerves have opposing effects. Also, each part of the autonomic nervous system may have different effects in different involuntary muscles. General sympathetic stimulation contracts the smooth muscle of the arteries in the gut. On the other hand the arteries in the skeletal muscles dilate. Both effects serve the same purpose, which is to prepare the body for exertion. Muscle cells of involuntary muscle can be more easily stretched than the cells of the voluntary muscle, and they can lengthen without causing an increase in tension in the muscle. These muscles respond more slowly than voluntary muscle to nerve impulses, and they do not tire.

In addition to responding to nerve impulses involuntary muscle, unlike voluntary muscle, can also contract of its own accord. This usually happens when the muscle is stretched beyond a certain point. Some involuntary muscles are also very susceptible to the action of circulating hormones. The womb, for example, is very sensitive to levels of oestrogens, pro- gesterone and oxytocin and this is important in labour and childbirth. When involuntary muscle is stimulated, by whatever means, the wave of contraction can easily spread throughout the muscle because the cells lie tightly together.