smooth muscle

smooth muscle The cardiovascular, gastrointestinal, genitourinary, and respiratory systems are composed mostly of hollow organs (tubular or sacular), which transport and/or store fluids (either liquids or gases) within the body. The walls of these organs contain smooth muscle, a type of tissue which enables them to constrict or dilate, in this way retarding or facilitating fluid movement as required. This is accomplished by the shortening or lengthening of the individual smooth muscle cells, which occurs in a co-ordinated fashion because the cells are electrically coupled by intercellular connections, known as gap junctions. Other structures in the body that contain smooth muscle include the myometrium — the muscular wall of the uterus — which is responsible for the rhythmic contractions of labour; the piloerector muscles, which cause skin hair to stand up; and the irises, which control the diameter of the pupils.

Smooth muscle thus subserves all internal, involuntary functions, except the movements of breathing and the beating of the heart. Many directly acting chemical agents affect its contraction, but most smooth muscle is also under the control of the autonomic nervous system; in some sites (notably most blood vessels) it is influenced only by the sympathetic component, and at others (for example in the gut and the iris) by dual, and sometimes opposite, effects of sympathetic and parasympathetic nerves.

As befits its many functions, smooth muscle at different sites is much more heterogeneous than skeletal or cardiac muscle. By creating diverse structural arrangements of smooth muscle and other associated cells, and at the same time varying the mechanisms that control contraction, evolution has achieved a remarkable diversity of smooth muscle-containing organs, each of which is designed to fill a unique functional niche.

Calcium and contraction

On a cellular level, however, all smooth muscles share many characteristics. When relaxed, the cells assume the shape of long, narrow spindles or worms. The cells are termed ‘smooth’ because they lack the regular bands or striations which are prominent in skeletal muscle fibres and cardiac muscle cells. Smooth muscle cells are capable of contracting dramatically, to half or less of their relaxed length. Contraction may be sustained, as in the smooth muscle cells present in the blood vessels or airways, or rhythmic, as in the cells of the myometrium and gastrointestinal tract. The main stimulus for contraction is a rise in the cellular concentration of calcium. This can be triggered by an impressive array of chemical signals, that differ depending on the type of smooth muscle involved, including a variety of neurotransmitters released at autonomic nerve endings.

As in striated muscles, contraction occurs because the rise in cellular calcium causes an interaction between cellular action and myosin filaments, although the arrangement of these filaments within the cells is not of a similar consistent pattern. Also, the mechanism whereby calcium stimulates this interaction in smooth muscle differs from that in striated muscle, in that it involves activation of a different signalling protein (calmodulin rather than troponin). Another important difference between smooth and striated muscle is that smooth muscle never becomes fatigued, because it uses metabolic energy much more efficiently than does striated muscle.

In common experience, some obvious manifestations of altered smooth muscle activity are the widening of the pupils in the dark when the radially arranged muscle of the iris contracts; asthmatic wheezing, when the smooth muscle in the walls of the bronchioles impedes airflow; and the phenomenon of erection, when blood vessel relaxation allows engorgement.

Self-regulating pipes

The functioning of each type of smooth muscle is intimately tied up with the organ or system of which it is a part, so that this type of tissue is perhaps best appreciated if one abandons the attempt to generalize and considers, for example, the blood vessels.

The arteries and veins are not merely conduits designed to convey blood passively to and from the capillaries. Rather, they exist in a dynamic state of partial constriction, regulated by the smooth muscle cells which form much of the vascular wall, where they are arranged in multiple layers embedded in a tough and elastic matrix of connective tissue. The cells wrap around the vessel in a low-pitch spiral, so that, when they shorten, the vascular lumen is constricted. The layers of smooth muscle cells are separated from the blood by a monolayer of flat, polygonal endothelial cells. These remarkable cells carry out multiple vital tasks, which include controlling the clotting of blood and releasing substances which influence the contraction and also the growth of the smooth muscle cells. The most important of these substances, nitric oxide, is a short-lived gas which reacts with the protein guanylyl cyclase in the smooth muscle cells, causing them to relax and lengthen. Nitric oxide release is controlled by many factors, including the friction exerted by the flowing blood, and also by hormones and other messenger molecules present in the blood.

The outer layer of blood vessels contains nerves of the sympathetic component of the autonomic nervous system, the activity of which is controlled by the brain. The sympathetic nerve endings are constantly releasing minute quantities of norepinephrine (also called ‘noradrenaline’), a neurotransmitter which stimulates the smooth muscle cells to shorten. The length of the smooth muscle cells, and therefore the diameter of the blood vessel, is largely determined by the ongoing balance between the opposing influences on constriction of nitric oxide and norepinephrine.

These factors account for the dynamic state of partial constriction of the blood vessels, of which the overall effect is to impose a net resistance to the flow of blood from the heart; along with cardiac output, this is an important determinant of blood pressure. This resistance can be varied by alterations in the levels of norepinephrine release and nitric oxide production; by a myriad of other factors such as local tissue acidity, the oxygen concentration in the blood, temperature; and also by other hormones which can stimulate or inhibit smooth muscle cell shortening.

Apart from variation in the overall net resistance to blood flow, the degree of constriction or relaxation varies from region to region in different physiological circumstances. For example, although during strenuous exercise the heart may increase its pumping rate by about five times, the rate of delivery blood to each organ does not increase by this amount. Instead, the combined effects of activation of the sympathetic nervous system, and the release of substances generated in the heart and working muscles, causes the arteries in the heart and muscles to dilate dramatically, while the arteries in the non-working muscle and the digestive organs constrict. In this way, the flow of blood and therefore of oxygen to the muscles and to the heart may increase by twenty- and five-fold respectively, while the flow of blood to the rest of the body, excepting the brain, actually falls. Conversely, at rest after a meal, it is the vessels of the digestive organs which dilate.

The humble workhorse

In the gut, the smooth muscle is responsible for the many types of motility — peristalsis, which moves the contents along; relaxation, which accommodates a meal in the stomach; various churning and mixing movements; and finally expulsion of faeces — assisted by voluntary action.

Analogous changes in the functioning of the smooth muscles embedded in other organs are needed to support an enormous variety of involuntary activities, ranging from childbirth and ejaculation to urination, digestion, and visual adjustment to darkness and light. Indeed, as it faithfully performs its various automatically controlled tasks, this humble cousin of the heart and voluntary muscles plays many vital but unheralded roles in shaping both the most dramatic and the most routine events of our lives.

Philip Aaronson

See also alimentary system; autonomic nervous system; blood vessels; lungs.