Excitation and Contraction of Smooth Muscle

Multi-Unit Smooth Muscle

Multi-unit smooth muscle is composed of discrete, separate, smooth muscle fibers. Each fiber operates independently of the others and often is innervated by a single nerve ending, as occurs for skeletal muscle fibers. Furthermore, the outer surfaces of these fibers, like those of skeletal muscle fibers, are covered by a thin layer of basement membrane–like substance, a mixture of fine collagen and glycoprotein that helps insulate the separate fibers from one another.

Important characteristics of multi-unit smooth muscle fibers are that each fiber can contract independently of the others, and their control is exerted mainly by nerve signals. In contrast, a major share of control of unitary smooth muscle is exerted by non-nervous stimuli. Some examples of multi-unit smooth muscle are the ciliary muscle of the eye, the iris muscle of the eye, and the piloerector muscles that cause erection of the hairs when stimulated by the sympathetic nervous system.

Unitary Smooth Muscle

Unitary smooth muscle is also called syncytial smooth muscle or visceral smooth muscle . The term unitary does not mean single muscle fibers. Instead, it means a mass of hundreds to thousands of smooth muscle fibers that contract together as a single unit. The fibers usually are arranged in sheets or bundles, and their cell membranes are adherent to one another at multiple points so that force generated in one muscle fiber can be transmitted to the next. In addition, the cell membranes are joined by many gap junctions through which ions can flow freely from one muscle cell to the next so that action potentials, or ion flow without action potentials, can travel from one fiber to the next and cause the muscle fibers to contract together. This type of smooth muscle is also known as syncytial smooth muscle because of its syncytial interconnections among fibers. It is also called visceral smooth muscle because it is found in the walls of most viscera of the body, including the gastrointestinal tract, bile ducts, ureters, uterus, and many blood vessels.

Slow Cycling of the Myosin Cross-Bridges

The rapidity of cycling of the myosin cross-bridges in smooth muscle—that is, their attachment to actin, then release from the actin, and reattachment for the next cycle—is much slower than in skeletal muscle. The frequency is as little as 1/10 to 1/300 that in skeletal muscle. Yet, the fraction of time that the cross-bridges remain attached to the actin filaments, which is a major factor that determines the force of contraction, is believed to be greatly increased in smooth muscle. A possible reason for the slow cycling is that the cross-bridge heads have far less ATPase activity than in skeletal muscle; thus, degradation of the ATP that energizes the movements of the cross-bridge heads is greatly reduced, with corresponding slowing of the rate of cycling.

Low Energy Requirement to Sustain Smooth Muscle Contraction

Only 1/10 to 1/300 as much energy is required to sustain the same tension of contraction in smooth muscle as in skeletal muscle. This, too, is believed to result from the slow attachment and detachment cycling of the cross-bridges, and because only one molecule of ATP is required for each cycle, regardless of its duration.

This low energy utilization by smooth muscle is important to the overall energy economy of the body because organs such as the intestines, urinary bladder, gallbladder, and other viscera often maintain tonic muscle contraction almost indefinitely.

Slowness of Onset of Contraction and Relaxation of the Total Smooth Muscle Tissue

A typical smooth muscle tissue begins to contract 50 to 100 milliseconds after it is excited, reaches full contraction about 0.5 second later, and then declines in contractile force in another 1 to 2 seconds, giving a total contraction time of 1 to 3 seconds. This is about 30 times as long as a single contraction of an average skeletal muscle fiber. However, because there are so many types of smooth muscle, contraction of some types can be as short as 0.2 second or as long as 30 seconds.

The slow onset of contraction of smooth muscle, as well as its prolonged contraction, is caused by the slowness of attachment and detachment of the cross-bridges with the actin filaments. In addition, the initiation of contraction in response to calcium ions is much slower than in skeletal muscle, as will be discussed later.

Maximum Force of Contraction Is Often Greater in Smooth Muscle Than in Skeletal Muscle

Despite the relatively few myosin filaments in smooth muscle, and despite the slow cycling time of the cross-bridges, the maximum force of contraction of smooth muscle is often greater than that of skeletal muscle, as much as 4 to 6 kg/cm ² cross-sectional area for smooth muscle in comparison with 3 to 4 kilograms for skeletal muscle. This great force of smooth muscle contraction results from the prolonged period of attachment of the myosin cross-bridges to the actin filaments.

Latch Mechanism Facilitates Prolonged Holding of Contractions of Smooth Muscle

Once smooth muscle has developed full contraction, the amount of continuing excitation can usually be reduced to far less than the initial level, even though the muscle maintains its full force of contraction. Furthermore, the energy consumed to maintain contraction is often minuscule, sometimes as little as 1/300 of the energy required for comparable sustained skeletal muscle contraction. This mechanism is called the latch mechanism .

The importance of the latch mechanism is that it can maintain prolonged tonic contraction in smooth muscle for hours, with little use of energy. Little continued excitatory signal is required from nerve fibers or hormonal sources.