Smooth muscle is usually arranged in sheets or layers, commonly oriented in different directions. The major physiological properties of these muscles are their intrinsic ability to contract spontaneously and their dual regulation by the autonomic nerves of the sympathetic and parasympathetic systems. See also Autonomic nervous system.

Cardiac muscle has many properties in common with smooth muscle; for example, it is innervated by the autonomic system and retains the ability to contract spontaneously. Presumably, cardiac muscle evolved as a specialized type from the general smooth muscle of the circulatory vessels. Its rhythmic contraction begins early in embryonic development and continues until death. Variations in the rate of contraction are induced by autonomic regulation and by many other local and systemic factors.

The cardiac fiber, like smooth muscle, has a central nucleus, but the cell is elongated and not symmetrical. It is a syncytium, a multinuclear cell or a multicellular structure without cell walls. Histologically, cardiac muscle has cross-striations very similar to those of skeletal muscle, and dense transverse bands, the intercalated disks, which occur at short intervals. See also Heart (vertebrate).

Skeletal muscle is also called striated, somatic, and voluntary muscle, depending on whether the description is based on the appearance, the location, or the innervation. The individual cells or fibers are distinct from one another and vary greatly in size from over 6 in. (15 cm) in length to less than 0.04 in. (1 mm). These fibers do not ordinarily branch, and they are surrounded by a complex membrane, the sarcolemma. Within each fiber are many nuclei; thus it is actually a syncytium formed by the fusion of many precursor cells.

The transverse striations of skeletal muscle form a characteristic pattern of light and dark bands within which are narrower bands. These bands are dependent upon the arrangement of the two sets of sliding filaments and the connections between them. See also Muscle proteins; Muscular system.

Muscle is the body's contractile tissue. ‘Contraction’, in the physiological sense, may involve shortening and change of shape, or it may generate force without any change in length. All contraction depends on physicochemical alterations in the molecules of protein filaments within the cells, resulting in the generation of force at linkages (cross-bridges) between two different kinds of filament. The main proteins involved, in the respective filaments of all types of muscle, are actin and myosin; and in all muscles the process is powered by breakdown of adenosine triphosphate, during which chemical energy is converted by the interactions between these proteins into the mechanical energy of contraction. To initiate the process, muscle cells require excitation, which leads to contraction by a sequence that crucially involves an increase in the concentration of free calcium ions inside the cell — a sequence termed excitation- contraction coupling.There are three main types of muscle in the body: skeletal, cardiac, and smooth. When skeletal muscles contract they either move parts of the body via their attachments to bones, or produce tension to oppose stretch or even to allow controlled lengthening. Cardiac muscle and smooth muscle, by shortening, reduce the capacity of hollow organs and tubes: thus cardiac muscle ejects blood from the heart; smooth muscle ejects urine from the bladder or the fetus from the uterus, moves the contents of the gut along, and influences the flow of blood to different regions by varying the diameter of blood vessels.Skeletal and cardiac are together known as striated muscles, because their fibres have a striped appearance under the microscope, due to the orderly arrangement of alternating ranks of interdigitating actin and myosin filaments within their cytoplasm. Smooth (unstriated) muscle does not show this: the two types of filament are mingled throughout the cytoplasm of the cells. Whilst cardiac and skeletal muscle have a structural resemblance, skeletal muscle can be under conscious control and is therefore also known as voluntary muscle whereas cardiac muscle and smooth muscle share the designation involuntary because their actions are never under direct conscious control. (In certain contemplative regimes, the subtle influence which may be achieved — such as on the heart rate — is an indirect consequence of a profoundly disciplined emotional state.) The voluntary/involuntary distinction implies differences also in control of the three types of muscle. Skeletal muscle is controlled through pathways in the nervous system that can be consciously activated, cardiac and smooth by the involuntary or ‘autonomic’ pathways. Each skeletal muscle fibre is called into action by release of transmitter from a terminal branch of a single axon from a motor neuron in the spinal cord; the point at which this nerve terminal contacts the muscle fibre is a specialized synapse, the neuromuscular junction. All muscle fibres controlled by this nerve are recruited together, and the grouping of a motor neuron plus its family of muscle fibres is said to comprise a ‘motor unit’. When transmitter is not being released, the muscle fibres are relaxed. Individual cardiac muscle cells by contrast are activated by electrical transmission of excitation from their neighbours; this excitation originates rhythmically at a pacemaker, even in the absence of nerve action, although normally the rate of firing is modulated by the release, close to the pacemaker site, of transmitters from autonomic nerves. Smooth muscles differ again: in some, notably in the uterus at term, excitation is electrical, starting at pacemaker sites, much as in the heart. In others, such as those controlling the diameter of a large blood vessel, excitation is by neurotransmitters released from autonomic nerve endings close to the cells, but not with structured synapses. The contraction/relaxation state of smooth muscle can also be modified by chemical agents other than neurotransmitters, released from neighbouring cells or circulating in the blood. In the autonomic control of involuntary muscle, there is at many sites the possibility of either excitatory or inhibitory neural action, according to the particular transmitter released, resulting in a two-way control system analogous to accelerator and brake. The heart, for instance, is slowed by one transmitter, yet speeded up by another; the stomach wall is contracted by one and relaxed by another.

See musculo-skeletal system. See also autonomic nervous system; cardiac muscle; motor neurons; skeletal muscle; smooth muscle.

The contractile cellular unit of skeletal muscle is the cylindrical fibre, composed of many myofibrils. Chemically, muscle consists of three main proteins, actin, myosin, and tropomyosin. Contraction is achieved by formation of a complex between actin and myosin. The muscle fibre is surrounded by a thin membrane, the sarcolemma; within the muscle fibre, surrounding the myofibrils, is the sarcoplasm. Individual fibres are separated by a thin network of connective tissue, the endomysium, and bound together in bundles by thicker sheets of connective tissue, the perimysium.

There are three main types of muscle: cardiac (heart) muscle, smooth muscle, and striated (or skeletal) muscle. They all contain cells specialized for contraction. The cells convert chemical energy from food into mechanical energy, producing tension and movement.Cardiac muscle is very odd in that it contracts rhythmically, even when removed from the body. This inherent rhythm is the basis of the heartbeat, variations of which are controlled by signals from nerves and hormones (e.g. adrenaline). Cardiac muscle is unique in never suffering fatigue. However, it will stop contracting if starved of oxygen or nutrients, or if the tissue fluid is of the wrong chemical composition.Smooth muscle occurs in many parts of the body (e.g. gut and womb lining) where it produces slow, long-term contractions. Although smooth muscles usually contract involuntarily, it is possible to bring them under conscious control by training. Infants learn quite quickly (but probably never quickly enough for parents) to control the smooth muscle of the anal sphincter. Unfortunately, some people, especially elderly women, have great difficulty maintaining control of these sphincter muscles and suffer from incontinence. Special exercises (see pelvic floor exercises) can help to improve control.Skeletal muscles are attached to bones (figures 42, 43, and 44). When they contract they pull bones closer together or enable parts of the body to resist external forces. They are responsible for locomotion and movements used in physical activities. Contractions are usually under conscious control, therefore the muscle is sometimes referred to as voluntary muscle. There are approximately 600 skeletal muscles, each consisting of contractile muscle fibres wrapped in connective tissue, and supplied with blood vessels and nerve fibres. A muscle fibre is a cell with many nuclei. Under the microscope, it appears to consist of bands of light and dark fibres. Each muscle block comprises between 10 000 and 450 000 fibres. See also muscle fibre types.Figure 42 Muscles: front viewFigure 43 Muscles: side viewFigure 44 Muscles: back view

nounThe state or quality of being physically strong: brawn, might, potence, potency, power, powerfulness, puissance, sinew, strength, thew (often used in plural). See strong/weak.Effective means of influencing, compelling, or punishing: force, power, weight. Informal clout. See over/under, strong/weak.verbTo force one's way into a place or situation: push, shove. See enter/exit, push/pull. Idioms:muscleTopHome > Library > Literature & Language > Idioms Idioms beginning with muscle:muscle inIn addition to the idiom beginning with muscle, also see flex one's muscles; move a muscle. Antonyms:muscleTopHome > Library > Literature & Language > Antonyms nDefinition: power, influenceAntonyms: impotence, powerlessness, weakness Dental Dictionary:muscle(s)TopHome > Library > Health > Dental Dictionary nAn organ that, by cellular contraction, produces the movements of life. The two varieties of muscle structure are striated, which includes all the muscles in which contraction is voluntary and the heart muscle (in which contraction is involuntary), and unstriated, smooth, or organic, which includes all the involuntary muscles (except the heart), such as the muscular layer of the intestines, bladder, and blood vessels.