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The tissue in the body in which cellular contractility has become most apparent. Almost all forms of protoplasm exhibit some degree of contractility, but in muscle fibers specialization has led to the preeminence of this property. In vertebrates three major types of muscle are recognized: smooth, cardiac, and skeletal.

Smooth muscle

Smooth muscle, also designated visceral and sometimes involuntary, is the simplest type. These muscles consist of elongated fusiform cells which contain a central oval nucleus. The size of such fibers varies greatly, from a few micrometers up to 0.02 in. (0.5 mm) in length. These fibers contract relatively slowly and have the ability to maintain contraction for a long time. Smooth muscle forms the major contractile elements of the viscera, especially those of the respiratory and digestive tracts, and the blood vessels. Smooth muscle fibers in the skin regulate heat loss from the body. Those in the walls of various ducts and tubes in the body act to move the contents to their destinations, as in the biliary system, ureters, and reproductive tubes.

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

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

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.
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World of the Body: muscle
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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.

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