Dictionary: muscle   (mŭs'əl)

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1. A tissue composed of fibers capable of contracting to effect bodily movement.
2. A contractile organ consisting of a special bundle of muscle tissue, which moves a particular bone, part, or substance of the body: the heart muscle; the muscles of the arm.
3. Muscular strength: enough muscle to be a high jumper.
4. Informal. Power or authority: put some muscle into law enforcement.

To make one's way by or as if by force: muscled into the conversation.

To move or force with strength: muscled legislation through Congress.

[Middle English, from Old French, from Latin mūsculus, diminutive of mūs, mouse.]

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.

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.

— Neil Spurway

— Sheila Jennett

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 view

Figure 43 Muscles: side view

Figure 44 Muscles: back view

noun

1. The state or quality of being physically strong: brawn, might, potence, potency, power, powerfulness, puissance, sinew, strength, thew (often used in plural). See strong/weak.
2. Effective means of influencing, compelling, or punishing: force, power, weight. Informal clout. See over/under, strong/weak.

verb

To force one's way into a place or situation: push, shove. See enter/exit, push/pull.

Idioms beginning with muscle:
muscle in

In addition to the idiom beginning with muscle, also see flex one's muscles; move a muscle.

Definition: power, influence
Antonyms: impotence, powerlessness, weakness

An 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.

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Major muscles of the human body. (1) frontalis, (2) occipitalis, (3) temporalis, (4) orbicularis of … (credit: © Merriam-Webster Inc.)

Contractile tissue that produces motion for functions, including body movements, digestion, focusing, circulation, and body warmth. It can be classified as striated, cardiac, and smooth or as phasic and tonic (responding quickly or gradually to stimulation, respectively). Striated muscle, whose fibres appear striped under a microscope, is responsible for voluntary movement. Most of these muscles are phasic. They are attached to the skeleton and move the body by contracting in response to signals from the central nervous system; contraction is achieved by the sliding of thin filaments (of actin) between thick ones (of myosin); stretch receptors in the tissue provide feedback, allowing smooth motion and fine motor control. The branched fibres of cardiac muscle give it a netlike structure; contraction originates in the heart's muscle tissue itself with a signal from the natural pacemaker; vagus and sympathetic nerves control heart rate. Smooth muscle, the muscle of internal organs and blood vessels, is generally involuntary and tonic; its cells can operate either collectively or individually (in response to separate nerve endings) and have different shapes. Disorders of voluntary muscle cause weakening, atrophy, pain, and twitching. Some systemic diseases (e.g., dermatomyositis, polymyositis) can cause muscle inflammation. See also abdominal muscle; muscle tumour; muscular dystrophy; myasthenia gravis.

For more information on muscle, visit Britannica.com.

1. Fleshy contractile tissue that moves parts of the body relative to each other. The three main types of muscle are cardiac muscle, smooth muscle, and striated muscle.

2. A body structure composed of numerous striated muscle cells wrapped in connective tissue, and supplied with nerve fibres and blood vessels. There are approximately 600 muscles in the human body.

Muscles: front view
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Muscles: back view
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Muscles: side view
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An organ composed of bundles of fibers that has the power to contract and hence to produce movement. Muscles are responsible for locomotion and help support the body, generate heat and perform a number of other functions. They are of two varieties: striated (or striped, voluntary or skeletal), which makes up most of the meat of a carcass, and smooth (unstriated), which includes all the involuntary muscle of the viscera, heart and blood vessels.
Skeletal muscle fibers range in length from a few millimeters to many centimeters. They also vary in color from white to deep red. Each muscle fiber receives its own nerve impulses, which trigger fine and varied motions. At the signal of an impulse traveling down the nerve, the muscle fiber changes chemical energy into mechanical energy, and the result is muscle contraction. At least two major types of muscle fiber have been identified by histochemical techniques: type I (red) fibers, which have a slow contraction; and type II (white) fibers, which have a fast contraction.
Some muscles are attached to bones by tendons. Others are attached to other muscles, and to skin, producing, for example, the skin twitch, the eye blink and hair erection. Parts of the walls of hollow internal organs, such as the heart, stomach and intestines and also blood vessels, are composed of muscles. See also muscular. For a complete list of named muscles

• agonistic m. — prime mover; a muscle opposed in action by another muscle, called the antagonist.
• antagonistic m. — one that counteracts the action of another muscle (the agonist).
• appendicular m. — one of the muscles of a limb.
• arrector pili m. — small, smooth muscle attached to the bulb of the hair which causes erection of the hair and compression of the attending sebaceous gland when it contracts.
• arterial m. — part of the tunica media; smooth muscle fibers arranged in a circular pattern around the lumen.
• articular m. — one that has one end attached to the capsule of a joint.
• axial m. — 1. muscles derived from the somites in the embryo.
— 2. the muscles around the vertebral column.
• m. biopsy — sample of living muscle obtained by excision or punch.
• cardiac m. — striated involuntary muscle with branched fibers and containing modified fibers which act as cardiac conducting cells.
• congenital m. defects — may be environmental, e.g. nutritional muscular dystrophy, or inherited, e.g. splayleg of piglets.
• congenital type II m. fiber hypertrophy — occurs in the hip joint musculature in German shepherd dogs but there is no detectable abnormality of gait.
• cutaneous m. — striated muscle that inserts into the skin.
• double m. — see myofiber hyperplasia.
• esophageal m. — the tunica muscularis of the esophagus in most domestic animals is mostly striated; in pigs, horses and cats there are small segments of smooth muscle; in birds the entire tunic is smooth muscle.
• extraocular m's — the six or seven voluntary muscles that move the eyeball: dorsal, ventral, medial and lateral recti, dorsal and ventral oblique, and retractor bulbi muscles.
• extrinsic m. — one that originates in another part than that of its insertion, e.g. those originating outside the eye, which move the eyeball.
• fast-twitch skeletal m. — two of the three types of skeletal muscle are pale in color and fast-twitch—type IIa (fast-twitch oxidative–glycolytic), type IIb (fast-twitch glycolytic). Type IIa fibers are fatigue-resistant, type IIb fatigue easily.
• m. fiber — see muscle (above).
• fixation m's, fixator m's — accessory muscles that serve to steady a part.
• hamstring m's — the biceps, semimembranosus and semitendinosus muscles. See also hamstring.
• intraocular m's — the intrinsic muscles of the eyeball.
• intrinsic m. — one whose origin and insertion are both in the same part or organ, such as those entirely within the eye.
• involuntary m. — see smooth muscle (below).
• iridial m. — layers of circular (sphincter) and radial (dilator) muscles. See also iris.
• jaw m. — muscles of mastication.
• laryngeal m. — muscles of the larynx.
• limb m. — 3, 13.4 muscles of the fore- and hindlimbs.
• masseter m. — the principal muscle of mastication.
• mylohyoid m. — muscles of the hyoid apparatus.
• m. neoplasms — of striated muscle—rhabdomyoma, rhabdomyosarcoma; of plain muscle—leiomyoma, leiomyosarcoma.
• m. nonstriated — see smooth muscle (below).
• orbicular m. — one that encircles a body opening, e.g. the eye or mouth.
• m.-paralyzing drugs — drugs which produce neuromuscular blockade, used as muscle relaxants during surgical procedures. Include d-tubocurarine, alcuronium chloride, pancuronium, vecuronium, atracurium besylate, succinylcholine.
• red m. — type 1 fibers predominate with slow contraction cycles and aerobic metabolism.
• m. rupture — the muscle may have torn away from its insertion, in which case the tendon will be slack, or it may be a complete or partial separation of the belly of the muscle, when the muscle will be swollen and hard. Structural and conformational changes may result, e.g. in rupture of the gastrocnemius muscle, and the hernias caused by rupture of the ventral abdominal muscles or the diaphragm.
• skeletal m's — striated muscles that are attached to bones and typically cross at least one joint. Called also voluntary or striated muscles.
• slow-twitch skeletal m. — type 1 skeletal muscle fibers are bright red and contain large amounts of myoglobin; not easily fatigued.
• smooth m. — plain or involuntary muscle which powers the internal organs and is controlled by the autonomic nervous system; slow contracting cycles and fatigue resistant. Two types listed, visceral and vascular.
• sphincter m. — a ringlike muscle that closes a natural orifice; called also sphincter.
• m. spindle — sensory end-organ attached to the perimysial connective tissue of the muscle.
• m. strain — soreness and stiffness in a muscle due to overexertion or contusion, especially in muscles that have not been conditioned for hard use; some of the muscle fibers may actually tear.
• striated m. — see skeletal muscles (above).
• synergic m's — those that assist one another in action.
• temporal m. — a significant muscle of mastication.
• m.–tendon junction — the union between connective tissue investing muscles and anchoring connective tissue.
• type I m. fiber — see slow-twitch skeletal muscle (above).
• type II m. fiber — see fast-twitch skeletal muscle (above).
• type II m. fiber deficiency — a relative deficiency of type II muscle fibers, with a predominance of type I fibers. An inherited defect in Labrador retrievers. Clinical signs include stunted growth, and muscle weakness and abnormal gait, which subside with rest, from an early age.
• voluntary m. — see skeletal muscle (above).
• white m. — consist of type II fibers; fast contraction fibers and aerobic metabolism are characteristic.
• yoked m's — those that normally act simultaneously and equally, as in moving the eyes.