Muscle Pump: From 1 Different Sources
The contraction and relaxation of the limb muscles that helps pump the low pressure venous blood from the extremities back to the central collecting system.
Muscular tissue is divided, according to its function, into three main groups: voluntary muscle, involuntary muscle, and skeletal muscle – of which the ?rst is under control of the will, whilst the latter two discharge their functions independently. The term ‘striped muscle’ is often given to voluntary muscle, because under the microscope all the voluntary muscles show a striped appearance, whilst involuntary muscle is, in the main, unstriped or plain. Heart muscle is partially striped, while certain muscles of the throat, and two small muscles inside the ear, not controllable by willpower, are also striped.
Structure of muscle Skeletal or voluntary muscle forms the bulk of the body’s musculature and contains more than 600 such muscles. They are classi?ed according to their methods of action. A ?exor muscle closes a joint, an extensor opens it; an abductor moves a body part outwards, an adductor moves it in; a depressor lowers a body part and an elevator raises it; while a constrictor (sphincter) muscle surrounds an ori?ce, closing and opening it. Each muscle is enclosed in a sheath of ?brous tissue, known as fascia or epimysium, and, from this, partitions of ?brous tissue, known as perimysium, run into the substance of the muscle, dividing it up into small bundles. Each of these bundles consists in turn of a collection of ?bres, which form the units of the muscle. Each ?bre is about 50 micrometres in thickness and ranges in length from a few millimetres to 300 millimetres. If the ?bre is cut across and examined under a high-powered microscope, it is seen to be further divided into ?brils. Each ?bre is enclosed in an elastic sheath of its own, which allows it to lengthen and shorten, and is known as the sarcolemma. Within the sarcolemma lie numerous nuclei belonging to the muscle ?bre, which was originally developed from a simple cell. To the sarcolemma, at either end, is attached a minute bundle of connective-tissue ?bres which unites the muscle ?bre to its neighbours, or to one of the connective-tissue partitions in the muscle, and by means of these connections the ?bre affects muscle contraction. Between the muscle ?bres, and enveloped in a sheath of connective tissue, lie here and there special structures known as muscle-spindles. Each of these contains thin muscle ?bres, numerous nuclei, and the endings of sensory nerves. (See TOUCH.) The heart muscle comprises short ?bres which communicate with their neighbours via short branches and have no sarcolemma.
Plain or unstriped muscle is found in the following positions: the inner and middle coats of the STOMACH and INTESTINE; the ureters (see URETER) and URINARY BLADDER; the TRACHEA and bronchial tubes; the ducts of glands; the GALL-BLADDER; the UTERUS and FALLOPIAN TUBES; the middle coat of the blood and lymph vessels; the iris and ciliary muscle of the EYE; the dartos muscle of the SCROTUM; and in association with the various glands and hairs in the SKIN. The ?bres are very much smaller than those of striped muscle, although they vary greatly in size. Each has one or more oval nuclei and a delicate sheath of sarcolemma enveloping it. The ?bres are grouped in bundles, much as are the striped ?bres, but they adhere to one another by cement material, not by the tendon bundles found in voluntary muscle.
Development of muscle All the muscles of the developing individual arise from the central layer (mesoderm) of the EMBRYO, each ?bre taking origin from a single cell. Later on in life, muscles have the power both of increasing in size – as the result of use, for example, in athletes – and also of healing, after parts of them have been destroyed by injury. An example of the great extent to which unstriped muscle can develop to meet the demands made on it is the uterus, whose muscular wall develops so much during pregnancy that the organ increases from the weight of 30–40 g (1–1••• oz.) to a weight of around 1 kg (2 lb.), decreasing again to its former small size in the course of a month after childbirth.
Physiology of contraction A muscle is an elaborate chemico-physical system for producing heat and mechanical work. The total energy liberated by a contracting muscle can be exactly measured. From 25–30 per cent of the total energy expended is used in mechanical work. The heat of contracting muscle makes an important contribution to the maintenance of the heat of the body. (See also MYOGLOBIN.)
The energy of muscular contraction is derived from a complicated series of chemical reactions. Complex substances are broken down and built up again, supplying each other with energy for this purpose. The ?rst reaction is the breakdown of adenyl-pyrophosphate into phosphoric acid and adenylic acid (derived from nucleic acid); this supplies the immediate energy for contraction. Next phosphocreatine breaks down into creatine and phosphoric acid, giving energy for the resynthesis of adenyl-pyrophosphate. Creatine is a normal nitrogenous constituent of muscle. Then glycogen through the intermediary stage of sugar bound to phosphate breaks down into lactic acid to supply energy for the resynthesis of phosphocreatine. Finally part of the lactic acid is oxidised to supply energy for building up the rest of the lactic acid into glycogen again. If there is not enough oxygen, lactic acid accumulates and fatigue results.
All of the chemical changes are mediated by the action of several enzymes (see ENZYME).
Involuntary muscle has several peculiarities of contraction. In the heart, rhythmicality is an important feature – one beat appearing to be, in a sense, the cause of the next beat. Tonus is a character of all muscle, but particularly of unstriped muscle in some localities, as in the walls of arteries.
Fatigue occurs when a muscle is made to act for some time and is due to the accumulation of waste products, especially sarcolactic acid (see LACTIC ACID). These substances affect the end-plates of the nerve controlling the muscle, and so prevent destructive overaction of the muscle. As they are rapidly swept away by the blood, the muscle, after a rest (and particularly if the rest is accompanied by massage or by gentle contractions to quicken the circulation) recovers rapidly from the fatigue. Muscular activity over the whole body causes prolonged fatigue which is remedied by rest to allow for metabolic balance to be re-established.... muscle
The muscle, unique to the heart, which comprises the walls of the atria and ventricles. It consists of long broadening cells (?bres) with special physiological characteristics which enable them to keep contracting and expanding inde?nitely.... cardiac muscle
Muscle that does not operate under a person’s conscious control. Involuntary muscle – also called smooth muscle, because the cells do not contain the striations that occur in VOLUNTARY MUSCLE – is found in blood vessels, the heart, stomach, and intestines. (See PARASYMPATHETIC NERVOUS SYSTEM.)... involuntary muscle
Muscle under a person’s voluntary control (see MUSCLE; VOLUNTARY MUSCLE).... skeletal muscle
Muscle under the ‘involuntary’ control of the autonomic nervous system (see MUSCLE; NERVOUS SYSTEM).... smooth muscle
Also known as skeletal muscle, this forms the muscles which are under a person’s conscious control. Muscles that control walking, talking and swallowing are examples of those under such control (see INVOLUNTARY MUSCLE; MUSCLE; NERVOUS SYSTEM).... voluntary muscle
A key enzyme system in the parietal cells of the mucosal lining of the stomach: hydrogen ions are produced which acidify the stomach’s secretions and convert pepsinogen to PEPSIN, an active participant in the digestion of food.... proton pump
These are drugs that inhibit the production of acid in the stomach by blocking a key enzyme system, known as the PROTON PUMP, of the parietal cells of the stomach. The drugs include omeprazole, lansoprazole and pantoprazole, and they are the treatment of choice for oesophagitis (erosion and stricture – see under OESOPHAGUS, DISEASES OF); for the short-term treatment for gastric ulcer (see under STOMACH, DISEASES OF) and DUODENAL ULCER; and, in combination with ANTIBIOTICS, for the eradication of Helicobacter pylori.... proton-pump inhibitors
See HEART, ARTIFICIAL.... cardiac pump
These drugs produce partial or complete paralysis of skeletal muscle (see under MUSCLE – Structure of muscle). Drugs in clinical use are all reversible and are used to help insert a breathing tube into the TRACHEA (endotracheal tube) during general ANAESTHESIA and ARTIFICIAL VENTILATION OF THE LUNGS. They may be broadly divided into depolarising and nondepolarising muscle relaxants. Depolarising muscle relaxants act by binding to acetylcholine receptors at the motor end-plate where nerves are attached to muscle cells, and producing a more prolonged depolarisation than acetylcholine, which results in initial muscle fasciculation (overactivity) and then ?accid paralysis of the muscle. The only commonly used depolarising drug is succinylcholine which has a rapid onset of action and lasts approximately three minutes. Non-depolarising muscle relaxants bind to the acetylcholine receptors, preventing acetylcholine from gaining access to them. They have a slower onset time and longer duration than depolarisers, although this varies widely between di?erent drugs. They are competitive antagonists and they may be reversed by increasing the concentration of acetylcholine at the motor end-plate using an anticholinesterase agent such as neostigmine. These drugs are broken down in the liver and excreted through the kidney, and their action will be prolonged in liver and renal failure. Other uses include the relief of skeletal muscle spasms in TETANUS, PARKINSONISM and spastic disorders. The drugs dantrolene and diazepam are used in these circumstances.... muscle relaxants
A device used to draw milk from the breasts in order to relieve overfull breasts during lactation, to express milk for future use, or to feed a baby who is unable to suckle.... breast pump
Sudden and involuntary contraction of a muscle. Muscle spasm is a normal reaction to pain and inflammation around a joint. Common causes are muscle strain, disc prolapse, and stress. Usually, the cause of the spasm is treated. Muscle-relaxant drugs may also be needed. (See also spasticity.)... muscle spasm
A machine that is used for the administration of a continuous, controlled amount of a drug or other fluid. The fluid is delivered through a needle that is inserted into a vein or under the skin.... pump, infusion
See lavage, gastric.... stomach pump
The natural tension in the muscle fibres. At rest, all muscle fibres are kept in a state of partial contraction by nerve impulses from the spinal cord. Abnormally high muscle tone causes an increased resistance to movement, spasticity, and rigidity. Abnormally low muscle tone causes floppiness (see hypotonia; hypotonia in infants).... tone, muscle
The muscle at the back of the upper arm. At the upper end of the triceps are 3 “heads”; 1 is attached to the outer edge of the scapula (shoulderblade), and the other 2 to either side of the humerus (upper-arm bone). The lower part of the triceps is attached to the olecranon process of the ulna (the bony prominence on the elbow). Contraction of the muscle straightens the arm. (See also biceps muscle.)... triceps muscle
Drugs that are used to delay the premature delivery of a fetus. Beta2-adrenoceptor stimulants, such as salbutamol, relax the muscle of the uterus and may postpone labour for days or weeks in at-risk pregnancies of 24–33 weeks’ gestation. Delay of premature labour for up to 48 hours allows time for corticosteroid drugs to be given to the mother to help the fetal lungs to mature.... uterine muscle relaxants
a muscle, such as any of those controlling movements of the eyeball, that has its origin some distance from the part it acts on. See also eye.... extrinsic muscle
a muscle that is contained entirely within the organ or part it acts on. For example, there are intrinsic muscles of the tongue, whose contractions change the shape of the tongue.... intrinsic muscle
an agent that reduces tension in voluntary muscles. Drugs such as *baclofen, *dantrolene, and *diazepam are used to relieve skeletal muscular spasms in various spastic conditions, parkinsonism, and tetanus. The drugs used to relax voluntary muscles during the administration of anaesthetics in surgical operations act by blocking the transmission of impulses at neuromuscular junctions. Nondepolarizing muscle relaxants, e.g. *atracurium besilate, cisatracurium, pancuronium, and rocuronium, bind to receptor sites normally occupied by acetylcholine; depolarizing muscle relaxants, e.g. *suxamethonium, mimic the action of acetylcholine but *depolarization is prolonged.... muscle relaxant
a specialized receptor, sensitive to stretch, that is embedded between and parallel to the fibres of striated muscles. These receptors are important for coordinated muscular movement. See also stretch receptor.... muscle spindle
either of two muscles that cover the outer surface of the anterior wall of the pelvis (the obturator externus and obturator internus) and are responsible for lateral rotation of the thigh and movements of the hip.... obturator muscle
*coronary artery bypass grafting (CABG) performed on a beating heart, i.e. without using *cardiopulmonary bypass (the pump refers to a *heart-lung machine).... off-pump cabg
see Kegel exercises.... pelvic-floor muscle training
a drug that reduces gastric acid secretion by blocking the *proton pump. Proton-pump inhibitors include esomeprazole, lansoprazole, *omeprazole, pantoprazole, and rabeprazole sodium; they are used for treating gastro-oesophageal reflux disease, peptic ulcer, and acid hypersecretion associated with *gastrinoma.... proton-pump inhibitor
see sternomastoid muscle.... sternocleidomastoid muscle
(sternocleidomastoid muscle) a long muscle in the neck, extending from the mastoid process to the sternum and clavicle. It serves to rotate the neck and flex the head.... sternomastoid muscle
a tissue comprising the bulk of the body’s musculature. It is also known as skeletal muscle, because it is attached to the skeleton and is responsible for the movement of bones, and voluntary muscle, because it is under voluntary control. Striated muscle is composed of parallel bundles of multinucleate fibres (each containing many myofibrils), which reveal cross-banding when viewed under the microscope. This effect is caused by the alternation of actin and myosin protein filaments within each myofibril (see illustration). According to the ‘sliding filament’ theory, when muscle contraction takes place, the two sets of filaments slide past each other, so reducing the length of each unit (sarcomere) of the myofibril. The sliding is caused by a series of cyclic reactions, requiring ATP, resulting in a change in orientation of projections on the myosin filaments; each projection is first attached to an actin filament but contracts and releases it to become reattached at a different site.... striated muscle
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