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mechanism (mek´a-nizm)

1. An arrangement or grouping of the parts of anything that has a definite action. 2. The means by which an effect is obtained. [G. mechane, a contrivance]
association m. the cerebral m. whereby the memory of past sensations may be compared or associated with present ones.
counter-current m. a system in the renal medulla that facilitates concentration of the urine as it passes through the renal tubules. See countercurrent exchanger, countercurrent multiplier.
defense m. 1. a psychological means of coping with conflict or anxiety, e.g., conversion, denial, dissociation, rationalization, repression, sublimation; 2. the psychic structure underlying a coping strategy; 3. immunological m. vs. non-specific defense m.
double displacement m. ping-pong m
Douglas m. m. of spontaneous evolution in transverse lie; extreme lateral flexion of the vertebral column with birth of the lateral aspect of thorax before the buttocks.
Duncan's m. passage of the placenta from the uterus with the rough side foremost.
gating m. 1. occurrence of the maximum refractory period among cardiac conducting cells approximately 2 mm proximal to the terminal Purkinje fibers in the ventricular muscle, beyond which the refractory period is shortened through a sequence of Purkinje cells, transitional cells, and muscular cells; gating m. may be a cause of ventricular aberration, bidirectional tachycardia, and concealed extrasystoles; 2. a m. by which painful impulses may be blocked from entering the spinal cord. Cf. gate-control theory.
immunological m. the groups of cells (chiefly lymphocytes and cells of the reticuloendothelial system) that function in establishing active acquired immunity (induced sensitivity, allergy).
ordered m. a scheme for substrate binding and product release for multisubstrate enzymes; for a two-substrate two-product enzyme with an ordered m., one particular substrate has to bind to the enzyme first followed by the other substrate; chemistry then occurs, products are formed and are released from the enzyme in a distinct order. More complex ordered schemes exist for enzymes having more than two substrates. Some of the dehydrogenases have such a m.ordered;
ordered on-random off m. a scheme for substrate binding and product release for multisubstrate enzymes; for a two-substrate two-product enzyme with this m., the individuals have to bind to the enzyme in a distinct order; however, once the products are formed they may dissociate from the enzyme in either order. It has been suggested that pyruvate kinase has such a mechanism. The random on-ordered off m. is simply the reverse of this m.
ping-pong m. a special multisubstrate reaction in which, for a two-substrate, two-product (i.e., bi-bi) system, an enzyme reacts with one substrate to form a product and a modified enzyme, the latter then reacting with a second substrate to form a second, final product, and regenerating the original enzyme. An example of such a m. is found in the aminotransferases. More complex ping-pong m.'s exist for enzymes having more than two substrates.double displacement m;
pressoreceptive m. the pressoreceptor system, especially of the carotid sinuses and aortic arch.
proprioceptive m. the m. of sense of position and movement, by which muscular movements can be adjusted to a great degree of accuracy and equilibrium maintained.
random m. a scheme for substrate binding and product release for a multisubstrate enzyme; for a two-substrate two-product enzyme with this m., either substrate can bind first and, after the reaction has taken place, either product can be the first to dissociate from the enzyme. Brain hexokinase has a random m. More complex random m.'s exist for enzymes having more than two substrates.
re-entrant m. the probable basis of most arrhythmias, requiring at least three criteria in the heart: 1. a loop circuit, 2. unidirectional block, 3. slowed conduction. Impulses enter the loop circuit and divide in both directions (blocked in one direction only), negotiate the loop circuit to the area of block where the slowed conduction has allowed the impulse to arrive at a time when the tissue proximal to the unidirectional block has recovered and will permit its passage in the opposite direction.
Schultze's m. expulsion of the placenta with the fetal surface foremost.


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