1. In electroencephalography, the presence of neurogenic electrical energy. 2. In physical chemistry, an ideal concentration for which the law of mass action will apply perfectly; the ratio of the a. to the true concentration is the a. coefficient (γ), which becomes 1.00 at infinite dilution. 3. For enzymes, the amount of substrate consumed (or product formed) in a given time under given conditions; turnover number. 4. The number of nuclear transformations (disintegrations) in a given quantity of a material per unit time. Units: curie (Ci), millicurie (mCi), becquerel (Bq), megabecquerel (MBq). SEE ALSO: radioactivity.
- blocking a. repression or elimination of electrical a. in the brain by the arrival of a sensory stimulus.
- insulinlike a. (ILA) a measure of substances, usually in plasma, that exert biologic effects similar to those of insulin in various bioassays; sometimes used as a measure of plasma insulin concentrations; always gives higher values than immunochemical techniques for the measurement of insulin.
- intrinsic sympathomimetic a. (ISA) the property of a drug that causes activation of adrenergic receptors so as to produce effects similar to stimulation of the sympathetic nervous system.
- nonsuppressible insulinlike a. (NSILA) plasma insulinlike a. not suppressed by antibodies to insulin and mostly present after pancreatectomy. Nonsuppressible insulinlike a. is mostly the action of polypeptide insulinlike growth factors IGF-I and IGF-II.
- optic a. the ability of a compound in solution (one possessing no plane of symmetry, usually because of the presence of one or more asymmetric carbon atoms) to rotate the plane of polarized light.
- plasma renin a. (PRA) estimation of renin in plasma by measuring the rate of formation of angiotensin I or II.
- pulseless electrical a. (PEA) SYN: electromechanical dissociation.
- specific a. 1. radioactivity per unit mass of the stated element or compound; 2. for an enzyme, the amount of substrate consumed (or product formed) in a given time under given conditions per milligram of protein; 3. a. per unit mass of the stated radionuclide.
- triggered a. one or a series of spontaneously generated heartbeats originating from an action potential that produces an after-depolarization which reaches activation threshold.

* * *

ac·tiv·i·ty ak-'tiv-ət-ē n, pl -ties
1) natural or normal function: as
a) a process (as digestion) that an organism carries on or participates in by virtue of being alive
b) a similar process actually or potentially involving mental function specif an educational procedure designed to stimulate learning by firsthand experience
2) the characteristic of acting chemically or of promoting a chemical reaction <the \activity of a catalyst>

* * *

ac·tiv·i·ty (ak-tivґĭ-te) [L. agere to do, drive] 1. the state of being active; the ability to produce some effect; the extent of some function or action. 2. a thermodynamic quantity that represents the effective concentration of a solute in a nonideal solution; if concentrations are replaced by activities, the equations for equilibrium constants, electrode potentials, osmotic pressure, boiling point elevation, freezing point depression, and vapor pressures of volatile solutes are converted from approximations that hold only for dilute solutions to exact equations that hold for all concentrations. The activity is equal to the product of the concentration and the activity coefficient, a dimensionless number measuring deviation from nonideality. Symbol a>. 3. for radioactivity, the number of disintegrations per unit time of a radioactive material, measured in curies or becquerels. Symbol A>. 4. in electroencephalography and electromyography, the presence of recordable electrical energy in a muscle or nerve. Called also electrical a. 5. optical a.

Medical dictionary. 2011.


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