activation

1. The act of rendering active. 2. An increase in the energy content of an atom or molecule, through the raising of temperature, absorption of light photons, etc., which renders that atom or molecule more reactive. 3. Techniques of stimulating the brain by light, sound, electricity, or chemical agents, in order to elicit abnormal activity in the electroencephalogram. 4. Stimulation of peripheral nerve fibers to the point that action potentials are initiated. 5. Stimulation of cell division in an ovum by fertilization or by artificial means. 6. The act of making radioactive. SEE ALSO: cross-section.

- amino acid a. the formation of the amino acyl adenylate derivative ( E.G., during protein biosynthesis).

- EEG a. the low voltage, fast pattern of attentive wakefulness.

- feedback a. inhibitory or antiinhibitory a. on an enzyme by an end product of a biochemical pathway in which that enzyme plays a part. For example, the a. of factors VIII and V by thrombin during blood clotting.

- feed-forward a. the a. or stimulation of an enzyme by a precursor of the substrate of that enzyme.

- gene a. the process of a. of a gene so that it is expressed at a particular time. This process is crucial in growth and development.

* * *

ac·ti·va·tion (ak″tĭ-vaґshən) 1. the act or process of rendering active. 2. the transformation of a proenzyme into an active enzyme by the action of a kinase or another proenzyme. 3. the process by which the central nervous system is stimulated into activity through the mediation of the reticular activating system. 4. the deliberate induction of a pattern of electrical activity in the brain in electroencephalography.