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Very rapid fluctuations in intensity and modality of emotions; seen in the affective reaction or in certain organic brain disorders.



in physiology, functional mobility; the speed of flow of elementary cycles of excitation in nerve and muscle tissues.

The concept of lability was introduced by the Russian physiologist N. E. Vvedenskii (1886), who considered its measure to be the greatest frequency of tissue irritation possible without a change in rhythm. A tissue’s lability reflects the time necessary to restore the tissue’s work capacity after an excitation cycle. The greatest lability is found in the processes of the nerve cells, the axons, which are capable of reproducing as many as 500 or 1,000 impulses per sec. The central and peripheral sites of contact, or synapses, are less labile (for example, a motor nerve ending can transmit no more than 100–150 excitations per sec to a skeletal muscle).

Inhibition of the vital activities of tissues and cells (for example, by means of cold or narcotics) decreases lability, since the restorative processes are retarded thereby and the refractory period prolonged. The lability value is variable. Thus, the refractory period is shortened in the heart under the influence of frequent stimuli: its lability is increased. This phenomenon is the basis for what is called the assimilation of rhythm. The concept of lability is important to an understanding of the mechanisms of nervous activity, and of the operation of the neural centers and analyzers under both normal and pathological conditions.

In biology and medicine, the term “lability” refers to instability or variability, such as of the psyche, the physiological state, the pulse, or the body temperature.


Vvedenskii, N. E. Poln. sobr. soch, vols. 1–2. Leningrad, 1951–52.
Ukhtomskii, A. A. Sobr. sock, vol. 2. Leningrad, 1951.
Golikov, N. V. Fiziologicheskaia labil’nost’ i ee izmeneniia priosnovnykh nervnykh protsessakh. Leningrad, 1950.


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