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in physiology, the suppression of activity in an organ or tissue in response to the excessively high frequency or intensity of an applied stimulus. The pessimum was first described in 1886 by N. E. Vvedenskii. While investigating impulse conduction in a neuromuscular specimen from a frog, he discovered that the intensification of tetanus, or continuous muscle contraction, that is produced by a gradual increase in the frequency or strength of a muscular stimulus is suddenly replaced by complete relaxation and inhibition of the muscle’s activity as the applied stimulus is accelerated or intensified (seeOPTIMUM).

Vvedenskii viewed the pessimum phenomenon in terms of his theory of parabiosis. According to this theory, the efficiency of nerve endings that transmit impulses to muscles drops sharply after an excitatory wave passes. Restoration of efficiency requires some time; for example, in a neuromuscular specimen that uses the gastrocnemius muscle of a frog, the refractory period is 0.02-0.03 sec. The time determines the functional capacity of nerve endings, that is, their lability. If the interval between stimuli is shorter than the required refractory period, that is, if it exceeds the requirements set by the lability of the nerve endings, a special type of nontransmissible excitation, called parabiosis, arises in the nerve endings. Parabiosis blocks the conduction of nerve impulses to the muscle and thus inhibits the muscle’s activity; this mechanism is a protection against overfatigue. The pessimum phenomenon is reversible: a decrease in the intensity of stimulation restores muscle contraction. A number of organs and tissues have been shown to exhibit a pessimum. Many investigators believe that the pessimum is the underlying mechanism in the nervous system’s regulation of bodily activity.


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