Impulse

IMPULSE (from Latin impulsus - push, stimulation), a process in the nervous system that brings innervated organs into a state of activity or inhibition. To all effector organs, impulses arrive via an efferent nerve. Under normal conditions, impulses in the efferent nerve appear exclusively under the influence of impulses arising in nerve centers or ganglion cells. Impulses in nerve centers are in turn caused either by reflex irritation (i.e., an impulse from an afferent nerve) or by automatic irritation, i.e., the direct action of metabolic products on centers. For an impulse to appear, the stimulus causing it must reach a certain threshold value, regardless of whether the stimulus acts on the peripheral endings of sensory nerves or directly on nerve centers. If one does not stand on the basis of the 'all or nothing' law, it should be recognized that subthreshold irritations can also change the excitability of tissue, i.e., change the conditions for the appearance of the next impulse. For the analysis of the concept of impulse, the representation of a single wave of excitation is extremely important. Such a wave is obtained under artificial conditions of nerve irritation by instantaneous single irritation (for example, by an inductive shock). A characteristic manifestation of such a single wave of excitation is the electrical reaction of the nerve and the innervated apparatus. Each single wave of excitation leaves behind changes in the state of tissue (refractory and excitatory phases). The study of the electrical reaction of tissue (so-called action currents) is the basis for the study of the nerve impulse. This research has shown that impulses coming from the nerve center are never, under natural conditions, a single wave of excitation, but consist of a series of successive waves of excitation. Thus, in so-called voluntary muscle contractions, 30-70 separate waves of excitation are recorded per second in the motor nerve and in the muscle. According to Einthoven, inhibitory impulses going to the heart via the n. vagus as a result of adequate reflex irritations also consist of a series of single waves of excitation. According to Adrian, the same is observed in afferent nerves with non-threshold and non-instantaneous irritation of sensory endings. Thus, it should be recognized that under physiological conditions, each impulse going to an effector organ consists of a series of successive waves of excitation; the exception is apparently tendon reflexes - Eigenreflexe of Hoffmann. The rhythm of excitation waves varies depending on the conducting apparatus. A nerve trunk is capable of conducting up to 500 waves of excitation per second. Untired nerve centers, according to Adrian and Cooper (1924), up to 250; a muscle is capable of perceiving up to 200 irritations per second. With a higher rhythm of irritations, it is transformed into a lower one (Vvedensky) and vice versa, a single strong irritation is transformed into a series of waves of excitation (Adrian). Apparently, the normal rhythm of nerve impulses does not exceed 150 excitations per second. From the above, it is obvious that for understanding the nerve impulse, the changes in state left behind by each wave of excitation are extremely important. If each successive wave of excitation falls into the refractory stage, this wave will not be able to give its effect. Vvedensky showed that excessively frequent waves of excitation cause phenomena of inhibition. Therefore, the result of the action of a nerve impulse is determined by the strength and frequency of the waves of excitation that constitute it. Within certain limits, the frequency of excitation waves in skeletal muscles exactly follows the rhythm of the nerve impulse. In the heart and in smooth muscles, generally in the area innervated by the autonomic nervous system, the effect in the innervated apparatus does not reflect the rhythm of the centrifugal nerve impulse. Regarding the passage of impulses through centers from the afferent part of the reflex arc to the efferent, it should be noted that centers possess a considerable ability to transform rhythms, to sum individual impulses and to delay them. Here too, the result of irritation can often be reduced to the frequency and strength of the incoming waves of excitation and to the state of the centers.