[ Pobierz całość w formacie PDF ]
.Nevertheless, the reactive effect in both stages alike must, in all probability, be regarded as a phenomenon forwhich nerve and muscle are jointly responsible.This view is borne out by the fact that the duration of the twostages of inhibition is largely determined by the character of the stimuli which affect the nerve.If, e.g., weapply a stimulus to a portion of nerve that lies within the sphere of operation of the anode of a constantcurrent, the duration of the inhibitory stages is considerably lengthened.We are now in a position to discuss the relation of the excitatory and inhibitory effects within the nerve fibre,in abstraction from the properties accruing to the reacting muscle.We may conceive of them as follows.Onthe occurrence of stimulation, excitatory and inhibitory effects are produced simultaneously.At first, theCHAPTER III.Physiological Mechanics of Nerve-Substance 45 Principles of Physiological Psychologylatter are very much the stronger.As time goes on, however, they increase more slowly, while the excitatoryeffects advance more quickly.Oftentimes, these last appear to maintain their ascendancy until the conclusionof the whole process.If however, the functional capacity of the: nerve is very high, the inhibitory effects mayagain acquire the upper hand for a brief period immediately following the conclusion of the twitch.This factindicates, at the same time, that the process is not entirely continuous, but that the rapid result produced incontraction by the excitatory effects is always followed by an inhibitory reaction.The release of excitationthus resembles a sudden discharge, in which the available forces are quickly consumed, so that for a shorttime the opposite effects are in the preponderance.Fig.30 is an attempt to show this sequence of events ingraphic form.Stimulation occurs at rr'.The curve ab represents the course of the excitatory, the curve cd ofthe inhibitory effects, measured by the magnitude of the downward directed (negative) ordinates of the curvecd.We assume that excitatory and inhibitory impulses are already present in the nerve, before the applicationof stimulus, but that they are in equilibrium.These pre-existent impulses we make proportional to theordinates xa and xc.The curve of inhibition is characterised by a rapid rise at its commencement, the curve ofexcitation by the gradual fall at its conclusion.What we term the functional capacity of a nerve is a functionat once of excitation and of inhibition.The more functionally capable the nerve, the greater is the efficacy ofboth the inhibitory and the excitatory forces contained within it.In the exhausted nerve, both alike arediminished, but the inhibitory in higher degree.Here, therefore, the irritability is enhanced, and the transitoryinhibitions at the conclusion of the twitch, which may perhaps be referred (as indicated by the dotted curvem) to an oscillatory repetition of the inhibitory process, are no longer observable.(c) After-effects of Stimulation: Practice and FatigueOur study of the changes of excitability which take place in a nerve during the process of stimulation hasshown two things: that the effect produced in the nerve disappears not abruptly but quite gradually, and that italways persists for a noticeable time after the conclusion of the twitch.We have now to consider anotherphenomenon, which evidently proceeds from the same causes.If several stimuli are successively applied atsuch intervals that each falls within the period of decline of the stimulation set up by its predecessor, theirritability of the nerve is increased.Indeed, under favourable conditions this increase may be so considerablethat a weak stimulus, which at first could not provoke any contraction at all finally releases a maximalcontraction.At the same time, the contractions become longer; while the longer after-effect shows furtherthat the course of the excitation has increased not only in intensity but also in duration [ Pobierz całość w formacie PDF ]