. Comparative physiology. Physiology, Comparative; Physiology, Comparative. NERVOUS CONDUCTION AND EXCITATION 149 shocks at an interval of 0-0036 second, the response to the single shock soon vanishes, while the response to the successive shocks persists (Fig. 37). At this stage we have therefore a summation of stimuli individually inadequate to elicit a response, and that this summation does not depend upon polarisation at the seat of stimulation is shown by several facts ; in particular, first, that it is obtained by alternating make and break shocks, in which case the effect of the second s
. Comparative physiology. Physiology, Comparative; Physiology, Comparative. NERVOUS CONDUCTION AND EXCITATION 149 shocks at an interval of 0-0036 second, the response to the single shock soon vanishes, while the response to the successive shocks persists (Fig. 37). At this stage we have therefore a summation of stimuli individually inadequate to elicit a response, and that this summation does not depend upon polarisation at the seat of stimulation is shown by several facts ; in particular, first, that it is obtained by alternating make and break shocks, in which case the effect of the second stimulus must be to prevent the persistence and extent of the polarisation due to the first; and second, that it is equally well seen when the successive stimuli are applied at different points along the course of the nerve. The interval which must elapse yig. 37.—(After Keith Lucas.) between successive stimuH if summation is to occur was found by Keith Lucas to have about the same range as the supernormal phase for con- duction in the nerve itself. Integrative Action o£ the Central Nervous System.—Con- siderations of the kind which have been advanced already open to us, in the words of Keith Lucas, " a whole range of possibilities in the regulation of nervous activity. According as we time impulses in the nervous system to follow one another at a shorter or a longer interval, we can make them less or more capable of being conducted through any regions of decrement which the system may contain. If there is a region of decrement such that normal impulse just cannot pass, then impulses of moderate frequency may pass it successfully, while impulses of high frequency may not only fail to pass it, but may by their frequency prevent other impulses finding their way ; The last sentence offers a possible interpretation of a very important phenomenon on which Sherrington lays emphasis in discussing the integrative action of the central nervous system. The normal or
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