68 HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



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FIG. 6. Tracings of intracellularly recorded spike potentials evoked by antidromic (.4) and mono- 

 synaptic (B) stimulation of a motoneuron, respectively. [From Coombs et al. (14).] The lower traces 

 shosv the electrically differentiated records. Perpendicular lines are drawn from the origins of the 

 IS and SD spikes, as indicated in the differentiated records, the respective threshold depolarizations 

 being thus determined from the potential records and indicated by horizontal lines labelled respec- 

 tively IS and SD. C. Diagram showing the lines of current flow that occur when a synaptically 

 induced depolarization of the soma-dendritic membrane electrotonically spreads to the initiaj 

 segment. 



at the levels of the horizontal SD arrow.s and is 

 approximately the same for the antidromically and 

 synaptically evoked spikes, as illustrated in figure 'oA 

 and B. Synaptic excitatory action thus generates an 

 SD spike not directly by its depolarizing action, but 

 only indircctK through the mediation of the IS spike 

 which lifts the depolarization of the SD membrane to 

 threshold by currents that flow in the reverse direction 

 to those drawn in figure 6C'. 



With normal motoneurons the threshold level of 

 depolarization has always been, as in figure 6.4 and B, 

 much higher for the SD membrane than for the IS 

 membrane. There has been a consideraljle range in 

 the threshold values for motoneurons that are shown 

 by their resting and spike potentials to be in good con- 

 dition. The IS threshold has ranged from 6 to 18 mv, 

 and the SD threshold from 20 to 37 mv (14). However, 

 for any one motoneuron the SD threshold has been 

 about two to three times the IS threshold. Several 

 other types of neurons in the central nervous system 

 also reveal a threshold difference between the IS and 

 SD membranes. The functional significance of these 

 distinctive threshold areas of neurons will be con- 

 sidered after synaptic inhibitory action has been 

 considered. 



The difference in threshold between the IS and SD 

 membranes must not be confused with the concept 



that membranes excited by chemical transmitter are 

 inexcitable electrically (cf. Grundfest, Chapter V). 

 This concept would be applicable merely to the sub- 

 synaptic areas of the SD ineitibrane and not to the 

 whole of that membrane. It should be noted that the 

 receptor membrane of the bare nerve ending in the 

 Pacinian corpuscle also appears to be inexcitable 

 electrically, though acting as a primary focus for 

 depolarizing the first node of the meduUated axon 

 (27; Gray, Chapter IV). There is some analogy here 

 with the SD membrane acting to depolarize the IS 

 membrane, so generating an impulse there; but the 

 analogy does not hold for subsequent e\ents because 

 the impulse in the IS membrane usually invades the 

 SD membrane, whereas with the Pacinian corpu.scle 

 there is no such antidromic invasion. 



Inhibitory Synaptic Action 



Strictlv, the concept of inhibition is restricted to 

 depressions of neuronal excitability which occur 

 independently of any conditioning excitatory synaptic 

 activity on that neuron, and also independently of any 

 depression of the excitatory .synaptic bombardment 

 that is employed in testing for the suspected inhibition. 

 It mav be noted that conditioning by large afferent 

 voUevs causes a fairlv prolonged depression in the size 



