EXCITATORY AND INHIBITORY PROCESSES 



311 



Fig. 20. The effect of inhibition on blocked antidromic impulses. Fast cell. 

 Resting potential 70 mV (unstretched). Intracellular electrodes filled with 

 0-6 M K2SO4. A (a). Antidromic impulse (a) alone (17 mV) blocked at the axon- 

 soma boundary region. A(b). Two inhibitory impulses set up depolarization in 

 relaxed cell, (a) is reduced and its time course accelerated. Dotted line indicates 

 control. A (c). Two antidromic impulses. (^1) during inhibitory train more 

 reduced than (al) after cessation of inhibitory stimulation. Note slower sweep. 

 B (a) and b (b). Preparation lightly stretched, (a) impulse again greatly reduced 

 by preceding train of seven inhibitory impulses at 500/sec. No appreciable 

 potential change was set up by inhibitory train alone, b (c). Antidromic impulses 

 placed at different times after inhibitory train revealed time course of inhibitory 

 effect, c (a) and c (b). Cell stretched, inhibitory impulse polarizes in c (b) and 

 again reduces the antidromic impulse, c (c). {a) moved later is less affected. 

 Voltage calibration same for all records. Large dots preceding antidromic 

 impulses are artefacts. (From Kuffler and Eyzaguirre, J. Gen. Phvsiol. 39 : 155- 



185, 1955.) 



blocked somewhere along the axon and its electronic spread is recorded at 

 the cell soma. The size of this electrotonically conducted impulse is markedly 

 reduced and its repolarization phase is accelerated. This effect seems to be 

 completely independent of any electric potential changes which might be 

 produced by the inhibitory impulses (Fig. 20). 



In order to explain this phenomenon one might think that inhibitory action 

 may change the properties of the soma-dendrite complex by increasing its 

 conductance and thereby changing the conditions for current flow. It is 

 possible that current flowing from the dendrites toward the blocked axonal 

 region is necessary either to propagate the impulse actively or to set up a 

 fairly large local response. The inhibitory action would tend to divert part of 



