66 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



tion indicates that each excitatory presynaptic impulse 

 generates in the postsynaptic neuron a potential 

 change of this same time course, and that the recorded 

 EPSP's of figure 4.4 to C are produced by a simple 

 summation of these elemental EPSP's. It thus pro- 

 vides an illustration of the classical concept of spatial 

 summation (72, 73). 



As shown in figure 4D to G, if the EPSP is increased 

 beyond a critical threshold level, it causes the neuron 

 to discharge an impulse, the latency being briefer the 

 larger the EPSP. In figure 4.E, F, G the increase of the 

 EPSP to above threshold was brought about by in- 

 creasing the size of the presynaptic volley, but, as 

 would be expected, the EPSP can also be made to 

 generate an impulse by conditioning procedures that 

 change the membrane potential towards the critical 

 threshold level. For example in figure 4/ to A' the 

 same EPSP as in figure 4// was made effective by the 

 operation of a background depolarizing current 

 which was commenced 1 2 msec, before and which 

 changed the membrane potential by the amount 

 shown in each record. The impulse is seen to arise 

 (at the arrows) at a total level of depolarization of 

 about 18 mv, which is made up in varying proportions 

 by the conditioning depolarization and the super- 



imposed EPSP. The threshold level of depolarization 

 may be attained also by superimposing an EPSP on 

 the depolarization produced by a preceding EPSP 

 (temporal summation), as is illustrated by Grundfest 

 (Chapter \', fig. i 7). 



All these investigations conform with the hypothesis 

 that synaptic excitatory action is effective in generat- 

 ing an impulse solely by the depolarization of the 

 neuron, i.e. by producing the EPSP (17, 28, 29, 44). 

 As far as the generation of an impulse by the EPSP is 

 concerned, the same processes obtain as with the 

 propagation of an impulse from one part of a neuron 

 to another. 



In order to produce the EPSP, the activated syn- 

 apses must cause an electric current to be generated 

 which depolarizes the postsynaptic membrane. Thus, 

 as shown in figure ^B, a current must flow inwards 

 immediately under the activated synapses, i.e. across 

 the subsynaptic membrane, in order that a return 

 current may flow outward across the remainder of the 

 postsynaptic membrane, so depolarizing it. When a 

 brief current pulse is applied across the membrane, it 

 builds up a potential difference that on cessation of 

 the current decays considerably faster than the EPSP 

 (12). Hence it is postulated that the current producing 



V/scc 



o 



NORMAL MEMBRANE 



3x|6'f 



6 



E SYNAPSES 



as low OS 



5x 10 n 



FIG. 5. A. The continuous line is the mean of several monosynaptic EPSP's, while the broken line 

 shows the time course of the subsynaptic current required to generate this potential change. B. 

 Diagram showing an activated excitatory knob and the postsynaptic membrane. .As indicated by the 

 scales for distance, the synaptic cleft is shown at 10 times the scale for width as against length. The 

 current generating the EPSP passes in through the cleft and inward across the activated subsynaptic 

 membrane. [From Coombs et al. (12).] C. Formal electrical diagram of the membrane of a motoneu- 

 ron with, on the right side, the circuit through the subsynaptic areas of the membrane that are 

 activated in producing the monosynaptic EPSP. Maximum activation of these areas would be 

 indicated symbolically by closing the switch. 



