HARRY GRUNDFEST 



135 



axon is absolutely refractory, after a directly evoked spike (29). Upon neural 

 (orthodromic) excitation, the giant nerve cell of Aplysia develops a p.s.p. 

 which is not evoked by electrical stimulation (12a, 178). As in other cases, the 

 p.s.p. can be elicited while the electrically excitable membrane is absolutely 

 refractory (12a). 



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I 50 mVl 



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15 msec 

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Fig. 7. Excitatory postsynaptic potentials in neurons. Left: Cat motoneurons. A, p.s.p.'s 

 in response to paired threshold orthodromic stimulation summate, at shorter intervals, 

 causing a spike. The lower trace in each pair shows the arrival of the presynaptic impulses 

 recorded extracellularly at higher gain. Note that this potential is also picked up by the 

 internal electrode, but the change in post-Junctional membrane potential is less than i mv. 

 B, the same experiment at lower amplification. Time is msec, (from ref. 20). Rigid: Responses 

 in the rabbit superior cervical sympathetic ganglion cell, (first column) to a single stimulus 

 of increasing strength (from below up) showing the subthreshold p.s.p., and decreased latencj' 

 of the spike with larger p.s.p.'s. Second column: The cell cannot generate a train of spikes at 

 frequencies of pre-synaptic stimulation above about 80/sec., but the p.s.p.'s are produced 

 and maintain the depolarization. Presumably sodium inactivation operates to block the 

 depolarized electrically excitable membrane of the cell, but does not affect the post-synaptic 

 (composed from ref. 66). 



Vertebrate neuronal synapses. Direct evidence is available (105, 163) that the 

 prolonged, graded response of the apical dendrites of the cat cortex (43, 44) is 

 a postsynaptic potential. Under the action of synaptic blocking agents the 

 dendrite response cannot be evoked by direct stimulation of the cortex. The 

 membrane, at least of the apical dendritic portion, is therefore inexcitable by 

 electric stimuli (fig. 6) although the soma and axon of the same cell are so 

 excitable and respond with characteristic spikes (e.g. 159, 160). Depolarizing 



