136 



PHYSIOLOGICAL TRIGGERS 



(excitatory) p.s.p.'s evoked by neural stimuli are recorded intracellularly in 

 motoneurons of cat (20, 49, 84) and toad (10) ; in cat cortical neurons (159, 160) ; 

 and in rabbit sympathetic ganglion cells (66). The p.s.p. of the latter develops 

 even when the cell does not respond with spikes to repetitive stimuli (fig. 7). 

 Antidromic activation of the motoneuron does not elicit the p.s.p. A hyper- 

 polarizing (inhibitory) p.s.p. also occurs in neurally stimulated motoneurons 

 (10, 20, 48, 50, 64 and fig. 8). The p.s.p.'s of cat and toad motoneurons and 

 turtle autonomic ganglion cells may be elicited during the falling phase of the 

 spike when the cells are absolutely refractory to electrical stimulation (10, 21, 



5mv. 



Fig. 8. Reversal of an in- 

 hibitory, hyperpolarizing p.s.p. 

 into a depolarizing, cat moto- 

 neuron. The initial deflection, M , 

 is the antidromic spike which did 

 not reach the soma. Following it 

 (.4) is a hyperpolarization gen- 

 erated by activation of inhibitory 

 post-synaptic membrane. Passing 

 current through the recording 

 electrode then increased the in- 

 ternal Cl^. Activation of the 

 inhibitory postsynaptic mem- 

 brane now led to a depolarizing 

 potential indicating outflow of CI". 

 C: Extracellular responses on 

 withdrawing the microelectrode 

 from the cell show the reversed M 

 spike, but not the p.s.p. (from 

 ref. 64). 



49, 136). The neurally evoked p.s.p. of toad motoneuron develops during 

 strong hyperpolarization of the cell (155). 



The neurohumorally, but not electrically e.xcitable transducer action of the 

 synapse may be schematized as follows: 



Stimulus -^ Transducer action -^ Electrogenic action 

 Specific transmitter agent — > Specific change in ionic -^ Depolarization or {4) 



permeability hyperjHjlarization 



c) Functional Value of Synaptic Electrical Inexcitability. It is interesting to 

 speculate about the jmrposive value of electrical inexcitability of the postsyn- 

 aptic membrane. The fields of electrical currents generated by active cell and 

 fiber masses of the central nervous system undoubtedly produce electrical 



