152 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



depolarizing electrogenesis is also evoked, although in 

 this case by stretch of the mechanosensory receptor 

 membrane (66, 67, 94). In other cells, notably neurons 

 of the vertebrate central nervous system (59, 60, 158, 

 159, 1 61-16 7) and some invertebrate muscle fibers 

 (73. 13O and neurons (33, 186), l:)oth types of p.s.p.'s 

 are found. 



The depolarizing p.s.p., being of the same sign as 

 the effective stimulus for electrical or local circuit 

 production of a spike, can also evoke the latter and is 

 therefore termed an " excitatory' p.s.p. (59, 60). The 

 spike arises when the p.s.p. is sufficient to depolarize 

 the adjacent electrically excitable, spike-generating 

 membrane to a critical firing level (fig. 3). The latter 

 varies among different cells and is of the order of 10 

 to 40 mv change from the resting level. The hyper- 

 polarizing p.s.p., by the same criterion, is an ' in- 

 hibitorv' p.s.p. However, these names are not always 

 appropriate. There are cells, like some electroplaques 

 or muscle fibers (cf. 95, 97), that generate depolarizing 

 p.s.p.'s but no spikes. The depolarizing p.s.p. there- 



fore may have nothing electrogenic to excite. Like- 

 wise, those gland cells which generate only hyper- 

 polarizing p.s.p.'s al.so have no spike to inhibit (cf. 

 fig. 20). On the contrary, the hyperpolarizing electro- 

 genesis of the gland cells is associated with actixity in 

 the form of secretion (146). 



When the two varieties of p.s.p.'s occur in a cell 

 \shich also generates spikes, they interplay with 

 excitatory and inhiljitory influences upon the elec- 

 trically excitable membrane. The inhibitory synaptic 

 action may occur independently of the magnitude 

 and even tiie sign of the inhibitory p.s.p. As will be 

 descriljed below (p. 160) this p.s.p. may be de- 

 polarizing under certain electrochemical conditions, 

 or the acti\'ity of the synaptic membrane may not 

 manifest itself as a potential. Nevertheless, when this 

 synaptic activity is pitted against a depolarizing 

 p.s.p. it always tends to decrease the magnitude of 

 the latter and thereby to diminish or block its ex- 

 citatory effect on the electrically excitaljle membrane. 

 In some cases, therefore, the term "inhibitory" p.s.p 



msec 



msec 



FIG. 3. Synaptic transfer from the p.s.p. to the spike. Intracellulai" recording, eel electroplaque. 

 Above: Increasing stimuU to a nerve produced a stepwise increase of the p.s.p. (.4 to C). A still larger 

 stimulus evoked a spike (Z) and £). Below: The p.s.p. first generates a local, graded response of the 

 electrically excitable spike-generating membrane. When the neural stimulus evokes a p.s.p. during 

 the absolute refractory period {A', B'), the response lacks this component of giaded activity of the 

 electrically excitable membrane. Later (C to G") the local response develops, grows, arises earlier 

 and fuses with the p.s.p. The combined response is seen in isolation in H'. This series of records was 

 taken at approximately ' 1 the amplification of the upper set. Baseline denotes the zero for the 

 resting potential and for the overshoot of the spikes. [From Altamirano el al. (4).] 



