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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 



ZOO 



MSEC 



FIG. 29. The time course of homosynaptic facilitation in a group of eel electroplaques. The testing 

 stimulus alone evoked responses shown by the horizontal lines at the end of the graph in B. At 

 various intervals after a conditioning stiinulus, the response to the test stimulus became larger than 

 this control value and gradually returned toward it. • : Facilitation without treatment with drug. 

 A : 1 2 min. after adding 50 >jg per ml physostigmine the cells became somewhat more excitable, 

 the whole curve of facilitation being lifted on the baseline of the larger response to the testing stim- 

 ulus in isolation. This effect presumably developed because of the anticholinesterase action of phy- 

 sostigmine; it is also produced by prostigmine. The two substances, however, have opposite synaptic 

 action, physostigmine being an inactivator of synaptic electrogenesis and prostigmine an excitant, 

 n ■ 64 min. later, the physostigmine had depressed synaptic excitability and the whole curve had 

 fallen. Expressed in percentile values of the response to the testing stimulus alone in each condition, 

 the three curves had essentially the same magnitudes and time courses (.4). [From .\ltamirano 

 et al. (6).] 



polarization remaining from the prior stimulus. This 

 is, indeed, the condition found experimentally (fig. 

 30^) in the electroplaques from the Sachs organ of 

 the eel. Both depolarizing p.s.p.'s being short, 

 facilitation occurs only during the first 2 msec. In 

 cells of the main organ, however, heterosynaptic 

 facilitation lasts some 50 to 75 msec. (fig. 3oi?) and 

 in this case the effect must be due to alteration of the 

 excitability of the synaptic membrane since pre- 

 synaptic interactions are ruled out. The different 

 behavior of the electroplaques in the two organs is 

 probably ascribable to difTerent spatial relations of 

 their synapses. If those in the electroplaques of the 

 main organ are closely spaced, diffusion of transmit- 

 ter from the sites activated b\- the conditioning 

 volley might affect the excitability of the synaptic 

 loci innervated by the second neural pathway (95). 

 The data presented above derive from a particu- 

 larly favorable structural configuration, a large 

 postsynaptic cell with an extensive responsive mem- 



brane (about 15 mm- in area) diffusely innervated 

 by several easily isolated nerve trunks. The experi- 

 mental conditions that obtain in nerve cells do not 

 usually perinit as clear a delineation between dif- 

 ferent spatial interactions. However, in the case of 

 cells with long dendrites, as in the cortex, it may be 

 expected that interaction between different axo- 

 somatic synapses will be greater than that between 

 these and the axodendritic. 



f) sp.'Kti.'^l sum.m.ation of converging p.^th\v.\ys. 

 Another variety of spatial summation is more fre- 

 quently noted in the central nervous system. This is 

 the case in which two widely separated neuronal 

 complexes eventually converge upon one or more 

 common paths. In that final common path the situa- 

 tion then reduces to a variant of the case discussed 

 above. These convergent types of interaction are 

 further complicated in the central nervous system 

 by the involvement of inhibitory p.s.p.'s. Spatial 



