72 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 1 



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MOTO 

 NEURONE 



FIG. g. A. Schematic drawing of tiie anatomical and physiological features of the direct inhibitory 

 pathway. It shows the events in the primary afferent hber, in its excitatory synaptic connections with 

 an intermediate neuron (I cell) and finally in the inhibitory synaptic connection of this neuron with a 

 motoneuron, where the inhibitory subsynaptic current is shown by a broken line and the IPSP by 

 a continuous line (cf. fig. 6A). B. Diagram summarizing the postulated sequence of events from an 

 impulse in a motor axon to the inhibition of a motoneuron. All events are plotted on the time scale 

 shown below and the corresponding histological structures are shown diagrammatically to the left 

 (note indicator arrows). The four plotted time courses are from above downwards for the following 

 events: the electrical response of impulse in motor-axon collateral; the electrical response evoked in a 

 Renshaw cell by the cumulatixe effect of acetylcholine at many synapses, showing impulses super- 

 imposed on a background depolarization; the IPSP generated in the motoneuron by the Renshaw 

 cell discharge; and the aggregate IPSP evoked in a motoneuron that is bombarded repetitively by 

 many Renshaw cells, which become progressively more asynchronous, so smoothing the latter part 

 of the ripple. The structural diagram to the left shows converging synapses on the Renshaw cell 

 and on the motoneuron. [From Eccles c/ at. C34)-j 



'excitatory neurons'. Conceptually, by this subdivision 

 of nerve cells into excitatory and inhibitory types, a 

 great simplification is produced in the physiology of 

 central synaptic mechanisms, for all branches of any 

 one neuron can be regarded as having the same 

 synaptic function, i.e. as being uniformly excitatory 

 or uniformly inhibitory. Terzuolo & Bullock (75) 

 give experimental evidence that this principle of 

 neuronal specificity does not hold for the cardiac 

 ganglion of Limulus. 



In attempting to understand the operation of any 

 neuronal system in the mainmalian central nervous 

 system, a useful provisional postulate would be that 

 all inhibitory cells are short-axon neurons lying in the 

 grey matter, while all transmission pathways including 

 the peripheral afferent and efferent pathways are 

 formed by the axons of excitatory cells. Such a postu- 

 late would be of most direct application in relation to 

 such simple problems as the modes of termination of 

 the descending tracts, but eventually it may be 

 applicable also to more complex situations in the 

 brainstem and even in the cerebellar and cerebral 

 cortices. In all these situations there is as yet no infor- 



mation on the structural features of the inhibitory 

 mechanisms. 



It will be sufficiently evident from the above 

 account of nerve cells that interactions between nerve 

 cells are attributed to synaptic contacts which operate 

 by a specific chemical transmitter mechanism. The 

 alternate postulate is that, at least in part, interaction 

 between neurons is attributable to the flow of electric 

 currents generated by active neurons. There is at 

 present no experimental evidence that the nervous 

 system ot \ertebrates operates in this way. The flow 

 of electric currents between neurons is far too small 

 to have any significant effect, even in experiments 

 using the unphysiologicai procedure of large syn- 

 chronous volleys. In contrast it should be mentioned 

 that some synapses in Crustacea do operate by elec- 

 trical transmission, there being special permeai:)ility 

 and rectification properties of the apposed synaptic 

 memijranes (47). Such a mechanism would have been 

 detected if it were operative at any of the central 

 synapses of vertebrates that have been systematically 

 investigated. 



