Nervous Systems 



803 



ranged with direct connections of ascending "floor" axons to the motoneu- 

 rones and indirect connections via many parallel interneurones (Fig. 302, A). 

 The late facilitation is maximal at about 1 msec. (Fig. 301). The time course 

 of this late facilitation is such that internuncial impulses are arriving at the 

 motoneurone at the peak of the facilitation, and the internuncial impulse 

 from volley one arrives at the same time as the direct impulse from volley 

 two. Prolonged facilitation can occur if there is a chain of successive inter- 

 neurones (Fig. 302, B). Facilitation can be altered by lesions to side path- 

 ways, and when motor responses are asynchronous the latencies of the late 

 waves can be altered by facilitation. In the oculomotor nucleus, then, sub- 

 maximal internuncial bombardment seems to account for late facilitation, 

 impulses from one indirect path adding to later ones from another and di- 

 rect path. Temporal summation is in this sense spatial summation. Such an 

 explanation of delayed facilitation is not applicable to centers like sym- 

 pathetic ganglia which lack interneurones. 



Curves of delayed facilitation have not been obtained for any non-mam- 

 malian nerve center. 



Excitatory axon — 



Motoneurone 



-Inhibitory axon 



Snnall Golgi 

 neurone 



Fig. iOi. Schema illustrating electrical theory of direct inhibition. It is assumed that 

 the small golgi neurone is excited subliminally and that its synaptic potential creates a 

 negative sink, causing outflowing current in the motoneurone and there producing a 

 region of anelectrotonus. From Eccles.^** 



Inhibition. In a series of classical experiments Sherrington demonstrated 

 that under certain circumstances reflex responses can be diminished by sen- 

 sory impulses. He proposed that a central inhibitory state (CIS) counter- 

 acts a central excitatory state (CES). Some afferent nerve fibers are inhibi- 

 tory, others excitatory. One afi^erent neurone can probably be excitatory and 

 inhibitory to different neurones, although this is not proved. Single inhibi- 

 tory fibers have been isolated in arthropod motor nerves (Ch. 16) and in 

 frog sensory nerves.^^^ Just as with facilitation, there are two times of syn- 

 aptic inhibition, early and delayed. Inhibitory impulses may prevent moto- 

 neurone stimulation if they arrive simultaneously with, or immediately be- 

 fore, the excitatory impulses. The two-neurone reflexes are thus inhibited 

 by afferents in specific ipsilateral sensory fibers.-^** 



A specific inhibitory substance might be liberated at synapses; a given 



