Nervous Systems 801 



one-to-one relay conduction of giant neurone systems is a relatively evolution- 

 ary development. 



When an interneurone or motoneurone is stimulated by two incoming vol- 

 leys separated by different times, the second volley is usually found to be 

 most effective at two periods after the first; these are periods of early and 

 of delayed facilitation. 



1. Early facilitation (spatially determined). Two afferent volleys converg- 

 ing over different axons on one motoneurone are most effective when syn- 

 chronous. One afferent volley alone may not be able to elicit a motor re- 

 sponse, but together they summate. If one volley is delayed after the first, it 

 becomes less effective as the excitatory process from the first decays. Vary- 

 ing the interval, then, between spatially converging subliminal impulses is 

 useful in showing the time course of the excitatory process. Temporal sum- 

 mation at one synapse (bouton) is impossible because of refractoriness. Spa- 

 tial summation is generally required for synaptic excitation. This is well 

 shown in the tail of the crayfish, where flexion of one sensory hair on a uro- 

 pod sends one impulse into the sixth abdominal ganglion; stimulation of sev- 

 eral, usually four, adjacent hairs is necessary to elicit one efferent (post syn- 

 aptic) impulse.^^"* In vertebrates it is not probable that a single impulse in 

 one afferent axon ever elicits a reflex response. 



In the superior cervical sympathetic ganglia of the cat two submaximal 

 volleys in different preganglionic branches sum maximally at synchrony, the 

 summation declines to zero at 4.5 msec.^-^' ^^"^ In the oculomotor nucleus of 

 the rabbit summation between two converging volleys declines to a minimum 



at\ cT 



.25 .5 .75 1 1.5 2 3 4 5 6 7 £ 



Fig. 301. Facilitation curve of motoneurones of oculomotor nucleus of rabbit. Motor 

 response in mv. to a test shock to presynaptic (floor) neurones at intervals in msec, 

 (abscissa) after a conditioning shock. uncT, Height of response of unconditioned test 

 shock. From Lorente de No.'"^ 



at 0.5 msec, separation^^^ (Fig- 301). In monosynaptic spinal reflexes of the 

 cat the facilitation falls to half the maximum of simultaneity at about 2.8 

 msec.^"* The early facilitation represents the time course of decay of the syn- 

 aptic transmitter. 



2. Delayed facilitation. The second type of facilitation represents enhanced 

 excitability of receiving neurones after the initial excitatory process has 

 passed; this facilitation can be detected by test volleys in either the same or 

 different incoming pathways. In flexor reflexes of the cat, for example, Eccles 

 and Sherrington^^^ observed a maximum response to a second afferent vol- 

 ley 12-20 msec, after an initial volley. The enhanced excitability after re- 



