800 Comparative Animal Physiology 



example, the synaptic potential reaches its peak in 0.6 msec; in fatigue this 

 local response does not change in latency but falls in height, the propa- 

 gated spike arising later on the synaptic potential."^^ In the cat spinal cord 

 the synaptic potential begins about 0.5 msec, after arrival of afferent im- 

 pulses; 0.05-0.1 msec, is needed for conduction in the small afferent fibers, 

 and the delay can be shortened by facilitation to 0.2-0.3 msec. Motor im- 

 pulses can be started only during part of the rising phase of the synaptic 

 potential, 1 msec, or less according to facilitation. ^^^ A synaptic delay is re- 

 quired to remove the intial anodal focus and permit catelectrotonic spread 

 of the local synaptic potential. ^^'^ 



Refractory Period. In general the absolute refractory period at synapses is 

 similar to the refractory period of the efferent nerve fibers. Measurements of 

 motoneurone refractory periods for several mammalian centers have been made 

 by preceding an afferent volley at varying time intervals by an anti- 

 dromic volley (impulses entering the center through the efferent neurones). 

 By this method the refractory period of the oculomotor nucleus is 0.5-0.6 

 msec.;^^^ that of the cervical sympathetic ganglion is about 2 msec, for the 

 fastest units.^^ 



Some centers fail to follow repetitive stimuli at lower frequencies than 

 do isolated axons, but this failure is not a direct measure of refractoriness. 

 In the crayfish, for example, transmission across the sixth abdominal ganglion 

 stops at 10/sec. while sensory impulses enter up to 100/sec.^^^ In fresh 

 preparations the junctions from giant fibers to motoneurones of the cray- 

 fish show no longer refractoriness than the giant fibers, but with slight fa- 

 tigue they are blocked at 30-50/sec.^^^ In the cockroach sixth ganglion the 

 synapse between cereal sensory fibers and ascending giant fibers follows up 

 to 400/sec.36i 



The refractory period of the giant fiber synapse in the stellate ganglion of 

 the squid is less than that of preganglionic fibers. Impulses can follow 

 across the junction at rates as high as 475/sec., and a local response, too 

 small for spike initiation, may be elicited with no absolute refractoriness. By 

 analogy with muscle, where the graded response shows no refractory pe- 

 riod, the synaptic potential also probably has no true refractory period. 



Facilitation. By synaptic facilitation is meant the requirement that more 

 than one incoming impulse is needed to excite a post-synaptic neurone. One 

 incoming impulse may be ineffective alone, but when accompanied by oth- 

 ers it is effective. Most synaptic transmission requires facilitation; there are, 

 however, some interneuronic junctions as in invertebrate giant fiber sys- 

 tems in which conduction is one-to-one, and no facilitation is required for 

 transmission.^^ 



Some of these junctions are non-polarized, as in the longitudinal con- 

 duction in giant fibers of annelids and crustaceans. Others are polarized, as 

 in the squid stellate ganglion and the connection between lateral giants and 

 motor fibers in the crayfish. Facilitation does appear, however, when these 

 systems are fatigued. 



In the coelenterates, facilitation in the nerve net is marked and often 

 necessary for the elicitation of a mechanical response. The excitatory process 

 persists for several seconds (see p. 595). It appears likely that primitive 

 interneuronic junctions required facilitation for transmission and that the 



