Nervous Systems 799 



volleys-^- (Fig. 300). Similarly in the hypoglossal nucleus deep electrodes 

 detect slow waves which can be separated experimentally from the axon im- 

 pulse, conduction in the soma being not more than one-tenth as fast as in 

 the axons."*'"''' These slow soma potentials can be observed when the cells are 

 excited antidromically. 



The ganglia of invertebrate animals are complex in organization. Soma po- 

 tentials lasting 30 msec, have been recorded from isolated cell bodies of the 

 visceral ganglion of the snail Aplysia. Their origin is not clear. ^**'' ^'' A closer 

 correlation between the histology and physiology of nerve centers is needed. 



Graded localized synaptic potentials precede the discharge of motoneu- 

 rones, and soma potentials are slower than axon spikes; the soma potentials 

 must have significant influence on the excitability of surrounding neurones. 



Synaptic Delay. At all interneuronic junctions an appreciable time is taken 

 for setting up an efferent impulse. Synaptic delays are longer in centers re- 

 ceiving impulses over slow afferent fibers than over fast ones. The older 

 measurements of "central time" in spinal reflexes give little information re- 

 garding synaptic delays because the number of interneurones interposed be- 

 tween aff^erent and motor neurones is not known. In two-neurone reflexes of 

 the cat Lloyd finds central delays of 0.7 msec; these can be reduced by fa- 

 cilitation to a minimum of 0.3 msec.^^' -^^ And in the oculomotor nucleus of 

 the rabbit, delays at synaptic junctions averaged 0.7 msec.-^- Delays in the 

 slower sympathetic ganglia are much longer. The superior cervical ganglion 

 of the cat shows delays of 3 and 6 msec, respectively, for two groups of 

 units, ^-^ and interneurones probably do not occur. In the stellate ganglion of 

 the squid a single synaptic junction of the giant fiber system has a delay of 

 0.5 msec, at 24°.^^ The sixth abdominal ganglion of the crayfish gives a gan- 

 glionic delay of 2-5 msec; this may include interneurones.^^^ In the last 

 abdominal ganglion of the cockroach some sensory fibers from the cerci 

 synapse with ascending giant neurones; the synaptic delay of this system is 

 0.6-1.5 msec, but there may be one interneurone.^^^ 



Several explanations of synaptic delay have been suggested: 



1. Axon branches are usually greatly attenuated; conduction is slow in 

 small fibers. This must be particularly important in sympathetic ganglia 

 where there is a basket-like arrangement of axon and dendrite branches. 

 When allowance is made for terminal slow conduction, the synaptic delay 

 is somewhat reduced. 



2. There may be asynchrony in arrival of impulses over the different 

 branches of an axon, hence time is consumed in their spatial summation. 



3. Motoneurones have a true excitation time, like the "utilization time" of 

 axons. 



4. Time is required for liberation and diffusion of acetylcholine or other 

 chemical transmitter from one membrane to another. Shortening of the de- 

 lay by facilitation may be explained by assuming that a subliminal quantity 

 of transmitter persists for a time so that a threshold concentration is at 

 tained sooner with a second volley. Diffusion is not a factor if transmission 

 is electrical. 



5. The synaptic potential rises at a certain rate and the time when an im- 

 pulse is started depends on when the synaptic potential reaches a critical 

 size, just as with end-plate potentials. In the squid giant-fiber synapse, for 



