798 Comparative Animal Physiology 



the ganglion. This graded synaptic potential is normally masked by the 

 propagated response. In the superior cervical ganglion of the cat this synap- 

 tic potential begins after a latency of 5 msec, reaches its peak at 10-20 

 msec, and falls to about 1/3 in 60-90 msec, many units rsponding out of 

 phase (Fig. 299).^-'* The synaptic potential is absent when antidromic im- 

 pulses enter the ganglion and is graded in size with the number of pre-gan- 

 glionic impulses; it can summate on repeated subthreshold stimulation. When 

 transmission to interneurones and motoneurones in the spinal cord is 

 blocked by nembutal a sensory volley sets up a synaptic potential which 

 can be picked up either by focal microelectrodes in the cord or conducted 

 electrotonically over ventral roots. In monosynaptic reflexes of cats the syn- 

 aptic potential recorded focally in the spinal cord shows a delay of 0.3 to 

 0.45 msec, and peak time of 0.8 to 1.5 msec. Similar graded potentials have 

 been recorded from the stellate ganglion of Loligo (Fig. 300). "^^^ ^^ The 



Fig. 300. Synaptic potentials and emergent action potential spikes from stellate gang- 

 lion of the squid. Lower record— spike in preganglionic fiber. Upper record— postgangli- 

 onic spike developing later and later out of synaptic potential as fatigue occurs finally 

 leaving only synaptic potential. From Bullock."*' ^'^ 



propagated postganglionic spike arises out of the synaptic potential, and 

 when motoneurones are fatigued only the synaptic potential remains. Di- 

 rect stimulation of the ganglion can elicit a synaptic potential, but antidromic 

 impulses cannot do so. It is postulated*'^ that the transmitter passes its peak 

 before the beginning of the synaptic potential, and that the synaptic poten- 

 tial must be of a critical size before the post-synaptic spike is evoked. 



SOMA RESPONSES. The soma of a central neurone refers to the non-axon 

 portions, i.e., the dendrites and cell body. The potentials which arise in the 

 soma appear to be slow compared with those in axons. Sympathetic gan- 

 glion potentials show several spikes corresponding to activity in diff^erent 

 cell groups, prolonged as compared with axon spikes, and they also show 

 slow negative and positive after-potentials^^^- -^^ (Fig- 299). Records from 

 the mammalian oculomotor nucleus show fast axon spikes followed by slowed 

 motoneurone potentials; the latter only are found in response to antidromic 



