402 G. ADRIAN HORRIDGE 



motoneurons of the worm are diverse and run to different muscles but in 

 other respects the situation in any one segment appears superficially to be 

 hke that in the crustacean heart ganglion when inhibited. 



In the crustacean heart ganglion there are nine motoneurons, about four 

 of which are normally spontaneously active, the others being relays which 

 only muhiply the number of impulses in each burst. Except round the smaller 

 cell bodies, the paired inhibitory axon branches apparently indiscriminately 

 in the clumps of neuropile into which also run dendrites from the nine 

 intrinsic neurons (Alexandrowicz, 1932). The inhibitory impulses affect all 

 the cells but not necessarily to the same extent, or in the same order in dif- 

 ferent preparations (Maynard, 1953, 1961). It looks very much as if the specifi- 

 cations given to the inhibitory axon for growth and formation of synapses 

 are not particular as to details of branching, number or destination of the 

 terminations or their actual site on the heart ganghon neurons. They appar- 

 ently spread to all the places where the ganglion cells have dendrites, and the 

 specificity of the inhibitory axon could, on present evidence, lie only in its 

 secretion of the appropriate transmitter substance. Similar considerations 

 apply to the acceleratory fibres. This type of widely ramifying termination is 

 perhaps particularly appropriate to this example where the regions of spon- 

 taneity are not necessarily static and are changed by stretching the prepara- 

 tion. The nuerons are also modulated by chemically addressed blood-borne 

 hormones. 



To this widespread inhibition in the heart ganghon the same general 

 considerations apply as to Heteroxenia, to the anemone, the jellyfish and the 

 worm. Other similar phenomena are known elsewhere in less detail, for ex- 

 ample the widespread changes in tone following removal of anterior ganglia 

 in many invertebrates, and taken together they suggest that a few axons act 

 more or less indiscriminately on the probability of firing of a group of moto- 

 neurons which normally fire together to initiate a co-ordinated movement. 

 Other examples are given below. There is no need to suppose a particular 

 wiring diagram for the general inhibitory or excitatory input; all that is re- 

 quired is that it should be adequately distributed in the volume of neuropile 

 where the motoneurons have their dendrites, and not necessarily anatomically 

 addressed to particular postsynaptic cells. 



The cerebral ganglion of the clam Mya initiates a sequence of motor 

 impulses in from ten to twelve axons of the anterior pallial nerve when the 

 connective between the posteriorly situated visceral and the anterior cerebral 

 ganghon is stimulated. One impulse in one pregangfionic axon is adequate 

 to give a patterned burst of postganglionic motor impulses. There is con- 

 siderable variation in the pattern seen in different preparations (Horridge, 

 1958, 1961). In the viscero-cerebral connective there is also at least one pre- 

 ganghonic inhibitory axon, which, when simultaneously stimulated, reduces 

 the probabihty of firing of the motoneurons with a similar effect on all in the 



