INHIBITORY NEURONS 5 



More centrally located inhibitors have been shown in a number of animals, 

 but the evidence that they are purely inhibitory is less. The work of Tauc (1958) 

 on inhibition in the ganglia of gastropods shows clearly that there exist 

 inhibitory fibers coming from the higher centers which give polarizing potentials 

 in cells of the pedal ganglion. In the crayfish, Hughes and Wiersma (1960) 

 obtained evidence that a specific inhibitory fiber runs from the brain to the 

 abdominal ganglia where it inhibits swimmeret movements. These rhythmic 

 movements can be elicited among other ways by the stimulation of one of 

 two interneurons originating in the brain, and rhythmic discharges remain 

 then present in the motor roots even when the sensory connections with the 

 periphery are severed in the abdominal region. By accepting the presence of 

 "pacemaker" cells which are brought into action by the two triggering fibers 

 and stopped by the inhibitory one, these phenomena can be explained. Note 

 the resemblance of the problems to those of other rhythmic reflexes, such as 

 the scratch reflex, which might possibly have similar explanation. In the cat 

 brain, Desmedt and Mechelse (1958) have recently found a thin tract from the 

 cortex to the cochlear nucleus which on stimulation inhibits click-evoked 

 responses, and they consider it as specifically inhibitory. This would thus be a 

 case in which the inhibitory interneurons were of a very "high" order. 



However, for such intracentral tracts the "pure" inhibitory nature may be 

 hard to prove, though for Renshaw cells it appears almost certain that they 

 do not have excitatory endings and the same may be true for other inhibitory 

 elements of the spinal cord at this level. In a number of other cases consider- 

 able doubt has arisen about this aspect. Thus Terzuolo and Bullock (1958) 

 consider that the extrinsic inhibitory fiber of the crustacean heart has dual 

 function on different cardioganglionic cells, inhibiting the pacemaker cells 

 and exciting the follower cells. It seems somewhat debatable that their 

 assumption that the excitatory eff"ect is caused by the same fiber, is justified. 

 For though the nerve contains a single inhibitory fiber, it may be asked if this 

 is the only fiber in it, and not accompanied by an accelerator. It would be of 

 importance to investigate this problem with the aid of the transmitter sub- 

 stance. 



Another instance that may eventually belong in this column is that of the 

 eccentric neurons of Linmlus eye. Here it appears as if the collaterals may 

 directly inhibit other eccentric cells. Interestingly enough though, it is possible 

 that they would be completely inhibitory in function, since according to 

 Wilska and Hartline (1941), their central eff'ect consists in an off" discharge in 

 the next elements. Thus Linmlus would develop "negative" prints, which looks 

 like a poor mechanism for vision, because of the resulting loss of information. 



The most direct evidence for two types of actions of the synapses made by 

 one neuron is that of the Mauthner's cells of fishes, as described by Retzlafif 

 and Fontaine (1958). Here the excitatory eff'ect on the motor neurons in the 

 tail can hardly be doubted, nor the inhibitory eff'ect on their partner of the 



