24 DAVID P. C. LLOYD 



monosynaptic reflex system? The answer is as unequivocally no as it is in 

 relation to inhibition of crustacean muscle fiber (Fatt and Katz, 1953). It 

 appears if the motoneurons are known to be partly depolarized (Lloyd and 

 Wilson, 1959) or if there is good reason to suppose they are, as when impaled 

 by intracellular microelectrodes. One might suppose that a background of 

 excitatory internuncial barrage could provide the appropriate conditions for 

 appearance. 



Secondly, can i.p.s.p. be regarded as the cause of inhibition? Again the 

 answer is no. A process that need not be present when an effect is registered 

 cannot be the cause of that effect. It can at best have a parallel relationship 

 to a common cause with the effect in question. But further, the time course 

 of i.p.s.p. does not conform with that required for a causal process of inhibi- 

 tion. It begins some considerable fraction of a millisecond after the onset 

 of inhibition (Lloyd and Wilson, 1959) if measurements of its latency are 

 correct, which presumably they are. It reaches a maximum of intensity in 

 1-5 msec (Coombs et al., 1955) to 20 msec (Eccles, 1955) whereas inhibition 

 is maximal at from 0-5 to 0-6 msec. This sort of temporal relation between 

 inhibition and potential change is exactly that found for crab muscle fiber 

 inhibition in 1953 by Fatt and Katz, and their statement: 



"It need hardly be emphasized that the 1-potential, as such, has no significance as an 

 inhibitory agent" (itahcs theirs), 



is as true for the spinal motoneuron as it is for the crustacean muscle fiber. 

 When potentials having the direction of increased polarization, which is 

 to say i.p.s.p.'s, are recorded, it seems best to think of them in terms of 

 repolarization in the sense used and discussed by Kolmodin and Skoglund 

 (1958) rather than in terms of hyperpolarization as is frequently done. 



Chemical Transmitter Hypotheses of Inhibition 



Once convinced of the hypothesis of chemical transmission Eccles (1953) 

 put forward three thinkable hypotheses to account for monosynaptic reflex 

 inhibition, the simplest and most acceptable of which assumed the same 

 transmitter at all endings with specialized "excitatory" and "inhibitory" 

 "subsynaptic" areas. 



Later, dissatisfied with these formulations, Eccles et al. (1956) sought to 

 interpose an interneuron in the direct inhibitory pathway, the sole purpose 

 for which was to act as a commutator from excitatory to inhibitory action. 

 In this formulation all primary afferent fibers would be excitatory and would 

 release an excitatory transmitter. Those whose function is to inhibit would 

 excite these interneurons which in turn would release an inhibitory trans- 

 mitter thereby inhibiting motoneurons. 



Unfortunately, tliis form of chemical hypothesis is not in harmony with a 



