INHIBITORY PATHWAYS TO MOTONEURONS 59 



depolarization of Renshaw cells by recurrent collaterals of motor axons 

 (Eccles, Fatt and Koketsu, 1954; Eccles, Eccles, Iggs and Lundberg, 1961); 

 but both the latency and the rising phase of the presynaptic depolarization 

 are much longer than with Renshaw cells. It would be surprising if a single 

 afferent volley resulted in such a slow onset of synaptic transmitter action. 



Alternatively it may be postulated that the presynaptic depolarization results 

 from electrical transmission from the activated intemiediate neurons. One 

 version of this postulate would be that, as suggested above, the field potential 

 was generated by the after-hype rpolarization of intemiediate neurons following 

 the discharge of an impulse, and that this field directly depolarized the la 

 presynaptic fibres. The orientation of the field would effectively give this 

 depolarization. Unfortunately, this version encounters grave difficulties both 

 because the potential field is much less than 1 mV and so would be insufficient 

 to produce a depolarization which would evoke a discharge of impulses in 

 the slightly cooled spinal cord. A further difficuhy is that anodal polarization, 

 and therefore depressed excitabihty, would be expected in the la afferent 

 fibres at the depth of the intemiediate nucleus and dorsal thereto, whereas 

 hyperexcitability extends right out to the dorsal root, though to a con- 

 tinuously diminishing degree. These two difficulties are both resolved by an 

 alternative version of the electrical hypothesis, it being postulated that the 

 potentials generated by the intemiediate neurons are selectively applied to the 

 la afferent fibres by two sets of synaptic contacts that have rectification 

 properties so as to allow current flow in the direction of the arrows (Fig. 8b). 

 By one set of connections the la afferent fibres have connections to inter- 

 mediate neurons that allow current to pass from the intermediate neuron 

 into the presynaptic collateral. By the other set of contacts the intemiediate 

 terminals form rectifying synapses with the Ta presynaptic terminals in the 

 ventral horn. These synapses selectively allow current to pass from the la 

 terminals into the intermediate axonal terminals. This explanation has the 

 merit of explaining why the observed field potential has the same time course 

 as the presynaptic depolarization. Furthermore, rectifying synapses have been 

 shown to exist by Furshpan and Potter (1959). 



However, it will be realized that these two alternative explanations are at 

 present speculative, though it is hoped that experimental evidence may soon 

 enable a discrimination to be made between them. At least there appears to 

 be good evidence that the central pathway for presynaptic inhibition involves 

 firstly the la and lb primary afferent fibres, then their synaptic relay in the 

 intermediate nucleus, but thereafter the situation is obscure. 



REFERENCES 



Araki, T., Eccles, J. C. and Ito, M. (1960). Correlation of the inhibitory post-synaptic 

 potential of motoneurones with the latency and time course of inhibition of mono- 

 synaptic reflexes. /. Phyxiol {London). 



