54 



J. C. ECCLES 



msec 



Fig. 5. Time course of the direct i.p.s.p. evoked by la impulses (a) and the lb 

 i.p.s.p. (b and c). Intracellular recording was made with microelectrodes filled 

 with 0-6mK2SO4. a was obtained from a biceps posterior-semitendinosus 

 motoneuron with stimulation of the quadriceps nerve, b and c show the i.p.s.p's 

 evoked in a gastrocnemius motoneuron by group I volleys in the nerves to 

 plantaris and flexor digitorum longus, respectively (Eccles, Eccles and Lundberg, 

 1957). Series d-h show intracellular i.p.s.p."s recorded as in a-c, but evoked by 

 antidromic volleys in the ventral root, which were progressively larger from d-g. 

 With G and h the volleys were maximal, but h was recorded at much slower sweep 

 speed (Eccles, Fatt and Koketsu, 1954). 



motoneurons by recurrent impulses in the axon collaterals of motoneurons 

 (Fig. 5d-h; Eccles, Fatt and Koketsu, 1954). The central latency for this 

 inhibitory effect indicates that there is no more than one interneuron in the 

 inhibitory pathway. At least one interneuron also appears to be interpolated 

 in the inhibitory pathways to motoneurons from groups II and III muscle 

 afferent impulses, and from cutaneous and joint afferents. When the central 

 latencies for the excitatory and inhibitory actions of group la and Tb muscle 

 impulses on the cells of origin of the dorsal and ventral spinocerebellar tracts 

 are compared, there is the same discrepancy of about 0-8 msec that was found 

 with the la action on motoneurons, so hkewise an additional interneuron on 

 the inhibitory pathway may be inferred. Recently also a descending volley in 

 the pyramidal tract of the monkey was found to have an inhibitory action with 



