INHIBITORY PATHWAYS TO MOTONEURONS 



53 



Fig. 4. Diagram showing time course of events during Ta inhibitory action on a 

 monosynaptic reflex splice. The pathways are shown to the left with the mono- 

 synaptic excitatory line in black and the la inhibitory line in interrupted black. 

 The remainder of the diagram is constructed both on the same time scale, as 

 shown above, and on the spatial scale of the diagram to the left, except that the 

 potential records are also shown in the conventional manner as rising from base 

 lines at the respective recording sites: dorsal root entry, DR; the motoneuron, M; 

 and the ventral root, VR. The slopes of the lines on the spatial-temporal co- 

 ordinates give the velocities, and delays at regions of junctional transmission are 

 given by lengths of the horizontals there. The spikes discharges by a and b are 

 shown propagating into the ventral root. An I afferent spike arriving synchron- 

 ously with E, as shown by records at DR, and having an equivalent length of 

 central pathway, is shown delayed for 0-75 msec at the intermediate cell (/C) 

 relay; and then after propagating to the motoneuron, having a further delay of 

 0-3 msec before initiating the i.p.s.p. after a total central latency of 1-35 msec. 

 This i.p.s.p. is just produced in time to delay or suppress (cf. Fig. 3k) all reflex 

 discharges after b (for example c). and so is just able to diminish the height of 

 the reflex spike as shown by the hatched area (Araki et al., 1960). 



remain virtually unchanged. With inhibitory action from group Tb afferent 

 volleys, the central latency is usually very little longer than with la inhibition 

 (cf. Fig. 5b with a) the pathway being consequently assumed to be disynaptic, 

 though sometimes the duration is long enough to indicate a trisynaptic 

 pathway (Eccles, Eccles and Lundberg, 1957). Moreover, the time course of 

 the i.p.s.p. is often sufficiently long (Fig. 5c) to indicate that either there are 

 relays through polysynaptic pathways or that there is a brief repetitive 

 discharge of the inhibitory neurons. More prolonged repetitive discharges of 

 inhibitory neurons and consequently a prolonged i.p.s.p. are produced in 



