Fig. 1.

Spinal proprioceptive pathways under study. A schematic representation of three sensory pathways transmitting inputs from muscle group I afferents to extensor motoneurons (Ext Mn) is shown to the left: the monosynaptic (stretch reflex) pathway (from group IA afferents originating in muscle spindles of extensors), the disynaptic inhibitory pathway (from group IB afferents of extensors originating in Golgi-tendon organs plus some group IA fibers), and the polysynaptic excitatory pathway (from group IB and IA afferents of extensors). In the acute spinal cat, this latter pathway shares interneurons with the network generating the excitatory locomotor drive in extensors (box E). Sample records of motoneuronal postsynaptic potentials used for measurements are on theright. a, The amplitude of monosynaptic EPSPs was measured at a latency of 1.4 msec (rising phase in this example; i.e., just before the onset of possible disynaptic components). b, The disynaptic IB inhibition was evoked by a short train of stimuli (6 pulses, 1.4–2.0 T, 200–300 Hz), and the IPSP amplitude was measured at the maximal negative deflection in the intracellular trace. Note that there were often monosynaptic EPSPs (six positive humps) overriding the inhibitory trough (dotted line). c, Polysynaptic excitation was evoked by a similar short train of stimuli, and the amplitude was measured at the maximal positive deflection (dotted line) underlying monosynaptic EPSPs.