Supraspinal facilitation of cutaneous poly synaptic early excitatory synaptic potentials in cat medial gastrocnemius moto neurons

Pinter M. J.; Burke R. E.; O&apos; donovan M. J.; Dum R. P.

The characteristics of postsynaptic potentials (PSP) produced an antidromically-identified medial gastrocnemius (MG) .alpha.-motoneurons was studied by electrical stimulation of low threshold (< 3 .times. T) distal limb cutaneous afferents in the sural (SUR) nerve in adult cats anesthetized with .alpha.-chloralose. The effects on SUR PSP of supraspinal conditioning stimulation of the contralateral red nucleus (RN) and pyramidal tract (PT) were also studied. In the majority of MG motoneurons, SUR afferents with electrical thresholds < 1.5 .times. T produced early excitatory synaptic potentials (EPSP) with minimum central latency of .apprx. 2.0 ms, suggesting activation of a trisynaptic segmental pathway with 2 interposed interneurons. Such early EPSP were often detectable with stimuli < 1.2 .times. T, as determined by recording the compound action potential in the sciatic nerve and from the 1st appearance of the N1 wave of the cord dorsum potential. Inhibitory synaptic potentials (IPSP) were regularly produced by SUR volleys of only slightly greater strength (often as low as 1.3 .times. T) and these had minimum central latencies of .apprx. 3.0 ms (.apprx. 1.0 ms longer than the earliest EPSP), suggesting a 3 interneuron central pathway. Repetitive stimulation of RN and PT regularly produced facilitation of both EPSP and IPSP components in the SUR response, suggesting that these supraspinal systems directly or indirectly excite some of the same interneurons that convey the SUR effects to MG motoneurons. When using very low strength SUR stimuli, PT conditioning produced relatively pure facilitation of the SUR EPSP but with larger SUR volleys, PT clearly facilitated both EPSP and IPSP. RN conditioning produced more parallel facilitation of SUR EPSP and IPSP. Supraspinal control of the polysynaptic pathway producing SUR EPSP is important because of earlier evidence that this pathway is differentially distributed to motoneurons of fast twitch vs. slow twitch MG motor units.