Evidence for restricted central convergence of cutaneous afferents on an excitatory reflex pathway to medial gastrocnemius motoneurons.

1. We previously reported that excitatory postsynaptic potentials (EPSPs) produced by low-threshold electrical stimulation of the caudal cutaneous sural nerve (CCS) occur preferentially and with the shortest central latencies in the medial gastrocnemius (MG) portion of the triceps surae motor nuclei. The present study employs the spatial facilitation technique to assess interneuronal convergence on the short-latency excitatory pathway from CCS to MG by several other ipsilateral hindlimb afferents [the lateral cutaneous sural (LCS), caudal cutaneous femoral (CCF), saphenous (SAPH), superficial peroneal (SP), posterior tibial (TIB), and posterior articular (Joint) nerves]. 2. Spatial facilitation of CCF EPSPs in MG motoneurons was demonstrated with conditioning stimulation of the LCS, CCF, SAPH, SP, and TIB nerves, but was most readily and consistently observed with CCF conditioning. Facilitation of CCS and CCF EPSPs was obtained in individual MG motoneurons with a wide range of condition-test intervals. 3. CCF EPSPs in MG motoneurons produced by twice threshold (2T) afferent stimulation had a mean latency of 4.8 ms and often appeared as slowly rising, asynchronous potentials. On the other hand, 2T CCS EPSPs had a mean latency of 2.8 ms and appeared as sharper rising, less variable depolarizations. The optimum condition-test interval for facilitation of CCS and CCF EPSPs was found to be 5.2 ms on average, with CCS stimulation delayed from that of CCF. The longer latency of CCF EPSPs and the finding that the minimum condition-test interval was on the order of 3.9 ms suggests that convergence occurs late in the excitatory CCF pathway to MG motoneurons. 4. Convergence between excitatory pathways to MG from CCF and CCS afferents is discussed with regard to the original observations of Hagbarth on the location of cutaneous receptive fields and excitation of ankle extensors. In addition, evidence for the segregation of these specialized reflex pathways from those involved in general flexion reflexes is discussed.