Differential control of short latency cutaneous excitation in cat FDL motoneurons during fictive locomotion.

Low threshold (less than or equal to 2 x T) cutaneous afferents in the superficial peroneal (SP) and medial plantar (PLNT) nerves both produce short-latency excitatory postsynaptic potentials (EPSPs) in flexor digitorum longus (FDL) alpha-motoneurons, with minimum central latencies (less than or equal to 1.8 ms) that indicate a disynaptic connection. However, SP and PLNT EPSPs in FDL motoneurons are differentially modulated during fictive stepping in decerebrate cats. The early components in SP EPSPs are systematically enhanced during the early flexion phase of fictive stepping (Schmidt et al. 1988) while those in PLNT EPSPs are markedly depressed during flexion. In addition, transmission in the PLNT----FDL pathway is enhanced during occasional step cycles in which the FDL displays firing during the extension phase. This enhancement affects only the trisynaptic components of PLNT EPSPs, is simultaneous with the extension FDL burst, and is not found in SP EPSPs. These results indicate that the SP----FDL and PLNT----FDL pathways are composed of entirely different sets of segmental last-order interneurons, each of which receives sensory information from contiguous, relatively limited regions of skin on the most distal parts of the hindpaw. Possible functional consequences of these interneuronal organizations are discussed.