[Characteristics of synaptic responses of motoneurons to ventrolateral funiculus stimulation in vitro]

By using the intracellular recording techniques, the electrophysiological characteristics of fast excitatory postsynaptic potentials (EPSP) evoked by ventrolateral funiculus (VLF) stimulation were analyzed in neonate rat motoneurons (MN) of spinal cord slices. The incidences of VLF-EPSPs was 80% (n = 28), among which 2 is preceded by inhibitory postsynaptic potential and 6 followed by slow EPSPs. Considering the distribution skewness of VLF-EPSP latency histogram, it was suggested that short- and long-latency EPSPs correspond respectively to mono- and poly-synaptic transmission respectively. The possible neurotransmitters mediating VLF-EPSPs were excitatory amino acids and non-NMDA receptors were critically involved in these synaptic transmissions, for both VLF-EPSP and glutamate-induced response were similarly and almost completely abolished by kynurenic acid and DNQX. Typical spatial summation of VLF-EPSPs and EPSPs evoked by ventral root stimulation were observed in the same recorded MNs. Most importantly, VLF-EPSPs were found to increase with hyperpolarizing and decreased with depolarizing the membrane, indicating that the descending fibers in the VLF may terminate on the soma or proximal dendrites of MNs, while the EPSPs evoked by the dorsal or ventral root stimulation were insensitive to membrane potential changes, indicating that the primary sensory afferents in the dorsal and ventral roots may contact the distal dendrites of the MNs. The results imply that the commanding signals conducting along the descending fibers in the VLF may directly modulate the MN's activities, whereas messages from the periphery may cause fine changes of the membrane potential subject to integration of the MNs.