Facilitatory and excitatory interaction between motoneurons of adjacent segments in the spinal cord of the frog

Abstract

Facilitation and excitation of motoneurons through antidromic impulses in ventral root fibers of an adjacent segment in the spinal cord of Xenopus laevis were investigated. The method of monosynaptic testing was employed. Orthodromically elicited monosynaptic ventral root reflex responses were strongly facilitated by antidromic volleys of impulses in neighboring ventral root fibers. The latency of this facilitatory effect was apparently too short to assume the presence of chemically transmitting synapses in the pathway. Electrical interaction between motoneurons seems to be the more likely explanation. In some preparations motoneurons were excited to the firing level in response to an antidromic stimulus only, when this was applied to neighboring ventral root fibers. This response was greatly facilitated by a preceding conditioning orthodromic volley. The latency of the response to the antidromic stimulus was relatively long and comparable to that of monosynaptic activation. These results might be accounted for by assuming synaptically mediated excitation through recurrent axon collaterals which terminate on neighboring motoneurons. The mechanism of this interaction between neighboring motoneurons therefore remains uncertain.