Moore, NJ; (2015) Recurrent excitation and inhibition in the Renshaw cell-motoneuron circuit of the lumbar spinal cord. Doctoral thesis , UCL (University College London).

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Abstract

Motor output from spinal motoneurons is influenced by interneuron networks in the ventral horn of the spinal cord. This thesis presents electrophysiologi- cal investigations of two separate but complementary aspects of the neuronal networks that influence this motor output. The first investigation focuses on inhibition of lumbar motoneurons. The second characterises the excitatory synapse formed by motoneuron axon collaterals onto Renshaw cells, which are interneurons that mediate recurrent inhibition onto motoneurons. Previous studies on neonatal rats have shown that inhibition of motoneu- rons is mediated a mixed GABAergic and glycinergic response. Whole- cell voltage-clamp recordings of spinal motoneurons obtained from juvenile (P 8 − 14) mice demonstrated that motoneuron inhibition is mostly mediated by glycine. GABA currents were not co-detected with glycine during this age range in the mouse. Further experiments, in which the relative content of pre-synaptic GABA and glycine was manipulated, showed that GABA is not co-released with glycine by premotor interneurons. Quantal analysis of paired recordings of pre-synaptic motoneurons and post-synaptic Renshaw cells showed that this excitatory synapse exhibits a large number of release sites and a high probability of release. This is suggestive of highly reliable synaptic transmission between the two cell types. Comparison of the number of release sites estimated from paired recordings with those estimated from responses evoked by ventral root stimulation revealed that on average six motoneurons project onto every Renshaw cell. We conclude that: • In mature animals motoneuron inhibition is mainly glycinergic. • The Renshaw cell to motoneuron synapse has a high efficiency of transmission. • The degree of convergence of motoneurons to Renshaw cells is very high. The last two conclusions suggest that firing in motoneurons pools reliably induces firing in the population of connected Renshaw cells.

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