Reorganization of Ia afferent synapses on motoneurons after peripheral nerve injuries

After peripheral nerve injuries patients lose and do not recover the stretch reflex which leads to altered locomotor function. The focus of this thesis is to investigate the structural integrity of the central connection between Ia afferents and alpha motoneurons that mediate the stretch reflex. The overall hypothesis is that the density and distribution of Ia synapses on motoneurons is altered after peripheral nerve injuries. Analysis of Ia afferent-motoneuron contacts, revealed by vesicular glutamate transporter 1 (VGLUT1) immunoreactivity, on the soma and dendritic arbor of motoneurons after peripheral nerve injuries revealed major reorganizations in the distribution and density of Ia synapses. Synaptic stripping of Ia afferent synapses occurred on the soma and proximal dendrites and appeared to be permanent even after reinnervation; in contrast, VGLUT1 synapses on distal dendrites were unchanged. In conclusion, after peripheral nerve injuries motoneurons are contacted by fewer Ia synapses and they are more distally located. This overall reorganization likely weakens the input and may contribute to stretch reflex anomalies.