Facilitation of sensory axon conduction to motoneurons during cortical or sensory evoked primary afferent depolarization (PAD) in humans

Sensory and corticospinal (CST) pathways activate spinal GABAergic interneurons with axo-axonic connections onto proprioceptive (Ia) afferents that depolarize these afferents (termed primary afferent depolarization, PAD). In rodents sensory-evoked PAD is produced by GABAA receptors at nodes of Ranvier in Ia-afferents, rather than at presynaptic terminals, and facilitates action potential propagation to motoneurons by preventing branch point failures, rather than causing presynaptic inhibition. Here we examined if PAD likewise facilitates the Ia-afferent mediated H-reflex in humans by evoking PAD with both sensory and CST stimulation. H-reflexes in several lower limb muscles were facilitated by prior conditioning from low-threshold proprioceptive, cutaneous or CST pathways, with a similar time course ([~]200 ms) to the PAD measured in rodent Ia-afferents. Long trains of repeated cutaneous or proprioceptive afferent stimulation produced long-lasting facilitation of the H-reflex for up to 2 minutes, consistent with the tonic depolarization of rodent Ia-afferents mediated by nodal 5-GABA receptors for similar stimulation trains. Facilitation of the conditioned H-reflexes was not mediated by direct facilitation of the motoneurons because isolated stimulation of sensory or CST pathways did not modulate the firing rate of tonically activated motor units in tested muscles. Furthermore, cutaneous conditioning increased the firing probability of a single motor unit during the H-reflex without increasing its firing rate at this time, indicating that the underlying excitatory postsynaptic potential (EPSP) was more probable, but not larger. These results are consistent with sensory and CST pathways activating nodal GABAA receptors that reduce intermittent failure of action potentials propagating into Ia-afferent branches. Key Points SummaryO_LIThe control of posture and movement requires peripheral sensory feedback, which was previously thought to be inhibited by specialized GABAergic neurons in the spinal cord. C_LIO_LIBased on new findings in rodents, we provide evidence in humans that sensory and corticospinal pathways that likely activate these GABAergic pathways facilitate, rather than inhibit, the flow of sensory feedback in afferents that carry information about body position, movement and effort. C_LIO_LIThese new findings of how sensory and descending pathways facilitate this sensory feedback to spinal motor neurons can now be applied to people with injury to the brain or spinal cord where these GABA neurons are affected, allowing us to understand how altered sensory control may affect residual motor function and the production of involuntary muscle spasticity. C_LI