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

STUDY PURPOSE: Sensory feedback to the spinal cord is essential for motor control. After injury to the central nervous system, such as spinal cord injury, the mechanisms that regulate sensory feedback become unbalanced leading to poor motor control, spasticity and pain. Unfortunately, our understanding of sensory feedback is relatively limited, even in the uninjured nervous system, making the treatment of motor control issues related to sensory feedback challenging. It was previously believed that sensory feedback was regulated by specialized GABAergic neurons that inhibited sensory inputs in the spinal cord, termed presynaptic inhibition. Recent findings in rodents shows that these GABAergic pathways actually facilitate conduction along afferents in the spinal cord by depolarizing the afferent at vulnerable branch points and reducing branch point failure. Based on these recent findings, we evaluated similar pathways in uninjured humans, with future hopes of evaluating these pathways in participants with spinal cord injury. DATA COLLECTED: The Ia - motoneuron pathway was measured using an H-reflex, evoked below half the maximum amplitude in either the soleus muscle (stimulation of the tibial nerve at the popliteal fossa) or the abductor hallucis (AbHal) muscle (stimulation of the tibial nerve at the popliteal fossa or at the medial malleolus). The peak-peak amplitude of the H-reflex was measured before (test) and after (cond) a conditioning stimulation to putative active GABAergic networks in the spinal cord and primary afferent depolarization (PAD) in the test (soleus or AbHal) afferents. Presynaptic effects of the conditioning stimulation on the test Ia afferents were discerned from postsynaptic effects on the test motoneurons using single motor unit and EMG analysis with the conditioning stimulation applied alone, thus giving information about the isolated postsynaptic effects of the conditioning stimulation. The probability of discharge of a tonically firing soleus single motor unit at the latency of the H-reflex (after tibial nerve stimulation) was also compared before and after a conditioning stimulation to putative evoked PAD. Specifically, the cutaneous branch of the superficial fibular nerve (SFN) was electrically stimulated on the top of the foot at an intensity just above perception threshold at various interstimulus intervals (ISIs) between 0 and 300 ms to assess short-lasting phasic PAD (200 Hz, 10 ms) and at longer intervals (out to 130 seconds) to assess long-lasting tonic PAD (0.2 and 2 Hz for 10s, 200 Hz for 500 ms) using the soleus H-reflex. Similarly, the common fibular nerve (CFN) was electrically stimulated (near the fibular head on the ipsilateral leg) at motor threshold (1.0 x MT) 0-300 ms before activation of the soleus H-reflex. A heteronymous H-reflex in the Biceps Hetero (stimulation of the tibial nerve supplying the medial gastrocnemius) was also conditioned by SFN and CFN stimulation. Activation of the corticospinal tract (CST) using transcranial magnetic stimulation (TMS; 0.9 x active motor threshold) of the contralateral motor cortex was used to condition the soleus and AbHal H-reflex and assess short-lasting phasic PAD. A peristimulus frequencygram (PSF) of the firing rate of a tonically discharging single motor unit in response to the putative PAD-evoking conditioning stimulation over many trials was created, with the time of the conditioning stimulation time-locked to 0 ms. The PSF was binned into 20 ms averages and the average firing rate at each bin was compared to the time when an H-reflex would have occurred at the various ISIs. A similar analysis was done for the EMG recorded during a small contraction. Self-facilitation of the soleus H-reflex was also measured by comparing H-reflexes evoked every 5 seconds. The average peak-peak amplitude of the first 7 H-reflexes were compared to the peak-peak amplitude of the next 7 H-reflexes. A more detailed description of the data collected can be found in the accompanying paper. DATA USAGE NOTES:

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