Antagonist tendon vibration dampens estimates of persistent inward currents in motor units of the human lower limb

Abstract

We can readily measure motoneuron discharge patterns in humans due to the one-to-one spike relation between motoneuron and muscle fiber action potentials, which allows us to make inferences about motor commands. Persistent inward currents (PICs), which provide gain control of motoneuronal output, are facilitated by monoaminergic input from the brainstem. This monoaminergic input is greatly diffuse, but resulting PICs are highly sensitive to inhibitory inputs. Antagonist muscle stretch, and thus Ia input from the antagonist decreases PIC magnitudes in the decerebrate cat. In the present study, we explored whether estimates of PICs are altered with vibratory input to antagonist muscles in humans. MUs of the tibialis anterior (TA), soleus (SOL), and medial gastrocnemius (MG) were discriminated using high-density surface electromyography and convolutive blind source separation. We estimated PICs using the paired MU analysis technique, which quantifies discharge rate hysteresis ({Delta}F) by comparing the discharge rate of a lower-threshold MU at the onset and offset of a higher-threshold MU. Participants performed isometric plantarflexion and dorsiflexion contractions to a peak of 30% of maximal voluntary contraction, with 10 s ascending and descending phases. In half of the trials, we applied vibration to the antagonist tendon and found that {Delta}F in agonist MUs decreased in the presence of vibration. These findings suggest that inhibition from the antagonist muscle, most likely Ia reciprocal inhibition, can reduce discharge rate hysteresis. This provides insights about non-invasive methods potentially capable of dampening PICs in hyperexcitable motoneurons, which are manifest in some neurological impairments. KEY POINTSO_LIPersistent inward currents in motoneurons amplify synaptic inputs and thus have a major impact on motor unit firing patterns. C_LIO_LIWe show that sustained vibration to the antagonist tendon reduces estimates of persistent inward currents ({Delta}F) of the contracting muscle in both the plantarflexors and dorsiflexors. C_LIO_LIThese findings provide evidence for the important role of sensory input in the control of persistent inward currents in the human. C_LIO_LIReciprocal inhibition may help refine neuromodulatory commands to tailor motor unit activation to diverse movement patterns and specific tasks, and loss of inhibition may exacerbate symptoms of neurological impairment. C_LI