Effect of standing posture on inhibitory postsynaptic potentials in gastrocnemius motoneurons

Abstract

This study examined the task dependence of sensory inputs on motoneuron excitability by comparing the inhibitory postsynaptic potential (IPSP) evoked by stimulation of the sural nerve between a standing postural task (Free Standing) and a comparable voluntary isometric contraction performed in a supine position (Lying Supine). We hypothesized that there would be a smaller IPSP in standing than in the supine position, based on the task dependence of the ankle plantarflexor activity on the standing task. Ten healthy participants participated in a total of 15 experiments. Single motor unit (MU) firings were recorded with both intramuscular fine-wire electrodes and high-density surface electromyography. Participants maintained the MU discharge at 6-8 Hz in Free Standing or Lying Supine while the right sural nerve was stimulated at random intervals between 1 and 3 s. To evaluate the reflex response, the firing times of the discriminated MUs were used to construct peristimulus time histograms and peristimulus frequencygrams. The sural nerve stimulation resulted in weaker inhibition in Free Standing than in Lying Supine. This finding is discussed in relation to the putative activation of persistent inward currents in standing posture and the task-dependent advantages of overriding inhibitory synaptic inputs to the plantarflexors to maintain the standing posture. NEW& NOTEWORTHY The task-dependent modulation of sensory inputs on motoneuron excitability in standing is not well understood. Evoking an inhibitory postsynaptic potential (IPSP) resulted in a smaller IPSP in gastrocnemius motoneurons in standing than in the supine position. Mildly painful sensory inputs produced weaker motoneuron inhibition in standing, suggesting an imperative to maintain ankle plantarflexion activity for the task of upright stance.

Estimating Exercise-Induced Changes in Human Neuronal Networks

“…As detailed in the section on "Reconsidering Functional Connections of the Human Neuronal Networks Using the PSF Method," several studies have already used the PSF method to revise movement-related neuronal networks' functional connection in the human nervous system. Besides my laboratory in Istanbul, five other motor control laboratories worldwide have started to use this technique to revise some "established" neuronal networks (41)(42)(43)(44)(45). Only after establishing the correct range of values for a neuronal network using PSF analysis can one quantify any change in them due to an exercise test.…”

“…With the PSF method used for investigating functional connections of neuronal networks in human subjects now well recognized (41)(42)(43)(44)(45), several studies have been conducted to reconsider the classical literature on several issues:…”