A novel method to estimate discharge-independent inhibition durations of spinal and brainstem circuits in humans

Direct recordings from human motoneurons are not feasible; therefore, researchers have developed indirect methods to estimate postsynaptic potential profiles, i.e. the functional inhibition or excitation, on firing motor units. Surface and intramuscular electromyography have shown that the duration of the functional inhibition varies depending on the neural circuit investigated and is influenced by stimulus intensity and muscle activity level. This study aimed to standardise the estimation of functional inhibition durations across three distinct spinal and brainstem circuits by leveraging the known dependence of inhibition duration on background motor unit discharge rate. We analysed data from previous rat brain slice experiments in which known currents were injected into regularly discharging motoneurons. Regression of injected inhibition duration against discharge rate revealed a strong predictive relationship when extrapolated, accurately converging on the known duration. Specifically, this regression yielded the actual inhibition duration at a discharge rate of 0.98 imp/s [range: 0, 5.91 imp/s]. Building on these findings, we conducted three inhibition paradigms in human volunteers, targeting the masseter inhibitory reflex, the cutaneous silent period and recurrent inhibition mediated by Renshaw cells. Using extrapolated correlation plots of motor unit discharge rate versus functional inhibition duration, we derived discharge rate-independent inhibition durations. All three circuits demonstrated longer inhibition duration ranges than previously reported. This standardised approach enables more accurate estimation of inhibition duration across various circuits, independent of discharge rate. It holds promise for clinical applications in the early diagnosis and monitoring of neurological disorders affecting inhibitory circuits.