How shunting inhibition affects the discharge of lumbar motoneurones: a dynamic clamp study in anaesthetized cats

Abstract

In the present work, dynamic clamp was used to inject a current that mimicked tonic synaptic activity in the soma of cat lumbar motoneurones with a microelectrode. The reversal potential of this current could be set at the resting potential so as to prevent membrane depolarization or hyperpolarization. The only effect of the dynamic clamp was then to elicit a constant and calibrated increase of the motoneurone input conductance. The effect of the resulting shunt was investigated on repetitive discharges elicited by current pulses. Shunting inhibition reduced very substantially the firing frequency in the primary range without changing the slope of the current-frequency curves. The shift of the I-f curve was proportional to the conductance increase imposed by the dynamic clamp and depended on an intrinsic property of the motoneurone that we called the shunt potential. The shunt potential ranged between 11 and 37 mV above the resting potential, indicating that the sensitivity of motoneurones to shunting inhibition was quite variable. The shunt potential was always near or above the action potential voltage threshold. A theoretical model allowed us to interpret these experimental results. The shunt potential was shown to be a weighted time average of membrane voltage. The weighting factor is the phase response function of the neurone that peaks at the end of the interspike interval. The shunt potential indicates whether mixed synaptic inputs have an excitatory or inhibitory effect on the ongoing discharge of the motoneurone.

How Much Afterhyperpolarization Conductance Is Recruited by an Action Potential? A Dynamic-Clamp Study in Cat Lumbar Motoneurons

“…Finally, the broad distribution of the AHP conductance suggests that motoneurons constitute a heterogeneous population in which the responsiveness to synaptic inputs varies from one cell to the other. A similar heterogeneity was found in a previous study (Brizzi et al, 2004) in which we showed that motoneurons exhibited large differences in their sensitivity to shunting inhibition.…”

“…This is the recommended mode for dynamic-clamp experiments (Brizzi et al, 2004;Prinz et al, 2004) because it allows for reliable measurements of the membrane potential, even when large currents are injected. Identification of motoneurons rested on the observation of an antidromic action potential after the electrical stimulation of their axon in a peripheral nerve.…”

Linear Summation of Cat Motor Cortex Outputs

“…We suggest that the interaction between cortical points involves a balance between excitation and inhibition such that the interaction results in near-linear summation of outputs. Detailed analyses of the effects of balancing excitation and inhibition on the firing properties of neurons have been done previously (Capaday, 2002;Chance et al, 2002;Ulrich, 2003;Brizzi et al, 2004;Meunier and Borejsza, 2005;Capaday and Van Vreeswijk, 2006). When a cortical point is disinhibited, the activation threshold of the in situ neurons is reduced and their firing rate increased in response to, for example, corticocortical inputs evoked by ICMS of a nearby point.…”

Mixed Mode Oscillations in Mouse Spinal Motoneurons Arise from a Low Excitability State

“…All motoneurons retained for analysis had a resting membrane potential more hyperpolarized than Ϫ60 mV and an overshooting action potential. All recordings and dynamic clamp experiments were performed using the discontinuous current-clamp mode (7-9 kHz) of the amplifier because it allows for reliable measurements of the membrane potential, even when large currents are injected (Brizzi et al, 2004). The input resistance was determined from the responses to a series of small-amplitude square current pulses (Ϫ1.5 to ϩ 1.5 nA) as in the study by Manuel et al (2009).…”