Synaptic Excitation in Spinal Motoneurons Alternates with Synaptic Inhibition and Is Balanced by Outward...

Area of Science:

• Neuroscience
• Computational Neuroscience
• Cellular Neuroscience

Background:

• Neurons in active networks can enter a high-conductance state, observed in spinal motoneurons during rhythmic motor behavior.
• Previously, this state was attributed solely to balanced inhibition and excitation, assuming no change in intrinsic conductance.

Purpose of the Study:

• To investigate the mechanisms underlying the high-conductance state in spinal motoneurons during functional network activity.
• To determine the contribution of intrinsic properties versus synaptic inputs to motoneuron firing patterns.

Main Methods:

• Electrophysiological recordings from red-eared turtle spinal motoneurons.
• Direct recording of synaptic currents during network activity.
• Analysis of membrane potential fluctuations and conductance changes near action potential threshold.

Main Results:

• Intrinsic outward rectification, mediated by voltage- and Ca2+-gated K+ channels, significantly contributes to the high-conductance state.
• Outward rectification facilitates spiking by focusing synaptic depolarization near the action potential threshold.
• Motoneurons are activated by out-of-phase excitation and inhibition peaks, while continuous low-level inputs may cause irregular firing.

Conclusions:

• Intrinsic outward rectification plays a critical role in balancing synaptic excitation and maintaining motoneuron membrane potential near spike threshold during network activity.
• The interplay between intrinsic properties and synaptic dynamics shapes neuronal firing patterns in active neural networks.