Motoneuron Excitability and Synaptic Modulation in Spinal Systems

Motoneurons represent the final common pathway through which all motor commands reach skeletal muscle. Their excitability is determined by a complex interplay of intrinsic membrane properties, synaptic inputs and neuromodulatory influences arising from brainstem and spinal circuits. Central to this interplay are persistent inward currents (PICs), voltage-sensitive conductances that amplify synaptic depolarisation and sustain firing beyond the initiating stimulus. Synaptic modulation via inhibitory circuits—including reciprocal and recurrent inhibition—serves to fine-tune the gain of motoneuron output, preventing runaway excitation. Neuromodulators such as serotonin and noradrenaline adjust the activation threshold and amplitude of PICs, thereby altering the input–output relationship of motoneurons across a wide dynamic range of motor tasks. Recent advances have elucidated molecular mechanisms underpinning bistability and plateau potentials in motoneurons, the contribution of PICs to motor unit discharge patterns, and non-pharmacological approaches to modulate spinal excitability. These insights carry implications for rehabilitation in spinal cord injury, spasticity management and the design of bioinspired neuroprosthetic systems.

Research from Nature Portfolio

Investigations into the molecular basis of motoneuron bistability have identified thermosensitive Na+-permeable channels as key drivers of activity-dependent plateau potentials. Genetic or pharmacological disruption of these channels abolishes bistable firing and impairs postural tone and locomotor amplification in animal models, demonstrating their essential role in sustaining motoneuron output under varying thermal or metabolic conditions. In parallel, human studies probing long-term involuntary muscle contractions reveal a modulatory role for serotonergic transmission in prolonged reflex activity. Blocking serotonin receptors accelerates the decay of sustained electromyographic activity following induced muscle contractions, confirming that monoaminergic mechanisms extend beyond immediate gain control to influence the time course of motoneuron excitability during prolonged tasks.

Motoneuron Excitability and Synaptic Modulation in Spinal Systems publication trend

The graph below shows the total number of articles in motoneuron excitability and synaptic modulation in spinal systems across all publications each year (not limited to Nature Index journals).

Technical terms

Persistent Inward Currents (PICs): Sustained depolarising currents in motoneurons generated by voltage-gated sodium and calcium channels that amplify synaptic inputs and support prolonged firing.

Neuromodulation: Regulation of neuronal excitability and synaptic efficacy by neurotransmitters acting on metabotropic receptors, notably serotonin and noradrenaline in spinal motor circuits.

Bistability: The capacity of motoneurons to switch between distinct firing and silent states, typically mediated by plateau potentials and intrinsic conductances.

Plateau Potential: A prolonged depolarised state of the motoneuron membrane maintained by persistent inward conductances, enabling sustained firing after transient excitation.

Reciprocal Inhibition: A form of synaptic inhibition in spinal circuits whereby activation of antagonist muscles inhibits motoneurons supplying the agonist, regulating force output and preventing co-contraction.

References

1. Supercomputer framework for reverse engineering firing patterns of neuron populations to identify their synaptic inputs. eLife (2024).
2. A geometric approach to quantifying the neuromodulatory effects of persistent inward currents on individual motor unit discharge patterns. Journal of Neural Engineering (2023).
3. Influence of serotonin on the long-term muscle contraction of the Kohnstamm phenomenon. Scientific Reports (2025).
4. Effects of reciprocal inhibition and whole‐body relaxation on persistent inward currents estimated by two different methods. The Journal of Physiology (2022).
5. Trpm5 channels encode bistability of spinal motoneurons and ensure motor control of hindlimbs in mice. Nature Communications (2021).