Trans-Spinal Direct Current Stimulation Targets Ca²⁺ Channels to Induce Persistent Motor Unit Responses

Area of Science:

• Neuroscience
• Motor Control
• Neuromodulation

Background:

• Trans-spinal direct current stimulation (tsDCS) is used to improve motor function after neurological injury.
• The underlying mechanisms of tsDCS on the motor system are not fully understood.

Purpose of the Study:

• To investigate the role of persistent inward current (PIC)-like responses in motoneurons in mediating tsDCS effects.
• To determine if tsDCS modulates Ca2+ conductances in motoneurons.

Main Methods:

• Recorded single motor unit activity in rat extensor and flexor carpi radialis muscles.
• Applied cathodal and anodal tsDCS (c-tsDCS; a-tsDCS) and administered L-type calcium channel blocker Nimodipine.
• Utilized a two-compartment neuronal model for computer simulation.

Main Results:

• Both c-tsDCS and a-tsDCS increased motor unit firing; c-tsDCS produced a longer-lasting persistent firing (165s) than a-tsDCS (27s).
• Nimodipine reduced firing during c-tsDCS and blocked the persistent response, while it blocked the short persistent response after a-tsDCS.
• Computer simulations replicated experimental findings, indicating c-tsDCS targets Ca2+ conductances.

Conclusions:

• Cathodal tsDCS augments motoneuron activity by targeting Ca2+ conductances, leading to persistent inward currents.
• This mechanism provides a basis for tsDCS in rehabilitation strategies to enhance muscle force and motor function post-injury.