Abstract
Spontaneous and evoked spinal activities interact to set the characteristics of emergent motor responses. Gamma motor neurons have feedforward and feedback functions in motor control, which are crucial for transforming motor commands into action. Meanwhile, the intrinsic excitability and functional connectivity of alpha motor neurons determine the accuracy of actions. In this study, we investigated the effects of trans-spinal direct current stimulation (tsDCS) on spontaneous and cortically evoked activity of well-isolated single units of gamma and alpha motor neurons in mice. We also investigated the effects of tsDCS on reflexive and locomotor actions. In general, motor neurons showed increased responses to cathodal tsDCS (c-tsDCS) and decreased responses to anodal tsDCS (a-tsDCS). These effects were observed for cortically evoked discharges and spontaneous firing rates of gamma motor neurons, cortically evoked discharges of larger alpha motor neurons, and spontaneous firing rates of smaller alpha motor neurons. An exception was that spontaneous firing rates of larger alpha motor neurons showed the opposite pattern of reduction by c-tsDCS and increase by a-tsDCS. Reflexive and voluntary behavior were also increased by c-tsDCS and reduced by a-tsDCS. Specifically, the amplitude and duration of crossed and tail pinch reflexes in decerebrate animals and the quality of ground and treadmill walking patterns in healthy awake animals showed this pattern. These polarity-specific changes in behavior could be attributed to polarity-mediated modulation of alpha and gamma motor neuron activity and spinal circuitry. The results reveal an important principle: effects of tsDCS on spinal motor neurons depend on current polarity and cell size.
Highlights
Direct current stimulation (DCS) offers a new method to manipulate ongoing behavior, subsequent plasticity, and learning
Results trans-spinal direct current stimulation (tsDCS) modulates spontaneous activity of gamma and alpha motor neuron ensemble In these experiments, tsDCS was applied for a short duration (20 sec)
Neurons were modulated differently by tsDCS, which implies that tsDCS affects neurons differently than their input source
Summary
Direct current stimulation (DCS) offers a new method to manipulate ongoing behavior, subsequent plasticity, and learning. Anodal trans-spinal DCS (a-tsDCS) increases the spontaneous firing rate of motor neurons and reduces their evoked responses. Cathodal trans-spinal DCS (c-tsDCS) does the opposite. This DC action changes the magnitude and direction of spinal plasticity (Aguilar et al 2011; Ahmed 2011). Increasing the spontaneous-to-evoked activity ratio impairs externally driven learning experiences, and decreasing this ratio increases the sensitivity of neurons to externally driven learning experiences. This pattern implies that the modification in a 2016 The Authors.
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