Paired Stimulation to Promote Lasting Augmentation of Corticospinal Circuits

Figure 1

Three paired stimulation models. The corticospinal tract (CST) connects motor cortex directly to the spinal cord. The termination of the CST is largely (>80%) onto interneurons in humans and exclusively so in rodents, but for simplicity it is depicted only as synapsing onto motoneurons. (a) Pre-post synapse model. Repetitive paired stimulation of a presynaptic neuron and its postsynaptic target modifies the strength of the synapse connecting them. The timing of pre-post synaptic neuron firing determines whether the synapse is made stronger or weaker. This is also termed Hebbian or spike-timing dependent plasticity. For corticospinal modulation, this strategy usually pairs motor cortex stimulation with back-propagating peripheral motor nerve stimulation [1, 77–79]. (b) Convergent model. Two (or more) presynaptic neurons converge onto a common postsynaptic target. For corticospinal modulation, this strategy may pair motor cortex stimulation with afferent sensory nerve stimulation [87, 94]. (c) Phasic during tonic model. Adding tonic direct current stimulation concurrently with phasic stimulation at one or more sites can augment corticospinal circuit responses [95, 98, 99]. In this schematic, CST activation of motor cortex is modulated by direct current stimulation of the spinal cord.

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