Traditionally, mapping the motor cortex requires electrodes to stimulate the brain and define motor output pathways. Although effective, electrode-based methods are labor-intensive, potentially damaging to the cortex and can have off-target effects. As an alternative method of motor mapping, we photostimulated transgenic mice expressing the light-sensitive ion channel channelrhodopsin-2 in predominantly layer-5 output cortical neurons. We report that optical stimulation of these neurons in vivo using a stage scanning laser system resulted in muscle excitation within 10-20 ms, which can be recorded using implanted electromyogram electrodes or by a noninvasive motion sensor. This approach allowed us to make highly reproducible automated maps of the mouse forelimb and hindlimb motor cortex much faster than with previous methods. We anticipate that the approach will facilitate the study of changes in the location and properties of motor maps after skilled training or damage to the nervous system.

Cortical motor maps are the basis of voluntary movement, but they have proven difficult to understand in the context of their underlying neuronal circuits. We applied light-based motor mapping of Channelrhodopsin-2 mice to reveal a functional subdivision of the forelimb motor cortex based on the direction of movement evoked by brief (10 ms) pulses. Prolonged trains of electrical or optogenetic stimulation (100-500 ms) targeted to anterior or posterior subregions of motor cortex evoked reproducible complex movements of the forelimb to distinct positions in space. Blocking excitatory cortical synaptic transmission did not abolish basic motor map topography, but the site-specific expression of complex movements was lost. Our data suggest that the topography of movement maps arises from their segregated output projections, whereas complex movements evoked by prolonged stimulation require intracortical synaptic transmission.

Continuous perineural infusions of local anesthetic are a safe and effective method for reducing post-amputation opioid analgesic medications after major lower limp amputation.