Figure 2.

Techniques for optogenetic interrogation of spinal circuitry. Opsins may be stimulated in four main ways to interrogate spinal circuitry. First, opsins can be expressed in primary afferent terminals, and peripheral stimulation can be used to stimulate or silence primary sensory neurons and observe real-time changes in behavioral output in vivo. Similarly, opsins can be expressed in neuronal populations within the spinal cord or brain, allowing for in vivo central stimulation of either spinal cord neuron populations or neurons involved in descending modulation, respectively. Finally, opsins may be expressed within primary afferents or spinal cord neuron populations, and then stimulation can occur in an in vitro slice preparation, often combined with electrophysiological recordings to measure the effect of activation or silencing of a given neuronal population on the excitability of the recorded neuron.