The schematic shows the effect of two types of artificial stimulations on a network of neurons (circles), which is activated when a rat with spinal cord injury pulls a lever with its affected limb. Column A shows the effect of vagus nerve stimulation on neural networks that are activated when the rat pulls that lever. In A1, when the rat performs the task, it activates neurons. Some of these are specific to this task (encircled by solid black lines), some of which are not (dashed black line). In A2, the vagus nerve (not represented) is stimulated (pink cloud and lightning bolts). The neurons previously activated are also modulated by this stimulation, and certain new neurons are activated (dashed red line). This combination of activation and modulation strengthens the connections between the neurons that previously fired together, which leads to a transformation of the network. Ultimately, this more robust network supports better physical performance by the rats. Column B shows the effect of spinal cord stimulation on the same neural networks. In B1, spinal cord stimulation is applied (blue cloud and lighting bolt) before the task takes place. When the task is performed (B2), the synapses between the groups of neurons that are involved are further reinforced, a result that is comparable to what is obtained with vagus nerve stimulation. Similar results may also be obtained by performing the stimulation during the task itself. Both vagus nerve and spinal cord stimulations depend on the subject performing a certain task that activates the networks which need reinforcing. However, how these two types of stimulations differ, and how they modulate neural connections, remains unclear.
IMAGE CREDITS: Figure courtesy of V Reggie Edgerton.