Intracortical mechanisms for the recruitment of motor cortex neurons.

Neurons project out of motor cortex to the spinal cord and to other targets. Not all projection neurons recruit in the same way during behavior, but instead recruitment patterns depend on the projection target of the neurons. The problem is to understand how neurons projecting to different targets are recruited selectively. We have investigated possible mechanisms for the recruitment of motor cortex neurons with electrophysiological approaches in anesthetized cats. To determine if neurons projecting out of motor cortex to different targets have selective input connectivity from extrinsic sources we electrically stimulated corticocortical, callosal and thalamocortical pathways. Subthreshold effects of input pathways were detected by monitoring latency variations of antidromic responses. Intracortical connections to identified output neurons were evaluated by cross-correlation and a new variation of the antidromic latency method. Output neurons in different layers along single electrode tracks usually had different inputs from extrinsic sources. Neurons in close proximity were most likely to share the same inputs, especially when they projected axons to the same target. These results support the conclusion that combinations of inputs from extrinsic sources could selectively recruit efferent neurons from separate cortical layers or from within groups of nearby neurons, according to the target of their axonal projections. In contrast, the data on intracortical connectivity suggest that common drive causes a more synchronous activation of nearby cortical neurons. Combining the conclusions on effects of inputs from extrinsic and intracortical sources leads to the speculation that motor cortex neurons that might at one time be recruited selectively by action of extrinsic afferent pathways to cortex could at another time be bound into synchrony by a common drive shared with their neighbours.