Reverse physiology in the vibrissal motor system

Lucas Herfst, Michael Brecht

Reverse physiology. Determining the role of single neurons in behavior is a central goal of our research. The function of single "peripheral" sensory- or motoneurons has been successfully described in terms of their stimulus/response or response/movement relationships. This descriptive approach has been less rewarding for determining the functional significance of activity in "central" cortical neurons and has led to rather incongruent ideas about the role of single cortical neurons. We confront this problem by stimulating single cortical neurons and testing the effect on movement, sensation and learning. This approach reverses conventional physiological research, where APs are studied as correlates of sensorimotor processing.

Single cell stimulation in the vibrissa motor cortex. Neuronal activity in the motor cortex is understood to be correlated with movements, but the impact of action potentials (APs) in single cortical neurons on the generation of movement has not been fully determined. Here we show that trains of APs in single pyramidal cells of rat motor cortex can evoke long sequences of small whisker movements. For layer-5 cells, we find that evoked rhythmic movements have a constant phase relative to the AP train, indicating that single layer-5 pyramids can reset the rhythm of whisker movements. Action potentials evoked in layer-6 pyramids can generate bursts of rhythmic whisking, with a variable phase of movements relative to the AP train. An increasing number of APs decreases the latency to onset of movement, whereas AP frequency determines movement direction and amplitude. We find that the efficacy of cortical APs in evoking whisker movements is not dependent on background cortical activity and is greatly enhanced in waking rats. We conclude that in vibrissae motor cortex sparse AP activity can evoke movements.

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