Learning in the Rodent Motor Cortex

Abstract

The motor cortex is far from a stable conduit for motor commands and instead undergoes significant changes during learning. An understanding of motor cortex plasticity has been advanced greatly using rodents as experimental animals. Two major focuses of this research have been on the connectivity and activity of the motor cortex. The motor cortex exhibits structural changes in response to learning, and substantial evidence has implicated the local formation and maintenance of new synapses as crucial substrates of motor learning. This synaptic reorganization translates into changes in spiking activity, which appear to result in a modification and refinement of the relationship between motor cortical activity and movement. This review presents the progress that has been made using rodents to establish the motor cortex as an adaptive structure that supports motor learning.

Keywords: motor cortex; motor learning; movement; neural circuits; plasticity; rodents.

Figure 1

Simplified schematic of the connectivity of the rodent motor cortex. Arrows represent major pathways. Activity from basal ganglia–recipient thalamus and frontal cortex can reach deep corticofugal cells directly, whereas motor-related thalamic nuclei and sensory cortical activity arrive indirectly through highly recurrent layer 2/3 networks. Color boxes indicate the soma locations of different types of output cells. Adapted with permission from Hooks et al. (2013) and Macmillan Publishers Ltd.: Nature Neuroscience, Weiler et al. (2008).

Figure 2

Schematic of the effects of learning on the motor cortex, with the naive stage on the left and the expert stage on the right. (a) Motor learning is characterized by increased task success and movement stereotypy, as illustrated here for a reaching task (black dotted lines, hand trajectories). (b) The motor cortex undergoes structural changes during motor learning, including a strengthening of local and long-range inputs ( gray arrows), a decrease in inhibitory connectivity to apical dendrites (red, inhibitory neuron), and dendritic expansion and clustered spine formation (orange circles) on movement-relevant neurons ( green neuron). (c) The population of neurons that exhibit movement-related activity is dynamic across learning and refines to a more consistent population (black triangles, inactive neurons; colored triangles, active neurons). (d ) Activity of neurons becomes more correlated with movement, and the relationship between population activity and movement is dynamic but becomes more consistent, as evaluated by comparing similar movements before and after learning ( yellow regions, periods of high muscle activity; colored bars, spikes of different neurons). Abbreviation: EMG, electromyograph.