FIG. 11.

Schematic representation of synaptic interactions among identified motor cortex neurons and the distribution of input fibers, based on observed correlations. Icons in the top half of the drawing indicate precentral cells within cytoarchitectonic layers (left). CM cells are represented as filled triangles, PT cells as unfilled triangles, and all other cells as circles. Unfilled and filled terminals signify excitatory and inhibitory synapses, respectively. Inputs from thalamic nucleus VL is shown projecting to 3 columns of precentral cells. Cortical inputs to motor cortex are represented by the fiber emerging from the right. Motoneurons of different extensor and flexor muscles are shown below, each receiving excitatory inputs from precentral CM cells. A spinal internuncial inhibitory neuron is shown to mediate disynaptic inhibition from a CM cell to an extensor motoneuron.