Spread of mono- and polysynaptic connections within cat's motor cortex

Summary1.Spread of synaptic effects within the gray substance of the motor cortex was examined in cats by delivering intracortical microstimulation (ICMS) through a tungsten electrode and recording postsynaptic potentials (p.s.p.'s) intracellularly from neighbouring cells.2.ICMS of 4 μa within layer I did not produce any p.s.p.'s in the surrounding neurons.3.ICMS of 4 μa at layers II, III and IV produced monosynaptic p.s.p.'s only in adjacent neurons. Their horizontal and vertical spread was limited to within 0.5 mm of the stimulating site. Ascending spread from the stimulating sites was not observed in these layers.4.Polysynaptic effects from layers II, III and IV were found at a distance up to 1.0 mm from the stimulating site both in horizontal and vertical directions. Inhibitory spread was wider than excitatory.5.ICMS of 4 μa in the deep layers (V and VI) produced both mono and poly synaptic activation of cells in these same layers as well as in the superficial layers (II–III). The effects of ICMS were mixed, but were predominantly inhibitory.6.The possible synaptic organization within the cortex is discussed and it is concluded that these neuronal connections as a whole constitute a columnar organization within the cortex with feedback circuits built into the system.