Intracortical distribution of axonal collaterals of pyramidal tract cells in the cat motor cortex.

Slow and fast pyramidal tract cells (Pt cells) from the cat motor cortex were identified antidromically and injected with horseradish peroxidase (HRP). The axonal collaterals of these cells were mapped following HRP histochemistry with benzidine di-hydrochloride. All cells, slow or fast, show a similar arrangement of their collaterals. A proximal axonal network of 0.5-0.8 mm in diameter delimits a local field of action for collaterals in layers V and VI. The tangential expansion of this local field corresponds to that of the basal dendritic domain of Pt neurons. Much longer collaterals running for millimeters in the lower gray or white matter were observed in all cells. They form at a cortical level a distal field of action for Pt neurons. Many of these long branches were traced to other regions of area 4 or toward other cytoarchitectonic areas. In one case a collateral was seen entering and dividing in area 3a. Due to limitations of the HRP technique most of these long branches could not be followed to their terminals. On the basis of the laminar distribution of Pt cell collaterals (mostly in layers V and VI) synaptic sites where recurrent excitation and inhibition are produced on Pt neurons are discussed.