Comparative properties of excitatory and inhibitory inter-laminar neocortical axons

Both types of axons had similarly slow conduction velocities (∼0.3 m/s), compatible with thin unmyelinated fibers. In addition, in both types of neurons, subthreshold changes of the somatic membrane potential affected the stimulus threshold for evoking an antidromic spike in layer 1, a distance of 600-800 μm. However, the axons differed considerably in their antidromic activation profiles. 1) The antidromic latency in RS neurons was highly consistent while some MCs display considerable activation-latency jitter; 2) RS neurons displayed a steeper increase in excitability to repeated 40 Hz stimulation; 3) RS neurons displayed a sharp, step-like antidromic activation threshold to both somatic voltage and stimulus intensity, while MCs displayed a gradual recruitment pattern. Morphological differences in the branching pattern of the two types of neurons may account for some of these distinctions. These results suggest differences among excitatory and inhibitory neocortical neurons in the computational tasks of their local axons.