Comparative properties of excitatory and inhibitory inter-laminar neocortical axons

In recent years it has become increasingly clear that variations in voltage-gated channels, as well as highly diverse geometrical properties, shape the way axons conduct action potentials to their terminals. Numerous cell types in the mammalian neocortex form a dense network of connections, and the properties of their axons may have an effect on the processing performed by this network. We studied the conduction properties of local, inter-laminar axons emanating from regular-spiking (RS) pyramidal neurons and Martinotti type inhibitory neurons (MCs) in layer 5 of the mouse barrel neocortex by comparing the patterns of their antidromic activation from layer 1. Both types of axons had similarly slow conduction velocities ( approximately 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 microm. 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.