Axonal conduction properties of antidromically identified neurons in rat barrel cortex

Physiological studies of the rodent somatosensory cortex have consistently described considerable heterogeneity in receptive field properties of neuro ns outside of layer IV, particularly those in layers V and VI. One such app roach for distinguishing among different local circuits in these layers may be to identify the projection target of neurons whose axon collaterals con tribute to the local network. In vivo, this can be accomplished using antid romic stimulation methods. Using this approach, the axonal conduction prope rties of cortical efferent neurons are described. Four projection sites wer e activated using electrical stimulation: (1) vibrissal motor cortex, (2) v entrobasal thalamus (VB), (3) posteromedial thalamic nucleus (POm), and (4) cerebral peduncle. Extracellular recordings were obtained from a total of 169 units in 21 animals. Results demonstrate a close correspondence between the laminar location of the antidromically identified neurons and their an atomically known layer of origin. Axonal properties were most distinct for corticofugal axons projecting through the crus cerebri. Corticothalamic axo ns projecting to either VB or POm were more similar to each other in terms of laminar location and conduction properties, but could be distinguished u sing focal electrical stimulation. It is concluded that, once stimulation p arameters are adjusted for the small volume of the rat brain, the use of an tidromic techniques may be an effective strategy to differentiate among pro jection neurons comprising different local circuits in supra- and infragran ular circuits.