Intracellularly labeled pyramidal neurons in the cortical areas projecting to the spinal cord. I. Electrophysiological properties of pyramidal neurons.

To study cortical motor control, we examined electrical characteristics of pyramidal neurons in the present report, and intra- or juxta-columnar connections of the pyramidal neurons to corticospinal neurons in the accompanying report. Pyramidal neurons were intracellularly recorded and stained in slices of rat motorsensory cortices (areas FL, HL and M1) where many corticospinal neurons were labeled retrogradely. They were morphologically classified into classical, star and other modified pyramidal neurons, and electrophysiologically into regular-spiking (RS), intrinsic bursting (IB) and irregular-spiking (IS) neurons on the basis of spiking pattern in response to 500 ms depolarizing current pulses. RS responses were further divided into RS with slow adaptation (RS-SA) and RS with fast adaptation (RS-FA). The electrical properties were associated with the laminar location of the neurons; RS-SA responses were observed frequently in layer II/III and less frequently in layers IV-VI, and IB and IS responses were exclusively found in layers V and VI, respectively. Interestingly, all layer IV neurons in area FL/HL were RS-FA star-pyramidal neurons, whereas layer IV neurons in area M1 were RS-SA classical pyramidal neurons. Although weak stimulation of areas FL/HL and M1 is known to elicit movement, these results suggest different information processings between the two areas.