Abstract
In an attempt to study the cortical control of motor systems, intra- or juxta-columnar connections of pyramidal neurons to corticospinal neurons of layer V in the rat motorsensory cortices were examined in brain slices by combining intracellular staining with Golgi-like retrograde labeling of corticospinal neurons. The pyramidal neurons were electrophysiologically classified into several types by the firing patterns in response to long depolarizing current pulses. Interestingly, all layer IV neurons were star-pyramidal neurons exhibiting phasic responses, whereas the vast majority of pyramidal neurons in layer II/III displayed tonic discharges in response to depolarizing current pulses. Morphologic analysis revealed that a large number (110-558) of axon varicosities of a pyramidal neuron located in layers II-VI were apposed to the dendrites of corticospinal neurons. In particular, the varicosities of a layer IV star-pyramidal neuron made 2- to 3-fold more appositions to the dendrites of corticospinal neurons than those of a pyramidal neuron in the other layers. The present results suggest that thalamic inputs are conveyed to corticospinal neurons preferentially via layer IV excitatory neurons, which have phasic response properties and might contribute to the initiation or switching of motor activity. In contrast, excitatory inputs from layer II/III seem to be integrated in corticospinal neurons with from layers IV-VI, and might be useful in maintaining the activity of corticospinal neurons by the tonic response properties of layer II/III pyramidal neurons. [Jpn J Physiol 54 Suppl:S48 (2004)]
