Intracellularly labeled pyramidal neurons in the cortical areas projecting to the spinal cord. II. Intra- and juxta-columnar projection of pyramidal neurons to corticospinal neurons.

Intra- or juxta-columnar connections of pyramidal neurons to corticospinal neurons in rat motorsensory cortices were examined with brain slices by combining intracellular staining with Golgi-like retrograde labeling of corticospinal neurons. Of 108 intracellularly labeled pyramidal neurons, 27 neurons were selected for morphological analysis by successful staining of their axonal arborizations and sufficient retrograde labeling of corticospinal neurons. Many varicosities of local axon collaterals of each pyramidal neuron were closely apposed to the dendrites of corticospinal neurons, suggesting the convergent projections of layer II-VI pyramidal neurons to corticospinal neurons. Particularly, the varicosities of a layer IV star-pyramidal neuron made two- to three-fold more appositions to the dendrites of corticospinal neurons than those of a pyramidal neuron in the other layers. Fifteen appositions were examined electron-microscopically and 60% of them made asymmetric axospinous synapses. The present results together with those of the preceding report suggest that thalamic inputs are conveyed to corticospinal neurons preferentially via layer IV star-pyramidal neurons with phasic response properties, and thereby might contribute to the initiation or switching of movement. In contrast, inputs with tonic response properties from the other layers seem to be integrated in corticospinal neurons, and might be useful in maintaining the activity of corticospinal neurons.