T. Kaneko

Department of Morphological Brain Science, Graduate School of Medicine, Kyoto University and CREST, Kyoto 606-8501, JAPAN.



To reveal the mechanism of information processing in the cerebral cortex, we focused on the morphological analysis of its intrinsic circuitry. Recently, we developed a retrograde neuronal tracing method with Golgi-like soma-dendritic filling. By combining this method with the intracellular staining method, local connections from pyramidal cells to corticospinal and corticothalamic projection neurons were examined in the motor cortical areas of the rat.

Layer III pyramidal neurons stained intracellularly were of the regular spiking type, showed immunoreactivity for glutaminase, and emitted axon collaterals arborizing locally in layers II/III and/or V. Nine of them were reconstructed for morphological analysis; 15.2% or 3.8% of varicosities of axon collaterals of the reconstructed neurons were apposed to dendrites of corticospinal or corticothalamic neurons, respectively. By con-focal laser-scanning and electron microscopy, some of these appositions were revealed to make synapses. These findings indicate that corticospinal neurons receive information from the superficial cortical layers 4-fold more efficiently than corticothalamic neurons. The connections were further examined by intracellular recording of EPSPs that were evoked in layer V and layer VI pyramidal neurons by stimulation of layer II/III. Most layer V pyramidal cells received monosynaptic inputs, whereas layer VI pyramidal cells admitted polysynaptic inputs from layer II/III.

By using the same technique, we investigated the axon collateral connections to corticospinal projection neurons in layer V of the motor areas. Corticospinal neurons received axon collateral inputs from pyramidal cells of all cortical layers except layer I. In particular, layer IV star-pyramidal cells sent 2- to 3-fold more axon collaterals to the corticospinal neurons than the other pyramidal cells. Interestingly, all the layer IV cells showed phasic responses to depolarizing current injection, whereas almost all layer II/III pyramidal cells showed tonic responses. Since layer IV neurons mainly received the cerebellar inputs through thalamic nuclei, these results suggest that cerebellar motor commands are preferentially transferred to corticospinal neurons through layer IV star-pyramidal cells, which have phasic response properties and might contribute to the initiation or switching of motor activity.