An early study by Langer and Lund (1974), using Golgi staining, revealed the cytoarchitecture of local circuits in the SC. According to this study, the local circuit in the superficial layers is composed of five neuron types: narrow-field vertical cells, wide-field vertical cells, piriform or stellate cells, horizontal cells, and marginal cells (Figure 2.2). Among these, wide-field vertical cells are major projection neurons connected to deeper layers of the SC (Mooney et al. 1988b) and lateral posterior nucleus of pulvinar (Lane et al. 1993). In addition, narrow-field vertical and marginal cells are also projection neurons targeted to the parabigeminal nucleus. Later immunohistochemical studies suggested that horizontal, stellate, and piriform cells are GABAergic based on the morphology of soma and proximal dendrites (Mize 1992). More recently, Endo et al. (2003a) showed that horizontal cells are GABAergic neurons by intracellular staining technique using biocytin in GAD67-GFP (green fluorescent protein) knock-in mice, in which GABAergic neurons are labeled with fluorescence of GFP.
The morphological characteristics of neurons in the intermediate layer are more heterogeneous. From the somatodendritic morphology, they are classified into multipolar-, pyramidal-, fusiform-, horizontal-, round-shaped, and wide-field vertical cells (Norita 1980; Ma et al. 1990). Their classification, however, appears to be less distinct than those in the superficial layer. The electrophysio-logical properties of SC neurons are also very heterogeneous. Based on the firing pattern to the depolarizing current step, they are classified into regular-spiking, late-spiking, burst-spiking, fast-spiking, and rapid adaptation types (Saito and Isa 1999). Among these, the regular-spiking type constitutes the majority of the neurons both in the superficial (50%; Endo and Isa, unpublished) and
horizontal narrow-field stellate horizontal wide-field vertical
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