Bipolar neurons in rat visual cortex: a combined Golgi-electron microscope study

Golgi-impregnated bipolar neurons in rat visual cortex have been examined by both light and electron microscopy. Bipolar neurons are encountered throughout layers II to V and are recognized by their spindle-shaped cell bodies and vertically elongate, narrow dendritic trees which may traverse the cortex from layer II to layer V. Although a single primary dendrite usually extends from each end of the cell body, two primary dendrites may extend from one pole, usually the lower one, and an additional short dendrite may emerge from one side. In the electron microscope gold-toned Golgi-impregnated neurons are seen to have folded nuclear envelopes and except at the poles of the cell body where the dendrites emerge, the nucleus is surrounded by only a thin rim of cytoplasm. Both the cell body and the dendrites form asymmetric and symmetric synapses. Usually the axon of a bipolar neuron arises from one of the primary dendrites and it soon assumes a vertical orientation, to either descend or ascend through the cortical neuropil. Some bipolar neurons have myelinated axons and only the initial portion is impregnated in Golgi preparations, but when they are unmyelinated the axons can be seen to form vertical plexuses and asymmetric synapses. Most commonly the terminals synapse with dendritic spines, some of which are derived from apical dendrites of pyramidal cells, but other terminals synapse with the shafts of apical dendrites, and with the cell bodies and dendrites of nonpyramidal cells. It is apparent that these bipolar neurons are the cells which others have shown to label specifically with antisera to vasoactive intestinal polypeptide (VIP), and it is suggested that the prime role of these cells in the cerebral cortex is to excite the clusters of pyramidal cells.