Histological and ultrastructural characterization of interfascicular neurons in the rat anterior commissure.

The histological, connectional, and ultrastructural characteristics of a peculiar neuron type in the rat anterior commissure (AC) are described. Since these cells are located among the axonal fascicles of the rostral and caudal parts of the AC, they are termed interfascicular neurons (IFN). In rapid-Golgi sections IFNs appeared in two forms: internuncial (i.e., short axon) and projection neurons (i.e., long axon). The axon of the internuncial neurons terminates upon neighboring IFNs. The projection neurons give rise to an axon which is either incorporated into commissural fibers or ramifies into 12-26 collaterals running laterally in opposite directions along commissural axons. Immunohistochemistry to microtubule-associated protein 2 combined with confocal microscopy showed that IFNs display short varicose dendrites which remain confined to the domain of the AC. The neuronal nature of IFNs was confirmed with the electron microscope on the basis of distinctive organelles and the presence of synaptic inputs. Small areas of neuropil surround some IFNs. These areas are composed of proximal dendrites, terminal axons, axo-shaft and axo-spinous synapses. Because IFNs with their afferents and efferents constitute sufficient elements to integrate neural inputs, it is proposed that they may be involved in processing nerve impulses proceeding from the bilateral cerebral structures connected by the AC.