The ultrastructural organization of the nucleus of the posterior commissure of rat.

An electron microscope study was carried out to clarify the ultrastructure of the nucleus of the posterior commissure (NPC) of rat which is thought to be related to the eye movements and oculomotor reflexes. The neurons were small or medium-sized, loosely arranged in an extensive neuropil. The nerve cell bodies showed a high nucleocytoplasmatic ratio, scanty endoplasmic reticulum and very few axo-somatic contacts. The synaptic boutons were very numerous in the neuropil, thus indicating the latter as the main target for inputs to the NPC. The majority of the synapses showed the typical morphological features of the excitatory synapses and so it was hypothesized that a mainly excitatory activity takes place in the NPC. Since the NPC, together with the nucleus of Darkschewitsch (ND) and the interstitial nucleus of Cajal (INC) belongs to the accessory oculomotor nuclei, a comparison of their ultrastructure was carried out.