Electron microscopic identification of reticulo-subthalamic axon terminals in the cat

Synaptic boutons emanating from axons of nucleus tegmenti pedunculopontinus origin were identified by electron microscopy in the neuropil of the subthalamic nucleus. Such boutons measure 1.5-3 microns, contain round synaptic vesicles and make asymmetrical axodendritic and axosomatic synaptic contacts with large subthalamic neurons. Very few contacts with vesicle-containing dendrites, and no contacts with the perikarya of the small neurons were observed. The present findings, in keeping with the relevant light microscopic and electrophysiologic data, furnish evidence for a substantial bilateral tegmenti pedunculopontinofugal projection that excites monosynaptically the relay subthalamic neurons.