PATTERNS OF CORTICOTHALAMIC TERMINATIONS FOLLOWING INJECTION OF PHASEOLUS-VULGARIS LEUKOAGGLUTININ (PHA-L) IN THE SENSORIMOTOR CORTEX OF THE RAT

The morphology and spatial distribution of terminals emitted by corticothalamic axons originating from the rat motor cortex (as defined by intracortical microstimulations) were studied using Phaseolus vulgaris leucoagglutinin (PHA-L) as an anterograde tracer. After PHA-L injection in the face, forelimb or hindlimb motor cortical areas, small and densely packed boutons (about 1-mu-m in diameter), en passant and terminaux, were seen in the ventrolateral nucleus of the thalamus and, more sparsely, in the reticular nucleus, the nucleus ventrobasalis and the posterior nucleus of the thalamus. A separate projection with giant boutons (5-10-mu-m in diameter), en passant and terminaux, terminated in the posterior nucleus of the thalamus exclusively. Giant boutons originated from corticothalamic axons distinct from those providing small boutons. The corticothalamic projection originating from the motor cortex has basic organizational properties comparable to previous data obtained in the auditory and somatosensory corticothalamic projection systems.