Ultrastructure of supraspinal dorsal root projections in the toad. II. The cerebellar granular layer.

Following section of the left dorsal roots, degenerating fibres and boutons were observed in the granular layer of the ipsilateral cerebellum. The degenerating terminals were identified as large en passant varicosities of mossy fibres contacting the dendrites of presumptive granule cells. They contained round synaptic vesicles and neurofilaments and established Gray type I contacts. The terminals initially underwent filamentous degeneration with neurofilamentous hypertrophy, swollen mitochondria and loss of synaptic vesicles. At later survival times (6--30 days) they acquired an electron-dense appearance due to an increase and clumping of the filamentous component. After injection of horseradish peroxidase into the left cerebellum, all ipsilateral spinal ganglia showed a few (2--3%) labelled cells, indicating that a primary afferent contribution to this pathway originated from each segment of the spinal cord.