Synaptic connections between spinal motoneurons and dorsal root ganglion cells in the cat

Light- and electron-microscopical horseradish peroxidase (HRP) studies have been employed in conjunction with a degeneration study in order to clarify the origin and axonal passage of afferent synaptic terminals in cat dorsal root ganglia. After injection of HRP into ganglia (C3) without involvement of the ventral roots and spinal nerves, a few ipsilateral spinal ventral horn neurons (C3) were retrogradely labeled with HRP. The labeled neurons were localized in the dorsomedial and the ventromedial nuclei. Following ventral rhizotomy of C3, the afferent terminals in the ganglia (C3) anterogradely degenerated and contained accumulated and disintegrated neurofilaments, depleted, aggregated and enlarged synaptic vesicles. Subsequent to an HRP and wheat germ agglutinin (WGA)-HRP-mixture injection into the dorsal neck or suboccipital muscles, many spinal motoneurons (C3) were labeled retrogradely with an HRP mixture. On the other hand, the afferent synaptic terminals in ganglia contained the membrane-bound and electron-dense bodies which were anterogradely labeled with an HRP mixture in addition to the normal synaptic elements. The present findings strongly suggest that some spinal motoneurons send their axon collaterals to the dorsal root ganglia, in which the terminals of the axon collaterals directly synapse with the dorsal root ganglion cells.