Propriospinal neurons with ascending collaterals to the dorsal medulla, the thalamus and the tectum: a retrograde fluorescent double-labeling study of the cervical cord of the rat.

Branching neurons with descending propriospinal collaterals and ascending collaterals to the dorsal medulla, the thalamus and the tectum were studied in the rat's cervical spinal cord (C1-C8), using the retrograde fluorescent double-labeling technique: Diamidino Yellow Dihydrochloride (DY) was injected in the cord at T2, True Blue (TB) was injected in the brain stem. DY-labeled descending propriospinal neurons were present in all laminae, except lamina IX. They were concentrated in lamina I, laminae IV to VIII, and in the lateral spinal nucleus, LSN. TB-labeled neurons projecting to the dorsal medulla were concentrated in lamina IV and the medial parts of laminae V and VI (probably representing postsynaptic dorsal column--PSDC--neurons), but were also present in lamina I, the LSN, the lateral dorsal horn, and in laminae VII and VIII. DY-TB double-labeled neurons giving rise to both a descending propriospinal collateral and an ascending collateral to the dorsal medulla were intermingled with the TB single-labeled neurons. About 4% of the descending propriospinal neurons gave rise to an ascending collateral to the dorsal column nuclei; these double-labeled cells constitute a sizable fraction (10%) of the PSDC neurons. TB-labeled spinothalamic and spinotectal neurons were located in lamina I, the lateral cervical nucleus (LCN), and LSN, the lateral lamina V, lamina VII and VIII, lamina X and in the spinal extensions of the dorsal column nuclei, predominantly contralateral to the TB injections. DY-TB double-labeled neurons were present throughout C1-C8 in the LSN, lateral lamina V, lamina VIII, ventromedial lamina VII, and lamina X. Only very few were observed in lamina I and the LCN, and none in the spinal extensions of the dorsal column nuclei. The double-labeled neurons constituted only a minor fraction of all labeled neurons; 3-5% of the spinothalamic neurons and about 1-7% of the spinotectal neurons were double-labeled. Conversely, only about 1% of the labeled descending propriospinal neurons gave rise to an ascending spinothalamic collateral, and even fewer (0.1 to 0.6%) to a collateral to the dorsal midbrain. The LSN displayed the highest relative content of branching neurons. Up to 20% of its ascending spinothalamic and spinotectal neurons and up to 8% of its descending propriospinal neurons were found to be branching neurons, indicating that the LSN constitutes an unique cell-group in the rat spinal cord.