MORPHOLOGY OF SINGLE AXONS OF TECTOSPINAL NEURONS IN THE UPPER CERVICAL SPINAL-CORD

Morphology of single axons of tectospinal (TS) neurons was investigate d by intraaxonal injection of horseradish peroxidase (HRP) at the uppe r cervical spinal cord of the cat. TS axons were electrophysiologicall y identified by their direct responses to stimulation of the contralat eral superior colliculus (SC). None of these axons responded to thorac ic stimulation at Th2. Three-dimensional reconstructions of the axonal trajectories were made from 20 well-stained TS axons at C1-C3. Cell b odies of these axons were located in the intermediate or deep layers o f the caudal two-thirds of the SC. Usually, TS axons had multiple axon collaterals, and up to seven collaterals were given off per stem axon [2.7 +/- 1.6 (mean +/- S.D.); n = 20]. Collaterals had simple structu res and ramified a few times mainly in the transverse plane. The numbe r of terminals for each collateral was small. These collaterals termin ated in the lateral parts of laminae V-IX, mainly in laminae VI, VII, and VIII. There were usually gaps free from terminal arborizations bet ween adjacent collaterals, because the rostrocaudal spread of each col lateral (mean = 700 mu m) was narrower than the intercollateral interv al (mean = 2,500 mu m). Seven of the 19 TS axons had terminals in the lateral parts of laminae V-VIII, with little projection to lamina TX, and the other 12 axons had terminals in lamina TX besides the projecti on to the lateral parts of laminae V-VIII. Axon terminals in lamina TX did not appear to make contacts with the somata or proximal dendrites of retrogradely labeled motoneurons, but contacts were found with the somata of counterstained interneurons in the lateral parts of laminae V-VIII. Three spinal interneurons (two in lamina VIII and one in lami na V at C1) that received monosynaptic excitation from the SC were sta ined, and their axonal trajectories were reconstructed. They had multi ple axon collaterals at C1-C2 and mainly projected to laminae VIII and IX, with smaller projections to lamina VII. Many axon terminals of th e interneurons were found in multiple neck motor nuclei, where some of them made contacts with retrogradely labeled motoneurons. The present finding provides evidence that the direct TS projection to the spinal cord may influence activities of multiple neck muscles, mainly via sp inal interneurons, and may play an important role in control of head m ovement in parallel with the tectoreticulospinal system. (C) 1996 Wile y-Liss, Inc.