Morphology of single axons of tectospinal and reticulospinal neurons in the upper cervical spinal cord

Single axons of tectospinal (TS) and reticulospinal (RS) neurons were stained with intraaxonal injection of HRP after electrophysiological identification, and their axonal trajectory was reconstructed at C1-C3 of the cat. TS neurons were located in the intermediate or deep layers of the caudal two-thirds of the superior colliculus (SC) and had multiple axon collaterals (up to seven collaterals) per stem axon). Collaterals had a simple structure, ramified several times mainly in the transverse plane, and terminated in the lateral parts of laminae V-VIII. More than half also had terminals in lamina IX. Terminals of TS neurons did not appear to make contacts with either the somas or proximal dendrites of retrogradely-labeled motoneurons in lamina IX, but clear contacts were found on counterstained interneurons in the lateral part of laminae V-VIII. Here, we examined three stained spinal interneurons receiving monosynaptic excitation from the SC. These interneurons had multiple axon collaterals mainly in laminae VII-IX, and made extensive contacts with retrogradely-labeled motoneurons of multiple neck muscles. Stem axons of single RS neurons receiving input from the contralateral SC ran in the ventromedial funiculus and gave off multiple axon collaterals to laminae VII-IX over at least several cervical segments. Their terminal boutons appeared to make contact with both the somas and proximal dendrites of retrogradely-labeled neck motoneurons. Single RS neurons made contacts with motoneurons of different neck muscles. These results provide evidence for functional synergies at the level of single RS neurons and spinal interneurons for neck movements. The present finding indicates that the direct TS projection to the spinal cord may influence the activity of multiple neck muscles mainly via spinal interneurons, and plays an important role in control of head movement in parallel with the tecto-reticulospinal system.