Innervation of multiple neck motor nuclei by single reticulospinal tract axons receiving tectal input in the upper cervical spinal cord.

Axons of reticulospinal neurons (RSNs) activated monosynaptically by stimulation of the contralateral superior colliculus (SC) were stained with intraaxonal injection of horseradish peroxidase in the cat upper cervical spinal cord. Stem axons of single RSNs gave rise to multiple axon collaterals to laminae IX, VIII and VII over a few cervical segments. Single RSNs made contacts with retrogradely labeled neck motoneurons of different neck muscles. Therefore, RSNs were regarded as mediating output of the SC to functionally different groups of neck muscles simultaneously. The result gave evidence of neural implementation of a functional synergy for a neck movement at a single neuron level.