Neuronal responses to horizontal neck deflection in the group X region of the cat's medullary brainstem.

Neuronal responses to natural stimulation of neck proprioceptors were studied in the region of the small cell group x in the dorsolateral medullary brainstem of slightly anesthetized and paralyzed cats. Stimulation consisted of horizontal trunk rotations about C1 with the head fixed in space. Out of 74 neurons recorded, 92% showed an increase in discharge rate with ipsilateral neck stretch and a decrease with contralateral stretch (Type N I responses); 8% showed the reverse pattern (Type N II responses). In the primary head-to-trunk position, almost all neurons had tonic activity that probably stemmed from prestretched neck proprioceptors. Responses to sinusoidal stimulation and position trapezoids showed a static (position-sensitive) as well as a dynamic (essentially velocity-sensitive) component. The relative weight of the two components varied considerably among the neurons. It was not possible to distinguish discrete neuronal populations on the basis of the dynamic characteristics. There was no evidence of a convergent input from other receptor systems, such as the horizontal canal system. Several neurons responded to "muscle tapping" and showed an increase of the velocity component following systemic injection of succinylcholine. We take this as evidence that they may receive input from muscle spindle receptors.