Reflex connexions of motoneurones of muscles involved in head movement in the cat.

1. The reflex connexions from muscle afferents and ventral root fibres to the motoneurones of the muscles biventer-cervicis, complexus, sternocleidomastoid, trapezius and splenius, the principal muscles involved in head movement in the cat, were studied with the technique of intracellular recording. 2. Electrical stimulation of homonymous muscle afferents of biventer-cervicis and complexus, sternocleidomastoid and trapezius, at strengths below 1.6 times threshold of the dorsal root afferent volley, produced monosynaptic e.p.s.p.s in the corresponding motoneurones. Recruitment of higher threshold muscle afferents produced additional p.s.p.s with longer central delays. 3. Stimulation of low-threshold muscle afferents did not produce any p.s.p.s in the motoneurones of the ipsilateral antagonist. Stimulation of higher threshold afferents evoked i.p.s.p.s with central delays longer than 1.6 msec, or mixed e.p.s.p.-i.p.s.p.s in the ipsilateral antagonist. 4. Mixed e.p.s.p.-i.p.s.p.s or i.p.s.p.s with central delays longer than 1.5 msec were evoked in trapezius motoneurones upon stimulation of high threshold afferents from biventer-cervicis and complexus, while stimulation of low-threshold biventercervicis and complexus afferents evoked no p.s.p.s in trapezius motoneurones. 5. Stimulation of contralateral low-threshold biventer-cervicis and complexus afferents evoked a sequence of i.p.s.p. disinhibition in sternocleidomastoid motoneurones, and vice versa, with central delays longer than 1.7 msec. 6. Stimulation of the deafferented biventer-cervicis, complexus, splenius, sternocleidomastoid and trapezius muscle nerves frequently activated interneurones in the ventral horn at monosynaptic central delays. Activation of homoynmous ventral root fibres rarely evoked p.s.p.s in biventer-cervicis, complexius, splenius or sternocleidomastoid motoneurones, while it produced disynaptic i.p.s.p.s in 80% of trapezius motoneurones. 7. It is concluded that Ia reciprocal inhibition and recurrent inhibition, two reflex circuits which are so prominent in limb segments of the spinal cord, do not play a major role in the generation of head movement. Rather, head movement may be primarily controlled from supraspinal centres.