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

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Abstract
The reflex connections from muscle afferents and ventral root fibers to the motoneurons 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. 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 EPSP [excitatory post-synaptic potentials] in the corresponding motoneurons. Recruitment of higher threshold muscle afferents produced additional PSPS with longer central delays. Stimulation of low-threshold muscle afferents did not produce any PSP in the motoneurons of the ipsilateral antagonist. Stimulation of higher threshold afferents evoked IPSP with central delays longer than 1.6 ms or mixed EPSP-IPSP in the ipsilateral antagonist. Mixed EPSP-IPSP or IPSP with central delays longer than 1.5 ms were evoked in trapezius motoneurons upon stimulation of high threshold afferents from biventer-cervicis and complexus, while stimulation of low-threshold biventer cervicis and complexus afferents evoked no PSP in trapezius motoneurons. Stimulation of contralateral low-threshold biventer-cervicis and complexus afferents evoked a sequence of IPSP disinhibition in sternocleidomastoid motoneurons, and vice versa, with central delays longer than 1.7 ms. Stimulation of the deafferented biventer-cervicis, complexus, splenius, sternocleidomastoid and trapezius muscle nerves frequently activated interneurons in the ventral horn at monosynaptic central delays. Activation of homoynmous ventral root fibers rarely evoked PSP in biventer-cervicis, complexus, splenius or sternocleidomastoid motoneurons, while it produced disynaptic IPSP in 80% of trapezius motoneurons. Ia reciprocal inhibition and recurrent inhibition, 2 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. Head movement may be primarily controlled from supraspinal centers.