The pathways responsible for the characteristic head posture produced by lesions of the interstitial nucleus of Cajal in the cat.

(1) Experiments were performed in cats to examine effects of lesion of the interstitial nucleus of Cajal (INC) on head posture and the responsible pathway. Unilateral INC lesions resulted in lateral tilt of the head to the opposite side, and bilateral INC lesions resulted in dorsiflexion of the head as reported earlier. Such characteristic head posture was produced by successful kainic acid injections as well as by electrolytic lesions, suggesting that it was not due to damage of nerve fibers passing through the INC, but was produced most probably by damage of nerve cells in the INC. Electromyographic (EMG) recordings in unilateral INC-lesioned cats showed that activity was higher in the ipsilateral than in the contralateral major dorsal neck muscles (biventer, splenius, complexus, and rectus), and also higher in the contralateral than in the ipsilateral obliquus capitis caudalis muscle. The pattern of EMG activity was basically similar either when the cats presented typical head tilt or when their head was fixed to the frame at the stereotaxic plane. Characteristic head posture resulting from INC lesions seems consistent with the head posture produced by activation of these muscles. (2) Interruption of the medial and lateral vestibulospinal tracts did not significantly influence head tilt that had been produced by INC lesions. Characteristic head tilt was produced by INC lesions after cats had received bilateral labyrinthectomies, bilateral lesions of most of the vestibular nuclei, and bilateral aspiration of the cerebellar vermis and most of the lateral vestibular nuclei, indicating that typical head tilt can be produced without the vestibular nuclei and cerebellar vermis. (3) The medial longitudinal fasciculus (MLF) was interrupted at different levels to cut the major descending fibers from the INC. MLF interruption at the caudal midbrain produced typical head tilt, although MLF cut at the caudal pons and medulla was ineffective. Bilateral parasagittal cuts lateral to the MLF in the pons produced severe dorsiflexion of the head, and a subsequent unilateral INC lesion produced no further head tilt. These results suggest that fibers originating in the INC, removal of which is responsible for the typical head tilt, run through the MLF in the midbrain, and leave it in the pontine level.(ABSTRACT TRUNCATED AT 400 WORDS)