Vestibular control of neck muscles in acute and chronic hemilabyrinthectomized cats.

Reflex activity evoked in neck extensor muscles by head movements in the sagittal plane (the sagittal vestibulocollic reflex (v.c.r.), Dutia & Hunter, 1985), was studied in decerebrate cats with acute or chronic loss of one vestibular labyrinth. After acute hemilabyrinthectomy, tonic electromyographic (e.m.g.) activity in the biventer cervicis muscle ipsilateral to the lesion was normal, while that in the contralateral muscle was abolished. Sinusoidal head movements in the sagittal plane (0.1-5 Hz, 1-10 deg peak to peak) caused reflex modulation of e.m.g. activity in the ipsilateral muscle, but did not evoke any response in the contralateral muscle. The phase (re head position) of the reflex response in the ipsilateral muscle was similar to that in a normal cat with intact labyrinths, while reflex gain was lowered by 2-8 dB below its value before hemilabyrinthectomy. Removal of the remaining labyrinth in acutely hemilabyrinthectomized animals restored bilaterally symmetrical tonic e.m.g. activity in the neck extensors. There was no e.m.g. modulation during head movements after bilateral labyrinthectomy. In chronic hemilabyrinthectomized cats (four to seven weeks), tonic e.m.g. activity in the neck muscles on both lesioned and intact sides was similar to normal. The gain and phase of the sagittal v.c.r. were also normal over a wide range of frequencies of head movement on both lesioned and intact sides. Interruption of the medial longitudinal bundle approximately 1 mm rostral to the obex did not abolish the bilaterally symmetrical compensated reflex response in either muscle, indicating that the descending axons in the medial vestibulospinal tract are not essential in mediating the normal v.c.r. response in compensated animals.