SPATIAL COORDINATION BY DESCENDING VESTIBULAR SIGNALS .1. REFLEX EXCITATION OF NECK MUSCLES IN ALERT AND DECEREBRATE CATS

Electromyographic activity of dorsal neck muscles and neck torques was recorded to study vestibulocollic, cervicocollic, and combined reflex es in alert and decerebrate cats during rotations of the whole body, t he body except for the head, and the head but not the rest of the body . Cats were rotated about many axes that lay in the frontal, sagittal, and horizontal planes using sinusoidal 0.25-Hz waveforms or sum-of-si nusoid waveforms. Robust electromyographic responses were recorded fro m six muscles, with response directionality that in most cases did not show strong dependence on the reflex tested or on other factors inclu ding exact neck angle, stimulus amplitude from 5 degrees to 60 degrees , and intact versus decerebrate state. Based on the strength of respon ses to rotations about all the tested axes, neck muscles could be char acterized by maximal activation direction vectors representing the axi s and direction of rotation in three-dimensional space that was most e xcitatory during reflex responses. Responses to rotations about axes t hat lay in a coordinate plane were predicted by a cosine function of t he angle between the axis under test and the maximally excitatory axis in the plane. All muscles were excited by the nose down phase of pitc h rotation and by yaw and roll away from the side on which the muscle lay. Biventer cervicis was best activated by rotations with axes near nose-down pitch, and its axis of maximal activation also had small, ap proximately equal components of yaw and roll toward the contralateral side. Complexus was best excited by rotations with axes nearest roll, but with large components along all three axes. Occipitoscapularis was best excited by rotations about axes near pitch, but with a moderatel y large contralateral. yaw component and a smaller but significant con tralateral roll component. Splenius was best excited by rotations with a large component of contralateral yaw, considerable nose-down pitch, and a smaller component of contralateral roll. Rectus major was best excited by rotations near nose-down pitch, but with a substantial cont ralateral yaw component and smaller contralateral roll component. Obli quus inferior was best excited by rotations with a large component of contralateral yaw, but with considerable contralateral roll and nose-d own pitch components. All muscles responded as though they received co nvergent input from all three semicircular canals. Vestibulocollic and combined reflex responses in alert cats and vestibulocollic, cervicoc ollic, and combined responses in decerebrate cats appeared to have the same directionality, as evidenced by insignificant shifts in maximal activation vectors. Cervicocollic responses in alert cats were inconsi stent and often absent, but appeared upon decerebration, suggesting th at higher centers suppress the cervicocollic reflex in intact animals. Decerebration and partial cerebellectomy had no significant effect on maximal activation directions, although electromyographic response ma gnitudes increased after each. The results suggest that common circuit s or strategies are used by neck stretch and vestibular-neck reflexes. The reflex excitation directions do not match the mechanical actions of the neck muscles but agree fairly well with previously published pr edictions of a mathematical model of neck motor control.