THE ORIENTATION OF THE CERVICAL VERTEBRAL COLUMN IN UNRESTRAINED AWAKE ANIMALS .2. MOVEMENT STRATEGIES

Previously we demonstrated a stereotyped resting posture of the head-n eck arrangement in a number of vertebrates: the cervical vertebral col umn is oriented vertically to form one portion of the partial S-shaped configuration of the entire spine. The present investigation quantifi ed the various strategies of head-neck movements employed by different mammalian species (humans, monkeys, cats, rabbits and guinea pigs) us ing cineradiography. At rest, bipeds and quadrupeds hold their heads a t the extreme point of flexion of the passive atlanto-occipital range of motion. In this posture, the horizontal semicircular canals are til ted upward from earth horizontal by 5 to 10 degrees and roughly parall el the plane determined by the two obliquus capitis posterior muscles. Furthermore, at this head position, the utricular maculae become orie nted earth-horizontally. In quadrupedal animals, head-neck movements i n the sagittal plane result from movement at the atlanto-occipital art iculation (head/C-1) and at the multi-articular cervico-thoracic junct ion (C-6-Th-3). Only very small flexion/extension movements occur with in the body of the cervical vertebral column (C-2-C-5). Lowering the h ead from the resting position is only possible by flexion at the C-6-T h-3 vertebrae. Raising of gaze from the resting position is only possi ble by extension of the head at the atlanto-occipital articulation. By contrast, sagittal plane head movements in bipeds are largely confine d to the cervico-thoracic junction. This is related to a significantly reduced range of motion of the atlanto-occipital articulation. In mon keys and humans, its range of motion is about 13 and 8-11 degrees, re spectively, compared to 105 degrees in rabbits. Our cineradiographic d ata demonstrated different strategies for head movements in the sagitt al plane between quadrupeds and bipeds. At one end of the spectrum, in the case of rabbits, there was no systematic relationship between hea d and neck orientation. Rabbits stabilized head posture by using the h ead-neck structure in a parallelogram fashion, which resulted in head posture being largely independent of cervical vertebral column orienta tion. In monkeys and humans, however, orientation of the head depended almost entirely on the orientation of the cervical vertebral column. In such case, head movements in the sagittal plane almost exclusively relied on the positioning of the cervico-thoracic junction. These diff erent strategies again correlate with the different ranges of motion o f the atlanto-occipital articulation. We suggest that vertebrates use mechanical constraints and preferred planes of action for head-neck mo vement control to simplify sensory-motor transformations subserving mo tor control and plasticity and to minimize neuronal operations.