The orientation of the cervical vertebral column in unrestrained awake animals

Abstract

The orientation of the cervical vertebral column was studied by X-ray photography of the region containing the head and the neck in nine unrestrained species of vertebrates (man, monkey, cat, rabbit, guinea pig, rat, chicken, frog, lizard). In addition, the orientation of the horizontal semicircular canals was measured in four species using landmarks on the skull. In all vertebrates studied, with the exception of frog and lizard, the general orientation of the cervical vertebral column was vertical when animals were at rest, and not horizontal or oblique as suggested by the macroscopic appearance of the neck. The posture of the animal, whether lying, sitting or standing, had little effect on this general vertical orientation, although some variability was noticed depending on the species. This finding prompted the definition of a resting zone, where the cervical column can take any orientation within a narrow range around a mean position. The cervical vertebral column composes part of the S-shaped structure of the entire vertebral column, with one inflection around the cervico-thoracic (C7/Th1) junction. This feature is already noticable in the lizard. The vertical orientation of the cervical vertebral column is interpreted to provide a stable and energy saving balance of the head. Furthermore, when the head is lowered or raised, the atlanto-occipital and cervico-thoracic junctions are predominantly involved, while the entire cervical column largely preserves its intrinsic configuration. The curved configuration of the cervico-thoracic vertebral column embedded in long spring-like muscles is interpreted to function as a shock absorber. At rest, animals did not hold their heads with the horizontal canals oriented earth horizontally all the time, but often maintained them pitched up by ca. 5 deg, as has been reported for man. At other times, presumably when the vigilance level increased, the horizontal canals were brought into the earth horizontal plane. The vertical orientation of the cervical column results in a vertical positioning of the odontoid process of the axis (second cervical vertebra, C2), which thus provides the axis of rotation for yaw movements of the head. This axis corresponds to that of the horizontal semicircular canals. The vertical organization of the cervical vertebral column in birds and mammals, whether the animal is quadrupedal or bipedal, points to a common organizational principle for eye and head movement systems.(ABSTRACT TRUNCATED AT 400 WORDS)

Stabilization and mobility of the head and trunk in vervet monkeys(Cercopithecus aethiops) during treadmill walks and gallops

“…Bonnet macaques and hanuman langurs, members of two separate phylogenetic subfamilies (Cercopithecinae and Colobinae, respectively) and who differ in body size and proportions, nevertheless practice comparable head and trunk movement patterns during walks and gallops (Dunbar et al, 2004). This finding is consistent with the combined experimental evidence from other studies that the neural mechanisms underlying dynamic posture (Dunbar et al, 1986), locomotion (e.g., Peters and Goslow, 1983;Vilensky and Gehlsen, 1984) and gaze (Vidal et al, 1986) are conservative in organization among tetrapods. Cercopithecus aethiops is not a phylogenetically distant species differing so dramatically in morphology and lifestyle that differences in sensorimotor capabilities would be expected.…”

Eye orientation during static tilts and its relationship to spontaneous head pitch in the laboratory mouse

“…This value accords with the observation that most species position their heads so as to hold the plane of the horizontal semicircular canal horizontal. A previous study of posture in mice concluded that they carry their horizontal canals oriented 15° upwards during rest and locomotion (Vidal, et al, 2004), while a study of Long-Evans rats yielded a value of 14° ( Rabbath, et al, 2001), and a comparative study of several mammalian and non-mammalian species found an average upwards pitch of about 5°, although there was a large degree of moment-to-moment variation (Vidal, et al, 1986).…”

“…It may be significant that the eye elevation of 29° falls close to the midpoint of the range of vertical eye positions achieved over the entire range of pitch orientations (figure 1a), suggesting that the animal normally adjusts its head to place the eyes at the center of the range of the tiltMOR. Alternatively, the tiltMOR may have been optimized by an evolutionary process so that the midpoint of the variation of eye elevation coincides with the average head orientation during ambulation, which in turn may be dictated by the constraints of skeletal geometry (Vidal, et al, 2004;Vidal, et al, 1986). It is also interesting that the horizontal resting angle of 64° occupies something of a special position in the plots of horizontal eye orientation versus tilt angle, coinciding, as it does, with the approximate temporal limit in response to roll and the nasal limit in response to pitch tilts.…”

Functional morphology of the primate head and neck

“…Radiographic studies of the head and neck in alert mammals (e.g., cats, guinea pigs, rats, and some monkey species) have demonstrated instead that the cervical region can display complex curvatures and is often maintained, at least in resting positions, in a more vertical alignment (Vidal et al, 1986). This morphology is argued to indicate a more energetically efficient biomechanical system analogous to that of a suspension bridge or inverted bow-and-string !…”

“…Another mechanism to consider is the curvature(s) in the cervical column. The presence of such curvature is an important reminder that the pronograde primate neck is not necessarily a simple beam held in a near-horizontal orientation out from the torso.Radiographic studies of the head and neck in alert mammals (e.g., cats, guinea pigs, rats, and some monkey species) have demonstrated instead that the cervical region can display complex curvatures and is often maintained, at least in resting positions, in a more vertical alignment (Vidal et al, 1986). This morphology is argued to indicate a more energetically efficient biomechanical system analogous to that of a suspension bridge or inverted bow-and-string !…”