Impairment of postural control in rabbits with extensive spinal lesions

Abstract

Our previous studies on rabbits demonstrated that the ventral spinal pathways are of primary importance for postural control in the hindquarters. After ventral hemisection, postural control did not recover, whereas after dorsal or lateral hemisection it did. The aim of this study was to examine postural capacity of rabbits after more extensive lesion (3/4 section of the spinal cord at T(12) level), that is, with only one ventral quadrant spared (VQ animals). They were tested before (control) and after lesion on the platform periodically tilted in the frontal plane. In control animals, tilts of the platform regularly elicited coordinated electromyographic (EMG) responses in the hindlimbs, which resulted in generation of postural corrections and in maintenance of balance. In VQ rabbits, the EMG responses appeared only in a part of tilt cycles, and they could be either correctly or incorrectly phased in relation to tilts. Because of a reduced value and incorrect phasing of EMG responses on both sides, this muscle activity did not cause postural corrective movements in the majority of rabbits, and the body swayed together with the platform. In these rabbits, the ability to perform postural corrections did not recover during the whole period of observation (< or =30 days). Low probability of correct EMG responses to tilts in most rabbits as well as an appearance of incorrect responses to tilts suggest that the spinal reflex chains, necessary for postural control, have not been specifically selected by a reduced supraspinal drive transmitted via a single ventral quadrant.

FIG. 1.

Experimental design. A–C: testing of postural responses to tilts. The animal was standing on 2 platforms, 1 under the fore limbs (P1) and 1 under the hind limbs (P2). Platform P2 could be tilted in the transverse plane (α is the platform tilt angle). The sagittal plane of the animal was aligned to the axis of platform rotation. Mechanical sensor S, positioned at the half-height of the body, measured lateral displacements of the caudal part of the trunk in relation to the P2 platform. D: schematic representation of the trajectory of the platform angle (α) and corrective movements of the caudal trunk (S).

FIG. 2.

Examples of postural responses before and after 3/4 section. A and B: kinematical and electromyographic (EMG) responses to tilts in control (A) and 21 days after 3/4 section (B; rabbit 60). C: kinematical and EMG responses 20 days after 3/4 section (rabbit 45). α, tilt angle of the P2 platform; S, lateral displacement of the caudal part of the trunk in relation to the P2 platform. The EMGs of the following muscles are presented: left (L) and right (R) vastus (Vast), gastrocnemius (Gast) and biceps femoris (Bic). White arrows in B indicate correct responses; black arrows indicate incorrect responses. Shaded column highlights half of the tilt cycles to facilitate comparison between curves.

FIG. 3.

Characterization of EMG responses to tilts in the badly compensated rabbit (60). All EMG responses in individual muscles (recorded in 1 trial) were classified into 3 categories—correct responses (extensor EMG is timed to the ipsilateral tilt, flexor EMG is timed to the contralateral tilt), incorrect responses (opposite phase relations), and no response. The relative number of responses in each category (%) is indicated.

FIG. 4.

Summary of responses to tilts in ventral quadrant (VQ) rabbits. The diagrams (A–E) show, for individual rabbits, the percent of correct responses, incorrect responses, and cycles with no responses (averaged over all recorded muscles of both limbs) as a function of postlesion time (day 0 is control). Days of testing when the postural motor responses were present or absent are indicated by black and white horizontal bars, respectively. Right: extent of the spinal cord damage in individual rabbits. The shape of the tissue that remained intact was determined from the photomicrograph taken at the center of the lesion (1st column) and projected on a scheme of the spinal cord section taken more rostrally (2nd column). In the photos, the lesioned area is delimited by a continuous white line, and the intact gray matter by a dotted line, the remainder corresponding to the intact white matter. Arrows indicate a position of the ventral fissure. In the schemes, the damaged area is shaded.

FIG. 5.

Effect of the 3/4 section on the postural configuration. A and B: postural configuration of the rabbit before (A) and after 3/4 section of the spinal cord (B), view from below. To characterize the position of each hindlimb, the following values were measured: the foot angle (ɛ), as well as the coordinates (x and y) of the rostral point of the foot in relation to the mid-body axes. C: foot angles in the hindlimbs of rabbit 61 in control (day 0) and on different days after the 3/4 section. D: foot position of rabbit 61 in control and on different days after the 3/4 section. E–G: foot angles (E), lateral (F), and anterior-posterior (G) foot position (mean ± SE) averaged over 5 animals. H: the height (Ht) of the hindquarters was measured in the standing position. I: the height as a function of time after lesion in individual rabbits (0 is control).