Stapley - 2009 - The pontomedullary reticular formation contributes to ... postural responses ... in the standing cat

Citation

Stapley PJ, Drew T. The pontomedullary reticular formation contributes to the compensatory postural responses observed following removal of the support surface in the standing cat. J Neurophysiol. 2009 Mar;101(3):1334-50. PUBMED

10 Word Summary

Postural responses are driven by a single descending command.

Abstract

This study was designed to determine the contribution of reticular neurons in the pontomedullary reticular formation (PMRF) to the postural responses produced to compensate for an unexpected perturbation. We recorded the activity of 48 neurons in the PMRF, including 41 reticulospinal neurons, to removal of the support surface under each of the four limbs in four cats. The perturbations produced robust postural responses that were divided into three periods: an initial postural response (P1) that displaced the center of vertical pressure over the two diagonal supporting limbs; a secondary response (P2) during which the cat restored a tripedal support pattern; and a prolonged tertiary response (P3) that maintained a stable posture over all three supporting limbs. Most (44/48) reticular neurons showed modified activity to perturbation of at least one limb and a majority (39/48) showed changes in activity to perturbations of more than one limb. A few (7/48) discharged to perturbations of all four limbs. Discharge frequency in neurons showing increased activity during P1 was relatively high (>100 Hz in 57% of the neurons responding to perturbations of either the left or right forelimbs, lFl and rFL) and of short latency (17 ms for the lFL and 14 ms for the rFL). Discharge activity in most neurons was sustained throughout P2 and P3 but at a reduced level. These data show that neurons in the PMRF discharge strongly in response to unexpected perturbations and in a manner consistent with a contribution to the compensatory responses that restore equilibrium.

Notes

• Postural responses come from a localized center, not independent control of each limb (Dufosse 1982).
• Two goals of the study
  • Analyze biomechanics of response to removal of support surface
  • Determine whether neurons in the PMRF discharged in a manner compatible with a role in compensating for the perturbation applied
    • Do neurons discharge to perturbations to all four limbs?
    • Do neurons discharge in the same manner for all four limbs?
• The PMRF is thought to modify muscle tone.
• When the paw drops, the cat initially rocks away from dropped paw, then balances on two legs, shifts weight to three remaining legs and then guides weight back to "center"