Physiological and circuit mechanisms of postural control

Abstract

The postural system maintains a specific body orientation and equilibrium during standing and during locomotion in the presence of many destabilizing factors (external and internal). Numerous studies in humans have revealed essential features of the functional organization of this system. Recent studies on different animal models have significantly supplemented human studies. They have greatly expanded our knowledge of how the control system operates, how the postural functions are distributed within different parts of CNS, and how these parts interact with each other to produce postural corrections and adjustments. This review outlines recent advances in the studies of postural control in quadrupeds, with special attention given the neuronal postural mechanisms.

Figure 1

Reactions of the postural system to different perturbations. (a) Postural task with lateral tilts of the support surface. The cat is keeping balance on two platforms (fore and hind) tilted together or independently. (b) Functional model of the postural system in the task with lateral tilts. Lateral stability of the anterior and posterior parts of the body (shoulder and hip girdles) is maintained by two relatively independent sub-systems. Each sub-system contains two controllers (for the right and left limbs) generating a part of corrective limb movement in response to sensory input from the same limb (red lines); another part is produced in response to influences from the contralateral limb (blue lines). Coordination between these subsystems is primarily based on influences of the anterior sub-system on the posterior one (green lines). (c) Postural task with removal of the support surface under the left forelimb (FL). (d) Postural reaction (forces and EMGs in individual limbs) to drop of FL, and timing of the events: 1, drop of FL; 1–2, latency of FR EMG; 1–3, latency of FR force; 2–4, displacement of the center of mass over two diagonal supporting limbs; 4–5, transition from bipedal to tripedal support pattern; 5–6, maintenance of a stable posture and equilibrium over three supporting limbs. (e) Configuration of the system generating the early EMG responses to drop of FL (period 1–3). Later (period 3–5) the sensory inputs to supporting limbs are changed. (f) Main components of the postural system in quadrupeds. Two closed-loop mechanisms participate in the postural control. Spinal circuits generate postural limb reflexes, and their effects are added to the effects of supraspinal commands, which are generated on the basis of sensory information, and transmitted by – the major descending tracts reticulospinal (RS), vestibulospinal (VS), corticospinal (CS), and rubrospinal (RbS).