Mechanisms underlying achievement of final head position

TLDR

The study investigates how the central nervous system terminates a motor sequence and maintains the resulting posture. The aim was to determine whether the termination and maintenance of a head movement rely on proprioceptive feedback or are centrally programmed. We used vestibulectomized monkeys with unexpected load disturbances to provoke proprioceptive responses, and we also cut cervical and upper thoracic dorsal roots to interrupt afferent input, observing the effects on final head position. The results showed that the central pattern establishing final head position is preprogrammed and not reset by proprioceptive impulses, and that the equilibrium point depends on motoneuron firing, muscle length‑tension, and passive elastic properties.

Abstract

The studies reported here are directed toward understanding some of the mechanisms whereby the central nervous system terminates a given phase in a motor sequence and maintains a newly acquired position. In particular, we investigated the extent to which the termination of a centrally initiated head movement in monkeys and the subsequent maintenance of posture depend on a readout of proprioceptive afferent input generated during the movement itself or are instead centrally programmed. We approached this question in two ways: first, using vestibulectomized, but otherwise intact monkeys, we applied load disturbances unexpectedly at the beginning and throughout centrally initiated head movements with the aim of provoking a proprioceptive response in all types of neck receptors and to observe the outcome of this stimulation on the head final position. In a second set of experiments, we interrupted the flow of afferent input by cutting cervical and upper thoracic dorsal roots and observed how the absence of proprioceptive feedback affects the achievement of final head position. The results indicated that the central pattern of neural impulses establishing final head position is preprogrammed and it is not reset by the afferent proprioceptive impulses generated during the intended movement. In addition, our findings are consistent with the view that final head position is an equilibrium point dependent on a number of factors, such as the firing rate and the recruitment of the alpha motoneurons, the length-tension properties of the muscles involved in posture, and passive elastic properties of external loads.