Sensorimotor control of the head-neck system

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Head-neck stabilization is inherently challenging even when stationary, requiring constant vigilance to counter the downward pull of gravity. It involves a highly complex biomechanical system comprised of a large mass (the head) balanced on top of seven vertebrae (the neck), that are in turn connected to a moving base (the torso). This multi-degree-of-freedom system is controlled by an equally impressive array of more than 30 bilateral muscle pairs that connect across various combinations of the skull, vertebrae and thorax. Visual and vestibular systems, together with neck somatosensory inputs are integrated by the central nervous system to provide an estimate of both the external world and an internal representation of ourselves. When our external (or internal) environments change, for example through unexpected or unpredictable disturbances, neck muscles make nimble corrections, through CNS control, to ensure the head remains upright and stable. However, for patients suffering from neck sensorimotor disorders, stable head posture is a constant challenge, often leading to complaints of neck pain and fatigue. This thesis investigates the contribution of different sensory feedback mechanisms – with particular focus on vestibular and proprioception sensory systems – to natural head-neck stabilization. The experimental methods developed and the research outcomes obtained from studying healthy controls are in turn used to investigate patients suffering from the neck movement disorder, cervical dystonia. This thesis draws two primary conclusions. The first conclusion is that healthy subjects substantially modulate vestibular and proprioception reflexes with the bandwidth and amplitude of torso perturbations, and that this modulation is dependent upon the presence of visual feedback and the descending motor command. In cervical dystonia patients, neck reflex dynamics were modulated across conditions in a manner similar to healthy controls, provided patients were engaged in a task to overcome the reorienting effects of the dystonia. The second conclusion is that the neurophysiological properties of the vestibular neck reflex indicate a highly robust and wide bandwidth influence of vestibular information over neck muscle control that differs markedly from other muscles throughout the body.