Sensory- motor control of head-neck musculature

The neural connections between the vestibular, visual, proprioceptive and voluntary inputs are essential for the control of neck posture and movements. One mechanism how this is achieved is by vestibulo-spinal reflexes, such as vestibulo-collic reflex (VCR), and cervico-collic reflexes (CCR). The main role of these reflexes is to stabilize the head either in space or relative to the trunk, respectively. The VCR stabilises the head in space while the CCR tends to re-align the head on the trunk. These two reflexes can work synergistically or an-tagonistically according to context and movement goals. The aim of this thesis is to investigate the sensory –motor control of the sternocleidomastoid muscle and the nature of the functional interactions between the vestibular system and neck muscles in healthy participants and in patients with bilateral vestibular loss. Specifically, the experiments performed were designed to a) investigate how head control is organised in healthy subjects and b) to examine how this functional interaction is modulated in patients with vestibular dysfunction. Firstly, a novel head-restraint paradigm is used in order to attempt to isolate a neck stretch reflex in the SCM muscle. Using this paradigm, the activation of the vestibular system is minimised. This permits differentiation between vestibular and neck muscles reflexes. Forehead skull taps and tendon taps were used to differentiate between vestibular-mediated response and stretch responses and vestibularless patients were also included to aid with assessing the contribution of stretch reflexes to head control. Secondly, the sensory-motor organization of the induced responses by tendon tap was investigated following vestibular caloric stimulation to see if it was modulated by vestibular input. Finally, in order to assess cortical aspects of neck control the cortico-spinal excitability of the sternomastoid muscle have been assessed, with non-invasive brain stimulation (transcranial magnetic stimulation), and visual stimulation (rotating disc) in healthy participants. To be able to better understand visual processing of the latter (motion visual stimuli).In the final part of the thesis the effects of visual motion stimulation on the excitability of the visual cortex, both in areas V1 and V5 were investigated. Applying tendon taps of the left sterno-mastoid (SM) relatively long latencies (32msec) EMG responses have been recorded from the ipsilateral and contralateral SCM muscles in con-tracted and relaxed conditions in both healthy and bilateral vestibular failure subjects (BVF). These latencies (32ms) indicating a long loop, possibly via the cortex or subcortical struc-tures would suggest a long loop reflex. These long loop responses are not modulated apply-ing caloric vestibular stimulation and suggest that although the vestibular system is activat-ed, the neural pathway between the vestibular system and the motor cortex is not activated by the tap since this is purely a spinal reflex? In addition the physiological mechanisms of head neck control are significantly influenced by the visual cues. Visual – vestibular and proprioceptive loops are probably involved in head-neck sensory motor control.Open Acces