The influence of neck muscle spindles in control of limb motoneuron excitability

Neck injury and pain are an increasing burden on society and they are sometimes associated with balance disturbances. It has been proposed that neck pain and injury result in abnormal somatosensory inputs from the neck and these have an impact on postural control. Among neck somatosensory receptors, the muscle spindles are garnering attention. It has been demonstrated that neck muscle vibration induces postural perturbations when executing postural tasks. These studies conclude that an artificial increase in the rate of discharge of neck spindle afferents causes these disturbances. However, it is also known that neck vibration induces a kinaesthetic illusion that results in an alteration of the egocentric reference frame. Therefore, the observed alteration in execution of postural tasks might be due to an altered perceptual experience rather than changes in motor outputs. If changes in neck muscle proprioceptive inputs alter motor outputs without being mediated by an altered reference frame, then this should be examinable by measuring changes in the amplitude of spinal reflexes. The aim of this thesis was to investigate whether activation of neck proprioceptors modulates 1) motoneuron excitability and / or 2) the excitability of fusimotor neurons of lower limb muscles in the relaxed human. To systematically alter the resting discharge of neck muscle proprioceptors, a muscle conditioning procedure that creates hold-long (slack) or hold-short (sensitised) muscle spindles or a vibratory stimulus was applied. The H-reflex or tendon jerk was recorded from the right soleus muscle. The results showed that there was no change in H-reflex amplitude after the two forms of muscle conditioning applied to neck rotator muscles. A vibratory stimulus applied to the dorsal neck muscles increased H-reflex amplitude recorded immediately after the onset of vibration; however, a prolonged vibratory stimulus did not potentiate the H-reflex suggesting no increase in excitability of alpha motoneurons. It was also shown that dorsal neck vibration did not activate fusimotor neurons since the amplitude of the hold-long tendon jerk evoked after neck vibration was not significantly different from its control. The results of these studies informed subsequent studies. It was suggested that the rate of neck muscle spindle afferent input shapes the vestibulospinal reflex. Therefore, it was hypothesized that reflex excitability of the limb muscles in response to simultaneous vestibular activation would be systematically altered when the rate of discharge of neck muscle proprioceptors is changed. Therefore, the final study investigated the interaction between neck proprioceptive and vestibular inputs using static whole body tilt and dorsal neck muscle conditioning. When the body was horizontal, dorsal neck muscle conditioning did not alter reflex amplitude. However, when the body was tilted, tendon jerk and H-reflex amplitude after hold-long dorsal neck muscle conditioning significantly increased in comparison to hold-short. It was concluded that CNS reliance on vestibular information for regulation of the excitability of the lower limb motoneurons changes depending on the quantity of neck proprioceptive inputs. Also it is suggested that the contribution of neck muscle afferents to the regulation of spinal reflex excitability is dependent on vestibular background inputs.