Proceedings of The Physiological Society

Physiology 2014 (London, UK) (2014) Proc Physiol Soc 31, PCB146

Poster Communications

Stretch reflexes in sternocleidomastoid muscle evoked by tendon tap

S. Nousi2, M. Gresty2, A. Bronstein2, P. Strutton1

1. Surgery and Cancer, Imperial College London, London, United Kingdom. 2. Academic Department of Neuro-Otology, Imperial College London, London, United Kingdom.

The neck muscle motorneurones are innervated by both vestibular and proprioceptive afferents which stabilise the head either on the trunk or in space. Since the majority of body manoeuvres activate both systems simultaneously it is difficult to differentiate their respective properties. The aim of this study was to isolate a neck stretch reflex by tendon tap with the head fixed in space in order to eliminate vestibular activation. The subjects were 17 healthy adults and 8 patients with bilateral vestibular failure. Subjects were seated in a semi-reclined position with forehead head restraint. Electromyographic (EMG) activity was recorded bilaterally from sternal (SM) and clavicular (CM) heads of SCM. The tendon of the left SM was tapped using a hand held mechanical device whilst the subject remained relaxed and during isometric neck flexion. In addition forehead skull taps were applied to evoke vestibular responses. Rectified EMG responses were analysed.Tapping of the left SM tendon evoked EMG responses of a similar latency in the ipsi- and contra-lateral SCM muscles in both groups when the neck muscle was relaxed (~32 msec) and isometrically contracted (~33 msec). There were no latency differences between subject groups or between different levels of muscle contraction. The areas of the EMG responses were significantly larger in healthy subjects than in patients. In contrast, forehead skull taps (vestibular responses) evoked responses in healthy subjects but not patients.Our data indicate that the tendon tap responses in all subjects were induced via neck muscle stretch afferents and not by activation of vestibular reflexes. The relatively long latencies found would suggest a long loop reflex. Presumably this stretch reflex is the substrate of the cervico-collic reflex which normally interacts with the vestibule-collic reflex for the control of head posture. The weaker stretch reflexes in labyrinthine defective subjects could be an adaptive dis-facilitation to reduce neck muscle induced head oscillations.

Where applicable, experiments conform with Society ethical requirements