Proceedings of The Physiological Society

Physiology 2014 (London, UK) (2014) Proc Physiol Soc 31, PCB145

Poster Communications

Effect of caloric irrigation on neck stretch reflexes

S. Nousi2, Y. Nigmatullina2, 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.

We recently showed that vestibular stimulation with cold caloric irrigation induced an increase in the excitability of the pathway to the contralateral sternocleidomastoid (SCM) muscle using transcranial magnetic stimulation (TMS) over the SCM motor cortex which was consistent with the direction of the putative head turn. However, whether this increase was cortical or spinal was not clear. We have now developed a technique to induce neck reflexes in this muscle by tapping the tendon of the sternal head of the SCM in a fixed head paradigm. We have used this to probe the spinal excitability of the neck muscles during caloric irrigation in the present study.Ten healthy subjects with no history of vestibular dysfunction were seated in a semi-reclined position with forehead head restraint and eyes open. Electromyographic (EMG) activity was recorded from the sternal head of the left SCM and subjects were instructed to maintain a low level of isometric neck flexion, with feedback of EMG provided, of approximately 20% of maximum voluntary contraction (MVC). Cold caloric irrigation was applied to the right ear for 40 seconds, after 20 seconds (and on confirmation of the induced nystagmus using a head mounted infra-red binocular video-oculography system) the tendon of the left SCM was tapped using a hand held mechanical device every 5 seconds; at least 25 taps were delivered. Thirty taps were also applied in a caloric-free condition; the order of conditions was randomised. EMG data were rectified and averaged. Latencies and areas of responses were analysed for differences between the two conditions and are expressed as mean±SD.Tapping of the SCM tendon evoked EMG responses, the latencies of which were not different between the two conditions (tap only 32.00±9.00ms; tap+caloric 29.84±5.92ms; t=-1.05, P=0.32). The areas of the responses were also not different (tap only 0.15±0.11 %MVC; tap+caloric 0.16±0.11 %MVC; t=-0.06, P=0.95).These data indicate that our earlier finding of the increase in size of the TMS-evoked responses during cold caloric irrigation is unlikely to be due to an increase in the spinal excitability of the reflex arc responsible for these tendon tap-induced responses. Further clarification of the underlying mechanisms responsible for the alteration in excitability (using paired pulse TMS) is underway.

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