Objective: To investigate whether postural reflexes evoked by both vestibular and axial stimuli are enhanced by instability.

Background: Instability has been reported to increase vestibular-evoked postural reflexes (1).  Postural reflexes are often investigated while subjects lean so that they are unstable but this also increases tonic EMG activity in the target muscles which itself can increase reflex responses (2).  We wished to investigate both vestibular and non-vestibular postural reflexes for the effects of instability while keeping tonic activity constant. We also wanted to deduce where any effect was occurring.

Method: 10 subjects with no balance issues were studied, in neutral stance, leaning forwards and leaning backwards.  In neutral stance, subjects were studied with toes or heels elevated to match levels when leaning.  EMG was recorded from tibialis anterior (TA) and soleus as well as over the cerebellum, infra-ocularly and for the sternocleidomastoids (Scm). Four impulsive stimuli from a mini-shaker were used, 2 applied to the midline: over C7 and to the upper sternum and 2 to the mastoids with the head rotated: to the posterior mastoid and to the anterior mastoid.  The first two depend upon proprioceptive afferents, the second two, vestibular.  All evoke short latency reflexes in lower leg muscles.

Results: Leaning tonic EMG levels were well matched during neutral standing with toes or heels elevated. C7 impulses (activating soleus) were significantly larger when leaning than with similar tonic activity during neutral stance but this did not hold for sternal stimulation and TA.  Both mastoid stimuli evoked larger responses with leaning than matched tonic activity but these were significant only at the P <0.1 level.  Ocular responses showed minimal changes, cerebellar responses were larger leaning forwards for axial stimuli and Scm responses larger when leaning backwards.

Conclusion: Instability selectively facilitates postural reflexes, including those to soleus following midline stimuli. Ocular responses show little change. The facilitation is unlikely to occur at the level of the afferents excited and is more likely to be mediated through changes in the descending brainstem volleys.

References: [1] Naranjo EN, Allum JH, Inglis JT, Carpenter MG. Increased gain of vestibulospinal potentials evoked in neck and leg muscles when standing under height-induced postural threat. Neuroscience 2015; 293:45-54, 2015. doi:10.1016/j.neuroscience.2015.02.026
[2] Matthews PBC. Observations on the automatic compensation of reflex gain on varying the pre-existing level of motor discharge in man. J Physiol 1986;374:73-90. doi: 10.1113/jphysiol.1986.sp016066

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