Shoulder girdle control; some mechanisms of function to dysfunction
Caroline Alexander, Imperial College Healthcare NHS Trust, UK
Fast conducting, group I muscles afferents originating in the forearm and hand evoke long latency, facilitatory reflexes to the scapulothoracic muscles, trapezius and serratus anterior in healthy subjects (Alexander and Harrison 2003). These transcortical reflexes are facilitated during tasks that require precise use of the hand (Alexander et al. 2005). Presumably this is in order to assist the stabilisation and appropriate positioning of the scapula upon the chest wall whilst doing a task with the hand. Interestingly these reflexes are similarly modulated by observation of this hand task alone (Alexander et al. 2004). In contrast to healthy subjects, these reflexes are delayed or in the main absent in people with various shoulder dysfunctions. In order to investigate why this might be the case, we examined this reflex pathway in 11 subjects suffering with non-traumatic shoulder instability (NTSI) and compared the results to a healthy population (n=15). The subjects with NTSI had a low to moderate degree of disability (Oxford Instability Shoulder Score = 25.1/60 ± 9.7).
With ethical approval and informed consent, we attempted to evoke this reflex using electrical stimulation of the ulna nerve at the elbow. Secondly, the conduction velocity of the effective afferents was recorded by additional stimulation of the ulna nerve at the level of the wrist. Thirdly, the descending control of trapezius was investigated using magnetic stimulation of the contralateral motor cortex to record the latency and threshold of the motor evoked potential (MEP) of trapezius. Finally, the motor response and H reflex of trapezius were recorded by electrically stimulating the spinal accessory nerve and the cervical nerve of C3/4 (Alexander and Harrison 2002). The long latency reflex from ulna nerve afferents to the lower fibres of trapezius had a longer latency (p<0.01) and was less frequently evoked (p<0.002) in subjects with NTSI when compared to the healthy group. However, this was not the case for this reflex evoked in upper trapezius (p=0.23 and 0.41 respectively). When we examined the reflex pathway, the conduction of the peripheral part of the pathway was not different to the healthy group. However, the corticospinal part of the reflex pathway did differ. The threshold to evoke an MEP in the lower fibres of trapezius was greater (p<0.006) and the latency of the lower trapezius MEP was longer (p<0.003). However, the threshold and latency of the upper trapezius MEP did not differ (p=0.21 and 0.11 respectively). How these physiological differences relate to the patient’s problems will be discussed. These results suggest that feedback mechanisms, some of which may assist scapula stability are not as proficient in subjects with NTSI. These changes seem to be due, at least in part, to an alteration in the corticospinal control of lower trapezius. This suggests that there are also implications for the voluntary action of this muscle (Alexander 2007).
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