Alexander, Caroline Martha; (2002) Reflex control of shoulder girdle muscles in humans. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Trapezius and serratus anterior are bilaterally located axial muscles. They work synergistically to rotate the scapula during movements of the arm. With its attachments onto the spine, trapezius has a role in the posture of the trunk. These functions require both independent and closely co-ordinated control, both between homologous muscle pairs and between each other. The investigations presented here explore the reflexes of these muscles as well as the reflex control a) between the homologous muscle pairs, b) between trapezius and serratus anterior and c) between arm afferents and both muscles. How the reflexes between arm afferents and the shoulder girdle muscles differ in those individuals with various shoulder dysfunctions has also been investigated. A mechanical tap to trapezius evoked a short latency reflex in the ipsilateral trapezius muscle. In addition, short latency facilitatory reflexes were also seen contralaterally. Electrical stimulation of the afferent nerve to trapezius also evoked short latency, facilitatory reflexes in both the ipsilateral and contralateral muscles. These reflexes would appear to be the equivalent of the Hoffmann reflex seen in the soleus muscle. Evidence is presented which suggests that muscle spindle afferents from the ipsilateral trapezius muscle monosynaptically activate the trapezius motoneurones contralaterally. The investigations of serratus anterior reveal contrasting results, reflexes in contralateral serratus anterior being of much longer latency. Exploration of the reflex connections between the synergistic trapezius and serratus anterior revealed bi-directional short and long latency reflexes. Electrical stimulation of the medial, ulnar and radial nerves of the arm, revealed long latency reflex connections from afferents originating as far distally as the hand to both trapezius and serratus anterior. Evidence is presented that suggests that these reflexes are transcortically mediated and that they are evoked by group I muscle afferents. These reflexes are delayed or absent in those subjects with shoulder dysfunctions.

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