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Brachial plexus

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The brachial plexus is formed from the ventral roots C5 to Th1, with small additions from C4 and Th2. The 5th and 6th cervical nerves unite to form the upper trunk, which continues on to become the major component of the lateral cord. The ventral root of the 7th cervical nerve becomes the middle trunk and together with portions of the upper and lower trunk continues on to become the posterior cord. Ultimately, the 8th cervical nerve and the 1st thoracic nerve unite to form the lower trunk, which together with portions of the middle trunk becomes the medial cord. The brachial plexus passes through the posterior scalenus gap relatively close to the surface between the scalenus anterior and scalenus medius muscles. A short distance away, it leaves the suprascapular nerve, which primarily supplies the shoulder joint, exiting from the upper trunk to the dorsal. The trunks subsequently divide to form the individual cords just above the clavicle. 

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The axillary artery is encased together with the cords in a common sheath which runs beneath the clavicle in the direction of the axilla. This neurovascular sheath should run from the deep cervical fascia to the axilla. Before reaching the axilla, the plexus divides into the following nerves:

  • Musculocutaneous nerve from the lateral cord 
  • Median nerve from the lateral and medial cords 
  • Ulnar nerve from the medial cord 
  • Radial, axillary and circumflex humeri nerves from the posterior cord.

It is assumed that differing septation within this common sheath is responsible for the varying susceptibility of the individual nerves to the effects of anaesthesia. This septation as well as the functional existence of a common fascial sheath have been the subject of discussion. Nonetheless, clinical practice and a number of studies have shown that there must be an anatomical basis for a single injection technique. Our own investigations on the cadaver did not reveal a common “tube”, rather a filigreed, hexagonal, certainly very easily penetrated septation.

 

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Sensory supply areas

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