The ‘plexus’ (from Latin for a ‘braid’) is like a railway junction allowing sensory and motor axons from the different segmental levels of the spinal cord, having exited from the spine, to cross and recross, to travel in the main emerging lines, and eventually, in their branches, to reach their final destinations. The brachial plexus is in the form of a large fissured sheet lying behind skin and muscle in the neck, above the collar bone. It resolves into three main nerve cords, which in turn branch to give rise to the peripheral nerves that are distributed via their branches throughout the limb. The three largest are the radial nerve, which courses down the outer (thumb) side of the arm; the ulnar, down the inner side; and the median, in between. These are ‘mixed’ nerves — they carry both motor and sensory fibres, and give off branches on their way to the hand; some branches also are mixed, some motor, some sensory. The constituent microbundles of nerve fibres (fascicles) may change their position within a nerve, thus allowing axons to be directed towards their final target within a tissue (a particular part of muscle, skin, or joint) in the final smallest branches. The longest axons in the median and ulnar nerves, for example, terminate in the sensory branches to the fingertips.
Close to the spine the brachial plexus is joined by fibres from the sympathetic ganglia of the autonomic nervous system; these are distributed to the periphery in the same way, innervating the blood vessels and the glands of the skin. Because of this, damage to any nerve originating from the plexus results in warm, dry skin in the area that it supplies, because the smooth muscle of the blood vessels relaxes in the absence of sympathetic tone, and sweating cannot be stimulated.
Whereas lesions of a peripheral nerve give rise to discrete functional disturbance with local weakness, paralysis, and numbness (for example in the case of the median nerve in the ‘carpal tunnel syndrome’ — or more fashionably a computer-typing-based repetitive strain injury), damage to the brachial plexus itself results in widespread loss of both muscle power and sensation throughout the arm and shoulder. Damage may arise from a penetrating missile, from traumatic amputation, or from excessive traction when the constituent spinal nerves are literally torn from their roots, thus causing additional symptoms of spinal cord injury.
"brachial plexus." The Oxford Companion to the Body. . Encyclopedia.com. (May 27, 2018). http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/brachial-plexus
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