THE CONTEXT
The passage describes plexus divisions and cords, followed by the origin, pathway and functions of sympathetic fibres supplying the upper limb.
Trunks, divisions and cords
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Root C7 forms the middle trunk. The roots and trunks constitute the supraclavicular part of the plexus.
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Each of the three trunks divides into ventral and dorsal divisions, which ultimately supply the anterior and posterior aspects of the limb. These divisions form the retroclavicular part.
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The lateral cord forms from the ventral divisions of the upper and middle trunks.
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The medial cord is formed by the ventral division of the lower trunk.
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The posterior cord forms from the dorsal divisions of all three trunks.
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The cords and branches constitute the infraclavicular part.
Postfixed plexus
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In a postfixed plexus, the T1 contribution is large, T2 is always present and C4 is absent.
Sympathetic origin and preganglionic pathway
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Sympathetic nerves for the upper limb arise from spinal segments T2 to T6. Most arterial vasoconstrictor fibres emerge from T2 and T3.
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Preganglionic fibres arise from lateral horn cells, leave the spinal cord through ventral nerve roots and enter the sympathetic chain through white rami communicantes.
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The fibres ascend in the chain and terminate in the middle cervical, inferior cervical and first thoracic ganglia.
Postganglionic pathway and effects
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Postganglionic fibres from the cervical and first thoracic ganglia reach nerve roots through grey rami communicantes.
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Fibres from the first thoracic sympathetic ganglion reach the T1 nerve roots.
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Sympathetic activity dilates the arteries of skeletal muscles.
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In the skin, sympathetic nerves have vasomotor, sudomotor and pilomotor functions.