Vnerves Brachial Plexus

A. Local molecular messages produced at the base of the limb bud guide the early nerve fibers into the limb bud; the muscles themselves do not provide, any specific target messages to the ingrowing nerve fibers.

B. Ventral primary rami from C5, C6, C7, C8, and Tl arrive at the base of the limb bud and join in a specific pattern to form the upper trunk, middle trunk, and lower trunk.

C. Each trunk will divide into posterior divisions and anterior divisions.

1. Posterior divisions grow into the posterior condensation of mesoderm and join to form the posterior cord.

2. Anterior divisions grow into the anterior condensation of mesoderm and join to form the medial cord and lateral cord.

Figure 19-2. Dermatome pattern in the adult upper limb and limb bud. The 90° lateral rotation of the upper limb bud causes the criginally straight segmental pattern of innervation in the embryo to be somewhat modified (i.e., "twisted in a spiral") such that the dermatome pattern in the adult is slightly altered. However, an orderly dermatome pattern can still be recognized in the adult if the upper limb is positioned in the parasagittal plane with the thumb pointing superiorly (as shown). The dermatomes from C4 can be counted distally down the superior border of the upper limb (arrow) to C7 at the middle finger and then back proxi-mally up the inferior border of the upper limb (arrow) to T2. (From Dudek RW, Fix JD: BRS Embryology, 2nd ed. Baltimore, Williams & Wilkins, 1998, p 224.)

Trapped Nerve

humb

Thumb

D. With further development of the limb musculature, the posterior cord will branch into the axillary nerve (C5, C6) and radial nerve (C5, C6, C7, C8, Tl), thus innervating all the muscles that form from the posterior condensation.

E. With further development of the limb musculature, the medial cord and lateral cord will branch into the musculocutaneous nerve (C5, C6, C7), ulnar nerve (C8, Tl), and median nerve (C5, C6, C7, C8, Tl), thus innervating all the muscles that form from the anterior condensation.

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