Brachial Plexus
The nerves entering the upper limb provide the following
important functions: sensory innervation to the skin and
deep structures, such as the joints; motor innervation to
the muscles; influence over the diameters of the blood vessels
by the sympathetic vasomotor nerves; and sympathetic
secretomotor supply to the sweat glands.
At the root of the neck, the nerves form a complicated
plexus called the brachial plexus. This allows the nerve fibers
derived from different segments of the spinal cord to be
arranged and distributed efficiently in different nerve trunks
to the various parts of the upper limb. The brachial plexus
is formed in the posterior triangle of the neck by the union
of the anterior rami of the 5th, 6th, 7th, and 8th cervical and
the 1st thoracic spinal nerves (Figs. 9.18 and 9.19).
The plexus can be divided into roots, trunks, divisions,
and cords (Fig. 9.18). The roots of C5 and 6 unite to form the
upper trunk, the root of C7 continues as the middle trunk,
and the roots of C8 and T1 unite to form the lower trunk.
Each trunk then divides into anterior and posterior divisions.
The anterior divisions of the upper and middle trunks
unite to form the lateral cord, the anterior division of the
lower trunk continues as the medial cord, and the posterior
divisions of all three trunks join to form the posterior cord.
The roots, trunks, and divisions of the brachial plexus
reside in the lower part of the posterior triangle of the neck
and are fully described on page XXX. The cords become
arranged around the axillary artery in the axilla (Fig. 9.15).
Here, the brachial plexus and the axillary artery and vein
are enclosed in the axillary sheath.
Cords of the Brachial Plexus All three cords of the
brachial plexus lie above and lateral to the first part of
the axillary artery (Figs. 9.15 and 9.20). The medial cord
crosses behind the artery to reach the medial side of the
second part of the artery (Fig. 9.20). The posterior cord
lies behind the second part of the artery, and the lateral
cord lies on the lateral side of the second part of the artery
(Fig. 9.20). Thus, the cords of the plexus have the relationship
to the second part of the axillary artery that is indicated
by their names.
Most branches of the cords that form the main nerve
trunks of the upper limb continue this relationship to the
artery in its third part (Fig. 9.20).
The branches of the different parts of the brachial
plexus (Figs. 9.19 and 9.21) are as follows:
■■ Roots
Dorsal scapular nerve (C5)
Long thoracic nerve (C5, 6, and 7)
■■ Upper trunk
Nerve to subclavius (C5 and 6)
Suprascapular nerve (supplies the supraspinatus and
infraspinatus muscles)
etc snell