When studying Figure 22–1, it can be seen that the fifth cervical
root (C5) and the superior parts of the sixth and seventh cervical roots (C6
and C7) form most of the superior and middle trunk and all of the lateral
cord of the brachial plexus (Figure 22–2). Fibers from the C5, C6,
and C7 roots then terminate in the dorsal scapular nerve, suprascapular
nerve, lateral pectoral nerve, musculocutaneous nerve, and part of the
median nerve (Table 22–1). Electrical stimulation of only the C5
root, as with the cervical paravertebral block,8,9
for example, therefore results in a motor response in any or all of the
muscles supplied by the above nerves (rhomboid muscles, rotator cuff
muscles, major pectoral muscle, biceps muscle, pronators of the forearm,
superficial flexors of the forearm, and first and second lumbricals of the
hand and abductor of the thumb).
It is, however, almost never possible to achieve pure C5 stimulation. With
cervical paravertebral block, the needle is aimed toward the C6
root.8 Electrical stimulation of the C6 and C7 roots
results in motor responses in the muscles supplied by nerves branching off
the posterior cord (Figure 22–3), that is, the deltoid muscle
(axillary nerve), extensors of the arm (radial nerve) (Table 22–2)
and also any combination of the above muscles supplied by the nerves
branching from the lateral cord (see Table 22–1).