The plexus consists of roots, trunks, divisions, cords and branches.
Roots
These are constituted by the anterior primary rami of spinal nerves C5, C6, C7, C8 and T1, with contributions from the anterior primary rami of C4 and T2.
The origin of the plexus may shift by one segment either upward or downward, resulting in a prefixed or postfixed plexus respectively.
In a prefixed plexus, the contribution by C4 is large and that from T2 is often absent.
In a postfixed plexus, the contribution by T1 is large, T2 is always present, C4 is absent, and C5 is reduced in size. The roots join to form trunks as follows:
Trunks
Roots C5 and C6 join to form the upper trunk. Root C7 forms the middle trunk. Roots C8 and T1 join to form the lower trunk.
Divisions of the Trunks
Each trunk (three in number) divides into ventral and dorsal divisions (which ultimately supply the anterior and posterior aspects of the limb).
These divisions join to form cords.
Cords
i. The lateral cord is formed by the union of ventral divisions of the upper and middle trunks (two divisions).
ii. The medial cord is formed by the ventral division of the lower trunk (one division).
iii. The posterior cord is formed by union of the dorsal divisions of all the three trunks (three divisions).
Sympathetic Innervation
1. Sympathetic nerves for the upper limb are derived from spinal segments T2 to T6. Most of the vasoconstrictor fibres supplying the arteries emerge from segments T2 and T3.
2. The preganglionic fibres arise from lateral horn cells and emerge from the spinal cord through ventral nerve roots.
3. Passing through white rami communicans they reach the sympathetic chain.
4. They ascend within the chain and end in the middle cervical, inferior cervical and first thoracic ganglia.
5. Postganglionic fibres from middle cervical ganglion pass through grey rami communicans to reach C5, C5 nerve roots.
6. Postganglionic fibres from inferior cervical ganglion pass through grey rami communicans to reach C7, C8 nerve roots.
7. Postganglionic fibres from first thoracic sympathetic ganglion pass through grey rami communicans to reach Th1 nerve roots.
8. The arteries of skeletal muscles are dilated by sympathetic activity. For the skin, however, these nerves are vasomotor, sudomotor and pilomotor.
Vasomotor: Constricts the arterioles of skin.
Sudomotor: Increases the sweat secretion.
Pilomotor: Contracts the arrector pilorum muscle to cause erection of the hair.
Branches
The roots value of each branch is given in brackets.
Branches of the Roots:
1. Nerve to serratus anterior (long thoracic nerve) (C5, C6, C7)
2 Nerve to rhomboids (dorsal scapular nerve) (C5).
3 Branches to longus colli and scaleni muscles (both C5-C8) and branch to phrenic nerve (C4).
Branches of the trunks:
These arise only from the upper trunk which gives two branches.
1. Suprascapular nerve (C5, C6)
2 Nerve to subclavius (C5, C6)
Branches of lateral cord:
1 Lateral pectoral (C5- C7)
2 Musculocutaneous (C5-C7)
3 Lateral root of median (C5-C7)
Branches of medial cord:
1. Medial pectoral (C8, T1)
2 Medial cutaneous nerve of arm (C8, T1)
3 Medial cutaneous nerve of forearm (C8, T1)
4 Ulnar (C7, C8, Tl). C7 fibres reach by a communicating branch from lateral root of median nerve.
5 Medial root of median (C8, T1)
Branches of posterior cord:
1 Upper subscapular (C5, C6)
2 Nerve to latissimus dorsi (thoracodorsal) (C6, C7,C8)
3 Lower subscapular (C5, C6)
4 Axillary (circumflex) (C5, C6)
5 Radial (C5-C8, T1)
In addition to the branches of the brachial plexus, the upper limb is also supplied, near the trunk, by the supraclavicular branches of the cervical plexus, and by the intercostobrachial branch of the second intercostal nerve. Sympathetic nerves are distributed through the brachial plexus. The arrangement of the various nerves in the axilla was studied with the relations of the axillary artery.
Reference: B.D. Chaurasia, Human Anatomy, Volume 1 (Upper Limb and Thorax)
