1 Brachial Plexus (E2)

• A "typical" spinal nerve exhibits bifurcation into a dorsal and ventral ramus.
• At each vertebral level, the dorsal ramus passes segmentally to the back.
• In the thoracic levels T2-T11, the ventral rami remain "segmental".
• In other regions, the ventral rami may fuse, bifurcate, and rejoin in a complex fashion, forming a "plexus".
• The dorsal and ventral rami of each spinal nerve are responsible for innervating a strip of skin referred to as a dermatome.

Brachial Plexus

• It is responsible for the motor and sensory innervation of the upper limb and its associated muscles.
• It is a vulnerable structure prone to injury during obstetric delivery, in accidents, and gunshot wounds.
• The basic pattern exhibits roots, trunks, divisions, and cords.
• The branches (nerves emanating from the plexus) are added to this arrangement.
• The plexus is formed from a series of roots, which are the ventral rami of C5 to T1.
• The subsequent plexiform arrangement allows neurons from several segmental levels of the cord to be distributed to the periphery in a single nerve.
• Roots: There are 5 roots (C5 through T1), which lie within the prevertebral musculature of the neck.
• The roots emerge into the posterior triangle between the anterior and middle scalene muscles.
• Trunks: Roots then unite to form trunks, which are named after their anatomical relationship to one another.
• Superior Trunk: The upper two roots unite to form the superior trunk.
• Inferior Trunk: The lower two roots unite and form the inferior trunk.
• Middle Trunk: The root from C7 continues on to become the middle trunk.
• Behind the clavicle, each trunk divides into an anterior and posterior division.
• In the axilla, the cords of the brachial plexus are formed by the union of the anterior and posterior divisions of the trunks.
• The cords are named after their relationship to the axillary artery.
• Posterior Cord: The posterior divisions of all 3 trunks unite to form the posterior cord that supplies the extensor muscles of the posterior aspect of the upper limb.
• Lateral Cord: The upper two anterior divisions unite to form the lateral cord.
• Medial Cord: The remaining anterior division (of the lower trunk) forms the medial cord, which supplies flexor muscles on the anterior aspect of the upper limb along with the lateral cord.

Branches from the Roots

• C5 gives off the last contribution to the phrenic nerve (C3,4,5).

Branches from the Cords

• Lateral cord- the musculocutaneous nerve (C5,6,7) and the lateral root of the median nerve (C6,7).
• Medial cord - the medial root of the median nerve (C8, T1) and the ulnar nerve (C8, T1).
• Posterior cord - the axillary nerve (C5,6) and the radial nerve (C5-8, T1).
• The axillary artery and axillary vein lie within the axillary sheath in the axilla.
• The cords of the brachial plexus are wrapped around the axillary artery.
• Five main nerves arise from the plexus: musculocutaneous, median, ulnar, axillary, and radial nerves.

Regional Summary

• Roots of the brachial plexus are located in the prevertebral muscular floor of the posterior triangle.
• The trunks pass through the posterior triangle.
• Divisions are located behind the clavicle (removed).
• The cords are positioned around the axillary artery in the axilla.

Explanation of the spinal nerve structure, including dorsal and ventral rami. Overview of the brachial plexus, its formation from roots C5 to T1, and its role in upper limb innervation. Common injuries of the brachial plexus are also covered.