Lecture Notes on the Brachial and Lumbar Plexuses

Brachial Plexus

• Formed by the anterior rami of C5-T1 spinal nerves (Fig. 14.9).
• Structures are large trunks.
• C5 and C6 unite to form the superior trunk.
• C7 forms the middle trunk.
• C8 and T1 unite to form the inferior trunk.
• Each trunk splits into an anterior and a posterior division to become cords
• The anterior division of the inferior trunk forms the medial cord.
• The anterior divisions of the superior and middle trunks unite to form the lateral cord.
• The posterior divisions of each trunk unite to form the posterior cord.
• Several nerves originate from the brachial plexus' cords and trunks.

Nerves Originating from the Brachial Plexus

Axillary Nerve

• Branch of the posterior cord.
• Serves structures near the axilla.
• Innervates the deltoid and teres minor muscles.
• Provides sensory innervation to the skin around the axilla.

Musculocutaneous Nerve

• Distal continuation of the lateral cord.
• Located in the lateral arm.
• Serves the anterior arm muscles that flex the forearm (e.g., biceps brachii).
• Provides sensory innervation to the skin of the lateral forearm.

Radial Nerve

• Distal continuation of the posterior cord.
• Located in the posterior upper limb.
• Serves the muscles that extend the forearm and hand.
• Provides sensory innervation to the skin in the lateral hand.

Ulnar Nerve

• Distal continuation of the medial cord.
• Commonly known as the "funny bone nerve."
• Begins posteriorly but crosses to the anterior side of the forearm around the medial epicondyle of the humerus.
• Superficial at the medial epicondyle, making it easily injured.
• Supplies certain muscles in the forearm that flex the hand.
• Innervates most of the intrinsic muscles of the hand.
• Provides sensory innervation to the skin over the medial hand.

Median Nerve

• Results from the fusion of portions of the medial and lateral cords.
• Travels approximately down the middle of the arm and forearm.
• Supplies most of the muscles of the forearm that flex the hand.
• Innervates certain intrinsic hand muscles.
• Provides sensory innervation to the skin over the anterior and lateral hand.
• Travels under the flexor retinaculum at the wrist.
• Can become trapped and inflamed under the flexor retinaculum, leading to carpal tunnel syndrome.

Lumbar Plexus

• Consists of the anterior rami of L1-L4 with a small contribution from T12 (Fig. 14.10).

Iliohypogastric and Ilioinguinal Nerves

• Small nerves originating from the lumbar plexus.
• Provide motor innervation to the transversus abdominis and internal oblique muscles.
• Provide sensory innervation to the pelvic area.