Normal Plexus and Nerve Anatomy

Cervical Plexus

The cervical plexus forms from the ventral rami of C1-C4 and variably a C5 minor branch. It has ascending superficial, descending superficial, and deep branches that supply the nuchal muscles, diaphragm, and cutaneous head/neck tissues.

Brachial Plexus

The brachial plexus (BP) forms from the ventral rami of C5-T1 and variably from minor branches of C4 or T2. The BP contributes to several nerves originating above the BP proper, including dorsal scapular nerve, long thoracic nerve, nerves to scalene/longus colli muscles, and a branch to the phrenic nerve. The remaining minor and all major terminal nerve branches arise from the BP proper.

Anatomically, the BP is classically divided sequentially (proximal to distal) into 5 segments: Roots/rami, trunks, divisions, cords, and terminal branches. The roots/rami originate directly from the spinal cord levels C5 to T1. The 3 trunks include the superior or upper (C5-C6), middle (C7), and inferior or lower (C8, T1) trunks. Important minor nerves arising directly from the trunks include the suprascapular nerve and nerve to subclavius muscle. Two divisions are described: Anterior division innervates anterior (flexor) muscles, and posterior division innervates posterior (extensor) muscles. No named minor nerves arise directly from the anterior or posterior divisions. The 3 cords include the lateral cord (anterior divisions of superior, middle trunks) that innervates anterior (flexor) muscles, the medial cord (anterior division of inferior trunk) that innervates anterior (flexor) muscles, and the posterior cord (posterior divisions of all 3 trunks) that innervates posterior (extensor) muscles. The cords branch to form several important named terminal peripheral nerve branches.

Clinically, the BP is divided into 3 discrete segments based on anatomic relationship to the clavicle. These include the supraclavicular (roots, trunks), retroclavicular (divisions), and infraclavicular (cords, terminal branches) plexus.

Lumbar Plexus

The lumbar plexus derives from the L1-L4 ventral rami and a minor branch from T12. Important named minor nerve branches include the iliohypogastric, ilioinguinal, genitofemoral, lateral femoral cutaneous (L2-L3), and superior (L4-S1) and inferior (L5-S2) gluteal nerves. Important major nerve branches include the femoral (posterior division L2-L4) and obturator (anterior division L2-L4) nerves (Table 2).

Lumbosacral Trunk

The lumbosacral trunk (LST) is derived from the ventral rami of L4 (minor branch) and L5, and it is easily followed on axial imaging as it transits along the ventral surface of the sacral ala to join the sacral plexus in the pelvis.

Sacral Plexus

The sacral plexus is composed of the LST, S1-S3 ventral rami, and a minor branch from S4. The sacral rami and LST converge into the upper sacral neural band (LST and S1-S3) that forms the sciatic nerve and the lower sacral neural band (S3-S4) that forms the pudendal nerve.