Brachial Plexus Anatomy

• A nerve plexus constitutes a network of intersecting nerves.

Brachial Plexus

• A large network of nerves innervates the upper extremity.
• It innervates intrinsic, appendicular muscles and hypaxial muscles
• Formed by the ventral rami of C5-C8 cervical spinal nerves and the T1 ventral ramus.
  • Ventral rami are mixed nerves, but carry more motor fibers than sensory fibers.
• Located in the posterior triangle of the neck, as well as in the axilla.
• Divided into regional components: roots, trunks, divisions, cords, and branches.

Roots of the Brachial Plexus

• Originate at the C5-T1 spinal levels and pass between the anterior and middle scalene muscles.
• The ventral rami that form as the spinal nerves exit the IVF and divide are roots.
• Small contributions to the plexus may come from C4 and T2.
• C5 innervates near the shoulder, C6 innervates near the elbow, C7 innervates near the wrist, and C8-T1 innervates the hand.
• Roots combine to form 3 trunks.

Trunks of the Brachial Plexus

• Names reference relative anatomic position.
• The superior (or upper) trunk is formed by the union of C5 and C6 roots.
• The middle trunk is a continuation of root C7.
• The inferior (or lower) trunk is formed by the union of C8 and T1 roots.
• Each trunk divides into an anterior and posterior division.

Divisions of the Brachial Plexus

• A total of 6 exist, formed from the trunks: three anterior divisions and three posterior divisions.
• Approximately at the level of the clavicle.
• Divisions unite unevenly to form 3 cords.

Cords of the Brachial Plexus

• Names reference location with respect to the axillary artery.
• The lateral cord is formed by fusion of the anterior divisions of the upper and middle trunks.
• The posterior cord is formed by fusion of all three posterior divisions (one from each trunk).
• The medial cord is simply the anterior division of the lower trunk.

Branches of the Brachial Plexus

• A total of 5 major branches form from the cords.
• Many small branches originate from the roots, trunks, divisions, and cords of the brachial plexus as well.

Branches of the Brachial Plexus's Posterior Cord

• It has 4 branches and terminates as it bifurcates into the last two: subscapular nerves, thoracodorsal nerve, axillary nerve, and the radial nerve.
• The axillary nerve branches off from the posterior cord and wraps posteriorly to the space.
• The radial nerve is much larger and continues the course of the posterior cord (posterior to the axillary artery).

Branches of the Brachial Plexus From Medial and Lateral Cords

• These divide and the portions closer to the axillary artery unite to give rise to the musculocutaneous, median, and ulnar nerves.
• They form a large "M".
• The musculocutaneous nerve is the branch of the lateral cord.
• The median nerve is formed by the combination of both lateral and medial cords. It is the central part of the "M."
• The ulnar nerve is the branch of the medial cord.

Supraclavicular Branches of the Brachial Plexus

• Emane from the roots and trunks which lie in the posterior triangle (superior to the clavicle).
• Originate from the C5 root as the dorsal scapular nerve
• Originate from the C5, 6, and 7 roots as the long thoracic nerve.
• Originate from the upper trunk as the suprascapular nerve.
• Originate from the upper trunk as the nerve to the subclavius muscle.

Infraclavicular Branches of the Brachial Plexus

• Emane from the cords and travel from within the axilla.
• Arise from the lateral cord: lateral pectoral nerve, lateral root of the median nerve, and musculocutaneous nerve.
• Arise from the medial cord: medial pectoral nerve, medial brachial cutaneous nerve, medial antebrachial cutaneous nerve, medial root of median nerve, and ulnar nerve.

Muscle Innervation of the Upper Extremity

• Medial and Lateral cord branches innervate preaxial muscles, including pectoral muscles, arm and forearm flexors, and intrinsic hand muscles.
• Posterior cord branches innervate most of the postaxial muscles, including superficial (hypaxial) muscles of the back, and arm and forearm extensor muscles.

Lumbosacral Plexus

• The union of lumbar and sacral plexuses.

Lumbar Plexus

• The lumbar plexus forms the upper portion of the lumbosacral plexus.
• It is formed by the ventral rami from the L1-L4 spinal levels within the psoas major muscle.
• Six branches of the lumbar plexus emerge from the lateral, anterior, and medial borders of the psoas major muscle.
• Four nerves emerge from the lateral border of the psoas major muscle, superior to inferior: iliohypogastric nerve, ilioinguinal nerve, lateral femoral cutaneous nerve, and femoral nerve.
  • Iliohypogastric nerve supplies the abdominal muscles and the skin of the lower anterior abdominal wall
  • Ilioinguinal nerve enters the inguinal canal and innervates the skin of the groin, and scrotum or labia majora
  • Lateral femoral cutaneous nerve innervates the skin over the lateral surface of the thigh.
  • Femoral nerve enters the thigh behind the inguinal ligament lateral to the femoral sheath and supplies the anterior thigh muscles.
• One nerve emerges from the anterior surface of the psoas major muscle as the genitofemoral nerve, which gives rise to two branches.
• One nerve emerges from the medial border of the psoas major muscle as the obturator nerve. The obturator nerve crosses the pelvic brim in front of the sacroiliac joint, passes behind the iliac vessels, leaves the pelvis through the obturator foramen, and supplies the medial thigh muscles.

Sacral Plexus

• It is formed from the ventral rami of lumbar spinal nerves L4-L5 and sacral spinal nerves S1-S3.
• The sacral plexus provides motor and sensory innervation to the posterior thigh, the foot, most of the lower leg, and part of the pelvis.
• The contribution to the sacral plexus from L4-L5 is known as the lumbosacral trunk.
• Anterior to the piriformis muscle and posterior to the internal iliac vessels and the rectum lies the sacral plexus.
• Seven branches of the sacral plexus leave the pelvis through the greater sciatic foramen.

Branches of the Sacral Plexus

• The uppermost branch, the superior gluteal nerve, supplies gluteus medius and minimus, and tensor fasciae latae muscles.
• The inferior gluteal nerve supplies the gluteus maximus muscle.
• Sciatic nerve is the largest branch of the plexus and the largest nerve in the body, is comprised of the tibial division and common fibular division.
• Other branches include: nerve to obturator internus, nerve to quadratus femoris, posterior cutaneous nerve of the thigh, and pudendal nerve.

Sciatic Nerve

• A Major Branch of the Sacral Plexus
• It does not give supply the gluteal region and runs deep to the gluteus maximus and biceps femoris muscles.
• It supplies muscles of the portion of the posterior thigh and then divides in this region into two nerves that innervate all the muscles of the leg and foot, the tibial nerve and common fibular nerve.

Terminal Branches of the Sciatic Nerve

Tibial Nerve

• The larger of the two terminal branches of the sciatic nerve.
• Supplies all of the muscles of the posterior compartment of the leg.
• It courses through the leg deep to the soleus muscle.
• Passes into the foot posterior to the medial malleolus and deep to the flexor retinaculum.
• Under the flexor retinaculum it divides into two terminal branches, the medial and lateral plantar nerves.

Common Fibular Nerve

• The lateral and smaller of the two terminal branches of the sciatic nerve.
• Ends just below the neck of the fibula within the head of the fibularis longus muscle by dividing into two terminal branches: deep fibular nerve and superficial fibular nerve.

Branches of the Common Fibular Nerve

Deep Fibular Nerve

• Descends the anterior aspect of the leg anterior to the interosseous membrane.
• It passes onto the foot deep to the extensor retinaculum.
• Supplies the muscles of the anterior compartment of the leg.

Superficial Fibular Nerve

• Descends the leg anterolateral to the fibula and becomes superficial in the distal third of the leg.
• It passes onto the foot superficial to the extensor retinaculum.
• Supplies the muscles of the lateral compartment of the leg.