Brachial Plexus Anatomy

Brachial Plexus: Overview

• Large nerve network innervating the upper extremity
• Innervates intrinsic, appendicular, and hypaxial muscles
• Hypaxial muscles are also innervated
• Ventral rami of C5-C8 cervical spinal nerves and the T1 ventral ramus form it
• Ventral rami are mixed nerves, containing more motor fibers than sensory fibers
• Located in the posterior triangle of the neck and the axilla

Brachial Plexus Components

• The Brachial Plexus is divided into regional components:
• Roots
• Trunks
• Divisions
• Cords
• Branches

Roots of the Brachial Plexus

• Originate at the C5-T1 spinal levels
• Pass between the anterior and middle scalene muscles
• Roots are the ventral rami, exiting the IVF and dividing
• Small contributions can come from C4 and T2
• Typically, C5 innervates near the shoulder
• C6 innervates near the elbow
• C7 innervates near the wrist
• C8-T1 innervates the hand
• Roots ultimately combine to form 3 trunks

Trunks of the Brachial Plexus

• Trunk names reference their relative anatomic position
• Superior (upper) trunk: formed by the C5-C6 root union
• Middle trunk: a continuation of the C7 root
• Inferior (lower) trunk: formed by the C8 and T1 root union
• Each trunk subsequently divides into anterior and posterior divisions

Divisions of the Brachial Plexus

• Six total divisions form from the trunks
• These consist of three anterior and three posterior divisions
• Divisions are located approximately at the level of the clavicle
• Divisions unevenly unite to form 3 cords

Cords of the Brachial Plexus

• Cord names reference their location relative to the axillary artery
• Lateral cord: the fusion of anterior divisions from the upper and middle trunks
• Posterior cord: the fusion of all three divisions (one from each trunk)
• Medial cord: simply the anterior division of the lower trunk

Branches of the Brachial Plexus

• Five major branches form from the cords
• Many smaller branches originate from the roots, trunks, divisions, and cords

Branches of the Posterior Cord

• The posterior cord has four branches, terminating as it bifurcates into the last two
• Subscapular nerves
• Thoracodorsal nerve
• Axillary nerve branches wrap posteriorly to the _______ space
• Radial nerve: larger, continuing along the course of the posterior cord (posterior to axillary artery)

Branches of the Medial and Lateral Cords

• Medial and lateral cords divide and the portions that are closer to the axillary artery, unite
• Resulting in the musculocutaneous, median, and ulnar nerves
• Forms a large "M"
• Musculocutaneous nerve: Branch of the lateral cord
• Median nerve: Formed by the combination of both lateral and medial cords centrally in the "M"
• Ulnar nerve: Branch of the medial cord

Supraclavicular Branches

• Emerge from roots and trunks in the posterior triangle, superior to the clavicle.
• Dorsal scapular nerve: From the C5 root
• Long thoracic nerve: From C5, C6, and C7 roots
• Suprascapular nerve: From the upper trunk
• Nerve to subclavius muscle: From the upper trunk

Infraclavicular Branches

• Emerge from cords and travel from within the axilla
• From the Lateral cord: Lateral pectoral nerve, lateral root of the median nerve, and musculocutaneous nerve
• 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 most preaxial muscles
• Includes pectoral muscles, arm/forearm flexors, and intrinsic hand muscles
• Posterior cord branches innervate most postaxial muscles
• Includes superficial (hypaxial) back muscles, and arm/forearm extensor muscles.

Lumbar Plexus

• The lumbar plexus forms the upper portion of the lumbosacral plexus
• It is formed by the ventral rami from spinal levels L1-L4
• Occurs within the psoas major muscle
• Six branches emerge from the lateral, anterior, and medial borders of the psoas major muscle

Lateral Border Branches

• Four nerves emerge from the lateral border of the psoas major
• Arrange from superior to inferior
• Iliohypogastric nerve supplies 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 inguinal ligament
• Runs lateral to the femoral sheath and supplies the anterior thigh muscles

Anterior/Medial Border Branches

• One nerve emerges from the anterior surface of the psoas major muscle
• Genitofemoral nerve: Gives rise to two branches
• One nerve emerges from the medial border of the psoas major muscle
• Obturator nerve crosses the pelvic brim in front of the sacroiliac joint
• Passes behind the iliac vessels and leaves the pelvis through the obturator foramen and supplies the medial thigh muscles

Sacral Plexus

• Formed from the ventral rami of lumbar spinal nerves L4-L5 and sacral spinal nerves S1-S3
• Provides motor and sensory innervation to the posterior thigh, foot, most of the lower leg, and part of the pelvis
• L4-L5 contribution is known as the lumbosacral trunk
• The sacral plexus lies anterior to the piriformis muscle
• Lies posterior to the internal iliac vessels and the rectum

Branches of the Sacral Plexus

• Seven branches leave the pelvis through the greater sciatic foramen
• Superior gluteal nerve- Supplies the gluteus medius and minimus, and the tensor fasciae latae muscles
• Inferior gluteal nerve- Supplies the gluteus maximus muscle
• Sciatic nerve
  • Is the largest branch of the plexus and the largest nerve in the body
  • It is comprised of two parts: tibial division and common fibular division
• Nerve to obturator internus
• Nerve to quadratus femoris
• Posterior cutaneous nerve of the thigh
• Pudendal nerve

Sciatic Nerve

• Does not supply the gluteal region
• Runs deep to gluteus maximus and biceps femoris muscles
• Supplies muscles of the posterior thigh
• Then divides into two nerves that innervate all muscles of the leg and foot

Terminal Branches of the Sciatic Nerve

• Tibial Nerve

  • Larger of the two terminal branches
  • Supplies all muscles of the posterior compartment of the leg
  • Courses through the leg deep to the soleus muscle
  • Passes into the foot posterior to the medial malleolus and deep to the flexor retinaculum
  • Divides under the flexor retinaculum into the medial and lateral plantar nerves

• Common Fibular (Peroneal) Nerve

  • Lateral and smaller of the two terminal branches
  • Ends just below the neck of the fibula within the head of the fibularis longus muscle
  • Divides 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
  • 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
  • Passes onto the foot superficial to the extensor retinaculum
  • Supplies the muscles of the lateral compartment of the leg