Brachial Plexus Structure and Function

The Brachial Plexus is a network of nerves that supplies the upper limb
It is divided into roots, trunks, divisions, cords and branches.
The five roots are the anterior rami of spinal nerves C5, C6, C7, C8 and T1.
Roots combine to form three trunks:
- Superior trunk (C5 and C6)
- Middle trunk (C7)
- Inferior trunk (C8 and T1)
Trunks split into six divisions:
- Three anterior and three posterior
Divisions regroup into three cords:
- Lateral cord (from superior and middle trunk anterior divisions)
- Posterior cord (from all posterior divisions)
- Medial cord (from inferior trunk anterior division)

There are five terminal branches and several pre-terminal or collateral branches
Terminal branches are:
- Musculocutaneous nerve (from lateral cord)
- Axillary nerve (from posterior cord)
- Radial nerve (from posterior cord)
- Median nerve (from lateral and medial cords)
- Ulnar nerve (from medial cord)

Originates from C5, C6 and C7 spinal nerve roots
Motor function:
- Coracobrachialis: assists with arm flexion and adduction
- Biceps brachii: flexes and supinates forearm
- Brachialis: flexes the forearm
Sensory function: supplies sensation to lateral forearm
Clinical significance:
- Injury can cause weakened flexion and supination of the forearm and sensory loss in the lateral forearm
- May be injured due to trauma or compression in the axilla

Origin: posterior cord (C5 and C6)
Course: passes through the quadrangular space, winds around the surgical neck of the humerus
Motor function:
- Deltoid: abduction of arm beyond 15 degrees
- Teres minor: assists with lateral rotation of the arm
Sensory function:
- Supplies sensation to the skin over the deltoid
Clinical significance:
- Commonly injured in fractures of the surgical neck of the humerus or anterior shoulder dislocations
- Motor deficits: weakness or paralysis of the deltoid leading to difficulty abducting the arm
- Sensory deficits: sensory loss or alteration over the deltoid region (regimental badge area)
- Quadrangular space syndrome can compress the nerve, leading to pain and weakness in the shoulder

Origin: lateral cord (C5, C6, C7) and medial cord (C8, T1)
Motor function:
- Pronator teres: pronates forearm
- Flexor carpi radialis: flexes and abducts wrist
- Palmaris longus: flexes wrist
- Flexor digitorum superficialis: flexes middle phalanges of fingers 2-5
- Flexor digitorum profundus (lateral half): flexes distal phalanges of fingers 2 and 3
- Abductor pollicis brevis: abducts thumb
- Opponens pollicis: opposes thumb
- Flexor pollicis brevis: flexes thumb
- Lumbricals (2 and 3): flexes metacarpophalangeal joints and extends interphalangeal joints of fingers 2-3
Sensory function:
- Supplies sensation to palmar aspect of thumb, index, middle and radial half of ring finger
- Also supplies sensation to dorsal aspect of distal phalanges of index and middle finger
Clinical significance:
- Injury can lead to weakness in hand and forearm muscles causing difficulty with fine motor movements, grasping, and pronation.
- Commonly injured:
  - In the carpal tunnel
  - At the elbow
  - In the forearm
  - During surgical procedures

Origin: posterior cord
Motor function:
- Triceps brachii: extends elbow
- Anconeus: extends elbow and stabilizes elbow joint
- Brachioradialis: flexes elbow and supinates forearm
- Extensor carpi radialis longus: extends and abducts wrist
- Extensor carpi radialis brevis: extends and abducts wrist
- Extensor digitorum: extends fingers 2-5
- Extensor digiti minimi: extends little finger
- Extensor carpi ulnaris: extends and adducts wrist
- Supinator: supinates forearm
- Abductor pollicis longus: abducts and extends thumb
- Extensor pollicis brevis: extends thumb metacarpophalangeal joint
- Extensor pollicis longus: extends thumb interphalangeal joint
Sensory function:
- Supplies sensation to dorsal aspect of hand and fingers, except the medial one and a half fingers
Clinical significance:
- Injury causes wrist drop (inability to extend the wrist), loss of sensation in the affected area, and weakness in forearm and hand muscles.
- Commonly injured:
  - At the humerus (e.g., midshaft fractures)
  - At the elbow
  - In the forearm

Origin: medial cord (C8 and T1)
Motor function:
- Flexor carpi ulnaris: flexes and adducts wrist
- Flexor digitorum profundus (medial half): flexes distal phalanges of fingers 4 and 5
- Hypothenar muscles (abductor digiti minimi, flexor digiti minimi, opponens digiti minimi): control the little finger
- Interossei muscles: adduct and abduct fingers
- Lumbricals (4 and 5): flexes metacarpophalangeal joints and extends interphalangeal joints of fingers 4 and 5
- Adductor pollicis: adducts thumb
Sensory function:
- Supplies sensation to the palmar and dorsal aspects of the little finger and the medial half of the ring finger
Clinical significance:
- Injury can result in claw hand deformity, altered sensation in the little finger and half of the ring finger, weakness in hand muscles, and difficulty with fine motor movements.
- Commonly injured:
  - At the elbow (funny bone)
  - At the wrist
  - In the forearm