Describe the formation, course and branches of ulnar nerve. Which muscles do each of the branches supply? Why is ulnar nerve called the "musician's nerve"?

Formation of the Ulnar Nerve

The ulnar nerve originates from the medial cord of the brachial plexus. Its nerve root fibers are primarily derived from the ventral rami of C8 and T1 spinal nerves, with occasional contributions from C7. This medial cord forms in the axilla, where it gives rise to the ulnar nerve, marking the beginning of its extensive course down the upper limb.

Course of the Ulnar Nerve

The ulnar nerve follows a distinct pathway from the axilla to the hand, traversing different anatomical compartments:

• In the Axilla and Arm:
  • The ulnar nerve descends along the medial side of the axillary artery and then the brachial artery in the anterior compartment of the arm.
  • Approximately at the mid-point of the arm, it pierces the medial intermuscular septum to enter the posterior compartment. Here, it is accompanied by the superior ulnar collateral artery.
  • It passes distally between the medial head of the triceps brachii muscle and the medial intermuscular septum.
• At the Elbow:
  • The nerve passes posterior to the medial epicondyle of the humerus, within the cubital tunnel (ulnar tunnel), where it is superficially located and can be easily palpated, often giving the "funny bone" sensation when struck. It gives off an articular branch to the elbow joint here.
• In the Forearm:
  • It enters the forearm by passing between the two heads (humeral and ulnar) of the flexor carpi ulnaris muscle.
  • It then runs deep to the flexor carpi ulnaris and superficial to the flexor digitorum profundus, traveling along the ulnar aspect of the forearm, medial to the ulnar artery.
• At the Wrist and Hand:
  • At the wrist, the ulnar nerve lies lateral to the tendon of the flexor carpi ulnaris.
  • It enters the hand superficial to the flexor retinaculum, passing through Guyon's canal (ulnar canal) alongside the ulnar artery.
  • Within the hand, it terminates by dividing into superficial and deep branches.

Branches and Muscle Supply

The ulnar nerve provides both motor and sensory innervation through its various branches. The primary motor branches supply muscles in the forearm and most of the intrinsic muscles of the hand. Sensory branches innervate the medial aspect of the hand and specific fingers.

Branches in the Forearm:

• Muscular Branches: These supply two muscles in the anterior compartment of the forearm:
  • Flexor Carpi Ulnaris: Flexes and adducts (ulnar deviation) the wrist.
  • Medial Half of Flexor Digitorum Profundus: Flexes the ring and little fingers at the distal interphalangeal joints.
• Palmar Cutaneous Branch: Provides sensory innervation to the medial half of the palm.
• Dorsal Cutaneous Branch: Arises approximately 5-7 cm above the wrist, passing posteriorly to innervate the skin on the dorsal surface of the medial one and a half fingers (little finger and medial half of the ring finger) and the associated dorsal hand area.

Branches in the Hand:

Upon entering the hand, the ulnar nerve divides into two terminal branches:

• Superficial Branch:
  • Primarily sensory, supplying the palmar surface of the medial one and a half fingers (little finger and medial half of the ring finger).
  • It also gives off a small motor branch to the Palmaris Brevis muscle, which helps deepen the hollow of the palm for grip.
• Deep Branch: This is the major motor branch in the hand, innervating most of the intrinsic hand muscles:

  Muscle Group/Muscle	Function
  Hypothenar Muscles:	Control movements of the little finger.
  Abductor Digiti Minimi	Abducts the little finger.
  Flexor Digiti Minimi Brevis	Flexes the little finger.
  Opponens Digiti Minimi	Opposes the little finger towards the thumb.
  Lumbricals (Medial Two):	Flex the metacarpophalangeal joints and extend the interphalangeal joints of the ring and little fingers.
  Palmar Interossei (3 muscles):	Adduct the fingers (bring them towards the middle finger).
  Dorsal Interossei (4 muscles):	Abduct the fingers (spread them apart).
  Adductor Pollicis:	Adducts the thumb.
  Deep Head of Flexor Pollicis Brevis (occasionally):	Flexes the thumb.

Why is the Ulnar Nerve Called the "Musician's Nerve"?

The ulnar nerve is colloquially known as the "musician's nerve" because of its critical role in enabling the fine motor control and dexterity required for playing musical instruments, particularly those demanding intricate finger movements, such as string instruments (guitar, violin), piano, and wind instruments. Its innervation of the majority of the intrinsic hand muscles, especially the interossei and medial lumbricals, allows for precise abduction and adduction of the fingers, as well as delicate flexion and extension at the metacarpophalangeal and interphalangeal joints. These movements are fundamental for:

• Chord formation and finger placement: Essential for guitarists, pianists, and violinists.
• Rapid and independent finger movements: Crucial for intricate passages in all instrumental playing.
• Grip strength and control: Important for holding instruments and bows with stability and sensitivity.
• Vibrato and nuanced expression: Requires subtle and controlled finger actions.

Musicians frequently maintain positions of extreme elbow flexion and wrist movements for prolonged periods, which can put significant stress on the ulnar nerve, especially where it passes through the cubital tunnel. This vulnerability to compression, leading to conditions like cubital tunnel syndrome, can cause numbness, tingling, weakness, and loss of dexterity in the small finger and half of the ring finger – symptoms that are particularly devastating for a musician's career. Therefore, the health and proper functioning of the ulnar nerve are paramount for musical performance, cementing its reputation as the "musician's nerve."