Makindo Medical Notes.com

The nerve supply to the hand is primarily provided by three major nerves: the median nerve, the ulnar nerve, and the radial nerve. Each of these nerves is responsible for both sensory and motor innervation, enabling the hand to perform a wide range of functions, from fine motor skills to powerful grips.

Central hand control

• The motor homunculus is a distorted human figure mapped onto the precentral gyrus of the brain, which is the primary motor cortex. It illustrates that body parts requiring precise, fine motor control, such as the hands, face, and tongue, occupy larger areas of the cortex.
• The hand, particularly the fingers, occupies a substantial portion of the motor cortex. This large representation reflects the complex and dexterous movements the hand can perform. Fine motor skills, such as writing, typing, playing musical instruments, and manipulating small objects, demand significant neural control and coordination.
• The density of motor neurons and the richness of the neural connections in this area support rapid and precise adjustments in hand movements.
• The large representation of the hand in the motor cortex is thought to be an evolutionary adaptation that enabled humans to use tools, create art, and perform other activities that require fine motor skills.

Brain to hand

• The corticospinal tract is the primary pathway for transmitting motor commands from the brain to the hand. It involves a sequence of upper motor neurons descending from the motor cortex through the brainstem and spinal cord , where they synapse with lower motor neurons that extend through the brachial plexus and peripheral nerves to the muscles of the hand.
• This intricate pathway ensures precise control and coordination of hand movements.

Median Nerve

• Pathway: Originates from the brachial plexus (C5-T1). Travels down the arm through the anterior compartment, passing through the cubital fossa. Continues down the forearm and enters the hand through the carpal tunnel.
• Motor Innervation:
  • Thenar Muscles: Controls most of the thumb movements (abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis).
  • Lumbricals: Innervates the first and second lumbricals, which flex the metacarpophalangeal joints and extend the interphalangeal joints of the index and middle fingers.
• Sensory Innervation: Provides sensation to the palmar side of the thumb, index, middle, and lateral half of the ring finger. Supplies the distal dorsal surfaces of these fingers.
• Clinical Relevance: Carpal Tunnel Syndrome: Compression of the median nerve at the wrist, leading to pain, numbness, and weakness in the hand.

Ulnar Nerve

• Pathway: Originates from the brachial plexus (C8-T1). Travels down the arm, passing posterior to the medial epicondyle of the humerus (funny bone area). Continues along the medial side of the forearm and enters the hand through Guyon's canal.
• Motor Innervation:
  • Hypothenar Muscles: Controls movements of the little finger (abductor digiti minimi, flexor digiti minimi brevis, and opponens digiti minimi).
  • Interossei Muscles: Adducts and abducts the fingers (palmar and dorsal interossei).
  • Lumbricals: Innervates the third and fourth lumbricals.
  • Adductor Pollicis: Adducts the thumb.
  • Deep Flexor Muscles: Innervates the medial half of the flexor digitorum profundus.
• Sensory Innervation: Provides sensation to the palmar and dorsal surfaces of the little finger and the medial half of the ring finger.
• Clinical Relevance: Cubital Tunnel Syndrome: Compression of the ulnar nerve at the elbow, leading to numbness and tingling in the ring and little fingers.
• Claw Hand: Severe ulnar nerve injury resulting in hyperextension of the metacarpophalangeal joints and flexion of the interphalangeal joints.

Radial Nerve

• Pathway: Originates from the brachial plexus (C5-T1). Travels down the arm through the posterior compartment, passing around the radial groove of the humerus. Divides into superficial and deep branches at the lateral epicondyle.
• Motor Innervation:
  • Extensor Muscles: Innervates the extensor muscles of the forearm, which extend the wrist and fingers (extensor carpi radialis, extensor carpi ulnaris, extensor digitorum, etc.).
  • Abductor Pollicis Longus: Abducts the thumb.
• Sensory Innervation: Provides sensation to the dorsal surface of the lateral three and a half fingers (excluding the fingertips) and the associated area of the hand.
• Clinical Relevance:Wrist Drop: Radial nerve injury resulting in the inability to extend the wrist and fingers.

The coordinated action of the median, ulnar, and radial nerves allows for the complex movements and sensory perception necessary for hand function. Understanding the specific pathways and innervations of these nerves is crucial for diagnosing and treating nerve injuries and related conditions.