Somatic Innervation #2: Dermatomes and Myotomes

Vocabulary flashcards covering definitions, key points, and clinical relevance of dermatomes, myotomes, spinal nerves, and associated testing.

Spinal nerve

A mixed nerve formed by the union of a dorsal and ventral root, carrying somatic sensory and motor fibers; 31 pairs exist in humans.

Dermatome

The area of skin innervated by sensory neurons from a single spinal segment or spinal nerve.

Myotome

A collection of skeletal-muscle fibers innervated by motor neurons from a single spinal segment or spinal nerve.

Mixed spinal nerve damage

Leads to sensory loss in its dermatome and weakness in muscles whose fibers are mainly supplied by that nerve (its myotome action).

Spinal cord lesion

Produces sensory and motor deficits at and below the affected dermatome and myotome levels.

Cervical spinal nerves

Eight pairs (C1–C8) emerging above their corresponding vertebrae, supplying neck, upper limb, and diaphragm regions.

Thoracic spinal nerves

Twelve pairs (T1–T12) innervating trunk and intercostal muscles; supply thoracic dermatomes.

Lumbar spinal nerves

Five pairs (L1–L5) serving lower trunk and anterior lower limb regions.

Sacral spinal nerves

Five pairs (S1–S5) innervating posterior lower limb, perineum, and pelvic structures.

Coccygeal spinal nerve (Co1)

Single pair supplying skin over the coccyx.

Somite

Embryonic paraxial mesoderm block giving rise to dermis and skeletal muscle of a specific spinal level.

Dermatome overlap

Adjacent dermatomes share skin regions, providing partial redundancy of cutaneous sensation.

Limb rotation effect

6th-8th embryonic week limb rotation causes adult dermatomes to spiral along limbs.

Key sensory point

Standardized skin site used clinically to test a specific dermatome’s sensation.

C2 key point

~3 cm behind the ear; tests C2 dermatome.

T4 key point

Nipple line at mid-clavicular line; tests T4 dermatome.

T10 key point

Skin at the umbilicus (mid-clavicular line); tests T10 dermatome.

L4 key point

Medial malleolus; tests L4 dermatome.

S1 key point

Lateral heel; tests S1 dermatome.

Deltoid muscle

Receives predominant C5 innervation; key test muscle for C5 myotome (arm abduction).

C5 myotome action

Arm abduction via the deltoid muscle.

C6 myotome action

Elbow flexion, primarily by biceps brachii and brachialis.

C7 myotome action

Elbow extension via triceps brachii.

C8 myotome action

Middle-finger flexion through flexor digitorum profundus (to digit 3).

T1 myotome action

Little-finger abduction via abductor digiti minimi.

L2 myotome action

Hip flexion accomplished chiefly by iliopsoas.

L3 myotome action

Knee extension through quadriceps femoris.

L4 myotome action

Ankle dorsiflexion via tibialis anterior.

L5 myotome action

Great-toe (1st toe) extension through extensor hallucis longus.

S1 myotome action

Ankle plantar-flexion via gastrocnemius and soleus.

Biceps tendon reflex

Checks C5–C6 spinal cord integrity.

Triceps tendon reflex

Assesses C7 spinal cord level.

Patellar tendon reflex

Evaluates L3–L4 spinal cord function.

Calcaneal (Achilles) reflex

Tests S1–S2 spinal cord integrity.

Trigeminal nerve (CN V)

Provides cutaneous sensation to the face; not part of spinal dermatomes.

Clinical significance of dermatomes & myotomes

Patterns of sensory loss and muscle weakness localize lesions to specific spinal nerves or cord levels.