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This article explains the dermatomes and myotomes of the human body, with an included dermatome map. Examining myotomes and dermatomes is a vital part of a thorough neurological examination, particularly when a patient has a spinal cord injury.


Embryology

Dermatomes and myotomes both arise from somites, which are divisions of the body of an embryo. Somites are paired structures which run cephalocaudally (along the head to toe axis of the body).

In the developing human embryo, somites form dermatomes and myotomes, as well as tendons, cartilage and sclerotomes (which form bone).


Dermatomes

Definition: “An area of skin supplied by a single spinal nerve”

If you imagine the human body as a map, each dermatome represents the sensory innervation of a particular spinal nerve.

It is important to bear in mind that the dermatomes of the head are supplied by branches V1, V2 and V3 of the trigeminal nerve.

The list below details nerve roots and the approximate area of skin that they provide innervation to:

  •  Cranial nerve 5 (trigeminal)
    • V1 – ophthalmic branch – top of the head
    • V2 – maxillary branch – nares to the top lip
    • V3 – mandibular branch – bottom lip to jaw
  •  C2 – occipital protuberance
  •  C3 – supraclavicular fossa
  •  C4 – acromioclavicular joint
  •  C5 – lateral antecubital fossa
  •  C6 – thumb
  •  C7 – middle finger
  •  C8 – little finger
  •  T1 – medial antecubital fossa
  •  T2 – the apex of the axilla
  •  T3- T12 – bands down the torso
  •  L1 – upper anterior thigh
  •  L2 – mid anterior thigh
  •  L3 – medial femoral condyle
  •  L4 – medial malleolus
  •  L5 – dorsum of 3rd metatarsal phalangeal joint
  •  S1 – lateral heel
  •  S2 – popliteal fossa
  •  S3 – ischial tuberosity
  •  S4-5 – perianal area

Dermatome map


Sensory examination

Light touch sensation

Assesses dorsal/posterior columns and spinothalamic tracts.

1. The patient’s eyes should be closed for this assessment

2. Touch the patient’s sternum with the cotton wool wisp to confirm they can feel it

3. Ask the patient to say “yes” when they are touched

4. Using a wisp of cotton wool, gently touch the skin (don’t stroke)

5. Assess each of the dermatomes

6. Compare left to right, by asking the patient if it feels the same on both sides

Pin-prick sensation 

Assesses spinothalamic tracts.

Repeat the previous assessment steps, but this time using the sharp end of a neurotip.

This information can also be found as part of the Geeky Medics guides to upper and lower limb neurological examinations.


Myotomes

Definition: “A group of muscles innervated by a single spinal nerve”

This list details some important myotomes’ nerve roots and the actions that their associated muscles produce:

  • C4 – shoulder shrugs
  • C5 – shoulder abduction and external rotation; elbow flexion
  • C6 – wrist extension
  • C7 – elbow extension and wrist flexion
  • C8 – thumb extension and finger flexion
  • T1 – finger abduction
  • L2 – hip flexion
  • L3 – knee extension
  • L4 – ankle dorsiflexion
  • L5 – great toe extension
  • S1 – ankle plantarflexion
  • S4 – bladder and rectum motor supply

For information about examining myotomes, see the motor sections of the Geeky Medics upper and lower limb neurological examination guides.


Plexuses

We can classify groups of nerves into plexuses.

  • Cervical plexus (C1-C4) – innervates the diaphragm, shoulders and neck
  • Brachial plexus (C6 – T1) – innervates the upper limbs
  • Lumbosacral plexus (L2-S1) – innervates the lower extremities

 


Editor

Eve Benfield