Myotomes & Dermatomes Of Human Body By DrRavi
Myotomes & Dermatomes Of Human Body are Beautifully described By DrRavi.
Myotomes
A myotome is a group of muscles innervated by a single spinal nerve.
This list details some important myotome 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: big 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 (C5 – T1): innervates the upper limbs.
Lumbosacral plexus (L2 – S1): innervates the lower extremities.
Dermatomes
A dermatome is an area of skin supplied by a single spinal nerve.
If you imagine the human body as a map, each dermatome represents the area of skin supplied with sensation by a specific nerve root.
Dermatomes of the head
Trigeminal nerve (CN V)
V1: ophthalmic branch – the lateral aspect of the forehead
V2: maxillary branch – the cheek
V3: mandibular branch – the lower jaw (avoid the angle of the mandible as it is supplied by C2/C3)
Other
C2: 1-2 cm lateral to the occipital protuberance
C3: the supraclavicular fossa in the midclavicular line.
Dermatomes of the upper limb
C4: over the acromioclavicular joint.
C5: the lateral aspect of the lower edge of the deltoid muscle (known as the “regimental badge”).
C6: the palmar side of the thumb.
C7: the palmar side of the middle finger.
C8: the palmar side of the little finger.
T1: the medial aspect antecubital fossa, proximal to the medial epicondyle of the humerus.
Dermatomes of the torso
T2: the apex of the axilla.
T3: the intersection of the midclavicular line and third intercostal space.
T4: the intersection of the midclavicular line and the fourth intercostal space at the level of the nipples.
T5: the intersection of the midclavicular line and the fifth intercostal space, horizontally located midway between the level of the nipples and the level of the xiphoid process.
T6: the intersection of the midclavicular line and the horizontal level of the xiphoid process.
T7: the intersection of the midclavicular line and the horizontal level at one quarter the distance between the level of the xiphoid process and the level of the umbilicus.
T8: the intersection of the midclavicular line and the horizontal level at one half the distance between the level of the xiphoid process and the level of the umbilicus.
T9: the intersection of the midclavicular line and the horizontal level at three-quarters of the distance between the level of the xiphoid process and the level of the umbilicus.
T10: the intersection of the midclavicular line, at the horizontal level of the umbilicus.
T11: the intersection of the midclavicular line, at the horizontal level midway between the level of the umbilicus and the inguinal ligament.
T12: the intersection of the midclavicular line and the midpoint of the inguinal ligament.
Dermatomes of the lower limb
L1: the inguinal region and the very top of the medial thigh.
L2: the middle and lateral aspect of the anterior thigh.
L3: the medial epicondyle of the femur.
L4: the medial malleolus.
L5: the dorsum of the foot at the third metatarsophalangeal joint.
S1: the lateral aspect of the calcaneus.
S2: at the midpoint of the popliteal fossa.
S3: at the horizontal gluteal crease (the horizontal crease formed by the inferior aspect of the buttocks and the posterior upper thigh).
S4/5: the perianal area.
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