Dermatomes, myotomes, reflexes, and plexuses
Dermatomes and myotomes
Dermatomes and myotomes are used to diagnose potential damage to a spinal nerve root. Review the dermatome map and myotome chart below.
Myotomes
- Upper extremity
- C1: cervical rotation
- C2-C4: shoulder elevation
- C5: shoulder abduction/elbow flexion
- C6: wrist extension
- C7: elbow extension/wrist flexion
- C8: digit flexion/thumb extension
- T1: finger adduction
- Lower extremity
- L1-L2: hip flexion
- L3: knee extension
- L4: knee extension/ankle dorsiflexion
- L5: ankle dorsiflexion/great toe extension
- S1: ankle eversion/ankle plantar flexion
- S2: ankle plantar flexion
Spinal reflexes
- C5: biceps reflex
- C6: brachioradialis reflex
- C7: triceps reflex
- L3-L4: patellar reflex
- L5: semitendinosus reflex
- S1-S2: Achilles reflex
Example of reflexes working in the spinal cord below:
Example: Localizing a nerve root lesion
A patient reports weakness with knee extension, decreased sensation over the anterior/medial thigh, and a diminished patellar reflex. Which nerve root does this pattern point to?
- Myotome: knee extension is primarily controlled by L3.
- Reflex: the patellar reflex tests L3-L4.
- Dermatome: the anterior/medial thigh corresponds to the L2-L3 dermatomes.
All three findings overlap at the L3 level, so L3 is the most likely nerve root involved. This is the same reasoning you’ll use clinically: a single finding can have several causes, but when the sensory, motor, and reflex findings all point to the same level, that level is your answer.
Answer: L3 nerve root
Brachial, lumbar, and sacral plexuses
The brachial, lumbar, and sacral plexuses are a network of nerves that form a connection between the peripheral nerves and the central nervous system. The peripheral nerves innervating all muscles are derived from these three plexuses.
Brachial plexus
Muscles innervated by the brachial plexus
The brachial plexus gives off a few proximal branches before ending in five terminal nerves. The tested skill is recognizing each terminal nerve and its headline muscle action, not memorizing every muscle it supplies.
Proximal branches
- Dorsal scapular nerve (C5): rhomboids and levator scapulae
- Long thoracic nerve (C5-C7): serratus anterior
- Suprascapular nerve (C5-C6): supraspinatus and infraspinatus
Terminal nerves
- Musculocutaneous nerve (C5-C7): biceps brachii, brachialis, and coracobrachialis (elbow flexion)
- Axillary nerve (C5-C6): deltoid and teres minor (shoulder abduction)
- Radial nerve (C5-T1): triceps brachii and the wrist/finger extensors, recalled using the BEAST acronym:
- B: Brachioradialis
- E: Extensor muscles of the wrist and fingers
- A: Abductor pollicis longus, anconeus
- S: Supinator
- T: Triceps brachii
- Median nerve (C5-T1): most anterior forearm flexors (e.g., pronator teres, flexor carpi radialis, flexor digitorum superficialis), the thenar muscles, and the 1st and 2nd lumbricals
- Ulnar nerve (C8-T1): flexor carpi ulnaris, the medial half of flexor digitorum profundus, and most of the intrinsic hand muscles
Lumbar plexus
Muscles innervated by the lumbar plexus (L1-L4)
The major muscles and sensory territory supplied by each nerve are listed below.
- Iliohypogastric nerve (L1): internal oblique and transversus abdominis
- Genitofemoral nerve (L1-L2): cremaster muscle (in males)
- Femoral nerve (L2-L4): iliacus, sartorius, and the quadriceps femoris group (knee extension)
- Obturator nerve (L2-L4): the hip adductor group and gracilis (hip adduction)
- Lateral femoral cutaneous nerve (L2-L3): sensory only, lateral thigh
- Saphenous nerve (L3-L4): sensory only, medial lower leg (a terminal branch of the femoral nerve)
Sacral plexus
Muscles innervated by the sacral plexus (L4-S3)
- Superior gluteal nerve (L4-S1): gluteus medius, gluteus minimus, and tensor fasciae latae (hip abduction)
- Inferior gluteal nerve (L5-S2): gluteus maximus (hip extension)
- Sciatic nerve (L4-S3): the largest peripheral nerve in the body, splitting above the knee into two terminal divisions:
- Tibial nerve: the hamstrings (except the short head of biceps femoris), the posterior leg muscles (e.g., gastrocnemius, soleus), and the intrinsic foot muscles (ankle plantarflexion)
- Common peroneal (fibular) nerve: the short head of biceps femoris, then splits into the deep peroneal nerve (anterior compartment, ankle dorsiflexion) and the superficial peroneal nerve (lateral compartment, ankle eversion)