Innervation of the lower leg

The lower leg (and especially the foot) have a pretty fancy pattern of skin innervation by the terminal branches. For example, the skin of the foot is innervated by 7 separate nerves:

  1. Superficial peroneal nerve
  2. Deep peroneal nerve
  3. Sural nerve
  4. Saphenous nerve
  5. Calcaneal branch of the tibial nerve
  6. Medial branch of plantar nerve
  7. Lateral branch of plantar nerve

Also good to keep in mind that the anterior compartment is innervated by the deep peroneal nerve, the lateral compartment by the superficial peroneal nerve and the posterior compartment by the tibial nerve.

Median Nerve Distribution

The median nerve provides sensory and motor innervation to the anterior compartment of the forearm and hand.

  • Motor branches
    • Pronator teres
    • Flexor carpi radialis
    • Flexor carpi sublimis
  • Anterior interosseus (motor)
    • Flexor pollicis longus
    • Flexor digitorum profundus to 2nd & 3rd fingers
    • Pronator quadratus
  • Palmar cutaneous
    • Sensory distribution: Skin over thenar eminence
  • Terminal motor
    • Abductor pollicis brevis
    • Opponens pollicis
    • Lumbricals: 1st & 2nd
    • ± Flexor pollicis brevis: Also innervated by ulnar nerve
  • Terminal sensory
    • Sensory to palmar surface of thumb, 2nd, 3rd & lateral 1/2 of 4th finger

Physical exam for different muscles supplied by median nerve

  • Flexor digitorum superficialis: patient flexes fingers at PIP joint against resistance
  • Flexor digitorum profundus: patient flexes fingers at DIP joint against resistance
  • Flexor pollicis longus (anterior interosseous nerve): flexes distal phalanx of thumb against resistance
  • Abductor pollicis brevis: patient abducts thumb at right angles to palm against resistance
  • Opponens pollicis: patient touches base of little finger with the thumb, examiner tries to pull  apart
  • 1st lumbrical interosseous: patient extends finger at the PIP joint against resistance with the MCP joint hyperextended and fixed

Lateral Spinothalamic Pathway

The Lateral Spinothalamic Pathway is an ascending spinal tract, carrying sensory information to the brain. It is typically depicted as a chain of three neurons: first-, second-, and third-order neurons.

This pathway mediates sensation of pain and temperature.

The first-order neurons in the pathway are located in the dorsal root ganglia at all spinal levels. Their axons ascend the tract of Lissauer, and synapse with second-order neurons.

The second-order neurons are located in the dorsal horn, and their axons immediately decussate via the ventral white commissure. These axons ascend the lateral funiculus and project to the ventral posterolateral (VPL) nucleus of the thalamus.

Some collaterals are sent to areas involved in arousal, namely the midbrain reticular formation, and the intralaminar nuclei of the thalamus (which then project to the caudatoputamen, and frontal and parietal cortex).

The third-order VPL neurons send axons through the posterior limb of the internal capsule to the somatosensory cortex (areas 3, 1, 2).

Lesions to the Lateral Spinothalamic Pathway

Spinal cord lesions affecting the Lateral Spinothalamic pathway result in contralateral sensory deficits below the lesion, because the pathway immediately decussates at the second-order neuron level.

Ventral Spinothalamic Pathway

There is also a Ventral Spinothalamic Pathway, that carries crude touch sensation. It is organized very similarly to the Lateral Spinothalamic pathway; however, it is less clinically-emphasized since the Dorsal Column Medial Lemniscus pathway is more important for touch sensation. If the Ventral Spinothalamic pathway is lesioned, touch sensation will only be minimally affected, as long as the dorsal column remains intact.