NEUROANATOMY OF THE LOWER EXTREMITY
Anatomy of the Nerve Roots of the Lower Extremity
At the thoracic, lumbar, and sacral levels, roots are numbered by the vertebral level below which they exit: The T1 root exits below the T1 vertebra (between the T1 and T2 vertebrae), the L1 root exits below the L1 vertebra (between the L1 and L2 vertebrae), and the S1 root exits below the S1 vertebra (between the S1 and S2 vertebrae). The spinal cord ends at the L1-L2 vertebral level and the lumbar and sacral roots must therefore descend to reach the vertebral levels at which they exit. These descending roots are referred to as the cauda equina (See Fig. 15-1).
The sensory supply to the anterior thigh is covered by L1, L2, and L3 in three diagonal stripes running from proximal/lateral to distal/medial. L4, L5, and S1 cover the anterior shin in vertical stripes from medial to lateral: L4 covers the medial knee, medial shin, and instep; L5 covers the anterior and lateral shin and dorsum of the foot; and S1 the covers the distal lateral calf and lateral aspect and plantar surface of the foot (Fig. 17–1). A mnemonic way to remember the dermatomes of the lower extremity is to place your hands on your hips pointing inward/downward and then pat the thighs three times moving distally toward the knee (L1, L2, L3). From the knee, point the hands directly downward toward the feet and pat the shins three times from medial to lateral (L4, L5, S1). This medial-to-lateral pattern continues on the foot with the medial foot (instep) supplied by L4, the lateral foot supplied by S1, and the majority of the dorsum of the foot supplied by L5 between the medial L4 and lateral S1 dermatomes.
Schematic showing dermatomes of the leg. Reproduced with permission from Waxman S: Clinical Neuroanatomy, 27th ed. New York: McGraw-Hill Education; 2013.
The posterior middle thigh and calf are supplied by S1 (laterally) and S2 (medially), and the S1 and S2 dermatomes are bounded by L3-L4 medially and L5 laterally. See also Fig. 15-2.
To learn the motor actions controlled by each root, practice moving through the sequence of tested muscle groups from proximal to distal in front, then from proximal to distal in back, naming the roots as you move the associated muscles (Fig. 17–2): L2-L3 (hip flexion), L3-L4 (knee extension), L4-L5 (dorsiflexion of the foot), L5-S1 (hip extension), L5-S1 (knee flexion), S1-S2 (plantarflexion of the foot). All root pairs in the movement sequence are overlapping and in sequence, but note that L5-S1 is repeated for both hip extension and knee flexion.
Schematic showing nerve roots supplying actions of the leg and foot.