The L5 nerve root is part of the lumbosacral plexus. It is
an important component of the sciatic nerve. The L5 nerve root causes ankle
dorsiflexion, which also comes from the L4 nerve root. The tibialis anterior is
the primary dorsiflexor of the ankle, and the innervation comes from the deep
peroneal nerve. Injury of the L5 nerve root can cause weakness of the tibialis
anterior muscle, and this can lead to a foot drop. The L5 nerve root also
causes dorsiflexion of the toes through innervating the extensor hallucis
longus and extensor digitorum longus, and this innervation comes from the deep
peroneal nerve. Of particular interest, is the extensor hallucis longus. Weakness
of the big toes extension is usually present when disc herniation affects the
L5 nerve root. So, when the L5 nerve root is affected, the extensor hallucis
longus could become weak. The tibialis posterior is an important muscle that
runs behind the medial malleolus, and its innervation comes from the posterior
tibial nerve (L4-L5). The function of the tibialis posterior is to invert the
foot, to assist in plantar flexion of the ankle, and to maintain the medial
longitudinal arch. The L5 nerve root also innervates the muscles that cause hip
extension, and the muscles are the hamstrings, which is innervated by the
tibial nerve, and the gluteus maximus which is innervated by the inferior
gluteal nerve. The hamstring muscles are also a major flexor of the knee. The L5
also innervates the hip abductors (gluteus medius and gluteus minimus), and the
innervation comes from the superior gluteal nerve, injury of L5 nerve root can
cause weakness of the hip abductors, and this can lead to Trendelenburg Gait. The
L5 nerve root is really an important nerve root that supplies a lot of muscles.
The L5 nerve root gives sensory innervation to the top of the foot. If you do
not remember anything about the L5 nerve root, try to remember that injury to
this nerve can cause weakness of the big toe extension, weakness of ankle
dorsiflexion (foot drop), and weakness of the hip abductor muscles which will
give you Trendelnburg Gait.
