L5 dermatome and nerve root

Lumbar spinal root

Where is the L5 dermatome?

The L5 dermatome covers the outer shin, the top of the foot and the big toe. Its ISNCSCI key sensory point is the top of the foot at the third metatarsophalangeal joint. Its key movement is big toe extension. The L4/L5 disc is the one that usually compresses L5, or the L5/S1 disc when a herniation is far-lateral.

L5 at a glance
Type	Lumbar spinal root
Key sensory point	The top of the foot at the third metatarsophalangeal joint
Key movement	Big toe extension
Tendon reflex	None tests it on its own
Usual disc	L4/L5, or a far-lateral L5/S1

Try in: Dermatome and Myotome Map Explorer →

L5 myotome: the muscles it supplies

L5 is a major root for 8 of the 11 muscles below and a minor root for the rest.

Muscles L5 supplies, and every root each takes
Muscle	From L5	All its roots
Gluteus medius	Major	L5; L4 and S1 in part
Gluteus minimus	Major	L5; L4 and S1 in part
Tensor fasciae latae	Major	L5; L4 and S1 in part
Gluteus maximus	Major	L5 and S1; S2 in part
Hamstrings	Major	L5 and S1; L4 and S2 in part
Extensor hallucis longus	Major, ISNCSCI key muscle	L5; L4 and S1 in part
Extensor digitorum longus	Major	L5; L4 and S1 in part
Fibularis longus	Major	L5 and S1
Erector spinae	Minor	T1 to L5 in part
Multifidus	Minor	T1 to L5 in part
Tibialis anterior	Minor	L4; L5 in part

Erector spinae and multifidus: supplied segment by segment by the dorsal rami, so each root reaches only its own short stretch.

L5 and the tendon reflexes

No tendon reflex tests L5 on its own.

Why an L4/L5 disc compresses L5

In the lumbar spine each root leaves below its own vertebra, high in the foramen and above the disc, so a typical posterolateral L4/L5 herniation misses the exiting L4 root and catches L5, which crosses the back of that disc on its way down to the next foramen. The level of the disc and the root therefore differ by one.

A far-lateral herniation pushes out into or beyond the foramen, where the exiting L5 root lies, so a far-lateral L5/S1 disc catches L5 rather than the root a typical one would.

What an L5 root lesion does

Sensation. Numbness or tingling in the L5 band. Usually a smaller patch than the whole band, because L4 and S1 overlap it: Sherrington found in 1893 that cutting a single root in monkeys left no skin entirely without feeling.
Strength. Big toe extension, the standard’s key movement for L5 (extensor hallucis longus). Also weaker where L5 is a major supply: gluteus medius, gluteus minimus, tensor fasciae latae, gluteus maximus, hamstrings, extensor digitorum longus and fibularis longus.
Reflexes. No tendon reflex depends on L5.

A teaching map. Root territories overlap and vary between people, and a root lesion is diagnosed by examination and imaging, not by a map. Numbness between the legs or around the anus, new bladder or bowel problems, or quickly spreading weakness need emergency care.

A way to remember L5

L5, the largest of the five toes.

Neighboring dermatomes

L4: the inner shin, the inner ankle and the inner edge of the foot
S1: the outer edge and sole of the foot, the heel and the lower back of the calf

Where published maps disagree

The top of the big toe is L5 in some maps and L4 in others.
The inner sole is given to L4, L5 or S1, depending on the map.

Dermatome and Myotome Map Explorer, with L5 lit on a front and back body map
Every dermatome, with the disc levels and key movements side by side
Lumbar Spine and Pelvis Explorer, the lower back’s vertebrae and the sacrum in 3D
Lower Limb Muscles Explorer, the leg’s muscles in 3D
Reflex Arc Simulator, the loop a tendon reflex runs through the spinal cord
Muscles Worked by Exercise 3D Explorer, a whole body colored by root, in its root mode

Sources. Key sensory points and key movements: International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI), American Spinal Injury Association, 2019 revision. Muscles: Aids to the Examination of the Peripheral Nervous System, Medical Research Council and the Guarantors of Brain, 5th ed. (2010), cross-checked with Kendall et al. (2005). Reflex roots: Walker (1990), Clinical Methods, 3rd ed., Table 72.1, and StatPearls, “Physiology, Deep Tendon Reflexes”; the main root as the MSD Manual’s “How to Assess Reflexes” names it. Overlap: Sherrington (1893), Phil Trans R Soc Lond B 184:641-763.