T12 dermatome and nerve root

Thoracic spinal root

Where is the T12 dermatome?

The T12 dermatome covers the lower abdomen above the groin crease, and the flank above the hip. Its ISNCSCI key sensory point is the midpoint of the inguinal ligament. ISNCSCI grades no key movement for T12. The T12/L1 disc is the one that usually compresses T12.

T12 at a glance
Type	Thoracic spinal root
Key sensory point	The midpoint of the inguinal ligament
Key movement	None graded: the standard grades C5 to T1 and L2 to S1
Tendon reflex	None tests it on its own
Usual disc	T12/L1

Try in: Dermatome and Myotome Map Explorer →

T12 myotome: the muscles it supplies

T12 is a major root for 5 of the 7 muscles below and a minor root for the rest.

Muscles T12 supplies, and every root each takes
Muscle	From T12	All its roots
Rectus abdominis	Major	T7 to T12
External oblique	Major	T7 to T12
Internal oblique	Major	T7 to L1
Transversus abdominis	Major	T7 to L1
Quadratus lumborum	Major	T12 to L3
Erector spinae	Minor	T1 to L5 in part
Multifidus	Minor	T1 to L5 in part

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

T12 and the tendon reflexes

No tendon reflex tests T12 on its own.

Why a disc rarely compresses T12

Each thoracic root leaves below its own vertebra and runs round the trunk as an intercostal nerve, which is why its dermatome is a band. A disc rarely presses on a thoracic root, since the ribs steady that part of the spine; shingles, which reactivates in one root’s ganglion, is the classic cause of pain in one band.

What a T12 root lesion does

Sensation. Numbness or tingling in the T12 band. Usually a smaller patch than the whole band, because T11 and L1 overlap it: Sherrington found in 1893 that cutting a single root in monkeys left no skin entirely without feeling.
Strength. Also weaker where T12 is a major supply: rectus abdominis, external oblique, internal oblique, transversus abdominis and quadratus lumborum.
Reflexes. No tendon reflex depends on T12.

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.

Neighboring dermatomes

T11: a band round the trunk below the umbilicus
L1: the groin, just below the inguinal ligament, and the upper buttock

Dermatome and Myotome Map Explorer, with T12 lit on a front and back body map
Every dermatome, with the disc levels and key movements side by side
Vertebral Column and Pelvis Explorer, the openings between the vertebrae that the roots leave through, 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.