T9 dermatome and nerve root

Thoracic spinal root

Where is the T9 dermatome?

The T9 dermatome covers a band round the trunk just above the umbilicus. Its ISNCSCI key sensory point is the ninth intercostal space, midway between T8 and T10. A disc rarely compresses T9; when one does, it is the T9/T10 disc.

T9 at a glance
Type	Thoracic spinal root
Key sensory point	The ninth intercostal space, midway between T8 and T10
Key movement	None graded: the standard grades C5 to T1 and L2 to S1
Tendon reflex	None tests it on its own
Usual disc	T9/T10, rarely

Try in: Dermatome and Myotome Map Explorer →

T9 myotome: the muscles it supplies

ISNCSCI grades no key movement for T9: it grades only C5 to T1 and L2 to S1.

T9 is a major root for 5 of the 7 muscles below and a minor root for the rest.

Muscles T9 supplies, and every root each takes
Muscle	From T9	All its roots
Intercostal muscles	Major	T1 to T11
Rectus abdominis	Major	T7 to T12
External oblique	Major	T7 to T12
Internal oblique	Major	T7 to L1
Transversus abdominis	Major	T7 to L1
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.

T9 and the tendon reflexes

No tendon reflex tests T9 on its own.

Why a disc rarely compresses T9

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 T9 root lesion does

Sensation. Numbness or tingling in the T9 band. Usually a smaller patch than the whole band, because T8 and T10 overlap it: Sherrington found in 1893 that cutting a single root in monkeys left no skin entirely without feeling.
Strength. Also weaker where T9 is a major supply: intercostal muscles, rectus abdominis, external oblique, internal oblique and transversus abdominis.
Reflexes. No tendon reflex depends on T9.

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

T8: a band round the trunk halfway from the breastbone to the umbilicus
T10: a band round the trunk at the umbilicus

Dermatome and Myotome Map Explorer, with T9 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.