T3 dermatome and nerve root

Thoracic spinal root

Where is the T3 dermatome?

The T3 dermatome covers a band round the chest at the third rib. Its ISNCSCI key sensory point is the third intercostal space, at the midclavicular line. A disc rarely compresses T3; when one does, it is the T3/T4 disc.

T3 at a glance
Type	Thoracic spinal root
Key sensory point	The third intercostal space, at the midclavicular line
Key movement	None graded: the standard grades C5 to T1 and L2 to S1
Tendon reflex	None tests it on its own
Usual disc	T3/T4, rarely

Try in: Dermatome and Myotome Map Explorer →

T3 myotome: the muscles it supplies

ISNCSCI grades no key movement for T3: it grades only C5 to T1 and L2 to S1.

T3 is a major root for 1 of the 3 muscles below and a minor root for the rest.

Muscles T3 supplies, and every root each takes
Muscle	From T3	All its roots
Intercostal muscles	Major	T1 to T11
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.

T3 and the tendon reflexes

No tendon reflex tests T3 on its own.

Why a disc rarely compresses T3

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 T3 root lesion does

Sensation. Numbness or tingling in the T3 band. Usually a smaller patch than the whole band, because T2 and T4 overlap it: Sherrington found in 1893 that cutting a single root in monkeys left no skin entirely without feeling.
Strength. Also weaker where T3 is a major supply: intercostal muscles.
Reflexes. No tendon reflex depends on T3.

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

T2: the armpit, the inner upper arm and a band across the upper chest and back
T4: a band round the chest at the nipples

Dermatome and Myotome Map Explorer, with T3 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.