T4 dermatome and nerve root

Thoracic spinal root

Where is the T4 dermatome?

The T4 dermatome covers a band round the chest at the nipples. Its ISNCSCI key sensory point is the fourth intercostal space, the nipple line. ISNCSCI grades no key movement for T4, which mainly supplies the intercostal muscles. A disc rarely compresses T4; when one does, it is the T4/T5 disc.

T4 at a glance
Type	Thoracic spinal root
Key sensory point	The fourth intercostal space, the nipple 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	T4/T5, rarely

Try in: Dermatome and Myotome Map Explorer →

T4 myotome: the muscles it supplies

T4 is a major root for 1 of the 3 muscles below and a minor root for the rest.

Muscles T4 supplies, and every root each takes
Muscle	From T4	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.

T4 and the tendon reflexes

No tendon reflex tests T4 on its own.

Why a disc rarely compresses T4

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 T4 root lesion does

Sensation. Numbness or tingling in the T4 band. Usually a smaller patch than the whole band, because T3 and T5 overlap it: Sherrington found in 1893 that cutting a single root in monkeys left no skin entirely without feeling.
Strength. Also weaker where T4 is a major supply: intercostal muscles.
Reflexes. No tendon reflex depends on T4.

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 T4

T4 at the teat pore.

Neighboring dermatomes

T3: a band round the chest at the third rib
T5: a band round the chest just below the nipples

Dermatome and Myotome Map Explorer, with T4 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.