T1 dermatome and nerve root

Thoracic spinal root

Where is the T1 dermatome?

The T1 dermatome covers the inner side of the arm and elbow, and the top of the inner forearm. Its ISNCSCI key sensory point is the inner side of the elbow crease, just above the medial epicondyle. Its key movement is little finger abduction. A disc rarely compresses T1; when one does, it is the T1/T2 disc.

T1 at a glance
Type	Thoracic spinal root
Key sensory point	The inner side of the elbow crease, just above the medial epicondyle
Key movement	Little finger abduction
Tendon reflex	Finger flexor reflex, in part
Usual disc	T1/T2, rarely

Try in: Dermatome and Myotome Map Explorer →

T1 myotome: the muscles it supplies

T1 is a major root for 3 of the 10 muscles below and a minor root for the rest.

Muscles T1 supplies, and every root each takes
Muscle	From T1	All its roots
Abductor digiti minimi of the hand	Major, ISNCSCI key muscle	T1; C8 in part
The other small muscles of the hand	Major	T1; C8 in part
Intercostal muscles	Major	T1 to T11
Pectoralis major, sternal part	Minor	C7 and C8; C6 and T1 in part
Pectoralis minor	Minor	C7 and C8; T1 in part
Flexor carpi ulnaris	Minor	C8; C7 and T1 in part
Flexor digitorum superficialis	Minor	C8; C7 and T1 in part
Flexor digitorum profundus	Minor	C8; C7 and T1 in part
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.

Tendon reflexes through T1

Tendon reflexes T1 carries, how each is tested and its roots
Reflex	Role of T1	Tap	Response	Roots
Finger flexor reflex	In part	The examiner’s fingers laid across the patient’s slightly bent fingers	The fingers curl	C8 and T1, mainly C8

Why a disc rarely compresses T1

T1 leaves the spine below the first thoracic vertebra. Most of it joins C8 in the brachial plexus and goes on into the arm and the small muscles of the hand, which is why its skin is on the inner arm and elbow rather than in a band round the trunk; only a small branch runs in the first intercostal space. A disc rarely presses on it, since the ribs steady that part of the spine.

What a T1 root lesion does

Sensation. Numbness or tingling in the T1 band. Usually a smaller patch than the whole band, because C8 and T2 overlap it: Sherrington found in 1893 that cutting a single root in monkeys left no skin entirely without feeling.
Strength. Little finger abduction, the standard’s key movement for T1 (abductor digiti minimi of the hand). Also weaker where T1 is a major supply: the other small muscles of the hand and intercostal muscles.
Reflexes. The finger flexor reflex may be reduced.

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

C8: the ring and little fingers, the inner edge of the hand and the inner forearm
T2: the armpit, the inner upper arm and a band across the upper chest and back

Dermatome and Myotome Map Explorer, with T1 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
Upper Limb Muscles Explorer, the arm’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.