C8 dermatome and nerve root

Cervical spinal root

Where is the C8 dermatome?

The C8 dermatome covers the ring and little fingers, the inner edge of the hand and the inner forearm. Its ISNCSCI key sensory point is the back of the little finger’s proximal phalanx. Its key movement is finger flexion, at the tip of the middle finger (flexor digitorum profundus). The C7/T1 disc is the one that usually compresses C8.

C8 at a glance
Type	Cervical spinal root
Key sensory point	The back of the little finger’s proximal phalanx
Key movement	Finger flexion, at the tip of the middle finger
Tendon reflex	Finger flexor reflex and triceps reflex, in part
Usual disc	C7/T1

Try in: Dermatome and Myotome Map Explorer →

C8 myotome: the muscles it supplies

C8 is a major root for 6 of the 12 muscles below and a minor root for the rest.

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

Tendon reflexes through C8

Tendon reflexes C8 carries, how each is tested and its roots
Reflex	Role of C8	Tap	Response	Roots
Finger flexor reflex	Main root	The examiner’s fingers laid across the patient’s slightly bent fingers	The fingers curl	C8 and T1, mainly C8
Triceps reflex	In part	The triceps tendon just above the point of the elbow	The elbow straightens	C6, C7 and C8, mainly C7

Why a C7/T1 disc compresses C8

There are eight cervical roots but only seven cervical vertebrae. Each root from C1 to C7 leaves above the vertebra of its number, so C8 leaves between the seventh cervical and first thoracic vertebrae, beside the C7/T1 disc, and a disc bulging backwards and to the side there presses on C8.

What a C8 root lesion does

Sensation. Numbness or tingling in the C8 band. Usually a smaller patch than the whole band, because C7 and T1 overlap it: Sherrington found in 1893 that cutting a single root in monkeys left no skin entirely without feeling.
Strength. Finger flexion, at the tip of the middle finger, the standard’s key movement for C8 (flexor digitorum profundus). Also weaker where C8 is a major supply: pectoralis major, sternal part, pectoralis minor, palmaris longus, flexor carpi ulnaris and flexor digitorum superficialis.
Reflexes. The finger flexor reflex is reduced or absent and the triceps 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

C7: the middle finger, and a strip down the back of the forearm
T1: the inner side of the arm and elbow, and the top of the inner forearm

Where published maps disagree

Maps disagree whether C7 reaches up the back of the forearm or stays in the hand.
The ring finger is C8 in some maps, C7 in others, or split between them.

Dermatome and Myotome Map Explorer, with C8 lit on a front and back body map
Every dermatome, with the disc levels and key movements side by side
Cervical Spine and Skull Base Explorer, the neck’s vertebrae, which the cervical roots leave between, 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.