C3 dermatome and nerve root

Cervical spinal root

Where is the C3 dermatome?

The C3 dermatome covers the neck, front and back, down to the collarbones. Its ISNCSCI key sensory point is the supraclavicular fossa, behind the collarbone at the midclavicular line. ISNCSCI grades no key movement for C3, which mainly supplies the levator scapulae. The C2/C3 disc is the one that usually compresses C3.

C3 at a glance
Type	Cervical spinal root
Key sensory point	The supraclavicular fossa, behind the collarbone 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	C2/C3

Try in: Dermatome and Myotome Map Explorer →

C3 myotome: the muscles it supplies

C3 is a major root for 1 of the 4 muscles below and a minor root for the rest.

Muscles C3 supplies, and every root each takes
Muscle	From C3	All its roots
Levator scapulae	Major	C3 and C4; C5 in part
Sternocleidomastoid	Minor	C2 and C3 in part
Trapezius	Minor	C3 and C4 in part
Diaphragm	Minor	C4; C3 and C5 in part

Sternocleidomastoid: its motor supply is the accessory nerve, cranial nerve XI; C2 and C3 add to it.
Trapezius: its motor supply is the accessory nerve, cranial nerve XI; C3 and C4 add to it.
Diaphragm: the phrenic nerve.

C3 and the tendon reflexes

No tendon reflex tests C3 on its own.

Why a C2/C3 disc compresses C3

Cervical roots leave the spine above the vertebra of the same number, so the C3 root leaves between the C2 and C3 vertebrae, right beside the C2/C3 disc. A disc bulging backwards and to the side presses on it there, which is why a C2/C3 disc gives C3 symptoms.

What a C3 root lesion does

Sensation. Numbness or tingling in the C3 band. Usually a smaller patch than the whole band, because C2 and C4 overlap it: Sherrington found in 1893 that cutting a single root in monkeys left no skin entirely without feeling.
Strength. Also weaker where C3 is a major supply: levator scapulae.
Reflexes. No tendon reflex depends on C3.

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

C2: the back of the scalp, behind the ear and the angle of the jaw
C4: the tops of the shoulders and a cape across the upper chest and back

Dermatome and Myotome Map Explorer, with C3 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
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.