C4 dermatome and nerve root

Cervical spinal root

Where is the C4 dermatome?

The C4 dermatome covers the tops of the shoulders and a cape across the upper chest and back. Its ISNCSCI key sensory point is the top of the acromioclavicular joint. The C3/C4 disc is the one that usually compresses C4.

C4 at a glance
Type	Cervical spinal root
Key sensory point	The top of the acromioclavicular joint
Key movement	None graded: the standard grades C5 to T1 and L2 to S1
Tendon reflex	None tests it on its own
Usual disc	C3/C4

Try in: Dermatome and Myotome Map Explorer →

C4 myotome: the muscles it supplies

ISNCSCI grades no key movement for C4: it grades only C5 to T1 and L2 to S1.

C4 is a major root for 2 of the 4 muscles below and a minor root for the rest.

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

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

C4 and the tendon reflexes

No tendon reflex tests C4 on its own.

Why a C3/C4 disc compresses C4

Cervical roots leave the spine above the vertebra of the same number, so the C4 root leaves between the C3 and C4 vertebrae, right beside the C3/C4 disc. A disc bulging backwards and to the side presses on it there, which is why a C3/C4 disc gives C4 symptoms.

What a C4 root lesion does

Sensation. Numbness or tingling in the C4 band. Usually a smaller patch than the whole band, because C3 and C5 overlap it: Sherrington found in 1893 that cutting a single root in monkeys left no skin entirely without feeling.
Strength. Also weaker where C4 is a major supply: levator scapulae and diaphragm.
Reflexes. No tendon reflex depends on C4.

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 C4

C3, 4 and 5 keep the diaphragm alive, and C4 is the cape over the shoulders.

Neighboring dermatomes

C3: the neck, front and back, down to the collarbones
C5: the outer upper arm, over the deltoid, down to the elbow

Dermatome and Myotome Map Explorer, with C4 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.