S4 dermatome and nerve root

Sacral spinal root

Where is the S4 dermatome?

The S4 dermatome covers the skin round the anus, with S5. Its ISNCSCI key sensory point is beside the anus, within 1 cm of its margin, tested with S5. No disc compresses S4 on its own: it lies inside the sacrum, reached only by a large central disc higher up.

S4 at a glance
Type	Sacral spinal root
Key sensory point	Beside the anus, within 1 cm of its margin, tested with S5
Key movement	None graded: the standard grades C5 to T1 and L2 to S1
Tendon reflex	None tests it on its own
Usual disc	None on its own: inside the sacrum, reached only by a large central disc higher up

Try in: Dermatome and Myotome Map Explorer →

S4 myotome: the muscles it supplies

ISNCSCI grades no key movement for S4: it grades only C5 to T1 and L2 to S1.

S4 is a major root for both muscles below.

Muscles S4 supplies, and every root each takes
Muscle	From S4	All its roots
Pelvic floor muscles	Major	S3 and S4; S2 in part
External anal sphincter	Major	S4 and S5; S2 and S3 in part

S4 and the tendon reflexes

No tendon reflex tests S4 on its own.

Why no disc compresses S4 on its own

The sacral roots run down inside the sacrum together, so a disc does not press on one alone. They carry sensation from the saddle area and supply the bladder, bowel and pelvic floor, which is why numbness between the legs with new bladder or bowel trouble is treated as an emergency: it can mean the whole bundle, the cauda equina, is compressed.

What an S4 root lesion does

Sensation. Numbness or tingling in the S4 band. Usually a smaller patch than the whole band, because S3 and S5 overlap it: Sherrington found in 1893 that cutting a single root in monkeys left no skin entirely without feeling.
Strength. Also weaker where S4 is a major supply: pelvic floor muscles and external anal sphincter.
Reflexes. No tendon reflex depends on S4.

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

S3: a ring round the anus on the inner buttocks, at the fold under the buttock
S5: the skin round the anus, with S4

Dermatome and Myotome Map Explorer, with S4 lit on a front and back body map
Every dermatome, with the disc levels and key movements side by side
Lumbar Spine and Pelvis Explorer, the lower back’s vertebrae and the sacrum 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.