S5 dermatome and nerve root

Sacral spinal root

Where is the S5 dermatome?

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

S5 at a glance
Type	Sacral spinal root
Key sensory point	Beside the anus, within 1 cm of its margin, tested with S4
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 →

S5 myotome: the muscles it supplies

ISNCSCI grades no key movement for S5: it grades only C5 to T1 and L2 to S1.

S5 is a major root for the external anal sphincter.

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

S5 and the tendon reflexes

No tendon reflex tests S5 on its own.

Why no disc compresses S5 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 S5 root lesion does

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

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

S4: the skin round the anus, with S5

Dermatome and Myotome Map Explorer, with S5 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.