Conus medullaris
Conus medullaris
Also known as: medullary cone
The tapered, cone-shaped lower end of the spinal cord, lying at about the L1–L2 vertebral level in adults. It contains the sacral and coccygeal segments that control the bladder, bowel and sexual function.
Overview
Below the lumbosacral enlargement the cord narrows rapidly into the conus medullaris. Its apex usually lies at the L1–L2 intervertebral disc in adults (normal range T12–L3) and at about L2–L3 in the newborn. A conus lying below L2–L3 in an adult suggests a tethered cord. From the apex, the pial filum terminale continues down to the coccyx, while the lumbar, sacral and coccygeal nerve roots descend around it as the cauda equina.
The conus contains segments S2–S5 and Co1. This includes the sacral parasympathetic nucleus (S2–S4), which drives detrusor contraction, defecation and erection, and Onuf's nucleus, the motor neurons supplying the external urethral and anal sphincters via the pudendal nerve. Lesions here therefore produce early and prominent sphincter disturbance.
- Location
Apex at the L1–L2 disc (T12–L3) in adults, within the dural sac, surrounded by the upper cauda equina roots.
Anatomy
- Relations
It lies at the L1 vertebral body, surrounded by the lumbar and sacral roots of the cauda equina, and continues inferiorly as the filum terminale.
Function
- Contains the spinal centres for micturition, defecation and erection (S2–S4 parasympathetic outflow).
- Contains the motor neurons of the external urethral and anal sphincters (Onuf's nucleus).
- Receives sensory input from the saddle (perineal) area via S3–S5.
Clinical significance
Conus medullaris syndrome
Compression at about the L1 level (disc, fracture, tumour) produces sudden, bilateral, fairly symmetrical deficits. Early bladder and bowel dysfunction is prominent, with saddle anaesthesia and erectile dysfunction, and pain is mild. There is a mix of UMN and LMN signs: the ankle jerk may be lost while the knee jerk is preserved. Contrast this with cauda equina syndrome, which is purely LMN, asymmetric and painful.
Tethered cord
A thickened or fatty filum terminale, or a spinal dysraphism, anchors the conus abnormally low. Growth then stretches the cord, causing back pain, gait disturbance, foot deformities and bladder dysfunction. Overlying cutaneous markers (hair tuft, dimple, lipoma) are clues.
Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.