What is the anatomy of the cauda equina (CE) and conus medullaris?

Updated: Jun 14, 2018
  • Author: Segun Toyin Dawodu, JD, MD, MS, MBA, LLM, FAAPMR, FAANEM; Chief Editor: Nicholas Lorenzo, MD, MHA, CPE  more...
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Answer

The cauda equina (CE) is a bundle of intradural nerve roots at the end of the spinal cord, in the subarachnoid space distal to the conus medullaris. Cauda is Latin for tail, and equina is Latin for horse (ie, the "horse's tail"). The CE provides sensory innervation to the saddle area, motor innervation to the sphincters, and parasympathetic innervation to the bladder and lower bowel (ie, from the left splenic flexure to the rectum). [100]

The nerves in the CE region include lower lumbar and all of the sacral nerve roots. The pelvic splanchnic nerves carry preganglionic parasympathetic fibers from S2-S4 to innervate the detrusor muscle of the urinary bladder. Conversely, somatic lower motor neurons from S2-S4 innervate the voluntary muscles of the external anal sphincter and the urethral sphincter via the inferior rectal and the perineal branches of the pudendal nerve, respectively.

Hence, the nerve roots in the CE region carry sensations from the lower extremities, perineal dermatomes, and outgoing motor fibers to the lower extremity myotomes.

The conus medullaris obtains its blood supply primarily from 3 spinal arterial vessels: the anterior median longitudinal arterial trunk and 2 posterolateral trunks. Less prominent sources of blood supply include radicular arterial branches from the aorta, lateral sacral arteries, and the fifth lumbar, iliolumbar, and middle sacral arteries. The latter contribute more to the vascular supply of the cauda equina, although not in a segmental fashion, unlike the blood supply to the peripheral nerves.

The nerve roots may also be supplied by diffusion from the surrounding CSF. Moreover, a proximal area of the nerve roots may have a zone of relative hypovascularity.


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