Conus medullaris (medullary cone) and cauda equina
The conus medullaris (medullary cone) is the cone-shaped terminal portion of the spinal cord. The tip of the conus medullaris is found between the L1 and L2 vertebra in the average adult. The conus medullaris is tethered to the coccyx by a fibrous cord called the filum terminale, which stabilizes the distal end of the spinal cord.
Distal to the conus medullaris is a collection of spinal nerve roots called the cauda equina, that emerges from the lumbosacral part of the spinal cord below the L1 vertebra and descends toward the coccyx. The cauda equina is translated from Latin into ‘horse’s tail’, and was so named due to its resemblance to the tail of a horse.
The conus medullaris and cauda equina give rise to spinal nerves that provide all motor and sensory innervation to the lower limbs, pelvis and perineum as well as parasympathetic innervation to the pelvic viscera.
This article will discuss the anatomy and functions of the conus medullaris and cauda equina.
Conus medullaris: Sacral segments of spinal cord (S2-S5), coccygeal segment of spinal cord
Cauda equina: Spinal nerve roots (L2-S5), coccygeal nerve (Co)
Motor: Lower limb muscles (L2-S2), pelvic floor and perineal muscles (S2-S4)
Sensory: Skin of lower limbs (L2-S3), perineum (S2-S4), coccygeal region (S4-S5, Co)
Parasympathetic (S2-S4): pelvic viscera (descending colon, sigmoid colon, rectum, urinary bladder, and reproductive organs)
- Definition and location
- Clinical relations
Definition and location
The conus medullaris is the tapering distal end of the spinal cord and consists of the sacral (S2-S5) and coccygeal spinal cord segments. Typically, the conus medullaris lies around the level of L1 vertebra in adults, but it may be as high as T12 vertebra or as low as L3 vertebra.
In early development, the entire length of the vertebral canal is occupied by the spinal cord, with spinal nerves exiting the vertebral canal at right angles through the intervertebral foramina. Over time, the vertebral column outgrows the spinal cord resulting in the relative ascent of the conus medullaris. Due to the increasing distance between the lumbosacral cord segments and their corresponding vertebrae, spinal nerves exiting the vertebral canal from these regions become progressively longer and more vertical. This loose bundle of spinal nerve roots of varying length emerging from the lumbosacral enlargement and conus medullaris form the cauda equina.
The cauda equina consists of the spinal nerve roots L2-S5 and the coccygeal nerve. It lies within the distal third of the vertebral canal and extends into the sacral canal. It occupies the lumbar cistern, which is an enlargement of the subarachnoid space containing cerebrospinal fluid (CSF).
Also extending distally from the apex of the conus medullaris is the filum terminale, a vestigial remnant of the distal part of the spinal cord . This thin, approximately 20 cm long filament of connective tissue is actually an extension of the pia mater that runs distally surrounded by the spinal nerve roots of the cauda equina. At the level of S2 vertebra, it becomes covered by the dura and arachnoid layers and attaches to the coccyx, anchoring the distal end of the spinal cord and spinal meninges.
The conus medullaris and cauda equina give rise to spinal nerves L2-S5 and the coccygeal nerve. The anterior rami of these spinal nerves contribute to the lumbar and sacral plexuses, which provide motor and sensory innervation to the entire lower limb, pelvic and perineal regions. Through their posterior rami, these spinal nerves supply the skin and deep muscles of the lower back, as well as sacral and coccygeal regions.
Additionally, the cauda equina provides parasympathetic innervation to the viscera of the pelvic cavity and perineum, including the urinary bladder, descending and sigmoid colon, rectum and both the internal and external reproductive organs. This parasympathetic innervation is also involved in the stimulation of erectile tissues of the external genitalia, enabling erection of the penis/clitoris.
The cauda equina, through its somatic and parasympathetic innervation, plays a crucial role in bladder control, control of the urethral and anal sphincters and the overall maintenance of continence.
Test your knowledge on the structures at the level of the cauda equina with this quiz.
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Cauda equina syndrome
Cauda equina syndrome is caused by a compression or irritation of lumbosacral spinal nerve roots, often due to lumbar disc herniation. The most common symptoms include low back pain that radiates into the sacral region and legs, and bilateral motor and sensory deficits, which present as asymmetric saddle anesthesia (S2-S5 dermatomes) and asymmetric lower limb weakness. Additional clinical features may include loss of bladder control (urinary retention), fecal incontinence and erectile dysfunction, which are often late findings and less frequent.
Conus medullaris syndrome
Conus medullaris syndrome, although quite similar, is clinically distinct from cauda equina syndrome. This syndrome is much less common and is typically caused by tumors or vascular anomalies within the spinal canal. It is characterized by symmetric saddle anesthesia and symmetric weakness in the lower extremities, with associated bladder dysfunction and bowel incontinence early on.
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