The spinal cord is the central communication pathway of the nervous system, transmitting signals between the brain and the rest of the body. Encased and protected by the vertebral column, this long structure extends downward from the base of the skull. It serves as the primary route for motor information traveling to the muscles and sensory information returning from the limbs and torso. Understanding the anatomy of the spinal cord is important, especially its lower boundary, where the solid neural tissue transitions into a collection of nerve fibers.
The Conus Medullaris
The tapered, cone-shaped end of the spinal cord is called the conus medullaris, a Latin term meaning “medullary cone.” This structure represents the termination of the central nervous system tissue within the vertebral canal. In most adults, the conus medullaris is located around the level of the first or second lumbar vertebra (L1 or L2).
The specific location of the termination is significant because the spinal cord is shorter than the vertebral column that protects it. The bony spine continues to grow during development while the spinal cord’s growth slows. Because of this difference, the sacral and coccygeal spinal cord segments, which control function in the lower body and pelvic organs, are housed within the conus medullaris in the upper lumbar spine. This short, conical segment is approximately three centimeters in length and is the site where the mass of neural tissue gives way to individual nerve roots.
The Filum Terminale
Extending directly from the tip of the conus medullaris is a thin, delicate strand of fibrous tissue known as the filum terminale, or “terminal thread.” This structure is not made of active neural tissue but is primarily composed of pia mater, which is one of the meningeal layers covering the spinal cord. Measuring about 20 centimeters long, the filum terminale serves the mechanical purpose of anchoring the spinal cord.
The strand descends downward, surrounded by a sheath of dura mater, until it ultimately fuses with the periosteum of the coccyx (tailbone). This attachment provides longitudinal support, stabilizing the entire spinal cord within the vertebral column and preventing excessive movement. The superior portion of the filum terminale is contained within the dural sac, while the lower portion extends outside of it to secure the cord.
The Cauda Equina
Below the conus medullaris, the spinal canal is occupied by a collection of individual spinal nerve roots known as the cauda equina. Its name, Latin for “horse’s tail,” describes the appearance of this bundle of nerves descending through the lower vertebral column. These are the lumbar, sacral, and coccygeal nerve roots that peeled away from the spinal cord higher up.
These nerve roots must travel down the spinal canal to exit the vertebral column at their appropriate intervertebral openings to innervate the lower limbs and pelvic organs. The cauda equina is considered part of the peripheral nervous system, distinct from the central nervous system tissue that terminated at the conus medullaris. The nerve roots are suspended in cerebrospinal fluid within the dural sac, which provides a protective cushion for the delicate fibers.
Clinical Relevance of the Spinal Cord’s End Region
The anatomical arrangement of the spinal cord’s end region is highly significant for medical procedures. Because the solid spinal cord tissue terminates at the L1 or L2 vertebral level, the space below this point is considered a safe zone for accessing the cerebrospinal fluid. This zone, which is only occupied by the floating nerve roots of the cauda equina, is the target for a procedure called a lumbar puncture, or spinal tap.
During a lumbar puncture, a needle is typically inserted between the L3/L4 or L4/L5 vertebrae, safely below the conus medullaris, to collect fluid for diagnostic testing. This allows clinicians to sample the fluid surrounding the brain and spinal cord to check for infections, inflammation, or other neurological conditions without risking damage to the spinal cord itself.
However, this region is susceptible to specific conditions. Cauda Equina Syndrome involves compression of the nerve roots. Another element is Tethered Cord Syndrome, where the filum terminale is abnormally tight, pulling the conus medullaris lower than its normal anatomical position.