Why the Spine’s Anatomy Allows for Safe Lumbar Punctures
This exploration of spinal cord anatomy is one of many deeper investigations into the structures, functions, and terminology of the body's systems and regions in the "Anatomy" course series.
How can a clinician insert a needle into the lower part of the spine to collect cerebrospinal fluid without puncturing the spinal cord?
The answer lies in a unique and important feature of spinal anatomy: the spinal cord ends before the vertebral column ends, leaving a section containing spinal fluid that can be safely accessed.
In the “Human Neuroanatomy” course, part of the “Anatomy” course series on Michigan Online, University of Michigan teaching professor Kelli A. Sullivan explains the gross anatomy of the spinal cord, including its regions, layers, and essential functions.
Taught by Michigan Medicine instructors, each self-paced course features interactive 3D scans of actual donor models — including the vertebral column and spinal cord.
Where Does the Spinal Cord End?
At birth, the spinal cord and vertebral column are approximately the same length. As the body develops, the vertebral bones continue to grow while the spinal cord remains the same length.
As a result, the adult spinal cord ends around the L1–L2 vertebral level. Its tapered end is called the conus medullaris.
The vertebral canal continues below the conus medullaris, and so do the nerve roots that also continue growing to serve the lower parts of the body.
What is the Cauda Equina?
Below the conus medullaris, descending nerve roots travel through the vertebral canal until they reach the appropriate locations to exit and become spinal nerves. Together, these roots form the cauda equina, a name derived from Latin for “horse’s tail.”
Since the nerve roots are no longer contained in the spinal cord, in the cauda equina they become a bundle of nerves surrounded by cerebrospinal fluid.
Sullivan likens the nerve roots in the cauda equina to spaghetti noodles floating in a pot of water. If you try to poke one with a utensil, the noodle will slip out of the way. This structure provides a safe location for a lumbar puncture.
The Anatomy of a Lumbar Puncture
The spinal cord is covered by the same meningeal layers, or protective membranes, found in the brain. The three meningeal layers found in the vertebral canal also continue below the conus medullaris, creating the surrounding membranes that hold cerebrospinal fluid.
This anatomical arrangement provides clinicians with a location to collect a sample. Sullivan explains that a lumbar puncture can be performed around the L4–L5 intervertebral space, safely below the end of the spinal cord.
A needle can enter the fluid-filled region of the cauda equina, where the nerve roots can move within the fluid, without damaging the nerves. Because the procedure occurs below the intervertebral area where the spinal cord ends, the needle is not being inserted into the spinal cord itself.
The collected cerebrospinal fluid can then be examined for indicators such as bacteria, white blood cells, or other substances when evaluating certain illnesses.
Connecting Structure with Clinical Practice
The essential functions of the spinal cord, its gross anatomy, and key vocabulary all combine into a framework for understanding real clinical procedures in this module.
The concepts in the “Anatomy” course series, taught by Sullivan, B. Kathleen Alsup, and Glenn M. Fox, follow the same approach to instructing on the body’s nine major organ systems and regions. Whether refreshing your knowledge of anatomy or starting your healthcare education, the “Anatomy” course series prepares you for your clinical experiences.
