The spinal cord is a long, thin bundle of nervous tissue that runs from the base of your brain down through your spine, serving as the main communication highway between your brain and the rest of your body. It measures about 45 centimeters (18 inches) in adults, roughly the diameter of a finger, and it handles everything from voluntary movements to automatic reflexes.
Basic Structure and Size
The spinal cord begins at the brainstem, just below the skull, and extends downward through a canal formed by the stacked bones of your vertebral column. Despite carrying signals that control nearly every part of your body, it’s surprisingly compact. In adults, it ends at about the level of the first or second lumbar vertebra, which sits roughly at your lower back, well above where the spine itself ends.
Below that endpoint (called the conus medullaris), the spinal cord doesn’t just stop abruptly. Instead, a bundle of individual nerve roots fans out and continues downward through the spinal canal. This collection of loose nerves is called the cauda equina, Latin for “horse’s tail,” because of its appearance. These nerve roots still exit between vertebrae to serve the lower body, but they’re no longer part of the cord itself.
How It’s Organized
The cord is divided into 31 segments, each giving rise to a pair of spinal nerves, one exiting to the left and one to the right. These segments break down into five regions:
- Cervical (neck): 8 pairs of nerves controlling the head, neck, shoulders, arms, and hands
- Thoracic (mid-back): 12 pairs serving the chest and upper abdomen
- Lumbar (lower back): 5 pairs supplying the lower abdomen, hips, and legs
- Sacral (pelvis): 5 pairs controlling the bladder, bowel, and parts of the legs
- Coccygeal (tailbone): 1 pair serving a small area of skin near the tailbone
The higher the segment, the more of your body it affects. This is why injuries higher on the spinal cord tend to cause more widespread loss of function than injuries lower down.
Gray Matter and White Matter
If you sliced the spinal cord in cross-section, you’d see two distinct types of tissue arranged in a specific pattern. At the center is a butterfly-shaped (or H-shaped) core of gray matter. This gray matter is made up of nerve cell bodies, the parts of neurons that process information. It’s where incoming signals get sorted, where reflexes get organized, and where motor commands get relayed to the appropriate muscles.
Surrounding the gray core is a thick layer of white matter. White matter is composed mostly of long nerve fibers (axons) coated in a fatty insulating substance called myelin, which gives it the white color. These insulated fibers act like bundled cables, carrying signals up toward the brain or down from it at high speed. The myelin coating is essential for fast, reliable transmission. Without it, signals slow dramatically or fail altogether, which is what happens in diseases like multiple sclerosis.
What the Spinal Cord Actually Does
The spinal cord performs two major jobs: relaying messages and processing reflexes.
For the relay function, signals travel in organized bundles called tracts. Ascending tracts carry sensory information upward, things like pain, temperature, touch, and the position of your limbs. Descending tracts carry motor commands downward from the brain, telling muscles when and how to contract. Some descending pathways control fine, deliberate movements like typing or picking up a cup. Others regulate posture, muscle tone, and the background coordination that keeps you balanced and moving smoothly. These posture and coordination pathways actually play an even larger role in everyday movement than the pathways for fine motor control.
The second job, reflex processing, is what makes the spinal cord more than just a cable. It can process certain signals and generate responses entirely on its own, without waiting for instructions from the brain. When you touch something painfully hot, for example, the pain signal enters the spinal cord, gets routed through a local circuit of nerve cells, and triggers a motor response that yanks your hand away. This all happens faster than the signal can reach your brain and come back, which is why you pull your hand away before you consciously feel the burn.
Protective Layers
The spinal cord is delicate tissue, and it sits inside multiple layers of protection. The vertebral column, the stack of bones most people call the spine, provides the outermost shield. Inside that bony canal, three membranes called meninges wrap the cord:
- Dura mater: the tough outer layer, closest to the bone
- Arachnoid mater: the middle layer, named for its web-like structure
- Pia mater: the delicate inner layer that sits directly on the cord’s surface
Between the arachnoid and pia layers is a fluid-filled space containing cerebrospinal fluid. This fluid acts as a liquid cushion, absorbing shocks and protecting the cord from impacts that make it through the bone. The same meninges and fluid also surround the brain, forming a continuous protective system for the entire central nervous system.
What Happens When It’s Injured
Unlike many other tissues in the body, the spinal cord has very limited ability to repair itself. Damage to the cord can result in permanent loss of movement, sensation, or both below the level of injury. The severity depends on two factors: where the injury occurs and whether the cord is fully or partially severed.
A complete injury means no signals pass through the damaged area at all, resulting in total loss of feeling and voluntary movement below that point. An incomplete injury means some connections survive, so varying degrees of sensation or motor function remain. Doctors classify injuries on a five-level scale ranging from complete (no preserved function) to normal. Most spinal cord injuries fall somewhere in the incomplete range, meaning some function below the injury site is retained.
Injuries to the cervical region can affect all four limbs and the trunk (quadriplegia), while injuries to the thoracic or lumbar regions typically affect the legs and lower body (paraplegia). Because the cord also controls bladder, bowel, and sexual function through its lower segments, injuries at almost any level can affect these systems as well.
The Spinal Cord vs. the Spine
People often use “spine” and “spinal cord” interchangeably, but they’re different structures. The spine is bone, a column of 33 vertebrae that provides structural support and protects what’s inside. The spinal cord is the nervous tissue running through the canal inside those bones. You can have spine problems, like a herniated disc or arthritis, without any spinal cord involvement. But when a spine problem narrows the canal enough to compress the cord, neurological symptoms like numbness, weakness, or coordination problems can develop.
The cord also doesn’t fill the entire length of the spinal canal. Since it ends around the first or second lumbar vertebra, procedures like lumbar punctures (spinal taps) are performed in the lower back precisely because there’s no cord there to accidentally damage, only the loose nerve roots of the cauda equina floating in cerebrospinal fluid.