The spinal cord is a dense column of nervous tissue made primarily of neurons, support cells called glia, and bundled nerve fibers wrapped in a fatty insulating material called myelin. It runs from the base of the brain down to roughly the first or second lumbar vertebra, measuring about 43 to 45 centimeters long in most adults and no wider than your little finger at its thickest point. Despite its small size, it contains billions of cells organized into two distinct tissue types, surrounded by protective membranes, fluid, and a dedicated blood supply.
Grey Matter and White Matter
If you sliced the spinal cord crosswise, you’d see a butterfly-shaped core of darker tissue surrounded by lighter tissue. The darker core is grey matter, and it contains the cell bodies of neurons, the parts that process and relay signals. This is where decisions about movement and sensation actually happen at the spinal level. The surrounding lighter tissue is white matter, and it gets its color from myelin, the fatty coating wrapped around long nerve fibers (axons) that carry signals up and down the cord.
Grey matter handles local processing. Motor neurons in the front portion of the grey matter send commands to your muscles. Sensory neurons entering through the back portion receive information about touch, temperature, and pain. Interneurons connect the two, enabling reflexes that happen before your brain even gets involved, like pulling your hand from a hot stove.
White matter, by contrast, is the long-distance communication system. Its axons are bundled into tracts that connect different levels of the spinal cord to each other and to the brain. Some tracts carry sensory information upward, others carry motor commands downward. The organization is precise: damage to a specific tract produces predictable losses in sensation or movement.
What Myelin Is Made Of
The white appearance of white matter comes from myelin, a sleeve of fat and protein that wraps around each axon like insulation around an electrical wire. When dried, myelin is 70% to 85% lipids (fats) and 15% to 30% proteins. Those lipids break down further into roughly 40% cholesterol, 40% phospholipids, and 20% glycolipids. In living tissue, myelin is 33% to 55% water.
Myelin does more than protect nerve fibers. It dramatically speeds up electrical signals by forcing them to jump between gaps in the coating rather than traveling continuously along the axon. At each gap, charged particles recharge the signal so it arrives at its destination without losing strength. Without intact myelin, signals slow down or fail entirely, which is what happens in conditions like multiple sclerosis.
The Cells Inside the Cord
Neurons get most of the attention, but they’re actually outnumbered by glial cells, the support staff that keep the spinal cord functioning. Several types of glia do very different jobs.
- Oligodendrocytes produce and maintain the myelin sheath. A single oligodendrocyte can wrap segments of multiple axons at once, making it essential for signal transmission throughout the cord.
- Astrocytes are star-shaped cells that maintain the chemical environment around neurons. They regulate neurotransmitter levels at synapses, control concentrations of important ions like potassium, and provide metabolic fuel. They also influence how synapses behave, making them active participants in signaling rather than passive bystanders.
- Microglia serve as the cord’s immune system. They patrol for injury and disease, clearing away dead cells and toxic substances. They also prune unnecessary connections between neurons during normal development and maintenance.
- Ependymal cells line the central canal, a tiny fluid-filled channel running through the middle of the cord. These cells produce cerebrospinal fluid, the same liquid that cushions the brain.
Protective Layers Around the Cord
The spinal cord itself is soft and delicate. Three layered membranes called meninges wrap around it for protection. The outermost layer, the dura mater, is tough and fibrous. The middle layer, the arachnoid mater, is thinner and web-like. The innermost layer, the pia mater, clings directly to the surface of the cord tissue.
Between the arachnoid and pia layers sits the subarachnoid space, which is filled with cerebrospinal fluid. This fluid acts as a liquid cushion, absorbing shocks from sudden movements or impacts. It also carries nutrients to the cord and removes waste products. Beyond the meninges, the vertebral bones of the spine provide the outermost layer of mechanical protection.
Blood Supply
The spinal cord receives its blood through three main arteries that run along its length. The anterior spinal artery travels down the front of the cord as a single, uninterrupted vessel. It supplies roughly the front two-thirds of the cord, including much of the grey matter. Two posterior spinal arteries run along the back, feeding the rear portions.
These arteries are reinforced by feeder vessels that enter at various levels along the spine. The largest of these feeders, sometimes called the artery of Adamkiewicz, typically enters between the T12 and L3 vertebral levels. In about 31% of people, this single feeder is the only radicular artery supplying the mid-to-lower spinal cord, which is why damage to it during surgery or trauma can have serious consequences. The anterior spinal artery itself is remarkably thin, averaging less than half a millimeter wide in the upper cord and just over one millimeter near where this main feeder joins it.
How It’s Organized Into Segments
The spinal cord is divided into 31 segments, each giving rise to a pair of spinal nerves that exit through gaps between the vertebrae. These segments are grouped into five regions: 8 cervical (neck), 12 thoracic (mid-back), 5 lumbar (lower back), 5 sacral (pelvis), and 1 coccygeal (tailbone). Each nerve pair carries both sensory fibers coming in and motor fibers going out, connecting a specific zone of the body to the cord.
The cord isn’t uniform in thickness. It bulges at two points where it handles the most complex wiring. The cervical enlargement, at around the C5 level, reaches about 13.3 millimeters across, because this is where nerves serving the arms and hands originate. The lumbar enlargement, near L3, widens to about 9.4 millimeters to accommodate the dense nerve supply to the legs. Between these two bulges, at the mid-thoracic level, the cord narrows to roughly 8.3 millimeters across, reflecting the simpler nerve demands of the trunk.
Putting It All Together
At its most basic, the spinal cord is a tightly organized cylinder of nervous tissue where neurons do the signaling, glial cells maintain the infrastructure, myelin enables high-speed communication, cerebrospinal fluid and meninges provide cushioning, and a thin but critical network of arteries delivers oxygen and fuel. Every component depends on the others. Damage to the myelin disrupts signaling even when neurons are intact. Loss of blood supply kills tissue within minutes. Compromise the meninges, and infection can reach the cord directly. The cord’s small size belies its complexity: packed into a structure no wider than a finger is the entire two-way communication highway between your brain and body.