Nerves are made of bundles of long, thin fibers called axons, wrapped in layers of fatty insulation and protective connective tissue, and threaded with tiny blood vessels that keep everything nourished. Think of a nerve like a cable: individual wires (axons) are grouped into bundles (fascicles), and those bundles are wrapped together inside a tough outer sheath. The smallest nerves contain just a handful of fibers, while the largest nerve in the body, the sciatic nerve running down each leg, measures about 20 mm wide near the hip.
Axons: The Core Signal Carriers
The functional heart of any nerve is the axon, a threadlike extension of a nerve cell that carries electrical signals. A single nerve can contain thousands of these axons running in parallel, each one transmitting its own signal independently. Some axons carry sensory information toward the brain, helping you feel touch, temperature, and pain. Others are motor axons, carrying commands from the brain out to muscles and glands so you can move and function. Many of the major nerves in your body, including all 31 pairs of spinal nerves, are “mixed” nerves containing both types.
Axons vary in diameter. Thicker axons transmit signals faster, and these tend to be the ones wrapped in the thickest layer of insulation. The thinnest axons, which carry slower signals like dull pain or temperature sensation, often have little or no insulation at all.
Myelin: The Fatty Insulation Layer
Most axons in the body are wrapped in a substance called myelin, a white, fatty coating that works like the rubber insulation around an electrical wire. Myelin is unusual compared to other biological membranes. It’s 70% to 85% fat by dry weight, with only 15% to 30% protein. A typical cell membrane is roughly half fat and half protein. That extreme fat content is what makes myelin such an effective electrical insulator: ions can’t easily pass through it, so the electrical signal stays contained within the axon instead of leaking out.
The fat molecules in myelin have especially long tails, ranging from 14 to 24 carbon atoms, which pack tightly together and reduce the membrane’s fluidity. This tight packing creates a rigid, nearly waterproof barrier around the axon. The result is that signals travel much faster in myelinated nerves, jumping from one small gap in the myelin to the next rather than crawling along the entire length of the fiber.
Myelin is produced by specialized support cells called Schwann cells. Each Schwann cell wraps itself around a short segment of a single axon, spiraling around it many times to build up the insulating layers. Axons thicker than about 1 micrometer get their own dedicated Schwann cell wrapping, while thinner axons are bundled together in groups within a single Schwann cell. These cells do more than just insulate. After a nerve injury, Schwann cells can transform into repair cells that guide regrowing axons back toward their targets, which is why peripheral nerves (the ones outside the brain and spinal cord) can sometimes heal after being damaged.
Fascicles: How Fibers Are Bundled
Axons don’t just run loose inside a nerve. They’re organized into distinct bundles called fascicles, each one functioning as a structural and functional unit. A fascicle contains a group of axons along with their Schwann cells, all embedded in a soft connective tissue matrix called the endoneurium. This inner tissue cushions the fibers and provides a stable chemical environment for signal transmission.
Fascicles aren’t random groupings. They’re organized so that fibers serving the same body region or function tend to travel together. A fascicle might contain the motor fibers heading to a particular muscle, or the sensory fibers coming from a specific patch of skin. This organization isn’t permanent, though. Individual nerve fibers can branch from one fascicle to another as they travel along the length of a nerve, changing which neighbors they’re bundled with.
Three Layers of Protective Wrapping
Each level of nerve organization has its own connective tissue layer, creating a system of protection from the inside out.
- Endoneurium: The innermost layer, filling the space between individual axons within a fascicle. It contains the Schwann cells and myelin that directly surround each fiber.
- Perineurium: A thicker, tougher sheath that wraps around each fascicle. This layer does several jobs at once. It absorbs stretching forces so the delicate axons inside aren’t damaged when you bend a joint or move a limb. It also regulates the pressure and chemical balance inside the fascicle, acting as part of a “blood-nerve barrier” similar to the blood-brain barrier. This keeps toxins and immune cells from freely entering the nerve tissue.
- Epineurium: The outermost coat, a layer of connective tissue that bundles all the fascicles together into the single structure you’d recognize as a nerve. It also carries the larger blood vessels that supply the nerve.
This layered design means a nerve can flex and stretch with your body’s movements without the internal fibers being crushed or torn. The connective tissue layers absorb mechanical stress while maintaining the stable internal environment that axons need to function.
Blood Supply Inside a Nerve
Nerves need a constant supply of oxygen and nutrients to keep firing, and they get it from a network of tiny blood vessels called the vasa nervorum, literally “vessels of the nerves.” These small arteries and veins run along the outside of the nerve within the epineurium, then send branches inward through the perineurium to reach the axons inside each fascicle.
Blood vessels and nerves are physically intertwined throughout the body. They travel together to reach almost every tissue, and they depend on each other: nerves control the blood vessel walls that regulate blood flow, while blood vessels deliver the fuel that nerves need to keep transmitting signals. When the blood supply to a nerve is disrupted, by compression, diabetes, or other vascular problems, the nerve fibers can begin to malfunction or die. This is one of the main mechanisms behind conditions like diabetic neuropathy, where long-term high blood sugar damages the tiny vessels feeding peripheral nerves.
Sensory, Motor, and Mixed Nerves
Not all nerves contain the same types of fibers. Sensory nerves carry signals toward the brain, giving you the ability to feel touch, taste, smell, and see. Motor nerves carry signals outward from the brain and spinal cord to muscles and glands, controlling movement and bodily functions. Most of the large nerves in your body are mixed nerves containing both sensory and motor fibers bundled together.
You have 12 pairs of cranial nerves originating from the brain, extending through the face, head, and neck. Some are purely sensory (like the optic nerve for vision), some are purely motor, and some handle both functions. The 31 pairs of spinal nerves branching from the spinal cord are all mixed nerves, carrying both sensory and motor fibers to and from the trunk and limbs. The sciatic nerve, the body’s largest, carries motor commands down to the muscles of the leg and foot while simultaneously relaying sensory information back up from the skin and joints below the knee.