A bundle of axons is called a nerve. More specifically, when axons are bundled together outside the brain and spinal cord, the bundle is called a peripheral nerve. When axons are bundled together inside the brain or spinal cord, the bundle is called a tract. Both structures do the same basic job: they carry electrical signals between different parts of the body and brain, functioning like biological cables that route information where it needs to go.
Nerves vs. Tracts: Location Changes the Name
The nervous system uses different terminology depending on where an axon bundle sits. In the peripheral nervous system, which includes everything outside the brain and spinal cord, bundles of axons are called nerves. The 12 pairs of cranial nerves emerge directly from the brain and handle functions mostly related to the head and neck. The spinal nerves branch out from the spinal cord and provide motor control and sensation to the rest of the body. The spinal cord itself is essentially a large, cylindrical bundle of nerve fibers enclosed in the spine that connects nearly all parts of the body to the brain.
Inside the brain and spinal cord (the central nervous system), bundles of axons are called tracts, columns, or pathways rather than nerves. The distinction is purely about location. The axons themselves work the same way regardless of where they are.
How a Nerve Is Built
A peripheral nerve isn’t just a loose collection of axons. It has a precise layered architecture, somewhat like a cable with wires bundled inside protective casings. Three connective tissue layers give the nerve its structure and protection.
The outermost layer, called the epineurium, is a dense sheath that wraps around the entire nerve. Inside it sit blood vessels, immune cells, and multiple smaller bundles called fascicles. Each fascicle is wrapped by the perineurium, a middle layer made of about 7 to 8 concentric rings of tissue. This layer is elastic and resistant to mechanical damage, which helps protect the delicate fibers inside from stretching or compression. The innermost layer, the endoneurium, surrounds each individual nerve fiber. It contains a fine network of tiny blood vessels that deliver oxygen and nutrients directly to the axons.
This layered design means a nerve can absorb a surprising amount of physical stress without losing function. Even if some fascicles are damaged, others may continue working, which is why partial nerve injuries sometimes produce unusual patterns of weakness or numbness rather than complete loss of function.
Fascicles: Bundles Within the Bundle
Inside a nerve, individual axons aren’t randomly scattered. They’re organized into fascicles, smaller subgroups that cluster fibers heading to similar destinations. This arrangement is called somatotopic organization, and it means fibers serving a particular area of the body tend to travel together through most of the nerve’s length. If you could look at a cross-section of a nerve under a microscope, you’d see these fascicles as distinct circles, each wrapped in its own protective sheath.
This internal organization has real clinical significance. A partial injury to a nerve can produce very specific deficits, affecting sensation or movement in one area while leaving neighboring areas untouched, because only certain fascicles were damaged.
What the Axons Actually Do
Axons within a bundle carry signals in one of two directions. Sensory axons (afferent fibers) carry information from the body toward the spinal cord and brain, reporting things like touch, temperature, pain, and the position of your limbs. Motor axons (efferent fibers) carry commands from the brain and spinal cord outward to muscles and glands, telling them when to contract or release chemicals.
Many peripheral nerves are mixed, meaning they contain both sensory and motor fibers running alongside each other in the same bundle. The sciatic nerve, which runs from the lower back down through each leg, is a good example. It carries both the signals that let you feel your foot on the ground and the commands that move your leg muscles.
Speed Depends on Size and Insulation
Not all axons in a bundle transmit signals at the same speed. The two biggest factors are the axon’s diameter and whether it’s coated in myelin, a fatty insulating layer produced by specialized support cells. In the peripheral nervous system, cells called Schwann cells wrap around axons to form myelin. In the central nervous system, a different cell type called oligodendrocytes does the same job.
The speed difference is dramatic. Small, unmyelinated axons carry signals at roughly 1 meter per second, about walking pace. The largest myelinated axons can transmit signals faster than 100 meters per second, comparable to a race car. This is why you feel a sharp pain almost instantly when you touch something hot (carried by fast myelinated fibers) but then experience a slower, burning ache a moment later (carried by slower unmyelinated fibers).
Axon diameters within a single nerve vary considerably. In the sciatic nerve, for instance, axon diameters range from less than 1 micrometer up to about 20 micrometers, with the average sitting around 5 to 7 micrometers. Motor fibers that control quick muscle contractions tend to be among the largest, often 15 to 20 micrometers in diameter.
Blood Supply to Nerve Bundles
Nerves are living tissue and need a constant supply of oxygen and nutrients to function. A dedicated network of small blood vessels called the vasa nervorum runs along and through peripheral nerves, branching from larger arteries in the limbs. These vessels don’t just feed the nerve itself. They share their blood supply with surrounding bone, muscle, skin, and connective tissue.
This shared supply means that when blood flow to a region is severely reduced, the damage isn’t limited to the nerve. Muscles, skin, and other tissues suffer at the same time. Certain spots along nerves are particularly vulnerable to reduced blood flow because they sit at the far edges of overlapping arterial supply zones. These “watershed zones” are where ischemic nerve damage is most likely to occur, similar to how droughts hit areas farthest from the water source first.
What Happens When a Nerve Bundle Is Damaged
One of the remarkable properties of peripheral nerve bundles is their ability to regenerate. Unlike axons in the brain and spinal cord, which have very limited repair capacity, peripheral axons can regrow after injury. The regeneration rate is relatively constant at about 1 millimeter per day, or roughly one inch per month. Clinicians can often track this regrowth by testing along the nerve’s path for a tingling sensation that advances over time as the regrowing axons extend further.
The practical implication is that recovery timelines depend heavily on how far the regrowing axons need to travel. An injury near the wrist might recover in weeks, while damage high up in the arm or leg could take many months, because the axons have to regrow the entire distance from the injury site to their target muscle or skin area. The protective connective tissue layers play a critical role in this process: when the endoneurial tubes remain intact, regrowing axons have a guide channel to follow, which dramatically improves the chances of reaching the correct destination.