Unlike muscles, which are highly elastic and designed to lengthen significantly, nerves are relatively inelastic and do not stretch in the same way. A nerve is an organized bundle of fibers, called axons, that functions as an electrochemical communication cable, transmitting signals between the brain, spinal cord, and the rest of the body. While nerves must adapt to the body’s wide range of motion, they achieve this dynamic motion primarily through movement relative to their surroundings rather than through internal elongation. This protective mechanism ensures the delicate axons within the nerve are shielded from excessive tension that could impair signal transmission.
How Nerves Move: Gliding and Sliding
Nerves accommodate the extensive movement of joints through a specialized process known as neurodynamics, which involves two primary mechanical actions: gliding and sliding. When a joint moves, the path the nerve must follow lengthens, and the nerve must travel through surrounding tissues to take up the resulting slack. Peripheral nerves, such as the median nerve in the arm, can glide or slide up to two centimeters relative to the adjacent muscle and connective tissue during a full range of joint motion.
Nerves initially utilize an internal slack mechanism, where the wavy or undulating path of the nerve fibers straightens out, taking up immediate tension. As the movement continues, the nerve slides longitudinally within its sheath, much like a rope being pulled through a tunnel, allowing it to move away from areas of increased compression or tension.
The physical movement of the nerve is not uniform along its entire length. Instead, movement is localized, with the strain being distributed unevenly, which is known as strain distribution. When one end of the nerve is pulled, say through wrist extension, the tension is absorbed over a longer segment of the nerve, protecting the most vulnerable points. In physical therapy, techniques like nerve flossing or nerve gliding are used to intentionally encourage this natural sliding motion to improve the nerve’s mobility and reduce irritation.
Structural Limits to Nerve Elongation
The internal structure of a nerve provides its limited ability to withstand tension and defines its maximum tolerance for elongation. The axons are bundled and protected by layers of connective tissue sheaths that provide mechanical strength. The outermost layer, the epineurium, is a thick, fibrous sheath that surrounds the entire nerve, acting as the primary tensile load-bearing structure.
Beneath the epineurium, the perineurium wraps bundles of axons into groups called fascicles, providing a tough, protective barrier that maintains a stable internal environment. The innermost layer, the endoneurium, surrounds each individual axon. These tough, collagen-rich layers are what resist the actual stretch, meaning the nerve’s ability to elongate is finite.
The true physiological limit to nerve elongation is not mechanical rupture but the compromise of the nerve’s blood supply, a condition called ischemia. Experimental studies have shown that an elongation of approximately 8% past the nerve’s resting length can reduce intraneural blood flow by up to 50%. Once the nerve is elongated by about 15%, the blood flow can cease completely, leading to a lack of oxygen and nutrients that damages the axons and impairs nerve function.
Consequences of Excessive Tension and Entrapment
When the structural limits of the nerve are exceeded, or when its normal gliding motion is restricted, the consequences can range from temporary dysfunction to permanent injury. Excessive tension, whether from a traumatic stretch or chronic strain, can physically damage the axons and their protective myelin sheaths. The loss of blood flow due to elongation or compression is the direct cause of nerve pathology, with prolonged ischemia leading to structural degradation.
A more common issue than acute stretch is nerve entrapment, or compression neuropathy, which occurs when surrounding tissues chronically squeeze the nerve. Conditions like Carpal Tunnel Syndrome or sciatica are examples where a nerve is trapped in a tight anatomical space, preventing its necessary gliding and sliding motion. This restriction increases internal tension and causes localized pressure, which in turn reduces blood flow to the affected segment of the nerve.
Symptoms of nerve distress due to excessive tension or compression typically manifest as sensory disturbances. These include numbness, a “pins and needles” tingling sensation called paresthesia, or a sharp, burning pain. If the nerve contains motor fibers, chronic tension can eventually lead to muscle weakness in the area supplied by that nerve.