Peripheral nerves are highly organized structures that facilitate communication between the central nervous system (CNS) and the body’s muscles, skin, and organs. These nerves are composed of bundles of nerve fibers, also called axons, that are essential for transmitting motor, sensory, and autonomic signals. Their unique anatomical design allows for efficient signal conduction while providing protection and support to the delicate nerve fibers. Understanding the detailed anatomy of peripheral nerves is essential for comprehending the mechanisms of peripheral nerve injury (PNI), particularly in the context of operative care.
Structure of Peripheral Nerves
Peripheral nerves are made up of bundles of parallel axons that vary in diameter, typically ranging from 0.3 to 22 µm. These axons are the primary functional units of the nerve, responsible for conducting electrical signals between the CNS and peripheral tissues. The axons are enclosed by layers of connective tissue, which provide both structural integrity and protection. The connective tissue layers of a peripheral nerve include the endoneurium, perineurium, and epineurium:
Endoneurium: The endoneurium is the innermost layer of connective tissue that surrounds individual axons. It is composed of a delicate network of collagen fibers and capillaries that provide the necessary nutrients and support to maintain the axon's function. This microenvironment is essential for the nerve's metabolic activity, as the endoneurial capillaries, together with the perineurium, form the "blood-nerve barrier," regulating the exchange of substances between the bloodstream and the axons.
Perineurium: Axons are grouped into bundles known as fascicles, and the perineurium is a robust connective tissue layer that encloses each fascicle. The perineurium acts as a protective barrier, maintaining the internal environment of the nerve fascicles and providing tensile strength. This layer also plays a crucial role in preserving the nerve’s microvascular integrity, preventing damage from external mechanical forces, and maintaining homeostasis within the fascicle.
Epineurium: The outermost connective tissue layer, the epineurium, surrounds the entire nerve trunk, providing mechanical support and housing larger blood vessels that supply the nerve. It consists of loose connective tissue that allows the nerve to stretch and move with the body’s motions, protecting the more delicate fascicles inside. The epineurium is further divided into an internal and external layer, with the external epineurium surrounding the entire nerve and the internal epineurium separating the individual fascicles within the nerve.
Axonal Structure and Myelination
Peripheral nerves contain both myelinated and unmyelinated axons. The myelinated axons are covered by Schwann cells, which wrap around the axon in multiple layers to form a myelin sheath. This sheath is composed of a fatty substance that acts as an insulating layer, facilitating rapid transmission of electrical impulses along the nerve fiber. The gaps between these myelin segments, known as nodes of Ranvier, allow for the efficient propagation of action potentials through a process called saltatory conduction, enabling faster nerve signal transmission.
Unmyelinated axons, while lacking the myelin sheath, are still supported by Schwann cells, which encase multiple axons within a single Schwann cell. These axons generally conduct signals more slowly than myelinated axons but are essential for transmitting signals related to autonomic functions and pain sensation.
Blood Supply and Metabolic Activity
Peripheral nerves are metabolically active tissues that require a continuous and well-regulated blood supply to function correctly. The blood supply to peripheral nerves is provided by a delicate network of blood vessels known as the vasa nervorum. This network originates from larger arteries and enters the nerve through the epineurium, extending into the perineurium and endoneurium to reach the individual axons.
The vasa nervorum is crucial for maintaining the metabolic needs of the nerve fibers. Any disruption to this blood supply can lead to localized ischemia within the nerve, potentially causing nerve injury without affecting the entire structure. Because peripheral nerves rely on this intricate blood supply for oxygen and nutrients, they are highly susceptible to ischemic damage, which can occur due to prolonged pressure, compression, or other operative factors.
Fascicular and Nerve Organization
The organization of fascicles within a nerve varies along its length. Fascicles repeatedly divide, exchange fibers, and rejoin, so the fascicular pattern of a nerve changes every few millimeters, and motor and sensory fibers travel mixed within the same fascicles until the nerve approaches its target. This plexiform arrangement is one reason a focal injury can produce a patchy deficit and why repair of a transected nerve rarely restores perfect function.
Fascicles also vary in number and size with the nerve's distance from the spinal cord, and the pattern is the opposite of what intuition suggests. Proximal structures such as the roots and trunks of the brachial plexus contain few, large fascicles with little connective tissue between them (neural tissue makes up about half of the cross-section), whereas distal nerves in the limbs are polyfascicular, with many small fascicles cushioned in proportionally more epineurium (neural tissue closer to one third). This organization affects how nerves respond to injury. A nerve with many small fascicles and abundant epineurium tolerates a needle or a focal compression better, because the needle is more likely to pass between fascicles and a localized insult spares part of the nerve; a proximal nerve with a few large fascicles is more exposed, and injury there tends to affect a wider territory.
