Peripheral nerve · Lobeworks/17

A peripheral nerve is a cable of axons outside the brain and spinal cord, bundled and sheathed in three layers of connective tissue, that carries sensation in from the body and commands out to muscles and glands; the nerves together make up the peripheral nervous system, against the central nervous system of brain and

Peripheral nerve. A peripheral nerve is a cable of axons outside the brain and spinal cord, bundled and sheathed in three layers of connective tissue, that carries sensation in from the body and commands out to muscles and glands; the nerves together make up the peripheral nervous system, against the central nervous system of brain and spinal cord.

Its build is nested like an electrical cable. Each axon, with its myelin from a Schwann cell, lies in a thin tube of connective tissue, the endoneurium. Groups of these fibres form fascicles, each wrapped in the perineurium, several layers of flattened cells that also act as a diffusion barrier. The epineurium binds the fascicles together with the blood vessels that feed them into the nerve seen in an operation. A single nerve usually mixes sensory and motor fibres, myelinated and bare, of very different speeds.

The central nervous system has none of these sheaths. Brain and cord are protected instead by bone, the meninges and the cerebrospinal fluid they float in, and their axons are myelinated by oligodendrocytes rather than Schwann cells.

A cut peripheral nerve can regrow. The stretch beyond the cut degenerates, and its Schwann cells line up into tubes inside the old endoneurium that guide new sprouts from the stump, at roughly a millimetre a day, so a nerve cut at the elbow takes many months to reach the hand. Cut tracts in the brain or cord do not regrow on their own, because oligodendrocyte myelin and the scar around a central lesion inhibit the growth that Schwann cells support.

Getting the fascicles to line up matters. Surgeons repair a nerve by stitching the sheaths end to end, and a motor fibre that grows into a sensory tube reaches the wrong target, which is part of why recovery is often incomplete.

A nerve is a cable in three jackets, and its insulation can rebuild.

The same Schwann cells that myelinate each fibre become the guide rails along which a cut fibre grows back, something the central nervous system cannot do.

Questions: What separates the central from the peripheral nervous system, and how is each protected? The central nervous system is the brain and spinal cord; the peripheral nervous system is every nerve outside them. They are protected in different ways. Peripheral nerves are cables in three connective-tissue jackets: endoneurium around each fibre, perineurium around each bundle and epineurium around the whole nerve. Brain and cord have none of those jackets: they sit inside bone, wrapped in the meninges and floating in cerebrospinal fluid, behind the blood-brain barrier. Why does a cut nerve in the arm regrow while a cut tract in the spinal cord does not? The difference lies largely in the glia. In a peripheral nerve, the Schwann cells beyond the cut clear the debris and line up into tubes inside the old sheaths, and they guide sprouts from the stump along the old path at roughly a millimetre a day. In the brain and cord, myelin is made by oligodendrocytes, whose myelin carries proteins that stop axon growth, and the scar that forms around a central injury blocks it further, so cut central axons do not cross the gap on their own.