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PH-NE16

Peripheral nerve disease

How peripheral nerves fail by losing axons or myelin: why metabolic neuropathies start in the feet, how immune attack strips myelin in Guillain–Barré syndrome, how pressure harms the median nerve, and how nerve conduction tells them apart.

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  • 6 steps · 10 questions
  • 3D anatomy, affected parts marked

By the end, you can

  • Describe the normal peripheral nerve: its sheaths, its myelinated and unmyelinated fibres, Schwann cells and the work of keeping a long axon alive.
  • Contrast axonal degeneration (Wallerian and dying back) with segmental demyelination, and relate each to its signs and its recovery.
  • Explain how diabetes, vitamin B12 deficiency and alcohol damage nerves, and why symptoms start in the feet.
  • Explain the immune attack in Guillain–Barré syndrome and CIDP, and the inherited demyelination of Charcot–Marie–Tooth disease.
  • Explain how compression of the median nerve in the carpal tunnel produces its symptoms and signs.
  • Interpret nerve conduction and the other key tests, and explain how the main treatments act on each mechanism.

What it is

Peripheral nerve disease (peripheral neuropathy) is damage to the nerves outside the brain and spinal cord: the motor, sensory and autonomic fibres running from the spinal roots to their targets. Nerves fail in two main ways: the axon degenerates (axonal neuropathy) or its myelin sheath is lost (demyelinating neuropathy). By distribution, disease may affect one nerve (mononeuropathy), several separate nerves (multiple mononeuropathy) or many nerves symmetrically (polyneuropathy). Disease of the spinal roots (radiculopathy) has its own lesson.

4 types to tell apart
Axonal neuropathies
The axon degenerates, usually from its far end: most toxic, metabolic and nutritional neuropathies (diabetes, alcohol, drugs) and CMT2. Recovery needs slow regrowth.
Demyelinating neuropathies
Myelin or Schwann cells are attacked while axons survive: Guillain–Barré syndrome (in its commonest form), CIDP and CMT1. Conduction slows or blocks; remyelination can bring quick recovery.
Entrapment and other mononeuropathies
Pressure where a nerve passes a tight spot (the carpal tunnel, the elbow, the head of the fibula), injury, or infarction of a single nerve.
Multiple mononeuropathy (mononeuritis multiplex)
Separate nerves fail one after another, often because vasculitis infarcts them; diabetes can do the same.

US spelling: fibres → fibers, fibre → fiber.

Normal function

The peripheral nerve and its long axons

A peripheral nerve is a cable of axons in three wrappings: the round the whole nerve, carrying its blood vessels (the ); the round each fascicle, a barrier rather like the blood–brain barrier; and the delicate round each fibre. Each axon is an extension of a cell body that lies in the spinal cord (motor neurones) or in a dorsal root ganglion (sensory neurones).

Large axons are wrapped in myelin made by Schwann cells: each Schwann cell, its nucleus visible as a , makes one segment (internode) of on one , and the short bare gaps between segments are the nodes of Ranvier. The impulse jumps from node to node (saltatory conduction), reaching about 50 metres a second or more in the largest fibres. These large fibres carry movement commands, touch, vibration and joint position sense; thin, lightly myelinated and unmyelinated fibres carry pain, temperature and autonomic signals more slowly.

An axon to the foot can be a metre long, yet its proteins and mitochondria are made in the cell body. Axonal transport along microtubules carries them down the axon and brings signals back, using a great deal of energy, and the longest axons have the most to supply.

Point to it on the model