Peripheral Nerves Explained: Structure, Function and Signals

Understanding how a peripheral nerve is built makes a great deal of otherwise confusing symptom information fall into place.

The basic structure

A peripheral nerve is a bundle of axons - long projections from neuron cell bodies - wrapped in connective tissue layers. Many axons are insulated by myelin, produced by Schwann cells, which allows saltatory conduction where the signal jumps between gaps in the sheath rather than travelling continuously. This dramatically increases conduction speed. Unmyelinated fibres conduct more slowly and typically carry dull pain, temperature and autonomic signals.

Three functional types

Sensory fibres carry information inward: touch, vibration, position sense, temperature and pain. Motor fibres carry commands outward to muscles. Autonomic fibres regulate involuntary functions including sweating, blood vessel tone, heart rate and digestion. A given nerve often contains all three, which is why nerve problems can produce sensory symptoms, weakness and autonomic changes such as altered sweating in the same region.

Why symptoms usually start distally

The longest axons are the most metabolically demanding to maintain, because everything the axon needs must be transported from the cell body along its entire length. Where a systemic process affects nerve health, the longest fibres tend to be affected first. Since the longest sensory axons run to the feet, this produces the characteristic distal, symmetrical, stocking-then-glove pattern seen in many generalised neuropathies. Any new or progressive symptom of this kind warrants medical assessment.

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