The peripheral nerves are made of bundles of nerve fibres, which can be regarded as living telephone wires. They are kept alive by their cell bodies. The cell bodies of the motor nerves lie in the spinal cord within the spinal column or around the base of the brain. The cell bodies of the sensory nerves are in bunches, called ganglia, connected to the nerve roots on the back of the spinal cord or brain stem.
The motor nerve cell body has a long fibre called an axon, which extends from the central nervous system to the muscles. The longest axons can be as much as a metre in length, for example the nerves to the muscles in the feet. The connection between the motor axon and the muscle fibre is a specialised nerve ending, which contains tiny packets of a chemical. The motor nerve stimulates the motor nerve ending to release the chemical and make the muscle fibre contract. If a peripheral neuropathy affects the motor nerves, the muscles become weak because they do not receive the messages to move.
The cell body of a sensory nerve has two axons. One goes into the spinal cord and delivers messages to the brain. The other come from specialised receptors in the skin, joints and muscles and transmits messages from them. The receptors sense changes in pressure, position, or temperature or pain. The receptor translates the stimulus into a nerve impulse. The sensory nerve fibres relay the impulses to the brain.
The fastest conducting nerve fibres are like telephone wires and have their own insulating sheaths. The sheaths are made of myelin, a fatty substance made by special cells, called Schwann cells. Nerve fibres conduct nerve impulses very quickly because the myelin sheath has gaps about every milimetre, which allow the nerve impulse to jump from gap to gap and travel faster. These fast conducting myelinated nerve fibres control rapid movement and allow fine touch discrimination. There are also many nerve fibres without myelin sheaths. These are called unmyelinated fibres and conduct nerve impulses more slowly. They signal pain and temperature and are important for the control of blood circulation and sweating.
Different types of peripheral neuropathy
Most types of peripheral neuropathy usually come on very slowly over several months or years, a clinical course called chronic. Sometimes a peripheral neuropathy comes on very rapidly over the course of a few days, which is called acute. Intermediate courses, about four to eight weeks, are called subacute.
A peripheral neuropathy often affects all the nerves more or less together. Because the longest nerves are the most vulnerable, the feet and then the hands are most affected. Such a symmetrical pattern, affecting the feet and hands more than the hips and shoulders, is called a symmetrical polyneuropathy (poly means many). If only one nerve is affected, the condition is called a mononeuropathy (mono means single). If several discrete nerves are affected, the condition is called a multiple mononeuropathy (the term ‘mononeuritis multiplex’ might also be used). Sometimes the nerve roots (the name for parts of the nerves next to the spinal cord) are also affected, which gives rise to a polyradiculoneuropathy (radiculo means root). Polyradiculoneuropathy occurs in the common form of Guillain-Barré syndrome (GBS) and in chronic inflammatory demyelinating polyradiculoneuropathy (CIDP).
A peripheral neuropathy usually affects sensory and motor nerve fibres together so as to cause a mixed sensory and motor neuropathy. Sometimes the autonomic nerve fibres are also affected. These control sweating, pulse, blood pressure, bladder, sexual and bowel function which may become affected. Sometimes a peripheral neuropathy just affects sensory nerve fibres, causing a pure sensory neuropathy. Finally the motor nerve fibres may be affected on their own, producing a pure motor neuropathy.
Nerve fibres may be damaged in four main ways:
- Most commonly, the delicate long axons lose their energy supply because of a chemical upset in the nerve cell body causing the axon to This is called an axonal neuropathy.
- Less commonly the problem lies in the insulating myelin This is called a demyelinating neuropathy.
- Vasculitis(inflammation of the blood vessels) may affect the nerves and cause a vasculitic
- Sometimes unusual chemicals or cells collect in the nerves and cause an infiltrative
Peripheral neuropathies do not affect the brain, vision, or the sense of smell. They almost never affect hearing and taste. Most sorts of peripheral neuropathy do not affect breathing or swallowing.