Motor and Sensory Neurography – How Nerve Function Is Tested

A nerve conduction study (neurography) checks whether nerves can properly carry electrical signals. Doctors distinguish between motor neurography, which looks at muscle control, and sensory neurography, which checks whether nerves still transmit sensations like touch or cold.

A Brief Look at the History

Even before World War II, doctors were able to measure nerve conduction for the first time. During the war, many soldiers suffered nerve injuries from bullets or shrapnel. Doctors observed that some soldiers had a visibly intact nerve, but the arm or leg was still paralyzed or numb at first. A small number recovered quickly (Grade 1: Neurapraxia). For most, recovery took much longer, even though the nerve still looked intact (Grade 2: Axonotmesis). The exact reasons were not fully understood back then. For others, the nerve was completely torn or severed (Grade 3: Neurotmesis). In these cases, it was clear why paralysis or numbness occurred.

Line drawing of a World War II field hospital with wounded soldiers and doctors providing treatment. — Many nerve injuries were treated in field hospitals during World War II – the Seddon classification is based on these observations.

To better describe these differences, the British surgeon H. J. Seddon developed an initial classification, which was later refined by Sunderland. This system was based on what surgeons could see during operations and what they could measure electrically at that time. Compared to the past, modern technology now allows us to see very detailed images inside the body without surgery. This imaging is important because the classification relies on seeing (imaging) and measuring (nerve conduction tests) – and both together enable a more reliable assessment and classification of the severity.

When Is Neurography Used?

A nerve conduction study helps check whether a nerve is damaged or compressed. It is often used for:

Nerve entrapments, such as carpal tunnel syndrome or cubital tunnel syndrome
Injuries after accidents, cuts or crush injuries
Inflammatory nerve diseases or nerve damage due to metabolic conditions (e.g. diabetes)
Monitoring recovery, to see whether a nerve is healing over time

Important to know: After an accident, doctors often wait 2–3 weeks before doing a nerve conduction test. This is because some nerve injuries (like axonotmesis) only become measurable after some time – this helps better assess the severity.

How Does the Test Work?

Small surface electrodes are placed on the skin. The nerve is stimulated with short, weak electrical impulses. At another point, or over the related muscle, the response is measured to see how quickly and strongly the signal arrives.

llustration of the electrodes used for nerve conduction studies (neurography / NCS / ENG). — Nerve conduction studies (neurography) are performed using a stimulation electrode and adhesive skin electrodes.

The test is safe. Some people feel slight tingling or a short muscle twitch – this can be uncomfortable for a moment but is usually well tolerated.

Example of motor testing of the median nerve

A specific nerve – for example, the median nerve, which runs through the arm into the hand – is briefly stimulated with a mild electrical impulse. This is done step by step at easily accessible points:

at the wrist,
in the elbow crease,
and at the inner upper arm.

Neurography motor test or motor nerve conduction study for the median nerve. — The left arm is shown without muscles. For the median nerve, stimulation is applied step by step at the wrist, elbow, and inner upper arm. This allows the nerve conduction speed and response to be measured.

The response to this stimulus is recorded using small adhesive electrodes placed on the skin. This may cause a slight muscle twitch – completely normal and harmless.

What Can the Test Show?

Neurography shows whether a nerve transmits signals normally. It can show if the conduction speed is slowed down or if the signal is weaker. This helps to find out whether a nerve is just compressed or more severely damaged.

Limits of Neurography

As useful as it is, nerve conduction testing also has clear limitations:

It is a functional test, not an imaging test. Whether a nerve is partially torn or completely cut cannot be reliably seen from this test alone. Ultrasound or MRI is usually needed as well.
Especially for sensory conduction, mainly the A-delta fibers (fast fibers for touch or cold) are measured. Pain fibers (C-fibers) are not directly tested – so the test does not show whether a nerve is causing pain.
Very small skin nerves, which branch into tiny parts near the body surface, are often difficult to measure with standard methods.
The central parts (spinal cord and brain) are not tested with neurography. For this, special SSEPs (somatosensory evoked potentials) can be used to check whether signals travel all the way to the brain.

Summary

Neurography is a valuable test to check if a nerve is working properly. It is safe, quick, and provides important clues – but it only shows function, not the nerve’s appearance. This is why it is usually combined with modern imaging, to help ensure an accurate diagnosis and a reliable assessment of the severity.