Nerve Conduction Velocity
Calculate nerve conduction velocity with v = d/t.
Learn how neurologists measure the speed of electrical signals along nerves.
The Formula
Nerve conduction velocity (NCV) measures how fast an electrical impulse travels along a nerve fiber. The formula is straightforward: divide the distance between two recording points by the time it takes for the signal to travel between them. Despite the simple math, this measurement is one of the most important diagnostic tools in neurology.
Normal nerve conduction velocities in humans typically range from 50 to 70 meters per second for motor nerves and 40 to 70 m/s for sensory nerves. These speeds vary depending on the nerve, the individual's age, body temperature, and whether the nerve fibers are myelinated. Myelinated nerves conduct signals much faster than unmyelinated ones because the myelin sheath allows the signal to jump between nodes of Ranvier in a process called saltatory conduction.
A nerve conduction study (NCS) is a clinical test where electrodes are placed on the skin over a nerve. A small electrical stimulus is applied at one point, and the response is recorded at another point a known distance away. The latency (time delay) is measured in milliseconds, and the distance is measured in millimeters or centimeters. Dividing distance by latency gives the conduction velocity.
Reduced conduction velocity is a hallmark of demyelinating conditions such as Guillain-Barre syndrome, chronic inflammatory demyelinating polyneuropathy (CIDP), and Charcot-Marie-Tooth disease. Axonal damage, on the other hand, tends to reduce the amplitude of the signal rather than the velocity. This distinction helps neurologists determine whether a nerve disorder affects the myelin sheath or the nerve fiber itself.
Temperature significantly affects NCV. Conduction velocity decreases by approximately 1.5 to 2.5 m/s for every 1 degree Celsius drop in temperature. This is why limbs must be warmed before testing to avoid falsely low readings.
Variables
| Symbol | Meaning |
|---|---|
| v | Nerve conduction velocity (m/s) |
| d | Distance between the two recording electrodes (m or mm) |
| t | Time (latency) for the signal to travel between the electrodes (s or ms) |
Example 1
A neurologist places electrodes 300 mm apart on a patient's median nerve. The signal takes 5.5 ms to travel between the points. Find the conduction velocity.
Convert units: d = 300 mm = 0.3 m, t = 5.5 ms = 0.0055 s
v = 0.3 / 0.0055 = 54.5 m/s
NCV = 54.5 m/s (within normal range of 50–70 m/s)
Example 2
A patient with suspected peripheral neuropathy has an ulnar nerve recording. The distance is 250 mm and the latency is 7.5 ms. Is the conduction velocity normal?
Convert units: d = 250 mm = 0.25 m, t = 7.5 ms = 0.0075 s
v = 0.25 / 0.0075 = 33.3 m/s
NCV = 33.3 m/s — significantly below the normal range, suggesting a demyelinating neuropathy
When to Use It
Nerve conduction velocity is used in clinical and research settings whenever the integrity of peripheral nerves needs to be assessed.
- Diagnosing carpal tunnel syndrome (compression of the median nerve at the wrist)
- Detecting demyelinating diseases like Guillain-Barre syndrome and CIDP
- Evaluating peripheral neuropathy caused by diabetes or other conditions
- Monitoring nerve recovery after injury or surgery
- Distinguishing between axonal and demyelinating nerve damage
- Research into nerve physiology and the effects of temperature or drugs on conduction