Nerve conduction velocity

The Nerve conduction velocity indicates the speed at which electrical stimuli are transmitted along a nerve fiber. By measuring the nerve conduction velocity, nerve functions can be checked and diseases affecting the nervous system diagnosed. The speed of transmission of electrical pulses is calculated by taking the distance between two points and the time required.

What is the nerve conduction velocity?

The nerve conduction velocity (NLG) describes the speed with which electrical impulses are transmitted along a nerve fiber to the brain. The average conduction speed of human nerves is in the range of 1 to 100 meters per second. How fast the nerves transmit the electrical impulses depends, among other things, on their nature. Thick axons surrounded by a medullary sheath transmit stimuli more quickly than thinner fibers or axons without a medullary layer.

In principle, however, every nerve fiber is conductive. This already results from their physical nature: Inside the nerve fiber membrane (axolemm), an insulating covering, there is a conductive salt solution (electrolyte). Through this electrolyte, electrical impulses are inevitably transmitted along the nerve fiber.

However, the nerve fiber membrane does not completely isolate and the saline solution inside has a high electrical resistance. As a result, there is a natural voltage drop along a nerve fiber during the transmission of electrical impulses. For this reason, the distance for the transmission of nerve impulses is limited and the action potentials are additionally passed passively along a nerve (by changing the ion permeability).

Function & task

Nerves have the function of either transmitting stimuli from the environment to the brain or transmitting commands from the brain to the muscles. So that this can happen without interference, the speed of the transmission of such stimuli must be right.

There are two different types of nerve conduction velocity: The velocity in the sensory and in the motor nerves. In addition to these two types, there are also vegetative nerves. The respective nerve conduction velocity can be measured with the help of electroneurography (ENG).

Motor nerves are responsible for controlling movements. To do this, they transmit stimuli from the brain to the corresponding muscles. The conduction velocity of the motor nerves is measured by two electrodes on the surface of the skin, which are placed directly over the corresponding nerve. The nerve is then stimulated several times by a weak electrical impulse. This can only be perceived by the patient as a slight tingling sensation or pulling. The speed of the stimulus transmission can be calculated from the distance between the electrodes and the time it took for the impulse to cover this distance.

Sensitive nerves, on the other hand, transmit stimuli that are perceived by the human sensory organs (e.g. touching an object with the skin) to the brain. No electrical stimulation is necessary to measure the conduction velocity of the sensory nerves. Otherwise, the measurement of the sensible nerve conduction velocity is based on the same principle as that of the motor.

The principle of nerve conduction also applies to the central nervous system in the brain and spinal cord.The axons in the brain are all myelinated, that is, surrounded by a medullary sheath. Only in this way can groups of nerve cells be synchronized over a relatively large distance, since myelinated nerves have a higher conductivity. Conversely, the myelination of the axons in the brain is the prerequisite for higher cognitive processes and is therefore only present in more highly developed living beings.

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Illnesses & ailments

Since healthy nerves react differently than damaged nerves, a measurement of the nerve conduction velocity can provide information if a number of different diseases are suspected. The method for diagnosing neuronal damage by measuring the conduction velocity is called electroneurography (ENG). In addition to the nerve conduction velocity, this also measures the amplitude and the refractory period. For example, electroneurography can provide information about whether a herniated disc needs to be treated surgically.

This method is also used when a single nerve is injured, for example by pinching. Even after a period of alcohol abuse, electroneurography is often used to examine the condition of nerves and the extent to which they have been damaged.

The measurement of the nerve conduction velocity is carried out particularly frequently when polyneuropathy is suspected. This disease affects several nerves of the peripheral nervous system, both sensory, motor and vegetative. In the affected nerves, there is usually a disorder of the insulating myelin sheath of the nerve itself or its process (axons). In the course of polyneuropathy, sensory disturbances or muscle weakness occur. The causes of the disease are usually deep and can range from deficiency or symptoms of intoxication in the body to infectious diseases and cancer. Furthermore, as a result of diabetes mellitus, the patient often develops a polyneuropathy.

A measurement of the nerve conduction velocity can also provide information in the case of carpal tunnel syndrome. In this syndrome, the median nerve is trapped in the wrist because the carpal canal does not offer enough space. The result is numbness or tingling in parts of the hand through to pain and muscle breakdown in the heel of the hand. In the case of carpal tunnel syndrome, too, ENG can clarify whether a surgical procedure is necessary.