Nerve conduction velocity indicates the speed at which electrical stimuli are transmitted along a nerve fiber. By measuring nerve conduction velocity, nerve function can be checked and diseases affecting the nervous system can be diagnosed. The speed of transmission of electrical impulses is calculated by the distance between two points and the time required.
What is nerve conduction velocity?
Nerve conduction velocity indicates the speed at which electrical stimuli are transmitted along a nerve fiber. Nerve conduction velocity (NLG) describes the speed at which electrical impulses are transmitted along a nerve fiber to the brain. The average conduction velocity of human nerves is in the range of 1 to 100 meters per second. How fast the nerves pass on the electrical impulses depends, among other things, on their composition. Thick axons surrounded by a medullary sheath conduct stimuli faster than thinner fibers or axons without a medullary sheath. In principle, however, every nerve fiber is conductive. This is already evident from their physical composition: within the nerve fiber membrane (axolemm), an insulating sheath, there is a conductive salt solution (electrolyte). Through this electrolyte, electrical impulses are inevitably transmitted along the nerve fiber. However, the membrane of the nerve fiber does not completely insulate and the salt solution inside has a high electrical resistance. Therefore, a natural voltage drop occurs along a nerve fiber during the transmission of electrical impulses. For this reason, the distance for the transmission of nerve impulses is limited, and action potentials are additionally transmitted passively (by a change in ion permeability) along a nerve.
Function and task
Nerves have the function of either transmitting stimuli from the environment to the brain or transmitting commands from the brain to the muscles. For this to occur without interference, the speed of transmission of such stimuli must be correct. Nerve conduction velocity is distinguished between two different types: The speed in the sensory nerves and in the motor nerves. In addition to these two types, vegetative nerves also exist. The respective nerve conduction velocity can be measured by electroneurography (ENG). Motor nerves are responsible for controlling movements. For this purpose, 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 is perceptible to the patient at most by a slight tingling or pulling sensation. 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. Sensory nerves, on the other hand, transmit stimuli perceived by the human sensory organs (for example, touching an object with the skin) to the brain. To measure the conduction velocity of the sensitive nerves, no electrical stimulation is necessary. Otherwise, the measurement of the sensory nerve conduction velocity follows the same principle as that of the motor nerve conduction velocity. The principle of nerve conduction also applies to the central nervous system in the brain and spinal cord. The axons located in the brain are all myelinated, i.e. surrounded by a myelin sheath. This is the only way that groups of nerve cells can be synchronized even over a relatively large distance, since myelinated nerves have a higher conductivity. Conversely, myelination of axons in the brain is the prerequisite for higher cognitive processes and is thus present only in more highly developed organisms.
Diseases and disorders
Because healthy nerves respond differently than damaged ones, measurement of nerve conduction velocity can provide information when some different diseases are suspected. The method for diagnosing neuronal damage by measuring conduction velocity is called electroneurography (ENG). In addition to nerve conduction velocity, this also measures amplitude and refractory period. Electroneurography can, for example, provide information on whether a herniated disc requires surgical treatment.This method is also used in the case of injury to a single nerve, for example, due to entrapment. Even after a period of alcohol abuse, electroneurography is often used to examine the condition of the nerves and the extent of their damage. Especially often the measurement of nerve conduction velocity is performed when polyneuropathy is suspected. In this disease, several nerves of the peripheral nervous system are affected, both sensitive and motor, as well as autonomic. In the affected nerves, there is usually a disruption of the insulating myelin sheath of the nerve itself or its process (axon). In the course of polyneuropathy, sensory disturbances or muscle weakness occur. The causes of the disease are usually deep-seated and can range from deficiencies or poisoning of the body to infectious diseases and cancer. Furthermore, polyneuropathy often occurs in patients as a result of diabetes mellitus. Measurement of nerve conduction velocity can also provide information in carpal tunnel syndrome. In this syndrome, the median nerve is pinched in the wrist because the carpal canal offers too little space. As a result, there is numbness or tingling in parts of the hand to pain and muscle atrophy in the ball of the hand. In carpal tunnel syndrome, ENG can also clarify whether surgical intervention is necessary.
