Nerve conduction study

The nerve conduction velocity ( NCV ) indicates how fast electrical impulses along a nerve fiber are transmitted. This end - as defined in physics - the quotient of the local difference and the time difference made ..

3.1 brain function

Physical Basics

Physically, there is a nerve fiber from an insulating sheath (nerve fiber membrane ) and a conductive content ( saline). An applied voltage is therefore - propagated by electrodynamic laws - as with any electrical cables. However, since the nerve membrane is only incomplete insulator and the electrolyte has a relatively high electrical resistance compared with, for example, a copper wire, it has come rapidly along the nerve fibers in a voltage drop. As such, these nerve impulses can only be transmitted over a very short distance. The propagation of action potentials along a nerve is therefore carried out in addition by changing the ion permeability through voltage-dependent ion channels of the nerve fiber membrane. This is a relatively slow, active, i.e., Metabolism of energy-consuming process. In contrast to a metal conductor, which transmits pulses of the speed of electric fields near the speed of light, the speed of typical mammalian nervous between 1-100 m / s Due to the involved molecular structures, there is also a significant temperature dependence. Within a physiological range, the nerve takes around 1-2 m / s per degree Celsius.

Thickness of the axons and nerve

→ Main article: Cardiac Conduction

Thickness axons transmit with higher conduction velocities than thin due to the favorable ratio between the membrane surface () and conductive volume (). Invertebrates have thicker axons, higher animals and man have nerve fibers ( for example, the human arm and leg nerves to the large muscles ) with an additional passive insulation by myelin cells wrap around the axon (called myelin sheaths ). This allows a significant increase in nerve conduction can be achieved. Only on the exposed nerve fiber membrane components between the insulating myelin sheaths ( Ranviersche Schnürringe ) must ( saltatory conduction ) as in an amplifier chain the signal to be amplified active.

In other words, while the signal line in the form of a traveling " wave" of ion fluxes progresses in nerve fibers without myelin sheath through the cell membrane, these jumps in the more highly developed nerve fibers with myelin sheath of a gap this envelope to the next. This allows the nerve by about a factor of 20 compared to a unmyelinated axons of the same thickness up to 180 m / s can be increased.

These principles of nerve conduction apply to all nerves throughout the animal kingdom, regardless of whether they or peripheral nervous system (for example, in the arms ) run in the central (brain) if it the motor (activation of muscles) or the sensory nerve conduction ( feedback from sensations ) are used.

Classification of line speed according to Erlanger / Gasser

Nerve conduction in the central nervous system

Also in the central nervous system - the spinal cord and brain - to find the same principles. Especially long tracks are myelin Controlling ( here the myelin is formed, however, by Oligondendrozyten and not by Schwann cells). The measurement of conduction velocities is done with evoked potentials and magnetic stimulation.

Brain function

In the brain of the higher animals, all axons are myelinated. The failures of the nervous coordination, which are present in a pathological degeneration of the myelin sheaths ( multiple sclerosis), are evidence that the higher nerve conduction velocity of myelinated axons is essential for the synchronization of groups of neurons over long distances. As such, the myelination is also a prerequisite for higher cognitive processes of the brain. However, it is a common fallacy that cognitive processes (ie learning ) lead to an increase in nerve conduction velocity: Neither are learning the myelin sheaths changed, nor would any changes have a direct impact on the data processing speed of the brain [ source missing]. Neurophysiological changes in the brain during cognitive processes play rather to the synapses that mediate the signal transition from one nerve cell to another. This in turn synapses are devoid of myelin.

Measurement

The measurement of nerve conduction is a neurophysiological standard examination in neurology. Here, but not the nerve a single nerve fiber is measured, but the sum of the responses of all fibers of a nerve. By definition, this is the fastest detectable response for the determination of velocity is used. In reality, the fibers of nerves transmit at different speeds, what with appropriate analysis can provide additional diagnostic information.

The measurement is performed by means of electrical impulse initiation / readout, measured along a nerve.

A special case is the measurement of motor conduction time. Since the measurable voltage changes of a nerve to the skin surface are very small and therefore error- prone, is aided by the motor nerves in it, namely to irritate the nerves, but derive the response of the muscle. Because muscle with many muscular fibers have a much higher measurable voltage (factor 1:1000) provide, this is easily possible. However, in the time between stimulus and muscle response (latency) is not only the Nervenleitzeit, but also the transmission time on the muscle of the neuromuscular junction ( about 0.8 ms), and the conduction time at the muscle fiber membrane (a few m / s). The total time is called motor conduction time. Through the nervous stimulus in two different places at constant diversion location over the muscle but a 'real ' nerve can then be determined by difference.

Figure 2: Measurement of motor conduction time. Stimulation of the median nerve with two poles of a gold contact electrode to the skin surface at the wrist directly to the body of the carpal tunnel; is measured with an over the musculus abductor pollicis brevis ( thenar muscle) glued electrode and a reference electrode on the thumb. Be set for the appeal, for example, a rectangular pulse with a pulse duration of 200 microseconds and a current intensity of between about 3 and 20 mA, depending on the location of the stimulation and thickness and texture of fabric.

Figure 3: Shown are the results of two stimuli. The upper trace starts on the left at the time of the stimulus ( the beginning of the rectangular pulse ). From there, the beam travels with the set write speed of, for example, 5 ms / notch to the right. In part 1 line (corresponding to a latency of 5 ms) can be seen (shown opposite to the Convention, by default negative voltages in electrophysiology are presented to the top) the onset of electrical muscle response as a downward deflection. This is marked with the left marker. In the bottom track to recognize a similar rash, but a little later (right marker). Apparently, the stimulating electrode was put forth from the muscle to the nerve. The time difference between markers results in the nerve conduction time. The spatial difference of the stimulus points is measured, for example, with a tape measure. The quotient of place and time difference then gives the nerve.

Indication

A common indication for measurement of nerve conduction is a suspicion of polyneuropathy. In this disease there is a fault in the insulation of the nerves ( myelin ), and / or the nervous extension ( Axon ). As a result of a injury impaired nerve conduction is measurable. In carpal tunnel syndrome, the local pressure can damage the insulation of the wrist Medianusnerven so that the distal motor latency is significantly prolonged.