Electromyography

The electromyography (or tomography ) ( EMG) is an electrophysiological method in neurological diagnosis, in which the electrical muscle activity is measured.

By means of concentric needle electrodes can be the potential fluctuations of single motor units derived. With special needles also individual muscle fibers can be recorded ( Einzelfasermyografie ). Also, measurements of potential changes on the skin surface electrodes are possible, but much less precise because this technique measures the sum of a whole muscle action potential or even several muscles.

The main application is the recognition of myopathies and neuropathies, that is the process of determining whether a disease muscular and / or has nervous causes.

• 2.1 Miniaturendplattenpotentiale
• 2.2 fibrillation potentials and positive waves
• 2.3 Faszikulationspotentiale
• 2.4 myotonic discharges

Generally

When performing an EMG electrical activity in the resting muscle ( spontaneous activity) and at different degrees arbitrarily contracted muscle ( muscle action potentials) is measured.

In medical diagnostics, electrical leave by the EMG statements about diseases of nerve and muscle cells do. Biomechanics in the relationships between the frequencies and the amplitudes of the recorded electrical signals and the power of a muscle to be examined in order to optimize for example, the movements of sportsmen.

The recording of nerve action potentials is called electroneurography ( ENG). It is usually subsumed under the generic term electromyography.

Signal derivation

Depending on the mechanical dimensions of the electrodes used can be a high spatial resolution and statements of individual muscle fibers or a low spatial resolution and summary statements about whole muscle groups achieve. For the derivation of the electrical signals of the muscle used for routine diagnostic EMG in neurology concentric needle electrodes. These needles consist of a central conductor (wire) around which an electrically insulating layer is applied, then around the outer metal shell ( tube) is fitted. A typical EMG needle is 50 mm long, an outer diameter of 0.45 mm and at the top has an oblique bevel which is insulated from the ground section of the oval metal sleeve the center conductor. The potential measurement is between center conductor and metal sheath. With this structure, especially near peak potential changes essentially are recorded up to a distance of 1-2 mm. For a very specific diagnosis is possible. When Einzelfaserelektromyografie with special needles the detected half radius is even only 0.2-0.3 mm. For special purposes monopolar electrodes are used. These electrodes consist of a mostly Teflon insulated needle whose metal is exposed only at the tip. The voltage is measured against a separate reference electrode (usually surface electrode on the skin). Surface electrodes detect already due to their mechanical dimensions of electrical activity to a few centimeters and beyond, but can not on the other hand reflect the activity of single motor units. Such electrodes are used for example in the sports physiology when it comes to determine the timing of the onset of muscle contraction.

Reinforcement

The potential source of EMG is the membrane potential of the muscle cell, inside -70 mV compared to the outside. Upon excitation of a muscle cell, for example about the motor end plate to ion channels open, leading to a brief (approximately 1 ms) and local reversal of the membrane potential. This change in potential is measured. Which potential difference of the input amplifier of the electromyograph detected thereby, depends on a large number of physical and physiological and ultimately pathological factors: the distance of the electrode from the signal source ( attenuation to the third power of the distance ), of the number and type of simultaneously active muscle fibers, the spatial arrangement of conducting and insulating tissue components (blood conductor = good conductors, fat = bad conductor ) and many other factors more. The result is a very complex electric field. The measurement result is determined by both capacitive and ohmic resistances. In the result, a signal is measured in a typical measuring arrangement for a medical neurological EMG, detects the potential in the range of 50 microvolts to several millivolts. The discharges of single motor units then present themselves as potential fluctuations of about 10 ms duration and contain frequency components up to a few kilohertz. There are electrically low-noise, voltage-free instrumentation amplifier with very high input resistance used (typically: 200 MOhm input impedance, noise level ( noise) 0.7 microvolts RMS, common mode rejection ratio> 100 dB).

But if, for example, in the exercise physiology not the internal structure of the muscular excitation to be detected, but rather the rough process of a muscle twitch, the EMG signal is often rectified and filtered electrically with a low pass. This results in an "integrated " and produces " smoothed " signal with a correspondingly low spatial and temporal resolution. The rectification is, however, a number often not true basic assumptions (it is just not a simple sine curve wave alternating voltage signal, such as the supply voltage). The result should therefore be interpreted with caution.

Documentation history

The representation of the EMG signal was initially on soot-blackened, rotating drums, the mechanical hands a track recorded (similar to seismology ). Later oscilloscope and used to store magnetic tape devices. Since the establishment of the wider digital storage technology standard PCs or laptops are used with color screens and appropriate printer.

Spontaneous activity

With complete relaxation of skeletal muscle no nerve action potentials pass over the nerves supplying the muscle. The muscle is flaccid. The muscle fibers of a healthy skeletal muscle does not receive excitatory nerve pulses, then show a constant membrane potential, which presents itself on the screen as a straight horizontal line undeflected. However, under certain, usually morbid conditions it comes to spontaneous activity, ie, to self-activity of the muscle fibers without initiation by causing nerve pulses. This spontaneous activity manifests itself in various forms, which can be distinguished in terms of frequency and amplitude values ​​. To separate from the spontaneous activity is the injection activity. It is generated with the use of needle electrodes and is explained by a temporary mechanical irritation of the muscular cell.

Miniaturendplattenpotentiale

Miniaturendplattenpotentiale, also called Endplattenrauschen, to have negative amplitudes of less than 50 microvolts and take 0.5 to 2 ms. They are formed at the contact points between axon and muscle cell by spontaneous transmitter release, but do not induce continuous -led action potential on the muscle cell membrane. You therefore do not indicate muscle damage. Endplattenbpotentiale take about 20 ms and are always above the threshold.

Fibrillation potentials and positive waves

Fibrillation potentials and positive waves arise in individual muscle cells and are showing a lack of innervation ( denervation ). Fibrillation take 1-5 ms, amplitudes up to several 100 microvolts, are mostly bi-or triphasic and occur strictly rhythmically (as opposed to discharge frequency modulated sound of motor units ).

Positive steep waves to take about 4 ms, have amplitudes up to several 100 microvolts, are biphasic with a characteristic shape and occur rhythmically at a frequency between 3 and 50 Hz.

Faszikulationspotentiale

Faszikulationspotentiale are generated by a motor unit. The origin of the excitement lies in the serving neuron. When damage to the innervating neuron can cause a depolarization of the nerve cell membrane, which reached as a continuous -led action potential the motor unit. The place of origin may far from the cell soma in the spinal cord ( motor neuron ) are or even distal to the terminal branches of the axon to the individual muscle fibers. Faszikulationspotentiale occur irregularly ( for example, every 1-30 s ) on. Fasciculations also point to a neuropathy.

Myotonic discharges

Myotonic discharges are high-frequency spike trains ( 60-150 per second) with a duration of one to two (or three) seconds and an amplitude of 10 microvolts up to 1 mV. They indicate a malfunction of the muscle membrane, the ion channels are damaged.

Action potentials of motor units ( MUAP )

Each nerve fiber branching after entering the muscle into several terminal branches that innervate each muscle fiber of the motor endplates. A single, continuously -led action potential of a single motor unit therefore resolves in several muscle fibers almost simultaneously from a depolarization ( action potential ) and, following a contraction. The sum of the depolarizations of a motor unit can be observed in the EMG as a characteristic rash on the monitor. The height of the rash are doing a rough measure of the number of innervated muscle fibers (large versus small motor unit ). The duration of the potential may provide clues as to whether all muscle fibers discharge synchronously or if there are delays in the terminal branches. In the evaluation of parameters such as age, type of muscle and muscle group must be observed. The results are compared to standard values ​​in order to make statements about possible diseases. The standard value for the duration in middle age of 8 to 10 ms and the amplitude of 1-3 mV. A special case of MUAP - examination is the interference pattern analysis, in which one measures the electrical activity of a voluntary maximum contracted muscle.