Motor evoked potentials
Motor evoked potentials ( MEP for short ) are electrical voltage changes in a muscle that are triggered by a targeted stimulus on a part of the motor system. MEP are therefore not triggered by a "motor" movement, so the term is misleading, but rather by a stimulus in the motor system.
properties
Motor evoked potentials are a special form of evoked potentials . In neurology, they open up possibilities for diagnosing the central motor system in the spinal cord and brain and complement the neurographic diagnostic possibilities in the peripheral motor system. Latency extensions can indicate damage in myelination . The amplitude and shape provide information on the number and distribution of the structures. The evaluation is usually carried out in a side-by-side comparison.
By transcranial magnetic stimulation of the motor cortex can be in humans and some higher mammals targeted a muscle twitch for example in the leg muscles trigger. Which target muscle contracts depends on which part of the primary motor cortex is stimulated. The excitation runs from the motor cortex in the direct corticospinal pathway, an important part of the voluntary motor system ("pyramidal pathway") to the motor neurons in the anterior horn of the spinal cord. From there it is conducted to the muscle in the efferent nerves and triggers a contraction. Diseases of the motor cortex as well as the corticospinal tract, but also of the motor neurons in the spinal cord and the peripheral motor nerves lead to characteristic changes in the measurable MEP. Therefore, MEP are used in diagnostics such. B. multiple sclerosis , amyotrophic lateral sclerosis (ALS) but also used in stroke and spinal cord diseases.
Muscle potentials that have been triggered by magnetic stimulation in other parts of the motor system are also referred to as MEP. Magnetic stimulation on the side of the lumbar spine can be used to stimulate individual nerve roots and measure the response in the muscles of the leg. This gives evidence of a disease z. B. the leg nerve plexus ( plexus lumbosacralis ).
Magnetic stimulation can also be used in places in the peripheral nervous system where classic electrical stimulation ( electroneurography ) cannot be used or would be too painful. For example, by stimulating the motor facial nerve ( facial nerve ) at the exit from the brain stem and measuring the twitching of the facial muscle in facial palsy, damage in the facial canal can be detected.
Development: Motor evoked potentials can in principle be triggered by any suitable stimulus. Routine diagnostics in neurology usually use electrical (electroneurography) or magnetic stimuli.
Time course: The runtime ( latency ) between the stimulus and the response of the muscle depends, among other things, on the structural and functional distance between the stimulus location and the measurement location, the conduction speed of the nerves involved and the functional state of the neurons in the switching stations. Typically, the latency between the stimulus in the motor cortex and the response in a leg muscle is 14-16 ms.
Measurement: The motor evoked potentials are measured as muscle action potential (see electroneurography ). The locations (sorted by stimulation object) are in the cortex, the facial nerve, the peripheral nerve and the spinal nerve root .
Individual evidence
- ^ Rüdiger Kramme: Medical technology. Springer Verlag, Berlin 2011. ISBN 9783642161872 . P. 189ff.
- ^ Peter Vogel: Course book Clinical Neurophysiology: EMG - ENG - Evoked Potentials. Georg Thieme 2011. ISBN 9783131594235 . P. 178ff.