The Motor evoked potential Sample Clauses

The Motor evoked potential. The MEP is a compound action potential recorded from the skin overlying a muscle, arising ~20 ms after TMS stimulation of the motor cortex (Xxxxxx and Xxxxx, 2009). The amplitude of the MEP reflects the efficiency of horizontally oriented axons, corticospinal neurons as well as the ease with which motor neurons and muscle fibres can be depolarised, (Kapogiannis and Xxxxxxxxxx, 2008). For hand muscles, the optimum field orientation is perpendicular to the precentral gyrus and tangential to the skull. One of the intrinsic hand muscles such as the First dorsal interosseous (FDI) is chosen, for the reasons highlighted earlier, ie: its large and relatively superficial topographical representation in the motor cortex, (Xxxx et al., 1998) thus requiring a lower intensity. As the exact location of gyri and sulci differ between individuals, time must be taken to find the hotspot in each participant, defined as the point on the scalp at which stimulation produces optimal motor evoked potentials MEPs. Physiologically the fundamental process of stimulating the muscle can be seen in Figure 10 and involves stimulation of the motor cortical area with horizontally oriented axons (cortical interneurons) synapsing with pyramidal tract neurons. The axons leave the pyramidal cell bodies entering the white matter converging in the corona radiate. In the pons, the tract is broken into many bundles by the transverse pontocerebellar fibers which merge in the medulla to form the pyramids. At the junction between the medulla and the spinal cord, 75-90% of the fibres cross at the decussation of pyramids and reside in the lateral white matter of the spinal cord. The pyramidal tract neurons excite alpha motor neurons which, in turn innervate skeletal muscle when action potentials travel along axons between the spinal cord and the hand muscle, where these motor neurons release acetylcholine at the neuromuscular junction, stimulating the target muscle, culminating in a contraction. In healthy subjects, the number of motor units active in the muscle and hence the size of the MEP, varies according to certain stimulus parameters, including the intensity of stimulation. The MEP is intrinsically variable in morphology from one stimulation to the next despite all parameters remaining constant, thus it is important to collect multiple trials of data for this as with all TMS parameters. Alterations in TMS measures have been demonstrated in a number of conditions including epilepsy, (Xxxxxx et a...