Utilizing Transcranial Magnetic Stimulation to Study the Human Neuromuscular System

The overall goal of this procedure is to perform transcranial magnetic stimulation to elicit motor evoked potentials. First in the flexor carpi radialis muscle, and then in the erector spine, a muscles. This is accomplished by first preparing the skin overlying the respective muscles for electromyographic recordings and biomechanically positioning the subject for transcranial magnetic stimulation.

The hotspot is identified as the portion of the stimulation location over the motor cortex that elicits the largest motor revoked potentials. The next step is to determine the motor threshold. At this point, single pulse transcranial magnetic stimulation is performed to quantify motor evoked potential amplitude and silent period duration.

Paired pulse transcranial magnetic stimulation is also performed to quantify intracortical facilitation and short and long interval intracortical inhibition. Ultimately in vivo indices of corticospinal and cortico cortical excitability in humans can be assessed via combined use of transcranial magnetic stimulation and electromyography. Generally individuals new to this method may struggle because it can be difficult to derive and understand the subtle technical aspects of TMS from a written description only and simply attempting to learn through a trial and error approach when experimenting with and testing human subjects poses many difficulties and challenges demonstrating the procedures.

In addition to myself will be David Gus Jr.A researcher working within the Ohio Musculoskeletal and Neurological Institute. Before beginning the TMS experiment, first screen the subject following the screening guidelines set forth by the Institute for Magnetic Resonance Safety Education and research exclude individuals with a family history of epilepsy or seizures. Next place surface electrodes over the subject's FCR muscles on the forearm using a bipolar electrode arrangements.

Electrodes should be located longitudinally over the muscle unshaved and a braided skin. Also place a roc cap to mark stimulation locations on the head. Now have the subject sit with the left arm resting in an extended position in a Biodex system four dynamo meter.

In this protocol, we will stimulate the right side of the brain, which controls the left arm to stimulate the FCR muscles, place a 70 millimeter figure eight TMS coil, tangential to the scalp and 45 degrees to the midline so that the induced current flows in a lateral posterior to medial anterior direction. Next, subtly move the TMS coil around in very small increments to determine the stimulation location that elicits the largest motor evoked potential for the FCR muscle. Once located, mark this area with indelible ink on either the scalp or a like cap.

Now quantify the motor threshold for the FCR by delivering single pulses at gradually increasing stimulation intensities until motor evoke potentials have peaked to peak amplitudes greater than 50 microvolts in more than 50%of the trials. To examine the motor evoked potential amplitude of the FCR deliver a single TMS pulse to the previously determined hotspot as an intensity equal to 130%of motor threshold and calculate the peak to peak amplitude. This outcome can be normalized to the maximal compound muscle fiber action potential observed following supra maximal electrical stimulation of the median nerve.

When a CMS pulse of the cortex is delivered during muscle contraction, it will produce a motor revoked potential followed by electrical quiescence before activity resumes. That is indicative of cortico corticospinal inhibition. This is commonly referred to as the silent period.

To quantify the FCR corticospinal silent period, deliver a single TMS pulse to the hotspot at an intensity equal to 130%of motor threshold while the study participant is performing a wrist flexion muscle contraction at 15%of maximal strength. To determine intracortical facilitation using paired pulse TMS for the FCR muscle first determine the stimulus intensity needed to elicit a motor evoke potential that is between 0.5 to one millivolt. Next, deliver a sub-threshold conditioning pulse, 70%of motor threshold 15 milliseconds before a supra threshold test pulse a a conditioning pulse delivered at this time period prior to the test pulse will increase or facilitate the amplitude of the motor evoked potential more than a single uncondition pulse of the same intensity.

Next, use paired pulse TMS to quantify short interval intracortical inhibition using the same procedure as described for measuring intracortical facilitation, with the exception that the inter stimulus interval between the two pulses should be reduced to three milliseconds. The conditioning pulse delivered at this time period prior to the test pulse will decrease or inhibit the amplitude of the motor revoked potential more than a single unconditioned pulse of the same intensity. To quantify long interval intracortical inhibition using paired pulse TMS deliver two identical supra threshold test pulses that are separated by 100 milliseconds.

Here for the FCR muscle, the motor evoked potential associated with the second pulse will be smaller or inhibited more than that associated with the first.Next. For the erector spining muscle stimulation have subjects sit in a swivel based chair with the thigh at 90 degrees relative to the trunk, the lower leg at about 45 degrees relative to the thigh, the lumbar spine in a neutral upright posture and the hand resting in the lap for the erector spine. A muscles use an electrode arrangement located longitudinally over the muscles at the L three L five vertebral level on shaved and braided skin to comfortably locate the cortical erector spine, a muscle stimulation location.

Here we use anthropometric measurements to identify the vertex of the skull, find the intersection of the skull in the sagittal plane between the nasion and Ian and the coronal plane between the trauss and mark this as the vertex to stimulate the erector spin. A muscles use a double cone coil, which has greater penetration depth, and hence can reach the representation of these muscles deeper in the homunculus. Here, the coil is positioned such that the current flows in an anterior to posterior direction.

The coil here is custom modified with a laser attachment system to assist us in subsequent repositioning of the double cone coil to determine the motor threshold for the erector spining muscles. The previously demonstrated method for the upper limb should not be used as stimulation to the back muscles is not well tolerated. Instead, begin the TMS protocol by delivering an initial single pulse at 50%of the maximum stimulator output to determine if this stimulus intensity is above or below motor threshold.

To examine the motor evoked potential amplitude of the erector spining muscles deliver a single TMS pulse to the vertex at an intensity 40 or 50%above the sub motor threshold intensity. These motor evoked potentials cannot be normalized because peripheral nerves innovating the ES muscles are not accessible to electrical stimulation. It should be noted that when stimulating the ES muscle group, several other muscle groups are also visibly and dramatically stimulated concomitantly, including the muscles of the lower extremity, which are represented within the same general region of the homonculus.

To use paired pulse TMS to determine intracortical facilitation for the erector spining muscle group, the conditioning pulse intensity should be set to the observed sub motor threshold. Intensity being either 40%or 50%of stimulator output, and the test pulse intensity should be set to 40%above the sub motor threshold level being 80%or 90%of stimulator output. Next to quantify short interval intracortical inhibition use the same procedure as described for measuring intracortical facilitation, with the exception that the inter stimulus interval between the two pulses should be reduced to three milliseconds when the experiment is complete, assist with the removal of the electrodes and like cap and be sure to thank your subject for participating.

The EMG traces seen here represent the motor evoked potential response to gradually increasing stimulus intensities represented as a percentage of stimulator output or so. Note that at the lower intensities, very small meps were elicited, but that at 32%so an MEP was elicited that reached motor threshold. The silent period is observed when a subject performs a slight contraction and a single stimulus is applied to the motor cortex.

The first part of the silent period is due to spinal cord inhibition, and the latter part is attributed to cortical inhibition, specifically GABA B receptors. Here we see the change in motor evoked potential sizes with paired pulse TMS of the FCR muscle measurement of short interval intracortical inhibition or SICI and intracortical facilitation or ICF can be seen. And here we see an example of EMG traces from the erector spin muscles and the measurement of short interval intracortical inhibition and intracortical facilitation.

Finally, here we see an example measurement of long interval intracortical inhibition or LICI. After watching this video, you should have a good understanding of how to perform single pulse and paired pulse TMS as applied to the flexor carp Redis muscle, as well as the erector spining musculature. Of note, these demonstrations in these muscle groups are only selected examples of TMS being used to study the human neuromuscular system.

Don't forget that working with human subjects can place individual's health at risk. It's important to ensure that it's safe for an individual to be exposed to a strong magnetic field, and care should always be taken when performing this procedure.