Know your spinal cord – Motor evoked potentials
Day forty-one in the know your spinal cord series. While the number of posts is going up, we made them easy to find by using our awesome neuroanatomy category! Maybe we did this a bit backwards, but it’s our series and this was the order we did it in. Yesterday we covered cervicomedullary motor evoked potentials, which is a subset of what we will be covering today. So again, slightly out of order, but hey let’s look at motor evoked potentials.
Now that we’ve pretty much covered the basics of the spinal cord (it only took ~40 posts), we can look at the tools we use to probe the circuitry of the cord to determine how it works and how things are routed within it. While there are a lot of different ways we can do this, including ex vivo or in vitro, since my work focuses on non-invasive in vivo ways, we will be primarily focused on the tools that let us do that. One broad category of tool is the motor evoked potential!
Say you are a new researcher and want to determine how the spine communicates. How could you do this without physically removing the spine and trying to trace the connections? The answer is thankfully given for us, we can send an electrical pulse down the cord which activates certain muscle groups. Let’s be clear right out the gate, there are many ways we can do this and each way tells us something a little different. More importantly, we can use different combinations of stimuli and locations to tell us even more about how the signal travels.
Let’s start from the top, literally. We can non-invasively and very accurately stimulate the motor cortex in the brain to create this signal. To do this we use something called transcranial magnetic stimulation, this sends an electric signal in a very localized way so we can perferentally stimulate the part that controls the hand for example. Below is an image showing just that. We stimulate the motor cortex and measure the latency, but more importantly we can do different things (put the arm in certain locations, make the person move while stimulating, etc.) to see how the amplitude and shape of the response changes in the presence of these changes.
As the name implies any sort of stimulus that stimulates the motor pathway and elicits a muscle response is a motor evoked response. So while we can do that using TMS, we can also do this using the cervicomedullary motor evoked potential method, or by stimulating parts of the spinal cord, using magnetic stimulation or electrical stimulus. Although for the spinal cord, magnetic stimulation does not work very well. The image below shows how we typically place the electrodes for transcutaneous spinal stimulation, depending on the location where we are stimulating both the positive and negative pads will be moved.
The advantage to stimulating a motor response is that it is painless. Although, when the intensity is high enough, the results may be a bit unsettling. When stimulus is applied people will often kick slightly or there arms may flail around somewhat. While this is normal and expected (after all we are exciting motor pathways), sometimes people find it a little odd to see their body move without there intention.
That gives us a brief introduction to the motor evoked potential! Now there is a subtlety here that some of you may have missed, we are sending signals from the brain/spine to muscles. Can we do the same in reverse? The answer is yes! Tomorrow we will introduce the somatosensory evoked potential and we can discuss how they are different, how they are similar, and how going the other direction can tell us completely different things.
Until next time, don’t stop learning!