Know your spinal cord – The V-wave

We’ve made it to day forty-three of our know your spinal cord series! While that is a lot of posts, we’ve made it super simple for you to find all of them with our neuroanatomy category. Lately, we’ve looked at several different tools in our spinal cord probing toolbox. We’ve seen all sorts of different ways to create a response, but we are still missing one important tool for our exploration into the unknown spinal cord world and that is what we are going to talk about today!

While we’ve seen a multitude of techniques to gather information from the spinal cord, they all had something in common. Aside from being electrically generated, they are all evoked. A fancy way of saying there is no volitional movement involved. Now this is good because there is some unknown variance to having a person perform volitional tasks that we need to take into account. However, without having a tool involving volitional movement, we still aren’t seeing how the spinal cord may respond to stimuli coming directly from a person (ie volitional movement).

(A), #1—electrical stimulus evokes a single afferent volley which recruits motoneurons for the H-reflex according to the size principle; #2—presynaptic inhibition can influence afferent input to the motoneuron; #3—tendon tap evokes multiple volleys which arrive at the motoneuron over 25 ms. (B), #1—a small number of motoneurons may discharge to produce F-waves after antidromic impulses reach their soma; #2—at rest, F-waves are likely to be limited to large motoneurons due to reflex activation of smaller motoneurons and collision with the antidromic volley prior to the soma. (C), #1—in motor axons conducting orthodromic impulses of voluntary drive, voluntary and antidromic impulses will collide; #2—reflex response which travels along motor axons cleared by the collision described in point #1 will contribute to the V-wave; #3—presynaptic inhibition can influence afferent input to the motoneuron. (https://doi.org/10.3389/fnhum.2013.00152)

Enter the V-wave, or volitional wave (shown above). First discovered in 1971, the V-wave is a pseudo combination of the H-reflex and F-wave. Like the F-wave, supramaximal electrical stimulation is applied, this forces an antidromic impulse (traveling the wrong way) up the neron of a motor axon, stimulating the spinal cord motor neuron pool, which then creates a orthodromic rebound pulse. Now, add a volitional motor command into this and you create the V-wave. 

The question may be why add a volitional component, what do we learn from this? When the motor axons involved in the voluntary contraction are activated, there is a collision between voluntary orthodromic and the evoked antidromic impulses which leaves these axons clear to transmit a reflex response to the muscle. This is why we consider it a pseudo H-reflex and F-wave type response. Conversely, motor axons not involved in the contraction, do not contribute to the production of the V-wave because any reflex response will either collide with the antidromic impulse or the antidromic impulse will reach the soma (cell body) first and leave these motoneurons refractory, making it more difficult (very difficult) for them to fire when the afferent (arriving to the spinal cord) volley reaches the motor pool. Below is an image showing what a V-wave looks like.

Evoked V-wave responses recorded in a representative participant during 5 repeated maximal voluntary contractions from two different sessions consisting of the same protocol. They are shown as Session 1 (A and C) and session 2 (B and D). (DOI: 10.1097/WNP.0000000000000560)

The V-wave is influenced by many factors, such as the strength of voluntary contraction and the range of maximal firing rates within a motoneuron pool. This adds a layer of difficulty in interpreting a change to the response and therefore raises questions about its usefulness as an independent measure of motoneuron excitability. Moreover, the motoneuron discharge rate reflects not only supraspinal input to the motoneuron but the response to all inputs which arrive at the motoneuron so the origin of an increase in V-wave size is uncertain. That does not stop us from trying to understand the response and there are plenty studies that use it, although to a lesser degree than some of the other more easily interpreted methods.

That is about all you need to know in the basics of the V-wave. Will we make it to fifty days of spinal cord knowledge? I’m not sure right now, but now that we’ve looked at some of the ways we probe the spinal cord, we can get into some of the research I’m doing and how it relates to all the fun stuff we’ve covered over the past couple of months.

Until next time, don’t stop learning!