Hi, I’m Sophia and I work in the Single Motor Unit Lab. As you are probably aware spasticity can lead to crippling and painful muscle spasms after a person has had a hemispheric stroke. Today I will be talking about the mechanisms of spasticity in hemispheric stroke. And serotonin’s contribution to changes in motoneuron behavior.
Why do we think serotonin may be greater post-stroke than in healthy intacts? The current theory is that higher cortical structures inhibit brainstem areas where neuromodulators, like serotonin and norepineophrine are made. In other words, if the higher cortical structures are damaged they release the brainstem area that supplies the spinal cord with neuromodulators, enabling it to produce more neuromodulators than normal. Lesions to the internal capsule like those seen in stroke could disrupt the pathways between the cortices and the brainstem as well. This talk is going to specifically focus on the monoamine serotonin, which is produced in the raphe nucleus of the brainstem. Here you can see the raphe nucleus, and these are the tracts that synapse onto the nucleus from higher cortical structures. This pathway is believed to be interrupted by a lesion to the internal capsule, like a stroke. You can then follow the raphe’s projection to the spinal cord.
This is another view of the brainstem, showing you where the raphe nucleus sits.
But how could more serotonin in the spinal cord lead to the spasticity and hyperreflexia we see post-stroke? Serotonin is able to activate G-protien coupled receptors. These receptors then trigger calcium and sodium channels that do not inactivate to open. Because these channels don’t inactivate a persistent inward current continues to depolarize the cell, amplifying inputs to the motoneuonr. These persistent inward currents are called PICs for short. Only the calcium channel current will be discussed here.
Neuromodulators can amplify inputs to the motonerons by up to 500% Serotonin increases the excitability of motoneurons by keeping calcium voltage-gated channels open, facilitating PICs. These channels stay activated as long as the motoneuron remains above threshold. This is unlike the action potentials generated for most inward currents, which inactivate with in 1 – 2 ms even with prolonged membrane depolarization. We think that these PICs are in part responsible for a lower motoneruon threshold and the hyperexcitability of motoneurons. The bottom trace of this picture is the depolarizing current applied to the cell. Even after the current has turned off the cell continues to fire, this behavior is caused by the PIC and you don’t see this continuous firing if serotonin is absent.
Spasticity is marked by prolonged activation of the muscle. Sustained contraction of the muscle may be a sign that there is too much serotonin in the system and it’s turning on PICs. PICs help muscles maintain contractions. BUT How do we test for all of this? How can we test if there is too much serotonin present? and how can we test for serotonin’s effect on motoneurons? We can use Cyproheptadine a drug that’s an antagonist to serotonin, it’s actually an inverse agonist, a special type of anatagoinst. It blocks serotonin from binding to receptors on Motorneurons and also blocks any constitutive activity of the receptors. It’s of most interest because it is known to specifically bind to the serotonin receptor subtype on motoneruons that is responsible for activating PICs.
We want to test serotonin’s effect on motoneuron behavior in a physiologically relevant way, meaning not in a petri dish but there’s nothing wrong with those techniques… We are just after a different set of questions. We use human subjects and the monosynaptic stretch reflex as a measure of motoneuron excitability. We use cyproheptadine that blocks serotonergic action on motoneurons. And we compare the effects of the cyproheptadine between both the spastic sides and contralateral sides of stroke survivors and intacts.
The monosynaptic stretch reflex can be elicited by tapping the tendon, and this is often clinically done by physicians to test the excitability of motneurons. Mattheiu Chardon in our lab has engineered the tendon tapping device you see here. The device is placed over the tendon of the biceps and delivers transient taps on to the tendon. This device can actually measure the displacement of the tapper into the tendon and the force exerted on the tendon. We believe it to be more accurate than whacking some one with a hammer repeatedly and guessing on the force and position at which the tendon was hit.
Here is a picture of some of our raw data. The top trace is the position of the tapper, as the trace trails downward, the tapper is applying more and more force on to the tendon, stretching it. The vertical lines represent the transient taps that the tendon tapper delivers to the biceps tendon. The bottom trace is surface EMG activity of the medial head of the biceps. At the start of the trace you can see little vertical blips corresponding to the transient taps of the tendon tapper, that is just mechanical noise. BUT as you move further along the trace where the tendon tapper is applying more stretch to the tendon you see actual EMG responses. The threshold for the reflex loop is the tendon tapper’s force at which the muscle is repeatedly excited. Stroke survivors have hyperexcitable reflexes, meaning their motoneurons respond to smaller forces than those of intacts. So a stroke survivor’s threshold may be more out here, where as an intacts threshold would be later, in this area. This is a really exciting technique because never before has the stretch reflex loop threshold been measured to so accurately.
Here is an example of some preliminary testing results. In this stroke survivor we see the baseline EMG trace, the blue trace, to be much higher before cyproheptadine is administered. As cyproheptadine begins to act in the central nervous system we begin to see differences in the EMGs. At 2 hours after the drug is administered, the green trace, the EMG signal’s threshold has increased, meaning the stretch reflex loop has become less sensitive to the tendon taps, and there is less EMG activity overall. At the five hour mark, the red trace, close to when cyproheptadine has peaked in the blood stream, we see no EMG response at all. What does this is telling us in laymen’s terms? As cyproheptadine is taking effect, it is blocking the serotonin in the system. This blockade reduces the motoneuron response to inputs, because PICs can no longer activate.
There are some considerations we need to make with this experiment. One being that Cyproheptadine is not a clean drug, it effects more than just the serotonin receptor on motoneurons, it also effects the H1 histamine receptor. So, some of the characteristics we attribute to the drug may not be due to its serotonergic actions. There are cleaner drugs, but they are not approved for human use in the US and are not widely available. The second consideration is that the tendon tapper doesn’t maintain a perfectly stable relationship to the subjects’ tendon; the tendon can slip into different positions in relation to the tapper. This slippage changes the amount of stretch placed on the muscle, which can lead to varying responses to the transient taps. Lastly we would like to assume that our measures of motoneurons hyperexcitability are comparable to current clinical measurements. For example, when a PT stretches the arm of a person to find the catch angle, is that way of stretching the muscle analogous to the way the tendon tapper stretches the muscle?
I would like to thank everyone in the SMU lab for their help and their patience. And thank you for taking the time to listen to me.
Serotonin and Stroke
Mechanisms of spasticity in
The contribution of serotonergic
changes in motoneuron behavior
Sophia del Rio, Single Motor Unit Lab
PI Zev Rymer
• We believe that serotonin plays a large role in
regulating motoneuron excitability.
• We believe that the amount of serotonin in
the spinal cord is greater post-stroke.
The raphe nucleus provides the spinal cord with
Serotonin effects g-protein coupled receptors,
which amplify inputs to motoneurons via
long-lasting activation of calcium and
Note: only calcium channel activation will be discussed
Serotonin calcium channels open PICs
continuous firing of motoneurons
Schwindt and Crill, 1977; Hounsgaard and Mintz, 1988; Heckman et al. 2005;
Perrier and Delgado-Lezama, 2005; image from: Heckman and Enoka, 2000
How do we test for serotonin’s
effect on motoneuron behavior?
• Human subject testing:
– Use the monosynaptic stretch reflex loop as a
measure of motoneuron threshold and
– Use cyproheptadine, a pharmacological agent that
blocks serotonergic action on motoneurons
– Compare the effects between the spastic side and
contralateral side of stroke survivors and intacts.
How do we measure motoneuron
threshold and hyperexcitability?
Preliminary results, subject WJ7125
Effect of Cyproheptadine:
Blue = before drug
Green = post drug 2 hours
Red = post drug 5 hours
• Cyproheptadine is not a clean drug.
• Technical limitations of the testing system,
Linmot tendon tapper
• Are the tendon tapper’s measurements
equivalent to clinical measurements of
• M Chardon, N Suresh, WZ Rymer. A new method for reflex threshold estimation in spastic muscle.
• CJ Heckman, MA Gorassini and DJ Bennett. Persistent inward currents in motoneuron dendrites:
implications for motor output. Muscle and Nerve 31: 135-156, 2005
• CJ Heckman and R M Enoka. Plateau potentials and rhythmic firing in motoneurons. Annual
motoneuron meeting. 2000
• J Houndsgaard and I Mintz. Calcium conductance and firing properties of spinal motoneurons in the
turtle. J Physio (Lond) 398: 591-603, 1988
• KC Murray, A Nakae, MJ Stephens, M Rank, J D’Amico, PJ Harvey, X Li, RLW Harris,EW Ballou, R
Anelli, CJ Heckman, T Mashimo, R Vavrek, L Sanelli, MA Gorassini, DJ Bennett and K Fouad.
Recovery of motoneuron and locomotor function after spinal cord injusry depends on constitutive
activity in 5-HT2c receptors. Nature Medicine 16: 694-701, 2010
• JF Perrier and R Delgado-Lezama. Synaptic release of serotonin induced by stimulation of the raphe
nucleus promotes plateau potentials in spinal motoneurnos of the adult turtle. J of Neurosci 25:
• P Schwindt and WE Crill. A persistant negative resistance in cat lumbar motoneurons. Brain Res
120: 173-178, 1977