Nociception is not pain, and the difference matters

One more piece of vocabulary has to be right before those three questions can be answered, because getting it wrong makes the rest of the paper look like it is describing something else.

Nociception is the signal a tissue sends when something is wrong with it. Specialized nerve endings called nociceptors detect damage, or the chemical conditions that go with damage, and they fire. That firing travels to the spinal cord whether or not the person notices anything.

Pain is what a person experiences. It is produced in the brain, after a great deal of processing, and it does not track nociception reliably in either direction. A tissue can be sending nociception with no pain reported at all. A person can be in severe pain with very little nociception arriving.

So a stimulus can be nociceptive without hurting. This paper depends on that distinction the whole way through. When it says a second nociceptive input, it does not mean hurting the patient, and the patient does not have to feel anything for the effect to appear.

The fibers carrying this traffic out of muscle and joint are the small ones, the group III and group IV afferents named earlier in the numbering scheme. Group III endings respond mainly to mechanical loading. Group IV endings respond to the chemical byproducts of tissue activity. Both are relevant later.

Question one: what makes a muscle test weak

Start with what is intact.

Under experimental pain, produced in the laboratory by injecting saltier-than-blood saline into a muscle, maximal voluntary force falls while the muscle's evoked twitch is unchanged, so the contractile machinery is working normally [10]. Evoked twitch means the researcher stimulates the muscle's own nerve electrically, bypassing the person's intention entirely, and measures the resulting contraction. If the muscle's own machinery had failed, that twitch would shrink. It did not. The same fall in strength under experimental knee pain has been measured with an instrumented dynamometer rather than a twitch [11].

Schabrun's recordings go further than that. Motor output dropped while the muscle's response to direct stimulation of its own nerve stayed flat, the peripheral volley at Erb's point was unchanged, and the spinal volley at C7 was unchanged in both amplitude and latency [12]. Those two volleys are recordings taken at two checkpoints on the sensory route, one over the brachial plexus at the base of the neck and one over the cord itself at the seventh cervical level, so the first reads the nerve bundle and the second reads what arrives at the cord. They confirm that the incoming signal from the arm was the same size and arrived at the same time as before. The muscle can fire. Its nerve conducts. The afferent signal arriving is the same size it was. Output falls anyway.

So the loss is happening between the signal arriving and the command leaving. That leaves the pool and the cortex above it, and both turn out to be involved. The pool comes first.

What produces the fall is a source in tissue generating nociception. Group III and IV afferents from that tissue enter the back portion of the spinal cord, called the dorsal horn, and drive inhibitory interneurons onto the motorneuron pools serving that region [13]. An interneuron is a short local nerve cell that connects one cell to another inside the cord. An inhibitory one makes the cell it connects to less likely to fire.

Now the gain matters, and this is where the two halves of the paper meet.

Shutting the persistent inward current off requires active inhibition arriving at the cell, and once inhibition does arrive the current gives way easily. A small fraction of the cell's inhibitory synapses can nearly abolish it [14,4]. That is a strange and important asymmetry. It is hard to build the gain up, because that needs monoamines delivered from the brainstem, and it is easy to knock it down, because a little inhibition arriving at the right place does it.

Low-threshold stimulation of a peripheral nerve carrying flexion reflex afferents can suppress it completely in an extensor motorneuron [1]. Flexion reflex afferents are the fibers that drive the withdrawal reflex, the one that pulls a hand off a hot stove. Nociceptive afferents are flexion reflex afferents. Steffens and Schomburg, recording in a decapitated high-spinal cat, delivered a nociceptive input first, which is what conditioning means, and found it reduced the direct one-synapse input running from the muscle's stretch receptors to the alpha motorneurons [13]. Nothing in that circuit requires a brain, and nothing requires the patient to feel anything.

That preparation is worth pausing on, because it settles something a reader will otherwise keep wondering about. The animal had no brain in the circuit at all. There was no consciousness, no attention, no expectation and no pain in any meaningful sense. Nociceptive input still turned motor output down. Whatever is happening here, it is not the patient deciding not to push.

That gives the size of the finding. A weak grade is a motorneuron pool that lost its gain.

There is a slower layer above it. Nociception raises inhibition inside the motor cortex and reduces the output that reaches the muscle when the cortex is stimulated [15,16], and changes of the same kind are found in people carrying a musculoskeletal injury [17,18]. The stimulation used there is transcranial magnetic stimulation (TMS), a magnetic pulse delivered through the scalp over the motor cortex, with the resulting twitch recorded from the muscle. Read that measure carefully. It is set both at the cortex and at the pool, so a smaller response can mean the cortex sent less or that the pool amplified less, and the measure cannot separate the two. That slower layer moves over minutes and is why chronic cases carry weakness that outlives the tissue.

A standing source acts in its own region while the patient sits still. It produces the baseline weak grade found on the first pass, and it does not reach across the body on its own [19,20]. That last point becomes important in a moment.

The exception is a muscle with no drive left to restore. Denervation, myopathy, motor neuron disease. In an outpatient orthopedic caseload it is rare, and history helps flag it [21].

Question two: why loading that muscle weakens everything else

Two steps, and the first is easy to miss.

Contraction turns the existing source up. The tissue with the condition in it is the tissue being loaded. Group III endings are mechanically sensitive and group IV endings respond to the metabolic byproducts of contraction, and sensitization drops the threshold of both. Sensitization means the endings have become easier to set off than they were when the tissue was healthy. So ordinary movement now fires endings that used to stay quiet. When a patient bends forward as if doing dishes, the painful low back is being loaded, and its afferent output climbs. Using an inhibited muscle raises the output of the driver that was already sitting there.

This is the step that makes the whole thing counterintuitive. Asking a patient to use the weak muscle is not a neutral act of measurement. It is an intervention that increases the very signal causing the weakness.

Above a certain traffic level the effect stops being regional. The spinoreticular tract carries nociceptive input to the reticular formation, a network in the brainstem, and the reticulospinal system carries commands back down from there to motorneuron pools throughout the body. Its reach is broad rather than confined to the region the input came from, so what the reticular formation does with the traffic arriving is felt well past that region, which is what a body-wide drop in previously strong muscles looks like [22,23,24].

Two published pieces sit close to this. Sidhu showed fatiguing contraction raising inhibition inside the motor cortex through group III and IV feedback, which is the afferent class doing exactly what the model needs, though the measurement was taken in the muscle doing the work [25]. Simis recorded motor cortex inhibition at a hand muscle in knee osteoarthritis patients, scaling with the knee's cartilage grade rather than with reported pain [26]. That one is remote inhibition from a joint source, measured. A knee changed something at a hand.

Neither study tested a fresh muscle during a held load, which is the measurement the model actually turns on.

Question three: why a second nociceptive input turns the weak muscle on

This is the part that sounds impossible, and it has a published mechanism.

Nociceptive afferents from anywhere on the body converge on the subnucleus reticularis dorsalis (SRD) in the caudal medulla, one of the relays for this effect sitting above the spinal cord. The medulla is the lowest part of the brainstem, where it joins the spinal cord. This nucleus receives nociceptive traffic from the entire body rather than from one region, which is the unusual thing about it.

It projects back down to every spinal segment and depresses the convergent dorsal horn neurons, the ones that take nociceptive and ordinary input together. Le Bars, Dickenson and Besson measured depressions of sixty to a hundred percent in those neurons in the rat, outlasting the stimulus by minutes [27,9]. Sixty to a hundred percent is close to a shutdown, and it happens at the first synapse, before the signal has gone anywhere.

So the effect of adding nociception somewhere else is that the cord stops passing along nociception generally, including the traffic that was already running from the original tissue.

The contact changes nothing in the first tissue. Those nociceptors keep firing at the rate they were firing beforehand. Less of that firing crosses the first synapse. Fewer inhibitory interneurons get driven. The gain switches back on and the muscle grades strong [28].

Read that sequence again, because it is the answer to Part 2 of the opening. Nothing was repaired. The shoulder that graded 4-/5 is in exactly the condition it was in a second earlier. What changed is how much of its nociceptive traffic is reaching the motorneuron pool, and with less inhibition arriving, the multiplier comes back.

There is a second route running at the same time. That nucleus is also wired into the reticulospinal neurons, and the descending damping system releases serotonin, noradrenaline and endogenous opioids. Those are the transmitters that set the gain. So one contact opens the gate and raises the gain setting, and the projection carrying the gain has no capacity to aim. Agonist and antagonist rise together [9,1].

Why the same contact does the opposite thing to a strong muscle

Here is the objection a careful reader should already be forming. If the contact raises gain everywhere, why does anything get weaker?

Because two things arrive at every muscle, not one.

Afferents from the added input also drive their own inhibitory interneurons, and that traffic reaches whatever pools it maps to. Two influences arrive at every muscle at once, and their ranking is known. Inhibition outweighs neuromodulation at the motorneuron, because so few inhibitory synapses abolish the current [1]. Neuromodulation here means the serotonin and noradrenaline that set the gain, as against the inhibition arriving at the cell directly.

Follow both cases. A muscle carrying its own standing source has a large suppression waiting to be lifted, and lifting it dominates everything else happening to it. A muscle with no standing source has nothing available to lift, so the arriving inhibition is the only change it sees. Same contact, opposite outcomes, decided by what each pool was already carrying. No part of the circuit has to identify a target for this to work.

That last sentence matters more than it looks. Nothing in this account requires the nervous system to know which muscle is the injured one, or to select a target, or to do anything clever. One input does one thing everywhere. The difference in outcome comes entirely from the state each pool was in beforehand.

Back to the two puzzles

Part 1. The right shoulder tests 4-/5 because there is a nociceptive source in that region, sending traffic that drives inhibitory interneurons onto that pool and knocks its gain down. The left tests 5/5 because it carries no such source.

Then a weight goes into the right hand with the arm held out. The affected side is now being loaded, its sensitized group III and IV endings fire harder, and its afferent output climbs. Above a certain level that traffic stops being a local matter, because the spinoreticular route carries it to the brainstem and the reticulospinal route carries the consequence back out to pools everywhere. The left shoulder, which had nothing wrong with it, drops to 4-/5.

Nothing happened to the left shoulder. It got caught in a body-wide change produced by loading the right one.

Part 2. A tender spot on the right side of the neck is a second nociceptive source. Palpating it adds nociceptive input, and the palpation is held while each retest is made.

Retest the right shoulder with the contact held and it grades 5/5. The added input engaged the medullary damping system, less of the original neck-and-shoulder traffic is crossing the first synapse in the cord, fewer inhibitory interneurons are being driven onto that pool, and the gain comes back.

Palpate the same spot, hold it, and retest the left shoulder, and it drops to 4-/5. That pool had no standing suppression to lift. For it, the added input is simply new inhibitory traffic arriving, and inhibition outweighs the gain effect at the motorneuron.

One contact. Two muscles. Opposite results, from the same mechanism, decided by what each side was carrying before the finger landed.

Four things this accounts for

1. The contact does not have to hurt, since the steps that produce the reversal run below the cortex.

2. The speed follows from the same anatomy, because conduction and synaptic gating take milliseconds and nothing has to circulate or remodel.

3. The strength lasts as long as the input does. The medullary damping runs while the input is running, and the original source transmits throughout. Take the input away and the muscle grades weak again. The weakness was never removed, only outvoted.

4. A wrong spot costs nothing. An input landing where there is no condition in the tissue drives the large myelinated touch fibers and generates no nociceptive traffic in the group III and IV fibers, so it sets off neither the inhibition that turns a pool down nor the medullary damping.

The third of those deserves emphasis, because it is the honest limit on the whole finding. The reversal is not a treatment and it does not fix anything. The source is still there, transmitting, the entire time. The damping system is simply outvoting it for as long as the input is there.

What is published, and what is not

Every link above is published. The chain has not been run end to end.

Nobody has measured force, or a manual muscle test grade, after adding a second nociceptive input. Taking nociception away has been measured that way, with force numbers, in studies of subacromial anesthetic injection and of draining an effused joint.

Adding it has been measured as inhibition instead, and the evidence there is a pair of experiments from Schabrun's group using the same muscle and the same injection of saltier-than-blood saline, differing in one thing. In the 2013 experiment a second input, electrical stimulation of the ulnar nerve at the wrist, ran throughout the motor recording, and motor output was not suppressed while the person was in pain; the suppression appeared only after the pain had resolved [30]. In the 2015 experiment no such input ran, and the suppression appeared during the pain itself [12].

Two parts of the account above go beyond what the literature has established. That the dorsal horn damping and the restored gain are the same event in a patient has never been measured at both ends. And the route from a held load to a body-wide drop, spinoreticular in and reticulospinal out, is the path the anatomy provides rather than an experiment anyone has run.

The mechanism is established physiology. That it produces a change in MMT grade has not been measured by anyone.

What would test this, and what would break it

The physiology is published. The clinical measurement is not. These are the results the account requires.

A dynamometer, not a hand grade. One already-weak muscle and one already-strong muscle, same person, same session. Hold the second input while force is recorded.

• A held second nociceptive input raises force in the weak muscle while it is held, and force returns to baseline when it is released. The muscle itself is unchanged.

• The same contact lowers force in the strong muscle.

• A non-nociceptive contact does neither.

• Reported pain is not required.

• Loading the weak muscle drops previously strong muscles elsewhere.

• Evoked twitch and the afferent volleys stay flat while voluntary force changes.

The account is wrong if the weak muscle does not gain force during a real second nociceptive input, if both muscles move the same way, if light touch on a clean spot produces the same flip, or if twitch and voluntary force rise and fall together.

A missed spot is not a failed test. The account already says a wrong spot does nothing. A confirmed rise is still not a treatment. The source was outvoted while the input was there, not removed.