Under normal circumstances, a nociceptive signal reflects the features of a noxious stimulus. With central sensitization, nociception reflects altered excitability of a spinal cord circuit.
In this way, central sensitization marks the first point when nociception becomes independent of the environment.
The Original Phenomenon, Play by Play
Over 35 years ago, Clifford Woolf formally confirmed a hypothesis that had been floating around in the pain literature for many decades. Pain physiologists had long suspected that nociceptive signals could somehow be amplified in the spinal cord, but they didn’t have the technology to understand how this could happen. A major challenge was differentiating spinal-specific effects from peripheral nociception and how the brain processes this nociceptive information.
Woolf later described how the idea for this seminal experiment came to him. Keep in mind that electrophysiologists like Woolf spent their days in tiny, pitch-black rooms, subtly moving electrodes over tissue as they listened to neuronal spike trains like this. So just imagine a man sitting in the dark, pinching rat toes, and listening to intermittent static…and actually learning something. Woolf said:
“I found that most cells responded only to pinch or noxious heat of one or more toes. Some, however, had very large receptive fields encompassing the whole leg and could be driven by innocuous mechanical stimuli… It took me several months of recording to finally realize that [the neurons with large receptive fields] were only recorded at the end of the day, after the repeated noxious stimulation…. This was my ‘eureka’ moment…cells that had somehow changed as a result of the repeated input I had applied.”
In other words, Woolf found that pinching rat toes all day provided just the right kind of repeated noxious stimulation that changed the rules of normal nociception. A toe pinch that evoked a modest withdrawal response in the morning created unexplainably large areas of hind limb hypersensitivity in the afternoon. Incredibly, normal (innocuous) stimuli applied to the hind limb also evoked withdrawal behaviors in the afternoon. He had somehow changed the way spinal interneurons were processing sensation!
If you want to feel a pang of sadness right now, listen to the spike train video again and reflect on your life’s accomplishments.
Deconstructing The Circuit
In science, there is a standard strategy for determining the conditions that are necessary and sufficient for a complex system to produce a behavior (like a rodent producing a “pain” behavior). You systematically inhibit parts of the system and determine whether behavior is affected.
This is like the way you diagnose computer problems: did the power supply die? Is there a loose screw jostling your motherboard? Are any wires frayed? If you replace a cable, is the function restored?
Or the way you troubleshoot a recipe gone wrong: Did you combine the ingredients in the right order? Was the oven preheated? Did you overmix the dough?
Like you, Woolf reasoned that if he wanted to understand the spinal cord’s unique role in pain hypersensitivity, he would need to control other confounding factors that also impact nociception: the brain and peripheral nociceptors.
Nitty Gritty Study Design
If details don’t interest you, skip to the final sections for an overview of study findings.
But if you want to get a strong grasp of mechanisms underlying central sensitization, and if you want to critically interpret the quantitative sensory testing (QST) studies that claim to measure central sensitization, read this section carefully.
Let’s start with the big picture and break down Woolf’s experimental approach:
1. The thermal injury.
One leg of each rat was exposed to 75ºC radiant heat for 60 seconds to create a thermal injury.
Why radiant heat?
When you’re interested in nociception, it’s ideal to have a “pure” stimulus. Radiant heat is appropriate for two reasons:
- C-fiber nociceptors are responsible for detecting noxious heat, so Woolf knew exactly what type of nociceptor was being activated.
- He didn’t want other types of sensory neurons to influence his results. A thermode placed on the rat’s skin would be a noxious heat AND pressure stimulus. Radiant heat takes pressure out of the equation. Woolf could then be certain that the subsequent effects would only be caused by C-fiber nociception.
Why 75ºC radiant heat for 60 sec?
I don’t know. This temperature is blistering hot, and a 60 second exposure time sounds brutal. I assume Woolf wanted to create an injury that was severe enough to clearly track the onset and gradual recovery over the next 24 hours.
2. The noxious stimuli.
Hot water very quickly warms the surface temperature of the skin in a uniform way. In contrast, the surface temperature of contact thermodes can sometimes differ from the temperature they are supposed to deliver.
A measure of heat hyperalgesia.
To a rat, von Frey hairs pressed against the skin is like someone poking you with the pointy end of an umbrella. Not pleasant. Both the rat and you will quickly withdraw when you are poked.
A measure of mechanical punctate hyperalgesia. (Note: punctate = pinprick, which is considered a subset of tactile or pressure hyperalgesia.)
3. The response.
Animals can’t tell us they are in pain, so we have to rely on their behaviors to infer it. Since Woolf was interested in spine-specific mechanisms, he chose a spinally-dependent behavior. He reasoned that the amount of noxious stimulation needed to produce a withdrawal reflex is also the amount of noxious input that usually gives rise to pain perception. So he narrowed in on the flexion withdrawal response, which is a fancy way of describing a rapid lift of the hind paw. Flexion is considered a nocifensive (guarding/avoidance) behavior that occurs before nociceptive information can even reach the brain. It’s a reasonable behavioral proxy measure for pain in rats.
Woolf then recorded directly from bicep femoris (lower) motor neurons, which enable flexion withdrawal behavior. Motor neurons are efferent neurons controlled by local spinal circuits. These neurons are responsible for the muscle contractions that withdraw the hindlimb from noxious input.
Specific to spinal cord circuits? CHECK.
Specific to noxious stimulation? CHECK.
Brain Be Gone (sort of)
Woolf first sought to determine if the spinal enhancement of nociception required the brain. To do this, he removed the cerebellums of rats (decerebration) in his experiment to accomplish three goals:
- Preserve the spinal and brain stem reflex behaviors (like flexion);
- Prevent advanced reflex integration by the brain (which can actually exaggerate spinal reflexes because they are no longer inhibited by the brain);
- Preserve the rats’ abilities to generate nocifensive behaviors (withdrawal behavior, vocalizations, stimulus orientation), which is necessary to infer that noxious stimuli are intense enough to cause pain.
Woolf wanted to ensure that intact and decerebrated rats showed similar baseline sensitivity (measured by hind limb withdrawal) to the noxious mechanical and thermal stimulation he would use for his experiments. Figures 2.1 and 2.2 confirm this.
The next day, the lateral edge of one hind paw of each rat was exposed to 75ºC radiant heat for 60 seconds. This thermal injury induced substantial mechanical punctate hyperalgesia—meaning that von Frey hairs elicited an exaggerated hind limb withdrawal response that normalized after 24 hours.
The thermal injury also induced heat hyperalgesia for about 3 hours, as measured by how quickly rats pulled their injured hind limbs out of a hot water bath.
Woolf summarized these findings as:
“Peripheral tissue injury in the decerebrate rat produces…changes in the threshold and responsiveness of the flexion reflex that parallel sensory disturbances found in man.”
Spiking a Different Tune
As Woolf sat in his dark room recording baseline motor neuron activity, he found that they are normally quiet (i.e., these motor neurons do not fire under normal circumstances). When he applied a noxious stimulus (toe pinch, what he does best), the motor neuron transiently responded with some intense neural spiking that roughly corresponded to the duration of his pinch. After the pinch was done, the motor neuron went quiet again.
It was a different story after the thermal injury. Woolf noted two important changes in motor neuron activity:
- The pinch on the injured hind paw (noxious stimulus) provoked a larger magnitude of neural activity. This was the first evidence of enhanced excitability of spinal interneurons, which is one of the three key features of central sensitization.
2. A low level of neural activity continued after the stimulus stopped. This was the first evidence of stimulus-independent spinal activity, which is also one of the three key features of central sensitization.
When Woolf looked at 25 different biceps femoris motor neurons to see if this was a generalized response, he realized that this ongoing neural activity was steadily increasing over the course of an hour!
He noticed that as the ongoing neural activity following the stimulus continued to increase, mechanical sensitivity steadily changed as well. He wondered…could mechanical (punctate) hyperalgesia somehow be related to the spontaneous activity he was observing?
Identifying the Culprits
Woolf wanted to know how different types of nociceptors were contributing to this effect. He used his knowledge of nerve structure and function to make this happen.
Different types of nociceptors transmit information at different rates (called conduction velocity). These transmission rates are controlled by the diameter of the nerve, which determines how quickly action potentials can be generated and therefore how quickly a nociceptor can fire. C-fiber nociceptors transmit information very slowly because they have small diameters and no myelin to speed up the signal. Nerves encased in myelin (like A-delta nociceptors) transmit information more quickly than unmyelinated C-fiber nociceptors. A-beta touch neurons transmit information most quickly due to their thick myelination. Julius & Basbaum (2001) have an older but still exceptional review of these principles.
For example, myelinated A-delta nociceptor signals reach the brain more quickly than signals from unmyelinated C-fiber nociceptors. This is why A-delta mediated sharp pain is perceived before C-mediated dull, burning pain: simple differences in travel time.
Woolf decided to differentiate the contributions of different fiber types by looking at time delay signatures. He needed a precise noxious stimulus—in this case, electrical current to the sural nerve (which innervates the biceps femoris). By varying the intensity of this electrical current, he was able to selectively stimulate different groups of sensory neurons and their time delay signatures:
- A-beta fiber activity was examined with repeated 100 micro-amp currents
- A-beta and A-delta fiber activity was examined with repeated 250 micro-amp currents
- A-beta, A-delta, and C fiber activity was examined with repeated 5 mA currents
[CONTENT STILL UNDER CONSTRUCTION…CHECK BACK SOON!]