Pain - The Transmission

In our previous blog post, pain was described as a reaction to actual and potential tissue damage. Actual tissue damage is easy to understand – a cut on your finger, a broken leg, etc. But what exactly is meant by potential tissue damage, and how does pain perception actually occur in your brain?

Sensors, known as receptors, are located throughout your body at the ends of your nerves, inside the neurons. These receptors specialize in specific stimuli and respond to them – for instance, to mechanical influences, like a blow; to thermal influences, like heat and cold; while others respond to chemical influences from the outside and from your body itself, such as nettles or lactate.

If you stub your little toe on a table, the mechanical receptors of the neuron in that area open up, and positively charged particles from outside rush in, triggering an electrical impulse. This impulse travels along the nerve pathways to your spinal cord, where it is relayed and travels up the spinal cord to your brain. These nerves also have specializations – certain pathways conduct at speeds of up to 150 km/h, while others conduct at just 1 km/h. This means that the information reaching your brain is very limited. Importantly, what is transmitted isn't "pain" as information, but rather "danger in this area." Pain itself is ultimately constructed by your brain, combined with information from your eyes (light receptors), ears (sound wave receptors), and nose (odor receptors) – this interplay is your primary warning system against potential danger.

The system works the same way with potential tissue damage. Imagine you hold your hand too close to a fire. The thermal receptors in your hand open up, and the information "temperature increase in the hand" is forwarded to your brain. You pull your hand back immediately! At this point, no tissue damage has occurred yet, but you perceive it as pain. Along with your memory, your brain anticipates the potential burn and sends the necessary signals to your muscles for a rapid response.

How can a slight bump sometimes cause no pain at all, and other times be very painful? Your entire alarm system is a bit more complex than described above. 

A neuron only works on an all-or-nothing principle. When receptors on the neuron open and electrical particles flow in, the neuron becomes stimulated. However, a certain level of stimulation must be reached for the neuron to transmit the impulse. This level is called the threshold – if it is crossed, the neuron fires a single action potential that travels along the nerve to your spinal cord.

Now, if the neuron is in a neutral or resting state and you gently bump your elbow against the doorframe, you most likely won't feel any pain. But suppose you already have a bruise on your elbow and someone touches it; in that case, you will very likely feel pain. This happens because the area around your elbow is already irritated. Warmth and chemical messengers from the inflammation have opened the thermal and chemical receptors of the neurons in that area, and electrical particles have entered – but the threshold hasn't been crossed yet. The light touch (a mechanical influence) becomes the straw that breaks the camel's back – an action potential is triggered.

Once the action potential reaches your spinal cord, it is relayed to a new nerve pathway heading to your brain. The peripheral nerve – in this example, from your elbow – releases a very specific mix of chemical substances into the gap between the ends of the peripheral nerves and the central nervous system (the spinal cord). The endings of the central nervous system, in turn, have neurons with specific receptors that are only opened by certain chemicals. Put simply: if the elbow nerve releases round chemicals, only spinal cord neurons that specialize in round chemicals will open – the lock-and-key principle. If the threshold of this new neuron is crossed, a new action potential travels up the spinal cord to your brain. Only then do you perceive the information as pain.

However, not every action potential makes it all the way to your brain. When signals are relayed from the periphery to the central nervous system, the first filtering of information takes place. If chemicals flood the gap, the system could become overstimulated – every neuron would get excited and trigger a new action potential. But nerve pathways descending from your brain also end in this gap. These nerves release a cocktail of happy hormones into the space, calming the situation down. Your brain effectively suppresses the transmission of new action potentials.

This natural cocktail can be up to 60 times stronger than any injection or painkiller. This explains why an ultra-marathon runner [1] who dislocated and popped his shoulder back in at kilometer 26 was still able to finish the remaining 160 km and win the race.

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Find out more on our chronic pain page.

References

[1] https://www.denverpost.com/2017/07/15/hardrock-100-2017-kilian-jornet/