Pain - The Transmission

In the previous blog, pain was described as a reaction to effective and potential tissue damage. Effective tissue damage is easy to understand – a cut on the finger, a broken leg, etc... But what is meant by potential tissue damage and how does pain perception occur in the brain?

Throughout our body, there are sensors called receptors located at the ends of our nerves, in the neurons. These receptors are specialized for certain stimuli and respond to them – for example, to mechanical influences like a blow; to thermal influences, like heat and cold; others react to chemical influences from outside and within the body, like nettles or lactate.

When you stub your little toe on the table, the mechanical receptors of the neuron in that area open and positively charged particles flow in, triggering an electrical impulse. This impulse is transmitted along the nerve pathways to the spinal cord, where the impulse is once again switched and flows along the spinal cord to the brain. These nerves also have specializations – certain pathways transmit at speeds of up to 150 km/h, while others transmit at only 1 km/h. This means the information reaching the brain is very limited. It is important here to note that “pain” is not transmitted as information, but rather “danger in this area”. The pain itself is eventually constructed by the brain, together with information from the eyes (light receptors), ears (sound wave receptors), and the nose (odor receptors) – this interplay is our first warning system against potential danger.

With potential tissue damage, the system works the same way. Imagine you hold your hand too close to the fire. The thermal receptors in your hand open and the information “temperature increase in the hand” is sent to your brain. You immediately pull your hand back! However, no tissue damage has yet occurred, but you perceive it as pain. Along with your memory, your brain sees the potential burn and sends the necessary signals for an appropriate reaction to your muscles.

How can a light bump sometimes not cause pain and other times be very painful? Our entire alarm system is a bit more complicated than described above.

The neuron knows only the function all or nothing. When the receptors on the neuron open and the electrical particles flow in, the neuron is 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 exceeded, the neuron sends out a single action potential, which reaches the spinal cord along the nerve.

If the neuron is in a neutral or resting state, and you lightly bump your elbow on the doorframe, you will most likely not feel any pain. However, if you already have a bruise on your elbow and are touched there, you will very likely feel pain. This is because the area around the elbow is already irritated. Warming and chemical messengers due to inflammation have opened the thermal and chemical receptors of the neurons in this area and electrical particles have entered – however, the threshold has not yet been exceeded. The touch (mechanical influence) is the final drop that makes the barrel overflow – an action potential is triggered.

If the action potential reaches the spinal cord, it is switched to a new nerve pathway that reaches the brain. The nerve from the periphery, in this example from the elbow, releases a very specific mixture of chemical substances into the area between the endings of the peripheral nerves and the central nervous system (spinal cord). The endings of the central nervous system in turn have neurons with respective receptors that are only opened by certain chemicals. Simply put, if the elbow nerve releases round chemicals, then only neurons of the spinal cord, which specialize in round chemicals, will open – the key-lock principle. If the threshold of the new neuron is exceeded, a new action potential is conducted over the spinal cord to the brain. Only then do we perceive the information as pain.

Not every action potential reaches the brain, however. When switching from the periphery to the central nervous system, the first selection of information occurs. If chemicals flow into the interspace, it can lead to an overstimulation of the system – every neuron would be excited and would trigger a new action potential. However, nerve pathways ending in this space also come from the brain. These nerves release a cocktail of happiness hormones into the interspace, calming the situation. The brain prevents the transmission of new action potentials.

This cocktail can be up to 60 times stronger than any injections or painkillers. This explains why an ultra-marathon runner [1], who dislocated and relocated his shoulder at the 26th kilometer, can run the remaining 160 km and win.

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References

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