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 in your finger, a broken leg, etc. But what exactly is potential tissue damage, and how does the brain perceive pain?

Sensors, known as receptors, are located all over our bodies at the ends of our nerves, in the neurons. These receptors are specialized to respond to specific stimuli – for instance, mechanical influences like a bump;thermal influences like heat and cold; still others respond to chemical influences from the outside and the body itself, such as stinging nettles or lactate.

When you stub your little toe on the table, the mechanical receptors of the neuron in that area open up, and positively charged particles from the outside rush in, triggering an electrical impulse. This impulse is guided along the nerve pathways to the spinal cord, where it is redirected and flows along the spinal cord to the 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 the brain is highly filtered. Crucially, what is transmitted isn't "pain" itself, but rather "danger in this area." The actual pain is ultimately constructed by the brain, combined with information from the eyes (photoreceptors), ears (sound wave receptors), and nose (olfactory receptors) – this interplay is our ultimate early warning system against potential danger.

The system works exactly the same way for potential tissue damage. Imagine holding 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 instantly! Even though no tissue damage has occurred yet, you perceive it as pain. Combined with your memory, your brain anticipates the potential burn and sends the necessary signals to your muscles for an immediate reaction.

How can a light tap sometimes cause no pain at all, and other times be highly painful? Our entire alarm system is a bit more sophisticated than described above. 

The neuron operates on an all-or-nothing principle. When the receptors on the neuron open and the electrical particles flow in, the neuron becomes excited. However, a certain level of stimulation must be reached for the neuron to pass the impulse along. This level is called the excitation threshold – if this is exceeded, the neuron fires a single action potential that travels along the nerve to the spinal cord.

If the neuron is in a neutral or resting state and you lightly bump your elbow on the door frame, you will likely feel nothing. However, let's assume you already have a bruise on your elbow and something touches it – you are highly likely to feel pain. This is because the area around the elbow is already sensitized. Heat and chemical messengers from the inflammation have already opened the thermal and chemical receptors of the neurons in that area, and electrical particles have entered – but the excitation threshold hasn't been crossed yet. The touch (a mechanical influence) is the final straw that breaks the camel's back – an action potential is triggered.

Once the action potential reaches the spinal cord, it is transferred to a new nerve pathway leading to the brain. In this example, the nerve from the periphery (the elbow) releases a very specific cocktail of chemical substances into the gap between the peripheral nerve endings and the central nervous system (spinal cord). The endings of the central nervous system also have neurons with matching receptors that are only opened by specific chemicals. Simply put, if the elbow nerve releases round chemicals, only spinal cord neurons specialized for round chemicals will open – the lock-and-key principle. If the threshold of the new neuron is exceeded, a new action potential is guided up the spinal cord to the brain. Only then do we perceive the information as pain.

However, not every action potential makes it to the brain. The first screening of information occurs right at the transition from the periphery to the central nervous system. If chemicals flood the gap, the system can become overexcited – every neuron would fire, triggering back-to-back action potentials. Fortunately, nerve pathways descending from the brain also end in this gap. These nerves release a powerful cocktail of feel-good hormones into the gap to calm the situation down. The brain effectively blocks the transmission of new action potentials.

This cocktail can be up to 60 times more powerful than any medical injection or painkiller. This explains how an ultra-marathon runner [1], who dislocated and popped his shoulder back in at mile 16, can keep running the remaining 100 miles and still win the race.

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References

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