The Hidden Mechanism Behind Pain Relief

Have you ever wondered why you instinctively rub your head after bumping it? Why you shake your hand after burning it? Why your parents caress your knee after you fell?

Or why somehow, it would feel good?

Alongside acknowledged functions such as cognition, movement and sensory perception, our brain has an incredible hidden capacity: the ability to deploy useful mechanisms that operate beneath our awareness. The previously mentioned secondary reactions to pain— rubbing, moving rapidly and caressing — are not bugs but features of our evolved nervous system. To understand these reactions, we must start with understanding their cause and working mechanism.

Understanding Nociception

All over our skin and deep tissues, we have different structures that respond to different kinds of stimuli. Some respond to touch, some to proprioception, and others to noxious or hamful stimuli. These are termed nociceptors and they are activated by the presentation of different stimuli — extreme temperatures, intense pressure, intense chemicals — which is why different things in the environment can make us feel pain.

But the simple activation of these nociceptors is not enough to induce the perception of pain. As first claimed by Descartes, pain is perceived by the brain. Signals must get to the brain in order to be felt. Two modes of transport are possible for noxious signals: they can travel through the faster Aδ axons (5 to 30 m/s) or the slower C axons (less than 1.0 m/s). Both these axons are considered to have small diameters, and are slower than other larger fibers at conducting signals.

Through these fibers, the signal is transmitted to the brain. But this doesn’t happen in a fixed direct-line connection. It’s a much more complex and dynamic process involving modulation.

Pain Modulation

Before being sent to the brain, the signals stop at the spinal cord. This relay is home to a central idea proposed by Ronald Melzack and Patrick Wall in 1965 that would revolutionize pain research: the Gate Control Theory of Pain. This theory suggests that the spinal cord contains a neurological “gate” that can either allow pain signals to continue to the brain (open the gate) or block them (close the gate).

In the spinal cord, small fibers that carry pain signals suppress the brake cells (inhibitory interneurons), which normally control the transmission cells responsible for sending pain signals to the brain. When these brake cells are inhibited, the transmission cells become more active, allowing pain signals to reach the brain. This is called opening of the gate. On the other hand, large fibers, which carry non-noxious signals like touch or movement, activate the brake cells, reducing the activity of transmission cells and preventing pain signals from reaching the brain. This is called closing of the gate.

In other words, small fibers increase the activity of transmission cells and pain signals, while large fibers decrease the activity of these cells, blocking pain signals. When both fiber types are active at the same time, they have opposing effects on pain transmission. This closing of the gate will decrease or abolish pain signals from traveling to the central nervous system, thereby allowing pain to be perceived less or not at all.

Thus, rubbing, moving rapidly and caressing are not useless reactions we have. They activate large fibers that will close the gate on pain signals.

Real-Life Applications

This revolutionary theory has been considered as the explanation of some of today’s pain therapies, like Transcutaneous Electrical Nerve Stimulation (TENS) or acupuncture.

TENS uses low-level electrical currents applied to the skin to help relieve pain. While its exact mechanisms are unclear, the idea is that TENS activates larger nerve fibers which “close the gate” on pain signals from smaller nerve fibers, preventing them from reaching the brain — just like the gate control theory suggests.

Acupuncture might work in the same way. When needles are placed at specific points on the body, they are hypothesized to stimulate these larger nerve fibers, thereby closing the gate once again.

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