1. What Happens When Doctors Touch the Brain During Awake Brain Surgery?
Some surgeries to remove brain tumors are performed while the patient is awake, known as awake craniotomy. Because the patient is conscious, they can describe what they feel when stimulated in their own words. Since pain is felt by the brain, one might expect that touching the brain with an instrument would cause intense pain. However, when a research group led by Fontaine examined records from these surgeries, they found something different. Mechanical stimulation caused pain in 93 instances across 53 patients, which affects about one-quarter of those who had surgery [1].
The key point is where the pain occurred. The stimulated areas recorded were not the brain tissue itself, but the membranes covering the brain and small blood vessels. It is generally stated that brain tissue itself does not feel pain [2]. So, what exactly are these "areas around the brain" where pain was felt?
2. Where Does the Pain Come From During a Headache?
The brain is wrapped in thin membranes. The tough outer membrane is called the dura mater, and the membrane that fits tightly against the surface of the brain is called the pia mater. Blood vessels run through various parts of these membranes. In Fontaine’s records, counting the stimulated areas where pain occurred showed 30 instances for the dura mater and 61 instances for the pia mater and small brain blood vessels. About two-thirds of the 93 cases (61 divided by 93 is approximately 0.66) involved the pia mater and small vessels [1]. The pain was reported as sharp, strong, and short, stopping immediately when the stimulation stopped.
Medical explanations list sensitive tissues in the head as including the dura mater (especially at the base of the skull), arteries of the dura mater, venous sinuses, large arteries at the bottom of the brain, and some cranial nerves. Headaches are not caused by damage to the brain itself, but by irritation of the blood vessels and nerves in the membranes surrounding the brain and nearby tissues, which the brain then receives as pain [2].
Since a study by Ray and Wolff in 1940, the pia mater and small brain blood vessels were thought to be insensitive to pain. Fontaine’s team states that these areas can also feel pain [1]. Because this was an incidental observation during surgery and the strength of stimulation was not measured, it is preliminary knowledge. However, our understanding of where pain occurs in the head is still being updated.
3. Do Pain Signals Travel to Only One Part of the Brain?
Pain signals are created when nociceptors (pain sensors) in the skin and other tissues react to stimuli that could damage tissue. The signal travels from sensory nerves through the spinal cord and thalamus to reach the cerebral cortex [3]. Is there just one "center" for pain located somewhere in the brain? Imaging studies (PET) examining human brain activity have identified areas that consistently activate during painful stimuli. These include the insular cortex and anterior cingulate cortex on the side opposite to the stimulus, the thalamus and premotor area on both sides, and the central part of the cerebellum [4].
Even if we say "the place for pain," multiple areas are involved. It is more like several rooms lighting up at once rather than just one room becoming bright. However, this describes where activity is noticeable based on images; the specific roles of each area are not yet fully determined.
4. Does the Brain Process a Pain's Location and Intensity Separately From Its Unpleasant Feeling?
Consider an injection. You know the location and intensity: "It stung here on my right upper arm." At the same time, you feel emotions like "I don't like this" or "I hope it ends soon." Are these two things separate? In classifications of brain areas involved in pain, the primary and secondary somatosensory cortices are associated with location and intensity (the sensory aspect). The anterior cingulate cortex, anterior insular cortex, and amygdala are associated with unpleasantness (the emotional aspect) [5].
Japanese review articles also state that pain is an experience consisting of a sensory side related to recognizing location and intensity, and an emotional side related to unpleasant feelings, noting that there are sensory and emotional pathways. However, the idea that these split neatly into two systems is a simplified view. The specific roles of each pathway are still not well understood [8]. It is best to understand that "intensity" and "unpleasantness" tend to be handled in different parts of the brain, but they are also experiences that interact with each other.
5. Is a Pain Sensor Reacting the Same as Feeling Pain?
The International Association for the Study of Pain (IASP), a group of pain researchers, defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage." This definition includes notes stating that "pain and nociception are different phenomena" and "pain is always a personal experience" [6].
Nociception is the function of sensors reacting to stimuli that could damage tissue. The sensor reacting is not the same as whether the person felt pain. Nociception and pain are different phenomena. Thinking "Pain equals sensor reaction" is not accurate.
This definition was revised in 2020 for the first time since 1979 [7]. The phrase "or resembling that associated with" was added during this revision [6].
Connecting these points, we see that while the brain receives pain, its own tissue usually lacks pain sensors. In headaches, pain from membranes and vessels reaches the brain, and multiple brain areas handle "location and intensity" separately from "unpleasantness."
6. How Can You Separate Your Memory of Past Pain Into Three Parts?
Try an exercise to separate past memories of pain. Recall a time you bumped into something. Write down on paper: 1) Where did it hurt? 2) How much did it hurt? 3) How unpleasant was it? The first two items relate to the sensory side, while the third relates to the emotional side. It is fine if one aspect remains more prominent than others depending on the person or situation.
If you are currently injured, show the injury to an adult before trying to remember past pain [8]. If you want to read more, Japanese review articles discuss the division between sensory and emotional systems. Use Chapter 4 of this article as a guide to compare how "location and intensity" and "unpleasantness" are divided. The definition of pain can be read on the IASP website [6].