Why do we feel pain? Who is less afraid of pain, men or women?

Produced by: Science Popularization China

Author: Clover Aoko (Doctor of Medicine)

Producer: China Science Expo

Women's pain has long been ignored

(Image credit: Daniel Stolle for Nature)

Pain is a common experience in our lives, whether it's a headache, a stubbed toe, or the pain of an injection, everyone experiences it to varying degrees.

However, it is curious whether men and women actually perceive pain differently . On the surface, there seems to be "evidence" on both sides. Women are impressively stoic during childbirth, while men seem to be more pain-free in physical adventures and warfare.

Through experimental research, scientists have revealed a more complex answer - there are indeed significant differences in pain perception between genders, but the biological, psychological and social factors behind it are far more complex than we imagined .

What is pain?

To understand pain, we first need to understand how it is produced. Pain is not a simple physical signal, but a complex processing of nerve signals by the brain .

Our bodies are full of pain receptors, which are specialized sensory cells designed to detect painful stimuli. These receptors are found in muscles, internal organs, and skin. Once the receptors detect a painful stimulus, they transmit signals to the brain through neural networks, ultimately triggering the perception of pain.

Pain receptors are divided into the following categories:

Thermal nociceptors: sensitive to extreme temperatures (such as above 45°C or below 5°C).

Mechano-nociceptors: sensitive to strong pressure, such as cuts or blows.

Polymodal nociceptors: can respond to mechanical, chemical or thermal stimuli.

Silent nociceptors: These are found primarily in the internal organs and are usually activated only in response to inflammation or chemical stimulation.

Nociceptors and spinal input to pain: Different nociceptors are connected to axon fibers; neurons in lamina I of the dorsal horn of the spinal cord receive input from myelinated (Aδ) nociceptive fibers and from unmyelinated (C) nociceptive fibers via interneurons in lamina II.

(Image source: "Principles of neural science")

Different types of pain signals are transmitted to the brain through nerves at different speeds. They are transmitted to the spinal cord via Aδ axons that conduct at speeds of 5 m/s to 30 m/s and C fiber axons that conduct at speeds less than 1 m/s. The nerve signals are transmitted to the brain through the spinal cord and finally reach multiple areas in the brain for processing, especially the insula and anterior cingulate cortex.

The anterior cingulate cortex and insula are the areas at the end of the pain information processing route.

(Image source: "Principles of neural science")

It is these brain areas that allow us to experience pain and trigger the emotional responses associated with it, such as anxiety, fear, or discomfort.

Gender differences in pain

Although the physiological mechanisms of pain are basically the same in all people, gender significantly affects each person's perception and tolerance of pain.

In a classic 1998 experiment, researchers tested 10 women and 10 men with heat stimulation. Not only did the women report higher pain levels, but their brains also responded more strongly than the men, showing that gender differences in pain perception do exist. Another study of more than 27,000 people in 19 European countries found that women were 10% more likely than men to report chronic pain. In fact, about half of chronic pain is more common in women than in men.

Of course, for sociocultural reasons, men are less willing to admit that they experience heat as painful, but that doesn't mean there aren't underlying physiological differences in how the two sexes experience pain.

Pain epidemiologists at the Seattle Children's Research Institute in Washington say that the impact of gender and gender characteristics on the experience of pain begins to emerge during adolescence . After puberty, men and women begin to secrete different sex hormones—men mainly secrete testosterone, while women secrete estrogen. Testosterone is associated with a higher pain threshold, and men usually perceive pain less severely and have a stronger tolerance when facing pain. Estrogen fluctuations make women more sensitive to pain, especially at certain stages of the menstrual cycle. For example, women are more likely to experience migraines before and after menstruation, and this pain usually subsides with menopause.

In addition, the role of the hormone prolactin in women cannot be ignored . Studies have found that women with prolactinomas are at higher risk of migraine attacks because prolactin can lower the activation threshold of pain receptors, a mechanism that is rare in men because most male neurons do not have prolactin receptors.

Prolactin receptors (green) are absent in most male neurons.

(Photo credit: Frank Porecca)

Remember the pain perception terminal in the brain mentioned earlier - the anterior cingulate cortex?

Neuroscientist Natalie Osborne used functional magnetic resonance imaging to study the interaction of the anterior cingulate cortex with other pain-sensitive parts of the brain in two chronic pain conditions : ankylosing spondylitis (an inflammatory bone disease more common in men) and carpal tunnel syndrome (more common in women). She found that there were differences in connectivity of the anterior cingulate cortex below the genu between men and women with and without chronic pain, and that these sex differences were also influenced by whether the disease occurred primarily in women or men.

Neuroscientist Natalie Osborne studies areas of the brain involved in pain processing.

(Image credit: Kirk Maile)

Animal studies have also revealed the existence of gender differences . Male and female mice rely on different cellular pathways in the process of pain transmission, males rely on microglia and females rely on T cells. This suggests that treatment for pain may need to vary by gender. The researchers mentioned, "In the past, people tended to only focus on the results of studies on male animals and then directly apply them to female animals, ignoring many important gender differences and details in pain research."

Although biological mechanisms suggest that women are more sensitive to pain in certain situations, society's perception of women's pain is biased, especially pain problems related to the reproductive system, such as menstrual pain, which are often seen as "normal" and ignored.

As scientific research progresses, the impact of gender on pain is gaining more attention, the number of pain studies involving male and female animals is increasing, and women's pain dilemma is gradually being taken seriously.

Pain perception under the influence of multiple factors

In addition to being influenced by gender and hormones, pain perception is also influenced by genetic factors.

CIP (Congenital Insensitivity to Pain) is a disease caused by mutations in the SCN9A gene, which encodes a sodium channel protein that controls the conduction of nerve signals. CIP patients cannot feel pain and seem to be "lucky", but in fact, they often suffer more serious physical damage due to their inability to detect injuries, such as undetected bruises, burns, and even shortened lifespans. This also reminds us that although pain is uncomfortable, it is an important protective mechanism.

Age is also an important factor affecting pain perception. As humans age, the threshold for pain increases, which means that stronger harmful signals are needed to alert the elderly that something is wrong, which increases the risk of injury. Conversely, their tolerance for pain usually decreases. This may be related to the decline of neurons in the brain that process pain.

Traumatic experiences can also have a profound effect on pain perception, with patients with post-traumatic stress disorder (PTSD) often reporting a heightened sense of pain. Even more rare is synaesthesia, in which patients experience exaggerated empathy for the pain of others after experiencing traumatic pain.

There are cases where such patients experience similar pain when they see others being injured. Amputees (most of whom lost their limbs in traumatic circumstances) experience phantom pain from seeing others experiencing pain, or even just from stories of those experiences.

Facing the pain can help relieve it better

Pain is an inevitable part of our lives, but how it is perceived and tolerated varies greatly from person to person. As we have discussed, pain sensitivity and tolerance are influenced by many factors—sex, hormones, genetic background, traumatic experiences, and age—all of which combine to create a unique pain experience for each person.

Since 2004, the International Society for the Study of Pain has designated the third Monday of October each year as World Pain Relief Day. Understanding the multidimensionality and individual differences of pain is the key to improving global pain management. Only by recognizing the differences in pain perception among different groups of people can we truly find solutions to those often ignored pains.

With the advancement of science, we have more tools to objectively measure pain, and explore pain assessment criteria and treatment methods through biomarkers and brain signal analysis. In the following article, we can discuss how to more accurately assess pain through scientific means, as well as the cutting-edge progress in the field of non-drug analgesic treatment in recent years.

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