It's not just the skin that can sense warmth and cold, but also the teeth

Author: Shurui Chen of Nagoya University

The surface of teeth is covered with a layer of hard enamel, which protects the internal tissues and enhances the ability to chew food. This layer of enamel is like a tooth armor. As long as the tissues under the armor are not touched, the teeth will not easily detect the taste of sour, sweet, bitter, spicy, cold and hot. However, when bacteria or acidic substances corrode the enamel, the internal parts are exposed, and even cavities are formed, which is called "tooth decay", the teeth will become very sensitive.

This is common sense that everyone has, but if you dig deeper into the matter, such as asking why teeth react to various stimuli and what kind of wonderful biochemical changes occur in the human body during this process, I'm afraid no one can give a decent answer.

Recently, an article published in "Science Advances" gave a preliminary view on the above question. It turns out that there is a complete system in our teeth that can sense cold and other stimuli, and this is why many patients with tooth decay feel pain when eating ice cream.

In fact, not only teeth, but also the human body is distributed with various temperature sensors, which report the collected temperature information to the body temperature regulation center in the brain, and the center then directs the body to make appropriate responses. The complexity and sophistication of this system can easily beat various high-precision sensors and robots, which really makes people marvel at the magic of evolution.

The human body's temperature regulation system is not just in the teeth, it's all over the body

Let's first look at how human tissues other than teeth sense changes in ambient temperature and maintain a constant body temperature. The body's temperature regulation system can be divided into three parts: temperature receptors, temperature regulation centers, and sweat glands and skin blood vessels controlled by them. This system varies from person to person, so some people are more afraid of cold and some are more afraid of heat.

The first step in regulating body temperature is to sense temperature. There are countless temperature receptors distributed in the human skin, mucous membranes, abdominal viscera, etc. When they are stimulated by cold or heat, nerve impulses are generated and pulse signals are sent to the brain. These receptors have different response temperature ranges, and the human body surface perception is divided into four types: cold, cool, warm, and hot.

For example, thermoreceptors are free nerve endings distributed in a network around the root sheath of hair follicles in the granular layer of the epidermis. When the skin temperature is around 35 degrees Celsius, the thermoreceptors are most sensitive, and the human body will feel warm. While cold receptors are small columnar spherical bodies located in the shallow layer of the skin. They are most active when the skin temperature is around 30 degrees Celsius, and the human body will feel cold.

The biological essence of these temperature receptors is actually a special ion channel - transient receptor potential channel (TRP channel). In different temperature ranges, the corresponding ion channel will open, and through the mutual conversion between electrical signals and chemical signals, it will eventually transmit nerve impulses to the body temperature regulation center in the hypothalamus, allowing it to sense the corresponding temperature.

The body temperature regulation center is divided into the heat production center and the heat dissipation center. After receiving transient cold and hot sensory stimulation, they will integrate the stimulation signals and then regulate the skeletal muscles, skin blood vessels and sweat glands, so as to promote or inhibit the process of heat production and heat dissipation to maintain a constant body temperature. When the ambient temperature is low, the excited cold receptors transmit nerve impulses to the heat production center, which in turn causes the skeletal muscle tension to increase and heat production to increase; at the same time, the heat dissipation center is inhibited, causing the skin blood vessels to contract, the sweat glands to stop secreting, and less heat dissipation. Conversely, when the ambient temperature is very high, the heat receptors and the heat dissipation center are excited, and the cold receptors and the heat production center are inhibited, so the skin blood vessels are dilated and the sweat glands secrete more.

Whether it’s pain or temperature, the human body’s sensory systems are very different

Similar to body temperature, pain is also an important part of vital signs. The human body uses pain to warn of diseases and injuries, thereby protecting the body. Pain receptors are distributed throughout the surface layers of the body such as skin, muscles, joints, and deep layers such as organs. They are composed of free nerve endings, but they are slightly different from temperature sensors in that they receive stimulation from chemicals released by damaged tissues, rather than generating nerve impulses in response to physical stimuli such as temperature changes.

When the body is exposed to harmful stimuli, tissue damage will cause damaged cells to release certain chemicals (such as prostaglandins, bradykinin, 5-hydroxytryptamine, H+, K+, etc.), which will excite pain receptors, subsequently generate afferent impulses, and send this signal to the first sensory area, second sensory area and other parts of the cortex, causing pain.

Miracle protein helps teeth sense cold

With the change of seasons, we can add or remove clothes according to the temperature, but teeth feel the temperature and often experience pain. This is actually an "unfathomable" topic. For example, why teeth convert temperature signals into pain signals is difficult for us to answer. In this newly published paper, researchers explained the molecules and cells involved in the system of tooth perception of cold. They found that odontoblasts contain cold-sensitive proteins TRPC5 and TRPC1 that can sense temperature drops, and they can ultimately convert the perceived cold into pain in the brain.

The difficulty of this study is that in order to understand the internal tissue of the tooth, the enamel must be broken without damaging the internal pulp, nerves and blood vessels. However, once the enamel is knocked off, the entire tooth will shatter, so the researchers used a clever method to solve this experimental observation problem. They took the mandible, teeth and dental nerves as a whole to record the nerve activity of the tooth after it came into contact with the cold liquid.

The study found that in the normal control group, cold stimulation caused nerve activity to become excited, indicating that the teeth sensed the cold; while in the experimental group whose teeth lacked TRPC5 cold-sensitive protein, the same stimulation did not cause nerve excitement. In other words, TRPC5 is an important way to perceive cold. And TRPC1 has a similar effect.

Subsequently, they separated TRPC5 from odontoblasts and expressed a large amount of TRPC5 on one cell. We can understand this process as increasing the TRPC5 content in a cell. As a result, the researchers observed that this cell was very sensitive to cold stimulation and could transmit stimulation signals to the brain more quickly and convert them into pain perception in the brain.

Researchers have proposed that the results of this study not only reveal the molecular mechanism of how teeth sense cold, but also that TRPC5 cold-sensing protein can undoubtedly serve as a good molecular target. By blocking or inhibiting the TRPC5 pathway, we can alleviate the troubles caused by tooth sensitivity and toothache. Of course, further research and development is needed for this idea to become a reality.

In essence, the human body's regulation of various signs is the use of chemical systems. When the concentration difference of chemical substances inside and outside the cell membrane is large enough, the change in charge brought about by it will be quickly transmitted to the central nervous system that plays the role of commander through neurons in the form of electrical signals. The central nervous system that receives this signal will integrate and process the information to determine the response measures that the human body should take in the current environment.

For the human body's sensory system, each signal is a specific prompt. We can feel the warmth of spring and the coolness of summer, and we can taste the sour, sweet, bitter and spicy flavors. More importantly, the rich and colorful senses help us detect and resist diseases as early as possible.