One of the things I love about medicine is that if you pay close attention, what we know is being constantly updated and expanded and sometimes rewritten. Take for example nerves. Who would have thought that new nerve cells that carry pain signals would still be being discovered in 2017? You would have thought that stuff ended at the turn of the 19th century, but if so, you would be wrong. This morning I’d like to introduce you to a new cell that may have implications for how we treat chronic pain.
Neurons: The Cells of Our Nervous System
Technically termed neurons, nerve cells are the basic units that make up the nervous system including both the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the spinal cord and brain, while the PNS consists of all the nerves that branch off of the CNS, such as those that branch off of the spinal cord and into the arms and legs. Our nerve cells’ main job is to send signals back and forth between the two systems. For example, if we stub our toe, nerve cells send a pain signal to our brain. There are many different types of nerve cells, and our focus today is on somatosensory nerve cells, particularly those that are stimulated by external sensations such as pain, heat and cold, and even touch.
Before we dive into new study, let’s look at some other fascinating studies on nerve cells that we’ve covered over the past year.
So Much of the Nervous System Is Still Unexplored
Medicine is constantly learning new things about the nervous system. As technology continues to advance at a rapid pace, most people think we know most everything there is to know about the functions of the human body, but the truth is, we’re probably closer to the 60% mark, and the nervous system may perhaps be the most unchartered body system of all. Just looking at pain, for example, there are so many different types of pain stemming from so many different types of nerve cells (neurons) that understanding each and every one is tough. But one study at a time, we’re learning more and more about the nerve cells that are associated with pain. Let’s review a few of the most recent studies.
- When we are in chronic pain, our pleasure-sensing nerve cells can actually switch gears, transmitting pain signals instead of pleasure signals as a response to even the gentlest touch.
- Microglial cells (I also call these Pac-Man cells), immune cells that live alongside our nerve cells and support and protect our central nervous system by cleaning up waste, can become hyperactive, sensitizing the nerves. Chronic pain conditions have been shown to result from sensitized nerves.
- The interaction of nerves, structure, and chemicals is complex, and any one or combination of the three can cause pain; therefore, we need a neuromechanochemical (neuro+mechanical+chemical ) model for pain. MRI images alone are poor tools for diagnosing pain because pain isn’t just about structure.
Now let’s take a look at the new study on nerve cells.
Painful-Hair-Pulling Study Discovers New Nerve Cell
In the new study, researchers were looking specifically at mechanical pain, which is the pain we feel due to some type of mechanical stimuli, such as a pinch or, in this case, hair pulling. This is typically a brief pain response that quickly subsides once the stimulus is removed (e.g., once the hair pulling has stopped).
Using mice as subjects, the study used optogenetics, which involves using a light-based technology on cells genetically engineered with a fluorescent protein. The protein flashes green as it comes into contact with calcium (i.e., calcium imaging) when the cells are activated. Activation occurs when the cells sense that a painful mechanical stimulus has been applied (e.g., whiskers pulled). In other words, when the whiskers were pulled, certain pain-sensing cells are activated, and the green-flashing proteins told researchers which cells responded.
Researchers discovered that the ends of the nerve cells responsible for pain with hair pulling (interestingly, even just with one hair being pulled) wound themselves around each and every hair follicle, so they named these new cells circ-HTMRs (circumferential high-threshold mechanical receptors). Researchers hope that the discovery of this new nerve cell will further the understanding of mechanical pain and chronic pain in the future.
The upshot? Again, it blows me away that we’re still discovering new nerve cells in 2017! This and many other studies like it really make it clear that we as physicians need to keep an open mind that to the fact that we don’t know all there is to know. Regrettably, after a decade plus of learning, most physicians believe that they have learned and know it all. Hopefully studies like this will show that they really don’t.