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Updated July 16, 2014.
How does your brain know when you feel pain? How does it know the difference between the soft touch of a feather and a needle prick? And, how does that information get to your body in time to respond? How does acute pain become chronic pain? These are not simple answers, but with a little explanation about how the nervous system works, you should be able to understand the basics.
The Nervous System
Your nervous system is made up of two main parts: the brain and the spinal cord, which combine to form the central nervous system; and the sensory and motor nerves, which form the peripheral nervous system.
The names make it easy to picture: the brain and spinal cord are the hub, while the sensory and motor nerves stretch out to provide access to all areas of the body.
Put simply, sensory nerves send impulses about what is happening in our environment to the brain via the spinal cord. The brain sends information back to the motor nerves, which help us perform actions. It’s like having a very complicated in and out box for everything.
Pain Begins with Nerves
Let’s say you step on a rock. How does a sensory nerve in the peripheral nervous system know this is any different than something like a soft toy? Different sensory nerve fibers respond to different things, and produce different chemical responses which determine how sensations are interpreted. Some nerves send signals associated with light touch, while others respond to deep pressure.
Special pain receptors called nociceptors activate whenever there has been an injury, or even a potential injury, such as breaking the skin or causing a large indentation.
Even if the rock does not break your skin, the tissues in your foot become compressed enough to cause the nociceptors to fire off a response. Now, an impulse is heading through the nerve into the spinal cord, and eventually all the way to your brain. This happens within fractions of a second.
Your Spinal Cord: The Middle Man
Your spinal cord is a complex array of bundles of nerves, transmitting all kinds of signals to and from the brain at any given time. It is a lot like a freeway for sensory and motor impulses. But your spinal cord does more than act as a message center: it can make some basic decisions on its own. These “decisions” are called reflexes.
An area of the spinal cord called the dorsal horn acts as an information hub, simultaneously directing impulses to the brain and back down the spinal cord to the area of injury. The brain does not have to tell your foot to move away from the rock, because the dorsal horn has already sent that message. If your brain is the body’s CEO, then the spinal cord is middle management.
How Your Brain Sees Pain
Even though the spinal reflex takes place at the dorsal horn, the pain signal continues to the brain. This is because pain involves more than a simple stimulus and response. Simply taking your foot off the rock does not solve all of your problems. No matter how mild the damage, the tissues in your foot still need to be healed. In addition, your brain needs to make sense of what has happened. Pain gets catalogued in your brain’s library, and emotions become associated with stepping on that rock.
When the pain signal reaches the brain it goes to the thalamus, which directs it to a few different areas for interpretations. A few areas in the cortex figure out where the pain came from and compare it to other kinds of pain with which is it familiar. Was it sharp? Did it hurt more than stepping on a tack? Have you ever stepped on a rock before, and if so was it better or worse?
Signals are also sent from the thalamus to the limbic system, which is the emotional center of the brain. Ever wonder why some pain makes you cry? The limbic system decides. Feelings are associated with every sensation you encounter, and each feeling generates a response. Your heart rate may increase, and you may break out into a sweat. All because of a rock underfoot.
Where it Gets Complicated
While it may seem simple, the process of detecting pain is complicated by the fact that it is not a one-way system. It isn’t even a two-way system. Pain is more than just cause and effect: it is affected by everything else that is going on in the nervous system. Your mood, your past experiences and your expectations can all change the way pain is interpreted at any given time. How is that for confusing?
If you step on that rock after you have a fight with your wife, your response may be very different than it would if you had just won the lottery. Your feelings about the experience may be tainted if the last time you stepped on a rock, your foot became infected. If you stepped on a rock once before and nothing terrible happened to you, you may recover more quickly. You can see how different emotions and histories can determine your response to pain. In fact, there is a strong link between depression and chronic pain.
When Acute Pain Becomes Chronic
In this scenario, after your foot healed, the pain sensations would stop. This is because the nociceptors no longer detect any tissue damage or potential injury. This is called acute pain. Acute pain does not persist after the initial injury has healed.
Sometimes, however, pain receptors continue to fire. This can be caused by a disease or condition that continuously causes damage. With arthritis, for example, the joint is in a constant state of disrepair, causing pain signals to travel to the brain with little down time. Sometimes, even in the absence of tissue damage, nociceptors continue to fire. There may no longer be a physical cause of pain, but the pain response is the same. This makes chronic pain difficult to pin down and even more difficult to treat.
Sources:
Hobson, Anthony R and Aziz, Qasim. Central Nervous System Processing of Human Visceral Pain in Health and Disease. News in Physiological Sciences. Vol. 18, No. 3. June 2003. pp 109-114
Pain Mechanisms. http://www.painclinic.org/aboutpain-painmechanisms.htm The Pain Clinic. Accessed March 16, 2009.
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