Referred Pain

Ah, the classic heart attack. Although you may not be a doctor, you've probably heard of a few heart attack warning signs, a common one being chest pain that radiates down your left arm. If you are reading this article to learn about heart attacks, here is where I must disappoint you. My focus will not be the sexy topic of myocardial infarction. Instead, let’s talk about why damage in one place can cause pain in another. Spoiler alert: no arm tissue was harmed in the making of that heart attack.
Our brains communicate with the rest of the body through a vast network of nerves, and the main highway for all that communication is the spinal cord. Nerves that bring information from the body to the brain are called “afferent nerves.” Now, no matter how far out in the boondocks these afferent nerves are coming from, sooner or later, they are going to have to merge onto the highway. As it turns out, the spinal cord is similar to a real highway in that it has a finite number of exits and entry ramps. So at the same entry ramp where nerves from the heart merge onto the spinal cord, there are arm nerves merging as well.
Now is where we will have both our eureka moment and also a little bit of hand-waving. Somehow (that was the hand-waving) nerves that come into the spinal cord through the same entry ramp seem to be able to get their signals crossed, so that when your heart is trying to tell the brain “ouch” by the time the arm neurons get to the brain, they’re shouting “ouch” too. Your brain has no way to know that the pain message didn’t originate from the arm. After all, it came along the nerve that represents the arm, so that’s where you perceive it.
When you have an injury or damage to one part of your body that causes the sensation of pain somewhere else, this is called referred pain. Although science still can’t tell you precisely how nerves that merge onto the spinal cord together get their signals crossed, now you know the basics on why heart attacks cause arm pain, and why gallbladder inflammation can make your shoulder hurt.

Photo Credit/Irina Blok

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