Whether it’s dull, sharp, burning or a pressure that could only be described as having a boa constrictor squeezing one of your joints, chronic arthritis pain is all too common. For the approximately 50 million Americans with some form of arthritis or a related disease, pain is a wily and persistent nemesis.

Like a fever or a rash, pain is a symptom, telling you that something is wrong in your body. If you touch a hot iron and burn your finger, you feel pain, telling you to pull your finger away from the heat. Similarly, pain in one of our joints tells you something is wrong. The joint tissues may be inflamed, or the cushioning cartilage and joint fluid may be so depleted that bones rub against one another with every movement.

“Pain is the natural protective response for the body,” says Doreen Stiskal-Galisewski, PhD, chair of the department of physical therapy at Seton Hall University in South Orange, N.J. Pain informs you that there is a problem in your body, so you can address the issue and take steps to heal it.

While its purpose – to alert us to a problem – is simple, pain is a complex phenomenon. Scientists are constantly working to learn more about the different types of pain and the different mental and physical causes of pain. Though research is ongoing, here’s a brief look at what they’ve discovered so far.

Classifying Pain By Its Duration

The first step to defining pain is to note its duration. Doctors classify pain as being either acute or chronic.

Acute pain lasts a short time, from a few seconds after a person burns or injures himself, or for a few weeks as in a back strain that subsides with basic treatments. Chronic pain is longer-lasting, occurring persistently or regularly for at least three to six months. Some chronic pain lasts for a lifetime.

Unlike the short-term, acute pain that results from an injury or brief illness, arthritis pain is chronic, or long-lasting.

Acute pain is typically easier to treat, says Robert Taylor, MD, a neurologist and medical director of the Ohio State University Medical Center for Palliative Care in Columbus. “Acute pain, by definition, is self-limited. It’s a short-term problem and will go away. We get rid of the pain for the short term” using mainly pain-relieving drugs, like analgesics or opiods.

Chronic pain is harder to treat and manage, Dr. Taylor says. “From a patient’s point of view, and even physiologically, chronic pain is a problem in and of itself. Chronic pain becomes its own problem. For many patients, it goes on indefinitely. And chronic pain produces changes in the central nervous system, neurological changes that make the pain more and more entrenched and harder, over time, to treat.”