The experience of pain is essential to humans. We live in a dangerous world and there are lots of things that can hurt us or kill us. The experience of pain warns our body-mind that damage is about to occur (or is occurring) and we immediately withdraw from whatever is causing the pain if we can. Almost all of this occurs at an unconscious level although there is definitely a conscious component to pain also. When we are injured pain forces us to rest the injured part until healing takes place. Once we have healed from the injury the pain goes away. It usually only lasts a few weeks at most.

People who can’t feel pain

To understand how important pain is to our survival, it is helpful to look at what happens when people are insensitive to pain. There is a rare condition called congenital insensitivity to pain. People who have this condition are able to feel touch normally, but are not able to feel pain at all. They sustain multiple injuries and rarely live past the age of 25. People with diabetes sometimes develop numbness in the feet and hands, called diabetic peripheral neuropathy. They are thus unable to feel pain in their feet. This leads to injuries and/or infections that they do not notice. It is common for diabetics with peripheral neuropathy to have amputations of toes or even feet because they can’t feel pain. Leprosy causes damage to skin nerves thus causing inability to feel pain. All of the deformities of leprosy are caused by injuries that are not painful to people with leprosy because their skin nerves are damaged.

Acute Pain

The kind of pain that both helps to keep us from being injured and helps us heal is called acute pain. There is another kind of pain that does not help us at all called chronic pain. More about chronic pain in the next post.

Pain Receptors

Humans (as well as other mammals) have four kinds of special pain receptors (nociceptors) in the skin. They only respond to damaging or potentially damaging input. There are mechanical nociceptors, which respond to pinching or pressure. There are temperature nociceptors that only respond to excessive heat. There are other temperature nocireceptors that only respond to excessive cold. Finally there are chemo nociceptors that respond only to damaging or potentially damaging chemicals.

There are also pain receptors in the intestines and the internal organs, mostly in the covering of those organs (called the peritoneum in the abdomen and the pleura in the lungs). These are much more difficult to study and not much is known about them other than that they are there and cause pain that is not very well localized.

Pain Pathways

There are three main pathways for pain in the nervous system:

• Lateral pathway. This pathway goes from the nociceptors through a structure called the thalamus and then directly to the somatosensory cortex. The somatosensory cortex is located in the side of the brain called the parietal area and contains a map of the whole body. Sensations on the right side of the body connect to the left somatosensory cortex and sensations on the left side of the body connect to the right somatosensory cortex. This reverse order is because all the motor and sensory nerves from the spinal cord cross in the lower part of the brain called the brainstem and connect to the opposite side of the brain. The lateral pain pathway tells us exactly where the pain is coming from , how intense it is, and also the character of the pain (stabbing, pinching, hot, cold, etc).
• Medial pathway. This pathway goes from the nociceptors to the thalamus and then to a part of the brain called the limbic system. This is the pathway to the emotion center of the brain. Pain thus causes an emotional response and lets us know how unpleasant the pain is. This pathway is where suffering starts to happen.
• Inhibitory pathway. This pathway goes from a structure called the amygdala in the brain down to the spinal cord and blocks some of the pain from the other two pathways. This pathway works by releasing endorphins that attach to our opioid receptors. That is why synthetic opioids like morphine help relieve acute pain. They don’t work very well for chronic pain, as we shall see later.

Because of these three different pathways it is possible to feel pain without suffering and to experience suffering without pain. When these three pathways are balanced, we feel acute pain when we should, we find the experience unpleasant, and the pain goes away when we don’t need it anymore. When they are unbalanced, we can experience chronic pain, that is pain that persists beyond when it useful to the body-mind.

Pain and Memory

The experience of severe pain is also perceived in the hippocampus, which is in the temporal lobes of the brain and where memories are created and sent to other areas of the brain for storage. Pain creates instant and strong memories, so that humans and other animals can avoid situations that led to the pain. If you touch the metal handle of a hot frying pan, the memory of that pain ensures that you will use a hot pad from then on!

Bottom Line

Pain is essential to our survival as a species. As a result we and other animals have developed special sensors (nociceptors) that detect input that could damage our bodies or are damaging our bodies. There are two pathways through which the nociceptors send their messages to the brain, the lateral and the medial pathways. The lateral pathway goes to the sensorimotor cortex and allows us to feel the location, intensity and character of the pain. The medial pathway connects to our emotions and lets us know how unpleasant the pain is. That pathway is also where suffering occurs. The descending inhibitory pathway reduces our experience of pain both when getting away is the priority (think soldiers with war wounds who don’t feel pain until later) or when we have healed and don’t need the pain anymore. Chronic pain happens when these three pathways are out of balance. More on chronic pain in the next post.