Pain and the Brain

All pain and other sensations, including those that are pleasurable or neutral, are routed to the brain through the nervous system. The peripheral nervous system is the most external and contains the sensory and motor neurons that control movement and surface types of pressure, touch, pain and sensations. The peripheral nervous system feeds into the central nervous system, which is comprised of the spinal cord and the brain. All of the peripheral nerves feed impulses through the body to the spinal cord, which carries the message up to the brain. The only part of the human body that can trigger the sensation of pain is the brain, but other parts of the central nervous system can trigger reflex responses that move your body away from the source of pain without the need to wait for the brain to respond.

Pain Receptors

The pain receptors in the body are called nociceptors. The specific mechanism by which the nociceptors, on an internal basis, respond to pain is not clearly understood. Researchers believe that it is more complex than just a single response pattern and may include variations in the extracellular matrix, cellular features and even sex hormones that may be signaling the nociceptors. 1 This complicated interplay and variation may account for the different levels of pain various parts of the body respond to and how some people have different pain thresholds than others.

Different nociceptors have different roles in responding to pain. Some respond to heat or cold that is outside of an acceptable range while others respond only to physical damage to the issue or a chemical that is causing a reaction in the cells that is harmful or damaging. The role of pain in this form is to instantly have the brain signal to the body to move, getting the tissue away from any source of damage. Various nociceptors may also be located deeper in the tissue which is most commonly seen in deep muscle and joint pain where the damage or injury to the tissue is internal, not from an external source.

Neuropathic Pain

Not all pain is triggered by nociceptors; it can also be triggered by damage to the nervous tissues. This is mostly found in the somatosensory system, which includes the nociceptors along with the ability to determine body position (proprioception) as well as taste and touch.  Since this is a much more generalized system there are multiple pathways for the message of this type of pain and discomfort to get to the brain, and there are several parts of the brain that may react. Most somatosensory information is processed in the parietal lobe; however, the sense of touch is controlled by the postcentral gyrus, which is linked to the parietal lobe.

Neuropathic pain is often interpreted by the brain to be a burning or tingling sensation, similar to a limb that has “fallen asleep” and has pins and needles when circulation is restored. However, with most neuropathic pain, there is no specific reason for the pain, but it can be be very intense and shooting or even more chronic in nature.

Getting the Message

The pain receptors, either the nociceptors or nerve tissues, send the pain message in the form of a chemical change that travels between neurons from the location of the injury or change through the peripheral nerves to the spinal cord. In the spinal cord, the dorsal horn may trigger a reflex response to create the movement away from the source of the pain depending on the message received. However, the signal that there is pain continues up the spinal cord until it reaches the thalamus, which is located right above the brainstem and in approximately the center of the brain.

The thalamus then routes the information to the respective part of the brain that handles that specific type of pain. In addition the thalamus may also send a message to the emotional center of the brain, the limbic system, which triggers an emotional response such as crying or becoming frightened or angry because of the pain. This also triggers a connection to other similar experiences that are catalogued in your memory.

The emotional response to pain changes based on a variety of factors. Individuals that are diagnosed with mood disorders, such as depression, have a very different response to even low levels of pain, leading researchers to look towards a more holistic way of treating the two. This is effective because of the overlap of the neurobiological mechanisms of serotonin and noradrenalin in both pain sensations and mood. 2

Responding To Pain 

Once the brain receives the message of pain it immediately triggers a response that is designed to prevent further injury. This is more complex than just a reflex and includes all the past history that is catalogued about what has happened when you have had a similar type of pain in the past. This can include triggering you to look for a sliver in the skin or to immediately rinse a burn under cold water.

If the nociceptors are no longer triggered by the presence of the pain, in this case a sliver, your brain stops receiving the message of acute pain. However, if you bang your finger or hit the wound site the nociceptors again send a message. This helps your body to heal and voids re-injuring the specific tissue. The brain, specifically the opioid system, responds to pain by releasing endogenous opioids, the endorphins, which block further pain messages to the brain about the same injury. The level of pain and the electrical activity in the brain contribute to the amount of analgesic efficacy of the opioids in the body. 3

However, in some injuries and possibly in fibromyalgia, deep muscle pain and joint pain leads to receptors continually firing for prolonged periods of time. This is known as chronic pain, and this constant firing of the neuropathic pain pathway or from the nociceptors causes the brain to constantly respond to the pain. Over time this changes the chemical balance in the brain and results in changes in the brain’s ability to respond appropriately to pain or to provide opioids that prevent the nociceptors from continually firing.

References

1 Hucho, T., & Levine, J. (2007). Signaling Pathways in Sensitization: Toward a Nociceptor Cell Biology. Neuron , 365-376.

2 Greden, J. (2009). Treating depression and pain. Journal of Clinical Psychiatry , e16-e18.

3 Stein, C., & Lang, L. J. (2009). Peripheral mechanisms of opioid analgesia. Current Opinion in Pharmacology , 3-8.

This article was originally published on July 11, 2012 and last revision and update of it was 9/7/2015