The Pain Story

Planted throughout our bodies are pain receptors waiting to be stimulated by the experience of tissue damage from burning, penetrating, tearing, crushing, fracturing, poisoning or freezing. Almost all of our tissues have a rich variety of these sense receptors, which are actually specialized nerve endings. They are all connected to nerves that run to the central nervous system, to pass on the information of injury. In this way we can avoid further injury through stopping use of an injured body part. Pain also causes us to withdraw reflexively from painful stimuli preventing extension of the damage. So rather than tormenting us, these pain receptors in our tissues literally save our lives and secure our well being. What happens when it all goes wrong, though? Why does pain come and stay? Why does pain remain severe or even worsen after the body has healed itself? Why, given similar injuries and treatments, do some people improve and others become worse? It is in answering these questions and others that the story of pain emerges.

Pain Receptors

Pain receptors are specialized nerve endings located throughout the body in most body tissues. They transmit pain from injury, disease, movement or environmental stress. These specialized nerve endings are stimulated by release of pain producing chemicals that arise from local blood vessels, connective tissue cells called fibroblasts and specialized blood cells in the tissues called macrophages. Once the nerve endings are stimulated by these chemicals they begin firing the nerves that are connected to them and send pain signals to the spinal cord and brain.

Nerves

Nerves are complex structures that carry electrical information throughout our bodies. Virtually every tissue is connected to the central nervous system through nerves. The nerves are composed of tens of thousands of nerve cells called neurons. The nerve cells have varied functions including muscle contraction and relaxation (Motor Neurons), temperature sense, position sense, vibratory sense, touch sense, pain sense. Additionally there are nerves that conduct specialized sensations such as hearing, seeing, smelling and tasting. Nerves are also involved in many activities that regulate the body’s functions automatically. Pain nerves often begin as special receptor endings implanted in various tissues. The receptors’ axons join together to form nerve fibers which in turn form smaller nerves that join with larger nerves. These continue to come together with other nerves as they approach the spinal cord. Just before they attach at different levels of the spinal cord the axons run into nerve cell bodies in an area known as the Dorsal Root Ganglion (DRG).These cell bodies send terminal axons to synapse with the cell bodies in the back part of the spinal cord and it is this way that pain is passed to the brain.

Spinal Cord

With rare exception nerve impulses from the body all must come into the spinal cord. Here the nerves synapse with spinal nerves that form tracts that run to the brain. Where pain is concerned, these tracts run through the spine to the part of the brain known as the Thalamus. The Dorsal Horn is the part of the spinal cord that receives painful nerve impulses. Here nerve axon terminals synapse with nerve cell bodies. It is here where the battle against chronic pain begins. If this part of the nervous system fails, pain can be greatly increased and difficult to bring under control. Failure can occur with NMDA receptor based wind-up, neuropathic pain and/or brain based modulating.

Brain

When pain signals arrive at the brain they are distributed to multiple regions, including the medulla, cerebellum, pons, limbic system and somatosensory cortex. At the same time the brain is also modulating incoming pain signals by sending down a countersignal from the cerebral cortex, diencephalon, midbrain, pons and medulla. Although the animation shows this to be occurring in order, this would best be understood as occurring simultaneously in a symphony of neuronal activity.

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