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.

Synapse

The synapse is the gap between the nerve axon terminals heading towards the spinal cord and the next nerve cells in the signal chain. In the case of pain these nerves synapse in the back part of the spinal cord called the Dorsal horn. Nerves convert electrical energy into chemical energy and back to electrical energy. This is the way all nerve cells in the body work. This animation shows a synapse enlarged many times to illustrate the presynaptic cell, synapse and postsynaptic cell.

Nerve Transmission

Presynaptic nerve dendrites receive an electrical signal.

It is converted into a chemical signal by release of neurotransmitters.

These chemicals connect with exact fitting post-synaptic receptors and make a new electrical signal.

Chemicals are released from receptors and broken down or transported back into the presynaptic cell.

Energy is provided by Mitochondria.

Acute Pain

Acute pain travels into the spinal cord along the appropriate nerve root. The nerve root splits into a front division and a back division. Among other functions, the nerve root carries pain to the Central Nervous System (the spinal cord and brain). The pain signal is passed to a short tract of nerve cells (Interneurons), which in turn synapse with a nerve tract that runs to the brain . From there it is sent out to the rest of the brain, connecting with thinking and emotional centers. A Modifier Pathway from the brain modifies pain at the synapses in the back part of the spinal cord. It is partially for this reason that acute pain is decreased rapidly after tissue injury.

Chronic Pain

Chronic pain occurs when the back part of the Spinal Cord has been bombarded by severe pain for a long period of time. The Interneuron adjusts and transforms to the Wind-up Nerve by adding fast pain receptors that amplify the pain signal and pass the amplified signal to the the nerve tract that runs up to the brain. From there the thinking and emotional centers of the brain receive a large pain signal and the mind perceives increasingly severe intractable pain. Modifier Pathways in the brain attempt to tone down the pain, but are defeated at the back part of the spinal cord by the Wind-up Nerve. The brain keeps receiving amplified pain signals and the mind continues to perceive unrelenting and severe pain

Muscle Spasm Pain

Muscle spasm pain can be quite severe. Pressure and Pain receptors in the muscle fibers become activated and a local inflammatory response is mounted. The increase in local inflammation serves to increase the pain and further irritate the muscle in spasm. This does not have to affect the whole muscle and can involve only some fibers, causing myofascial trigger points. These are places in a muscle that go into spasm and inflame the covering of the muscle locally. These can be exquisitely painful and send referred pain to other parts of the body

Inflammatory Pain

Inflammatory pain causes the receptors in the inflamed tissue to transmit the pain signal to the nerve that supplies the affected area. The inflammatory response is a complicated local reaction, involving numerous body chemicals, blood vessel changes, temporary or permanent tissue damage and potential spread to other sites. This can be an acute process or it can become chronic. All body structures in the inflamed area become inflamed themselves, including nerve endings. This can complicate the pain picture.

Nerve Pain

Neuropathic pain is the result of damage to the sensory part of a nerve. Nerves carry varied information to the spine and brain. They are made up of many nerve cells bundled together. When a nerve becomes damaged it can affect many types of function, including muscle control, temperature sense, position sense and pain. Pain is often the first sign of nerve damage. The nerve attempts to repair itself and forms microscopic nerve endings that fire off in rapid and prolonged ways. The result is a greatly increased pain signal arriving at the central nervous system. Additionally, because the presynaptic nerve is damaged, receptors used to turn down the pain on the presynaptic cell are also prevented from working. The result is excruciating and intractable pain sent to the brain and perceived by the mind, with no chance of Central Nervous System modification. This becomes even more complicated when the damage occurs or spreads to the part of the nervous system that normally controls automatic functions such as breathing and heart rate

Wind-up Pain

NMDA receptors are formed after the back part of the spinal cord (Dorsal Horn) is bombarded with pain for an extended period of time. These are receptors that use a very fast neurotransmitter and they cause a marked increase in the amount of pain transmitted to the brain. They also prevent opioids from working to their full effectiveness.

Central Pain

Central Pain is a controversial topic that has strong believers and detractors. Most pain specialists believe that central pain exists, but disagree as to what causes it to occur. The idea is that pain can cause central nervous system augmentation either through chronic stimulation, severity of injury, location of injury, chemical alterations, burst activity in specific locations, sympathetic nervous system coupling, or loss of balance between ascending and descending pathways. Central pain is believed to be caused by a variety of injuries to the peripheral nerves, the Dorsal Root Ganglion, Spinal Cord, Spinothalamic Tract, Thalamus or other brain structures.

Facet Neurotomy

If the facet joints(zygapophyseal joints) are suspected of being the source of some or all of the spine related pain, these can be injected with anesthetic and corticosteroids to see if the pain can be blocked or if an element of the pain can be blocked. A successful block should lead to blocking of the medial branches of the Dorsal nerve root that are fed by the facet joint. Subsequenty if there is a positive response, the medial branches can be disrupted with heat applied to the end of the needle. The medial branches only transmit pain signals from the facet joints. When these nerves are burned the signal will be interrupted. They will grow back, but this usually takes several months to a year and can result in long term pain relief, with repeat procedures as indicated

Medication Treatment

Medications for treating pain come from varying classes. Most uses of pain medication are off label, i.e. physicians have used medications approved for other types of treatment, to treat pain. These off label medications include anti-inflammatories, antidepressants, anxiolytics, anticonvulsants, cardiovascular agents, local anesthetics, and general anesthetics. Pain presents a complex picture of multiple alterations to the peripheral and central nervous system, with loss of normal symmetry between the spinal cord and the brain. The general rule of treating acute pain with one or two agents is reasonable, but chronic pain must usually be treated with an approach of rational polypharmacology. Coordinating this with psychotherapy, social interventions, non-invasive approaches, interventional invasive approaches and surgery may be necessary to control severe chronic pain states.