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The Physiology of Pain

Pain -- Receptors -- Spinal Cord -- Brain -- Nerve
A Short Course in Pain Physiology 101

Pain is not simple, and it can be quite complex to treat. We have pain receptors throughout the body. When our body feels pain, whether from touching a hot stove or from low back pain, the pain signals nocioceptors, or pain receptors. Hardly a second passes, and the spinal cord and brain begin responding.

The brain processes the pain stimulus, and sends an impulse down the spinal cord to the nerve in pain. It happens in microseconds. We remove our finger from the hot stove. We temporarily dull the pain with an anti-inflammatory medication, and call our doctor for an appointment.

The body's pain receptors are:

  • Mechanical: a stretch gone too far.
  • Thermal: extreme heat or cold
  • Chemical: can be externally ingested, or body products released during trauma or inflammation. (Lactic acid, for example, causes muscle pain after heavy exercise.)

There is acute pain and chronic pain. If you prick yourself with a needle, you know exactly where the pain is, and it's instantaneous. The pain does not spread to other parts of the body. It can usually be treated with a local anesthetic or painkiller.

Chronic pain is pain that continues longer than would normally be associated with a certain type of injury. Back pain is an example of chronic pain, and is best treated by an interdisciplinary team that can break the pain cycle.

Physiological pain takes several forms, depending on the pain source and nocioceptors, or pain-detecting neurons.

  • Cutanenous pain: Caused by injury to the skin or superficial tissues. The pain-detecting neurons are just below the skin because of high concentration of nerve endings. The pain is localized and short-term. Examples: paper cut, first-degree burn, exterior wound.
  • Somatic pain: Comes from ligaments, tendons, bones, blood vessels and nerves. There are few somatic pain receptors in these areas, so the pain is a dull pain of longer duration. Examples: sprains, broken bones.
  • Visceral pain: Visceral neurons are within body organs and cavities. Pain receptors in these areas are even more diffuse, so pain is felt as an ache over a longer period of time. Visceral pain is difficult to localize, and is often called "referred pain." This means the sensation is unrelated to the injury site. Myocardial ischemia (loss of blood flow to a part of heart muscle tissue) can be felt in the upper chest, or as an ache in the left shoulder, arm or hand).
  • Phantom Limb Pain: The sensation of pain from a limb that has been amputated, or where a person no longer feels physical pain signals (i.e., quadriplegic).
  • Neuropathic pain: Can begin through injury or disease to the nerve tissue. This can disrupt the sensory nerves from transmitting correct information to the thalmus. The brain interprets painful stimuli even though there is no obvious or physiological cause for pain.

Common Causes of Pain by Body Region

  • Head and Neck
    • Jaw (trauma, temporal arteritis)
    • Ear (infection, trauma)
    • Eye (Irritant in eye, glaucoma, trauma)
    • Head (Migraine, headaches, cancer, cerebral aneurysm, sinusitis, meningitis
    • Neck -- Trauma
  • Thorax
    • Chest (Heart attack, pancreatitis, hernia.
    • Shoulder (Cholecystitis, trauma)
    • Breast (perimenstrual, cancer, trauma)
    • Back (cancer, trauma)
  • Abdomen
    • - Left and upper right: peptic ulcer disease, gastroenteritis, hepatitis, pancreatitis, abdominal aortic aneurysm
    • - Left and lower right: appendicitis, pelvic inflammatory disease (women), diverticulitis, kidney stone, colorectal cancer
    • - Back: Muscle strain, cancer, lumbar disc herniation, degenerative disc disease.
  • Limbs
    • Arm: Heart attack, muscle strain
    • Leg: Deep vein thrombosis, peripheral vascular disease, muscle strain, lumbar disc herniation, sciatica
  • Joints
    • Small joints: osteoarthritis, rheumatoid arthritis, Lupus
    • Large joints: hip and knee, trauma
    • Back: Degenerative disc disease, cartilage loss, ankylosing spondylitis, inflammatory bowel disease

The brain has no pain receptors, and is not sensitive to pain. A headache comes from the membrane surround the brain and spinal cord (the dura mater). Stimulating these dural nocioceptors is thought to produce headache pain.

Commonly, aspirin is taken to relieve headache pain. Its active ingredient is acetylsalicylic acid -- a synthetic compound derived from salicin, found in the bark of the willow tree. Aspirin blocks the production of substances called prostaglandins, which the body produces when a cell in the body is injured. By design, aspirin relieves symptoms of pain, fever and inflammation.

Perception of Pain

Pain is sensed when the nocioceptors are stimulated. Pain signals are transmitted through sensory neurons in the spinal cord. These neurons release glutamate, an excitatory transmitter that relays signals from one neuron to another. The signals go to the thalmus, where you perceive pain. From here, the signal travels to the somatosenory cortex in the brain, where you are made fully aware of the degree and severity of pain.

Controlling pain

Depending on the degree and severity of pain, management can start with non-steroidal anti-inflammatory agents, or rubbing the pain (which stimulates other nerve fibers that delay attention from the pain nerve fibers). Natural opoids in the brain are released to bind receptors in the pain pathway to block transmission and reception of pain. An endorphin is an example of a pain blocker. The release of endorphins allows runners or cyclists to "push on" despite muscle pain.

For chronic pain, specialists take a health history, and perform a physical exam. Some types of pain can be blocked by medications, implanted neurotransmitters or nerve blocks. Some types of pain -- herniated discs or tumors, for example, may require surgery for pain relief.

Pain: Our Survival Mechanism

While pain is never pleasant, pain stops an individual from an activity associated with pain. Pain can alert us that injury is imminent. Pain is part of the healing process, since we protect injuries to avoid further pain.