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Anatomy and Physiology of Pain

Pain is a warning signal intended to protect you from harm.  It is part of your instinctual mechanism of survival.  Pain causes a reflex withdrawal which is directed by the nervous system.  Your nervous system is able to discern between stimuli that may be harmful and those that are not harmful and respond appropriately without your even knowing that it happened.

The nervous system is divided into two major components.  These are:

1) The central nervous system made up of the brain and the spinal cord; and
2) The peripheral nervous system made up of all of the other nerves

Information is transmitted to and from the brain via nerves.  Types of nerves include sensory nerves, motor nerves, interneurons and glia.  Sensory nerves carry messages from the body to the brain.  Messages are returned to the body from the brain by motor nerves.  Graphic depiction of the flow of information from denrited to axonThe sensory and motor nerves are located outside of the brain and spinal cord.  Separate nerve fibers distinguish temperature, vibration, pressure, position, 2-point discrimination, light touch, and pain.  It is the sensory nerves that trigger the simple reflex that pulls us away from pain.  Most sensory and motor nerves are enclosed in a myelin sheath that acts as a superconductor for impulses in the nerve fiber.
The individual types of pain are as follows:

Nociceptive Pain - specific pain receptors are stimulated. These receptors sense temperature (hot/cold), vibration, stretch, and chemicals released from damaged cells.

Somatic Pain - a type of nociceptive pain. Pain felt on the skin, muscle, joints, bones and ligaments is called somatic pain. The term musculo-skeletal pain means somatic pain. The pain receptors are sensitive to temperature (hot/cold), vibration, and stretch (in the muscles). They are also sensitive to inflammation, as would happen if you cut yourself, sprain something that causes tissue damage. Pain as a result of lack of oxygen, as in ischemic muscle cramps, are a type of nociceptive pain. Somatic pain is generally sharp and well localized - if you touch it or move the affected area the pain will worsen.

Visceral Pain - a type of nociceptive pain. It is felt in the internal organs and main body cavities. The cavities are divided into the thorax (lungs and heart), abdomen (bowels, spleen, liver and kidneys), and the pelvis (ovaries, bladder, and the womb). The pain receptors - nociceptors - sense inflammation, stretch and ischemia (oxygen starvation).

Visceral pain is more difficult to localize than somatic pain. The sensation is more likely to be a vague deep ache. Colicky and cramping sensations are generally types of visceral pain. Visceral pain commonly refers to some type of back pain - pelvic pain generally refers to the lower back, abdominal pain to the mid-back, and thoracic pain to the upper back.

Nerve Pain or Neuropathic Pain

Nerve pain is also known as neuropathic pain. It is a type of non-nociceptive pain. It comes from within the nervous system itself. People often refer to it as pinched nerve, or trapped nerve. The pain can originate from the nerves between the tissues and the spinal cord (peripheral nervous system) and the nerves between the spinal cord and the brain (central nervous system, or CNS).

Neuropathic pain can be caused by nerve degeneration, as might be the case in a stroke, multiple-sclerosis, or oxygen starvation. It could be due to a trapped nerve, meaning there is pressure on the nerve. A torn or slipped disc will cause nerve inflammation, which will trigger neuropathic pain. Nerve infection, such as shingles, can also cause neuropathic pain.

Pain that comes from the nervous system is called non-nociceptive because there are no specific pain receptors. Nociceptive in this text means responding to pain. When a nerve is injured it becomes unstable and its signaling system becomes muddled and haphazard. The brain interprets these abnormal signals as pain. This randomness can also cause other sensations, such as numbness, pins and needles, tingling, and hypersensitivity to temperature, vibration and touch. The pain can sometimes be unpredictable because of this.

Sympathetic Pain

The sympathetic nervous system controls our blood flow to our skin and muscles, perspiration (sweating) by the skin, and how quickly the peripheral nervous system works.

Sympathetic pain occurs generally after a fracture or a soft tissue injury of the limbs. This pain is non-nociceptive - there are no specific pain receptors. As with neuropathic pain, the nerve is injured, becomes unstable and fires off random, chaotic, abnormal signals to the brain, which interprets them as pain.

Generally with this kind of pain the skin and the area around the injury become extremely sensitive. The pain often becomes so intense that the sufferer daren't use the affected arm or leg. Lack of limb use after a time can cause other problems, such as muscle wasting, osteoporosis, and stiffness in the joints.  This is the type of pain associated with Complex Regional Pain Syndrome a/k/a Reflex Sympathetic Dystrophy.