Thermoceptors, the principle receptors for temperature, and nociceptors, which interpret the sensation of pain, are classified as free nerve endings. The skin and the internal tissues hold several million of these free nerve endings which discern pain and temperature. Those free nerve endings which are prone to pick up temperatures that are considered to be cold are positioned closer to the skin’s surface. They also out number the receptors of heat by about ten to fifteen times throughout any portion of the skin.

Damage tissue can kick pain receptors into high gear, as can all sorts of stimuli recognized as painful. The nervous tissue of the brain is devoid of these pain receptors, and they are spread throughout the visceral organs with great scarcity. While these particular free nerve endings are the ones who are designed to respond to painful stimulus, any coetaneous pain receptor will respond to pain if the painful stimulus is strong enough. Pain receptors are planted throughout the body to help the body stay alive and avoid dangerous and potentially life threatening situations.

Free nerve ending are not likely to extend any accommodation as other coetaneous receptors, and thus if a stimulus continues, the free nerve endings will continuously send the information to the central nervous system until the stimulus is thereby removed. Chemical stimulation is one of the most compelling stimuli when it comes to pain receptors. However, incidents such as muscular fatigue or muscular spasms, or the lack of the appropriate blood supply to any given organ can also cause the pain receptors to respond vehemently.

Pain is directed through the central nervous system via impulses carried on sensory neurons to the spinal cord. These impulses continue to the thalamus and then onto the lateral spinothalamic tract belonging to the spinal cord. The somatesthetic region of the cerebral cortex receives the final impulses, where the pain can be interpreted. The cerebral cortex expands on the basic information the thalamus is able to respond to. The thalamus interprets an understanding that there is pain while the cerebral cortex interprets what type of pain, the cause, and if there is an instant knee-jerk reaction that will alleviate the pain, like pulling a hand away from the flame.


There are two basic types of pain that are classified by sensation. Somatic pain and visceral pain indicate the type of pain the body is experiencing. When pain receptors along the skin are stimulated, the body senses what is considered to be superficial pain, which refers to location, not severity of the pain. Somatic pain refers to the pain felt from deep within the body, such as joint pain, skeletal muscles, or tendons. Visceral pain receptors respond to the stimulation of situations that cause visceral pain, such as joint injury, surgery, or even muscle pulls and tears. The neural pathways that relate the necessary information are known to be quite exacting, sending the sensation of pain directly back to the affected area.

However, in the instance of referred pain, visceral pain may not be discerned by the location, and may be perceived from other somatic locations. Referred pain remains nearly completely consistent from one individual to the next, and can be a useful diagnostic tool when discerning organ malfunctions based on reports of pain. For instance, back pain may be triggered by back problems, or can be related to serious kidney diseases. A heart attack might be perceived initially as pain in the specific quadrant of the chest, but then the pain travels down the arm. Stomach ulcers can feel as though they are stemming from the epigastric area of the torso.

Medical science has not uncovered many of the mysteries related to referred pain, however, there is an obvious relation to the development of the tracts within the spinal cord. Some of the nerve pathways are shared between the visceral organs and the coetaneous receptors. Each group of receptors shares numerous pathways which can trigger coetaneous responses rather than visceral responses.

Phantom pain is pain which occurs despite the obvious lack of free nerve endings. Patients who have experienced the loss of a limb or digit often experience pain where the appendage used to be. This is often triggered by the brain’s confusion, as impulses of the nerve endings in the remaining portion of the amputated limb are still functioning, the brain sends back the signal that the pain is originating from the coetaneous portion of the severed and now removed section.
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