Referred pain

# Overview
Referred pain is a very unpleasant sensation localized to an area separate from the site of the causative injury or other painful stimulation.  Often, referred pain arises when a nerve is compressed or damaged at or near its origin. In this circumstance, the sensation of pain will generally be felt in the territory that the nerve serves, even though the damage originates elsewhere. 

# Examples
A common example is spinal disc herniation, in which a nerve root arising from the spinal cord is compressed by adjacent disc material.  Although pain may arise from the damaged disc itself, pain and/or other symptoms will also be felt in the region served by the compressed nerve (for example, the thigh, knee, or foot).  Relieving the pressure on the nerve root may ameliorate the referred pain, provided that permanent nerve damage has not occurred.
A similar mechanism may be responsible for some instances of the phantom limb syndrome in amputees.
In another classic example of referred pain, male patients who are suffering a myocardial infarction (heart attack) feel pain in their left arm.  Another example of referred pain is the common "ice cream headache" or "brain freeze" which happens when the trigeminal ganglion is indirectly stimulated from cold food on the roof of the mouth. Another example is pain from an inflamed gall bladder which may refer pain to the right shoulder and pain from a herniated cervical disc referring pain down one or both arms into the hands.  In addition, tooth pain may refer pain that should be localized to the affected tooth to the opposite side of the mouth as opposed to actually feeling pain in the tooth with the cavity or abscess.

# Pathophysiology
In cases of damage to viscera, referred pain may be due to convergence of visceral nerves that innervate the damaged organs with somatic nerves that innervate sections of skin.  Because a neuron from the organ and one from the skin may form a synapse with the same projection neuron in the dorsal horn, input from either neuron will be interpreted the same way by it and all neurons further up the pathway.  Since the brain is more "accustomed" to receiving sensation from the peripheral structure than from the viscera, it may interpret the pain as originating from the former.  Thus there is an array of diseases that cause damage to organs and which produce characteristic patterns of pain in unrelated places in the body's periphery.
Despite the proliferation of literature on the mechanisms of referred pain, it is a process that is still not well understood.
Interestingly enough, there also seems to be some sort of pattern to referred pain in the symptoms associated with various disorders, for instance, many people are familiar with a physical symptom being associated with emotional distress, nausea, headaches, etc. but, there is a very minor chance that a person with a heart condition will have a tooth ache and no other obvious symptoms at all.  Other examples include joint pain associated with a kidney infection or a digestive disorder being felt in a headache.  It is not fully understood why these symptoms occur the way they do.

# Theories of referred pain
1. Covergence theory- The nerves from the visceral structures and the somatic structures to which pain is referred enter the CNS at the same level and converge on the same spinothalmic neurons. Since somatic pain is far more common than visceral pain, therefore when the same afferent pathway is stimulated by signals that originate in visceral afferent nerves, the signal that reaches the somatosensory cortex is identical and is interpreted as having arisen within the somatic area.
2.Facilitation theory- The afferent impulses from visceral structures produce subliminal fringe effects that lower the exctability threshold of spinothalamic neurons which recieve afferent fibres from somatic areas. Therefore, any slight activity in the pathways transmitting pain impulses from somatic regions, and which normally would die out within the spinal cord, is facilitated thus reaches conscious levels.