There are several proposed mechanisms for<unk> . Currently there is no definitive consensus regarding which is correct. The cardiac general visceral sensory pain fibers follow the sympathetics back to the spinal cord and have their cell bodies located in thoracic dorsal root ganglia 1-4(5). As a general rule, in the thorax and abdomen, general visceral afferent (GVA) pain fibers follow sympathetic fibers back to the same spinal cord segments that gave rise to the preganglionic sympathetic fibers. The central nervous system (CNS) perceives pain from the heart as coming from the somatic portion of the body supplied by the thoracic spinal cord segments 1-4(5). Classically the pain associated with a myocardial infarction is located in the mid or left side of the chest where the heart is actually located. The pain can radiate to the left side of the jaw and into the left arm. Myocardial infarction can rarely present as<unk>  and this usually occurs in people with diabetes or older age. Also, the dermatomes of this region of the body wall and upper limb have their neuronal cell bodies in the same dorsal root ganglia (T1-5) and synapse in the same second order neurons in the spinal cord segments (T1-5) as the general visceral sensory fibers from the heart. The CNS does not clearly discern whether the pain is coming from the body wall or from the viscera, but it perceives the pain as coming from somewhere on the body wall, i.e. substernal pain, left arm/hand pain, jaw pain. Convergent-projection This represents one of the earliest theories on the subject of<unk> . It is based on the work of W.A. Sturge and J. Ross from 1888 and later TC Ruch in 1961. Convergent projection proposes that afferent nerve fibers from tissues converge onto the same spinal neuron, and explains why<unk>  is believed to be segmented in much the same way as the spinal cord. Additionally, experimental evidence shows that when local pain (pain at the site of stimulation) is intensified the<unk>  is intensified as well. Criticism of this model arises from its inability to explain why there is a delay between the onset of<unk>  after local pain stimulation. Experimental evidence also shows that<unk>  is often unidirectional. For example, stimulated local pain in the anterior tibial muscle causes<unk>  in the ventral portion of the ankle; however<unk>  moving in the opposite direction has not been shown experimentally. Lastly, the threshold for the local pain stimulation and the<unk>  stimulation are different, but according to this model they should both be the same. Convergence-facilitation Convergence facilitation was conceived in 1893 by J MacKenzie based on the ideas of Sturge and Ross. He believed that the internal organs were insensitive to stimuli. Furthermore, he believed that non-nociceptive afferent inputs to the spinal cord created what he termed "an irritable focus". This focus caused some stimuli to be perceived as<unk> . However, his ideas did not gain widespread acceptance from critics due to its dismissal of visceral pain. Recently this idea has regained some credibility under a new term, central sensitization. Central sensitization occurs when neurons in the spinal cord's dorsal horn or brainstem become more responsive after repeated stimulation by peripheral neurons, so that weaker signals can trigger them. The delay in appearance of<unk>  shown in laboratory experiments can be explained due to the time required to create the central sensitization. Axon-reflex Axon reflex suggests that the afferent fiber is bifurcated before connecting to the dorsal horn. Bifurcated fibers do exist in muscle, skin, and intervertebral discs. Yet these particular neurons are rare and are not representative of the whole body. Axon-Reflex also does not explain the time delay before the appearance of<unk> , threshold differences for stimulating local and<unk> , and somatosensory sensibility changes in the area of<unk> . Hyperexcitability Hyperexcitability hypothesizes that<unk>  has no central mechanism. However, it does say that there is one central characteristic that predominates. Experiments involving noxious stimuli and recordings from the dorsal horn of animals revealed that<unk>  sensations began minutes after muscle stimulation. Pain was felt in a receptive field that was some distance away from the original receptive field. According to hyperexcitability, new receptive fields are created as a result of the opening of latent convergent afferent fibers in the dorsal horn. This signal could then be perceived as<unk> . Several characteristics are in line with this mechanism of<unk> , such as dependency on stimulus and the time delay in the appearance of<unk>  as compared to local pain. However, the appearance of new receptive fields, which is interpreted to be<unk> , conflicts with the majority of experimental evidence from studies including studies of healthy individuals. Furthermore,<unk>  generally appears within seconds in humans as opposed to minutes in animal models. Some scientists attribute this to a mechanism or influence downstream in the supraspinal pathways. Neuroimaging techniques such as PET scans or fMRI may visualize the underlying neural processing pathways responsible in future testing. Thalamic-convergence Thalamic convergence suggests that<unk>  is perceived as such due to the summation of neural inputs in the brain, as opposed to the spinal cord, from the injured area and the<unk>  area. Experimental evidence on thalamic convergence is lacking. However, pain studies performed on monkeys revealed convergence of several pathways upon separate cortical and subcortical neurons.
Referred pain
Pain out of proportion
Pain tolerance
Referred itch
Pain wind-up
Pain and pleasure
Baricity
Pain