When quantitatively examined, spinal dorsal horn neurons with visceral inputs are located in the dorsal horn of the spinal cord (lamina I, II, V), the intermediolateral cell column and sacral parasympathetic nucleus (pelvic nerve), and in lamina X. These neurons have multiple, convergent inputs from other viscera, from joints, from muscle, and from cutaneous structures. Convergent receptive fields for these neurons are therefore large with diffuse inputs. This is considered the basis of referral of visceral sensation to somatic sites (e.g. myocardial ischemia typically radiates to the left shoulder and upper arm; the pain is not felt at the source - the heart). In contrast, neurons with exclusively cutaneous input are commonly identified in the spinal dorsal horn, in particular from glabrous skin. Taken together, these results suggest an imprecise organization of visceral primary inputs that would be consistent with an imprecise localization by the central nervous system. Viscero-visceral convergence and secondary hyperalgesia are common enough phenomena that, when coupled with the baseline diffuse character of visceral sensations, there prove to be diagnostic difficulties for both patients and physicians when the possibility of more than one pathology exists.
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