Editing Referred pain (section)

==Laboratory testing methods==
Pain is studied in a laboratory setting due to the greater amount of control that can be exerted. For example, the modality, intensity, and timing of painful stimuli can be controlled with much more precision. Within this setting there are two main ways that referred pain is studied.{{citation needed|date=February 2021}}

===Algogenic substances===
In recent years several different chemicals have been used to induce referred pain including bradykinin, [[substance P]], [[capsaicin]],<ref name="pmid10666547">{{cite journal |vauthors=Witting N, Svensson P, Gottrup H, Arendt-Nielsen L, Jensen TS |title=Intramuscular and intradermal injection of capsaicin: a comparison of local and referred pain |journal=Pain |volume=84 |issue=2–3 |pages=407–12 |year=2000 |pmid=10666547 |doi=10.1016/S0304-3959(99)00231-6|s2cid=27403703 }}</ref> and [[serotonin]]. However, before any of these substances became widespread in their use a solution of hypertonic [[saline (medicine)|saline]] was used instead. Through various experiments it was determined that there were multiple factors that correlated with saline administration such as infusion rate, saline concentration, pressure, and amount of saline used. The mechanism by which the saline induces a local and referred pain pair is unknown. Some researchers have commented that it could be due to osmotic differences, however that is not verified.<ref name=ArendtNeilsen/>

===Using electrical stimulation===
[[Intramuscular]] electrical stimulation (IMES) of muscle tissue has been used in various experimental and clinical settings. The advantage to using an IMES system over a standard such as hypertonic saline is that IMES can be turned on and off. This allows the researcher to exert a much higher degree of control and precision in terms of the stimulus and the measurement of the response. The method is easier to carry out than the injection method as it does not require special training in how it should be used. The frequency of the electrical pulse can also be controlled. For most studies a frequency of about 10&nbsp;Hz is needed to stimulate both local and referred pain.{{Clarify|date=November 2016}}<ref name="pmid14499428">{{cite journal |vauthors=Kosek E, Hansson P |title=Perceptual integration of intramuscular electrical stimulation in the focal and the referred pain area in healthy humans |journal=Pain |volume=105 |issue=1–2 |pages=125–31 |year=2003 |pmid=14499428 |doi=10.1016/S0304-3959(03)00171-4|s2cid=24594646 }}</ref>

Using this method it has been observed that significantly higher stimulus strength is needed to obtain referred pain relative to the local pain. There is also a strong correlation between the stimulus intensity and the intensity of referred and local pain. It is also believed that this method causes a larger recruitment of nociceptor units resulting in a spatial summation. This spatial summation results in a much larger barrage of signals to the [[posterior horn of spinal cord|dorsal horn]] and [[brainstem]] neurons.<ref name=ArendtNeilsen/>