the spinal cord created what he termed "an irritable focus". This focus caused some stimuli to be perceived as referred pain. 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 referred pain 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 referred pain, threshold differences for stimulating local and referred pain, and somatosensory sensibility changes in the area of referred pain. Hyperexcitability Hyperexcitability hypothesizes that referred pain 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 referred pain 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 referred pain. Several characteristics are in line with this mechanism of referred pain, such as dependency on stimulus and the time delay in the appearance of referred pain as compared to local pain. However, the appearance of new receptive fields, which is interpreted to be referred pain, conflicts with the majority of experimental evidence from studies including studies of healthy individuals. Furthermore, referred pain 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 referred pain is perceived as such due to the summation of neural inputs in the brain, as opposed