The thalamus and cortex contain nociceptive neurons that are activated by nociceptive deep input from muscles and joints. Most of these neurons have convergent inputs from skin and deep tissue, but small proportions of neurons respond only to noxious stimulation of muscle and tendon.113,114115 In the thalamus, such neurons are located in the ventrobasal complex, in the posterior complex114 and in the medial nucleus.116 Similarly, the somatosensory cortex contains a large proportion of neurons that respond to noxious stimulation, and a small proportion of these neurons is driven by deep input.7,117 In polyarthritic rats, a large proportion of neurons in the ventrobasal complex respond to movements and gentle pressure on to inflamed joints and often long-lasting discharges were noted, whereas only few neurons respond to these stimuli in normal rats. Some neurons also displayed paroxysmal discharges. Furthermore, neurons in the nucleus centralis lateralis acquire input from the inflamed joint which is not present in normal ani-mals.118 Similarly, neurons in superficial cortical layers that do not respond to joint stimulation in normal rats start to respond to joint stimulation in polyarthritic rats.119,120 These findings indicate substantial neuroplas-ticity at the thalamocortical level that may contribute to inflammatory deep tissue pain. It is unknown whether these alterations mirror the altered spinal processing or whether additional elements of neuroplasticity are generated in the thalamus and cortex themselves.
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