The Role of the Thalamus in the Nociception

Long it has been recognized the thalamus as the entry gate to the brain for the sensory input transmission from the periphery. In the relay of the nociceptive input to a painful sensation a combination of processes in the periphery, spinal cord and brain are involved, but not completely understood. The thalamus has a very rich nuclear organization; more than 20 nuclei have been identified, and it can be classified from an evolutionary perspective, from the interconnectivity of the fibers and from the cytoarchitectonic and anatomic-functional organization. The thalamic nuclei of the medial line conform two independent endogenous systems that exert some control over the nociceptive input: the medial dorsal nucleus that participates in the descendent facilitation of the development of mechanical allodynia, and the ventral medial that specifically mediates the descendent inhibition of thermal hyperalgesia. More recently, the periventricular nucleus, located in the medial line, has been recognized to have modulatory effects in the nociceptive pathway. In this way the thalamus along with other areas of the cerebral cortex play a crucial role in the integration of the sensitive information. The interconnectivity of the cortico-thalamic projections prompts a complex network of communication that paired with the several neurotransmitters result in the integration of the painful response. In the present chapter, we will discuss the distal components along with the sensitive receptors and the interconnectivity relevant to the nociception focusing on the modulatory role of the thalamus.