Electrophysiological evidence for a role of the anterolateral quadrant of the spinal cord in the transmission of noxious messages to the thalamic ventrobasal complex in the rat.

Responses to noxious mechanical and thermal stimulation applied to the hindpaws were recorded extracellularly from the same neurons of the ventrobasal complex of the rat thalamus (VB) before and after lesions of various areas of the cervical cord in order to determine the pathways carrying the afferent messages. It was demonstrated that lesions of the dorsal and dorsolateral portions of the cord failed to eliminate the VB neuronal responses to noxious stimulation. By contrast, lesion of one anterolateral quadrant eliminated the responses to a noxious stimulation applied to the hindpaw contralateral to the lesion. This occurred whether the lesion was ipsilateral or contralateral to the recording site. From the present study and the data in the literature, it is concluded that the fibers of the spino-thalamic tract which are completely crossed in the spinal cord, travel in the anterolateral quadrant and project directly onto the VB, are involved in the transmission of noxious messages from the cord to the VB neurons. This conclusion indicates that the VB neuronal responses to noxious stimulation of the hindpaw ipsilateral to the recording site depend on the spinothalamic projection to the opposite ventrobasal complex. This therefore suggests that some noxious messages which reach a particular VB neuron are conveyed via the opposite VB and the existence of a thalamo-cortico-thalamic loop is discussed.