The nucleus of the solitary tract: an integrating station for nociceptive and cardiorespiratory afferents.

Painful stimuli can evoke dramatic responses in the cardiovascular and respiratory systems. We have assessed the role of both the sympathetic and parasympathetic nervous system in mediating the reflex tachycardia that accompanies somatic nociception. We describe a major role for the nucleus tractus solitarii (NTS) as a site for integrating nociceptive and cardiorespiratory afferents. Since cardiorespiratory and nociceptive afferents terminate in the NTS, this nucleus offers a powerful opportunity for central modulation. We show that the NTS plays a major role in mediating the reflex tachycardia evoked by somatic noxious stimulation. Similar noxious stimulation attenuates the cardiac component of the peripheral chemoreceptor reflex and inhibits the peripheral chemoreceptor-evoked excitatory synaptic response of some NTS neurones. The functional interpretation we propose is that by depressing homeostatic reflexes at the NTS, noxious stimulation-evoked cardiorespiratory changes can be expressed and maintained, which may be essential for the survival of the animal.