ABSTRACT
Mental appraisal of pain underlies suffering, but pain begins as an affective bodily feeling that signals an urgent need, like itch or hunger. All feelings from the body have a characteristic hedonic affect and are conjoined with autonomic activity and behavioral motivation that are aimed to restore homeostatic balance. Small-diameter fibers from all tissues terminate on lamina I neurons along the outer edge of the superficial dorsal horn. Consistent with their autonomic origin, lamina I neurons convey the interoceptive signals of small-diameter sensory fibers exclusively to the spinal autonomic cell columns and the brainstem homeostatic integration sites. A key finding was that in anthropoid primates the discrete lamina I interoceptive sensory channels are conveyed directly to the forebrain. This phylogenetically novel, anatomically distinct homeostatic sensory pathway surmounts the ancient mammalian brainstem homeostatic system, and it parallels a direct NTS forebrain projection also found only in primates.
