Anatomic evidence of nociceptive inputs to primary somatosensory cortex: Relationship between spinothalamic terminals and thalamocortical cells in squirrel monkeys

This study examined anatomic pathways that are likely to transmit noxious and thermal cutaneous information to the primary somatosensory cortex. Anterograde and retrograde labeling techniques were combined to investigate the relationship between spinothalamic (STT) projections and thalamocortical neurons in the squirrel monkey (Saimiri sciureus). Large injections of diamidino yellow (DY) were placed in the physiologically defined hand region of primary somatosensory cortex (hSI), and wheat germ agglutinin‐horseradish peroxidase (WGA‐HRP) was injected in the contralateral cervical enlargement (C₅T₁). Both DY‐labeled neuronal cell bodies and HRP‐labeled STT terminal‐like structures were visualized within single thalamic sections in each animal. Quantitative analysis of the positions and numbers of retrogradely labeled neurons and anterogradely labeled terminal fields reveal that: (1) ventral posterior lateral (VPL), ventral posterior inferior (VPI), and central lateral (CL), combined, receive 87% of spinothalamic inputs originating from the cervical enlargement; (2) these three nuclei contain over 91% of all thalamocortical neurons projecting to hSI that are likely to receive STT input; and (3) these putatively contacted neurons account for less than 24% of all thalamic projections to hSI. These results suggest that three distinct spinothalamocortical pathways are capable of relaying nociceptive information to the hand somatosensory cortex. Moreover, only a small portion of thalamocortical neurons are capable of relaying STT‐derived nociceptive and thermal information to the primary somatosensory cortex.