SPINOTHALAMOCORTICAL INPUTS NONPREFERENTIALLY INNERVATE THE SUPERFICIAL AND DEEP CORTICAL LAYERS OF SI

Using a combined anterograde and retrograde tracing technique, we exam ined the distribution pattern of the thalamocortical cells which proje cted to superficial layers of the hand region of the primary somatosen sory cortex (hSI), and quantitatively analyzed the retrogradely labele d cells which putatively contacted terminals of the spinothalamic trac t (STT) in the squirrel monkey and the macaque. Less than 25% of the s uperficial hSI projecting cells were putatively contacted by terminals of cervical enlargement spinothalamic neurons. These cells were prima rily located in ventroposterior lateral, ventroposterior inferior and centrolateral nuclei. Although the number of superficial hSI projectin g cells numbered less than 20% of the total hSI projecting cells, thei r patterns of location and their proportion of overlap with STT termin als within each thalamic nucleus were similar. It is suggested that th e spinothalamic nociceptive information input to the cortex equally ac cesses both superficial and deep SI.