Spinal neuronal collaterals to the intralaminar thalamic nuclei and periaqueductal gray.

The existence of spinal neuron collaterals projecting to the intralaminar thalamic nuclei (ILN) and the periaqueductal gray (PAG) was determined in the rat using double-labeling, fluorescent, retrograde axoplasmic transport techniques. Distinctively double-labeled neurons, although not numerous, were found in the entire extent of the spinal cord. More double-labeled neurons were observed in the lumbosacral enlargement than in other cord segments. The laminar origin of the ILN and PAG projecting neurons were found primarily in the contralateral reticular portion of V, medial VII and the nucleus of dorsolateral funiculus. In addition, ipsilateral lateral cervical and central cervical nuclei also exhibited double-labeled neurons. This finding suggests that the pathway of medial thalamus projecting to the paleospinothalamic tract is very complex because the tract has triple connections, i.e. direct, collateral and indirect.