Spinothalamic and spinomedullary neurons in macaques: A single and double retrograde tracer study

Abstract

The location and perikaryal size of spinothalamic cells and of spinal neurons projecting to the dorsal column nuclei has been compared in adult monkeys ( Macaco fascicularis) in which horseradish peroxidase has been injected in either the ventrobasal complex or the dorsal medulla. Spinothalamic neurons are found mainly in lamina I and in the lateral portions of laminae IV-VI throughout the side of the cord contralateral to the thalamic injection. Spinomedullary neurons are most ipsilateral to the medullary injection, in medial portions of laminae IV-VI throughout cervical levels but laterally, in the same laminae, in lumbar segments. At these levels, spinothalamic and spinomedullary neurons are also found along the lateral border of the ventral horn; neurons in this location, labelled by either injection, are large (40–65 μm in diameter), and are indistinguishable from ‘spinal border cells’ at the origin of the ventral spinocerebellar tract. Computer-assisted measurement of perikaryal size reveals that the mean values for dorsal horn neurons of the two systems differ significantly, although a sizeable fraction of both neuronal populations have identical cytological characteristics, including perikaryal size. A double-labelling strategy employing horseradish peroxidase and [ 3H]-apo-horseradish peroxidase has been applied (a) to identify simultaneously cells of origin of the two ascending systems and to appreciate better their topographical relationships, and (b) to answer the question of whether at least some of these spinal neurons have axons which, by way of collateral branching, reach both targets. The distribution of neurons at the origin of the two ascending tracts in animals with injection of the two tracers is identical to that revealed in animals in which only horseradish peroxidase has been injected. While primarily single-labelled neurons are found throughout the cord, some large dorsal horn neurons in both brachial and lumbosacral enlargements are labelled by both cytoplasmic horseradish peroxidasepositive granules and by reduced silver grains on the overlying emulsion. These double-labelled cells are interpreted as having an axon which gives origin, presumably at spinal cord levels, to collaterals ascending to both the ipsilateral medulla and the contralateral thalamus. Systematic evaluation of the extent to which ascending pathways to various supraspinal targets originate from common spinal neurons may provide new insights into the functional mechanisms of ascending spinal systems.