Origin of the rubrospinal tract in neonatal, developing, and mature rats

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Abstract
This investigation describes the origin of the rubrospinal tract in neo‐natal (1‐10 days old), developing (15‐20 days old), and mature (2‐4 months old) rats studied by using the horseradish peroxidase (HRP) method of tracing neuronal connections. HRP was administered in the cervical or lumbosacral segments of the spinal cord either in the crystal or solution form. The results showed that the rubrospinal tract extended to the lum‐bosacral part of the spinal cord at birth. There appeared to be no difference in the pattern of labelled rubrospinal (RS) neurons following the admin‐istration of HRP in the cervical or the lumbosacral cord segment of the neonatal, developing, and mature rats. In rats of these three age groups, labelled neurons were found bilaterally in the red nucleus, with a contra‐lateral predominance, and they were found in both the parvicellular and magnocellular portions of the red nucleus. There was a somatotopic ar‐rangement in the labelled RS neurons: Those projecting to the cervical cord segments were located in the dorsal and dorsomedial regions of the red nucleus and those projecting to the lumbosacral cord segments were located in the ventral and ventrolateral regions of the nucleus.