Unmyelinated corticospinal axons in adult rat pyramidal tract. An electron microscopic tracer study

The aim of the present study was to provide experimental ultrastructural evidence for a corticospinal component in the adult rat pyramidal tract (PT). For this purpose, the entire sensorimotor and frontal cortex of the left hemisphere was labelled using the anterograde tracer horseradish-peroxidase (HRP). Six months old rats were sacrificed 24 or 48 h after implantation of 6-8 HRP-gels. The detection of anterogradely transported HRP at the cervical as well as the lumbar intumescence was carried out as described earlier (J. Histochem. Cytochem., 35 [1987] 623-626). Our results demonstrate the occurrence of labelled myelinated as well as labelled unmyelinated axons within the adult rat PT at both spinal cord levels analyzed. This implicates that at least part of the unmyelinated profiles in the adult rat PT belong to fibres originating in the cortex and therefore must be interpreted as corticospinal axons. The findings are discussed in the light of their physiological significance.