We describe the distribution of “descending” interneurons in late Xenopus laevis embryos after retrograde filling with horseradish peroxidaze via their ipsilateral, descending axons in the spinal cord. These multipolar neurons, with dendrites spread throughout the marginal zone, form a longitudinal column extending from midtrunk spinal cord into the brainstem to the level of the vagus. In the hindbrain these neurons are part of the uncrossed reticulospinal projection. They are most numerous in the caudal brainstem, their density falling by half at the eighth postotic segment.
To examine their possible role in swimming we reduced the population of descending interneurons by making progressive transections of the brainstem or spinal cord at the first to fifth postotic segments: These led to progressive reduction in the initial frequency of fictive swimming in immobilized embryos, even when the brainstem was divided sagittally. Transecting the spinal cord at the fourth postotic segment did not reduce initial frequency rostral to the lesion. The effects of these lesions on the duration of fictive swimming episodes were similar.
The results suggest (1) that descending interneurons could provide excitatory drive during swimming and (2) that some reticulospinal and spinal interneurons may form single homogenecus populations.