Segmental and propriospinal projection systems of frog lumbar interneurons

Abstract

Spinal interneuronal networks have been implicated in the coordination of reflex behaviors and limb postures in the spinal frog. As a first step in defining these networks, retrograde transport of horseradish peroxidase (HRP) was used to examine the anatomical organization of interneuronal circuitry in the lumbar spinal cord of the frog. Following neuronal degeneration induced by spinal transection and section of the dorsal and ventral roots, HRP was placed at different locations in the spinal cord and the positions of labeled neuronal cell bodies plotted using a Eutectics Neuron Tracing System. We describe four spinal interneuronal systems, three with cell bodies located in the lumbar cord and one with descending projections to the lumbar cord. Interneurons with cell bodies located in the lumbar cord include: (1) Lumbar neurons projecting rostrally. Those projecting to thoracic segments tended to be located in the lateral and ventrolateral gray and in the lower two-thirds of the dorsal horn, with projections that were predominantly uncrossed. Those projecting to the brachial plexus and beyond were located in the dorsal part of the dorsal horn (uncrossed) and in the lateral, ventrolateral, and ventromedial gray (crossed). (2) Lumbar neurons with segmental projections within the lumbar cord. These neurons, which were by far the most numerous, had both uncrossed and crossed projections and were distributed throughout the dorsal, lateral, ventrolateral, and ventromedial gray matter. (3) Lumbar neurons projecting to the sacral cord. This population, which arose mainly from the dorsal horn and lateral or ventrolateral gray, was much smaller than in the other systems. Neuronal density of some of these populations of lumbar interneurons appeared to vary with rostrocaudal level. Finally, a population of neurons with cell bodies in the brachial and thoracic segments that projects to the lumbar cord is described. The most rostral of these neurons were multipolar cells with uncrossed projections, while those with crossed projections were confined almost exclusively to the ventral half of the cord. The distribution of spinal interneurons reported here will provide guidance for future studies of the role of interneuronal networks in the control of movements using the spinal frog as a model system.

Nitric oxide synthase in the spinal cord of the frog, Xenopus laevis

“…These cells could include interneurons participating in segmental and propriospinal projection systems. However, such neurons were not found exclusively at this location, but also in the dorsal horn and the lateral field (Schotland and Tresch 1997).…”

Identification of Multiple Subsets of Ventral Interneurons and Differential Distribution along the Rostrocaudal Axis of the Developing Spinal Cord

“…Different classes of cardinal INs have been described in the developing ventral spinal cord [36], [37], [58], [59], [60]. However, most of these populations have been characterized at a single rostrocaudal level, and the respective distribution of these embryonic neuronal populations along the rostrocaudal axis of the spinal cord has not been extensively described.…”

Metamorphosis-Induced Changes in the Coupling of Spinal Thoraco-Lumbar Motor Outputs During Swimming inXenopus laevis

“…Furthermore, the establishment of longitudinal projections from the lumbar to thoracic cord regions is also required to provide the necessary anatomical substrate for coupling dorsalis efferent commands to the hindlimb motor circuitry. While such ascending propriospinal pathways remain to be identified in postmetamorphic Xenopus, in Rana pipens, lumbar interneurons have been previously labeled that project directly into the thoracic spinal cord (Schotland and Tresch 1997). In Xenopus juveniles, furthermore, these lumbo-thoracic projections appear to be homolateral because a hemicord section in vivo that disconnected the thoracic circuitry of that side from the lumbar cord region suppressed activity of the corresponding dorsalis muscle during swimming but did not affect hindlimb kicking-timed activation of the dorsalis on the intact contralateral side.…”