Defining classes of spinal interneuron and their axonal projections in hatchling Xenopus laevis tadpoles

Abstract

Neurobiotin was injected into individual spinal interneurons in the Xenopus tadpole to discern their anatomical features and complete axonal projection patterns. Four classes of interneuron are described, with names defining their primary axon projection: Dorsolateral ascending and commissural interneurons are predominantly multipolar cells with somata and dendrites exclusively in the dorsal half of the spinal cord. Ascending interneurons have unipolar somata located in the dorsal half, but their main dendrites are located in the ventral half of the spinal cord. Descending interneurons show bigger variance in their anatomy, but the majority are unipolar, and they all have a descending primary axon. Dorsolateral commissural interneurons are clearly defined using established criteria, but the others are not, so cluster analysis was used. Clear discriminations can be made, and criteria are established to characterize the three classes of interneuron with ipsilateral axonal projections. With identifying criteria established, the distribution and axonal projection patterns of the four classes of interneuron are described. By using data from gamma-aminobutyric acid immunocytochemistry, the distribution of the population of ascending interneurons is defined. Together with the results from the axonal projection data, this allows the ascending interneuron axon distribution along the spinal cord to be estimated. By making simple assumptions and using existing information about the soma distributions of the other interneurons, estimates of their axon distributions are made. The possible functional roles of the four interneuron classes are discussed.

Persistent Responses to Brief Stimuli: Feedback Excitation among Brainstem Neurons

“…6 B) (see details in Appendix, Modeling methods). This network modeled a 0.4 mm length of CNS where neuron numbers (180), longitudinal distributions, and axon lengths were based on anatomical measurements (Roberts, 2000;Li et al, 2001). Excitatory and inhibitory interneurons (iINs) and motoneurons (MNs) were modeled as single compartments.…”

“…The number of MNs and excitatory (hdIN) and reciprocal iINs per 100 m CNS length (Table 2) and axon lengths were set to mimic the distributions found anatomically (Yoshida et al, 1998;Roberts et al, 1999;Li et al, 2001). They were the same on both sides of the model.…”

Primitive Roles for Inhibitory Interneurons in Developing Frog Spinal Cord

“…Neurons were observed using a 100ϫ oil immersion lens and traced using a drawing tube or photographed. To compensate for shrinkage during dehydration, all measurements in this study have been corrected by multiplying by 1.28 (Li et al, 2001).…”

“…At the time of hatching from the egg, the Xenopus tadpole spinal cord may have as few as seven anatomical classes of neurons with different functions in reflex responses and swimming (Roberts, 2000;Li et al, 2001). These are shown diagrammatically in Figure 1, where their functions are listed.…”

Role of type-specific neuron properties in a spinal cord motor network

“…These neurons can be divided into 8 distinct classes on the basis of clear anatomical differences (in dorso-ventral soma position, dendrite distribution and axonal projections; Roberts, 2000;Li et al, 2001). Nothing is known about the function of one class of possible neural canal receptor neuron (Dale et al, 1987).…”