ORCID

0000-0002-8090-9577

Date of Award

4-24-2024

Author's School

Graduate School of Arts and Sciences

Author's Department

Biology & Biomedical Sciences (Neurosciences)

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

In vertebrates, spinal interneurons are essential for the initiation and propagation of locomotor activity with each class of interneurons serving its own unique role. Ipsilateral interneurons function to control locomotor speed; whereas, commissural interneurons aid in left – right alternation. To date, most studies have been aimed at studying the impact of each individual class of interneurons onto motor neurons. However, not much work has revolved around studying interneuron – interneuron connectivity among the various cardinal classes. Furthermore, it remains unknown how the connectivity of interneurons changes along their entire axonal reach. This thesis aims to address the developmental patterns and structure of interneuron connectivity along the spinal cord. Among the various types of interneurons, two major groups of ipsilaterally projecting interneurons emerge from the same progenitor cell yet diverge into distinct cell types due to differences in Notch signaling. V2a (NotchOFF) neurons and V2b (NotchON) neurons provide glutamatergic and GABAergic/glycinergic input onto motor neurons, respectively. Given their shared origin, it was important to evaluate their developmental connectivity because previous work showed that sisters neurons exhibit stereotypic patterns of connectivity. In mammals, sister neurons assemble into shared microcircuits, whereas in Drosophila, Notch-differentiated sister neurons integrate into distinct circuits. Using an in vivo labeling approach, we identified pairs of sister V2a/b neurons born from individual Vsx1+ progenitors in the zebrafish spinal cord. We used paired whole-cell electrophysiology and optogenetics to reveal that sister V2a/b neurons do not communicate with each other, receive input from different presynaptic sources, and connect to distinct targets. These results resemble the divergent connectivity in Drosophila and represent the first evidence of Notch-differentiated circuit integration in vertebrates. The scarcity of shared targets revealed potential differences in connectivity between V2a and V2b neurons. Prior research on the mapping of V2b postsynaptic targets revealed a connectivity preference for short range targets, but to date, there has been no systematic assessment of V2a postsynaptic targets. Direct assessment of V2a postsynaptic targets using optogenetics revealed that connections from V2a neurons are weighted to longer ranges, explaining the lack of shared targets. Not only did V2a neurons elicit more EPSCs onto target neurons at longer ranges ( > 4 muscle segments away from the target neuron), the strength of the evoked EPSC events were larger than any of the observed local connections. Given the scarcity and strength of local connections, it is unlikely that V2a neurons function as the recurrent and rhythmogenic source in the unit burst generator model, but instead, the propensity to form long range connections could result from the need to ensure propagation of an excitatory wave down the spinal cord during locomotion and provide patterned input, such as directing turns.

Language

English (en)

Chair and Committee

Martha Bagnall

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