Synaptic strength in the leech heartbeat central pattern generator: Animal to animal variability and stereotypy Pubblico

Abstract

This study seeks to characterize, as quantitatively as possible, the strength of each synapse within the leech heartbeat central pattern generator and to examine the animal-to-animal variability and stereotypy that can underlie a circuit whose output is functionally stable. A central pattern generating circuit (CPG) that produces a reliable, stereotyped output in isolation is an excellent candidate for the study of the inner-workings of neural networks. The use of invertebrates is advantageous because of the small number and large size of constituent neurons, and their identifiability.

The importance of both intrinsic properties of neurons and their synaptic interactions in shaping network output has been well illustrated. Intrinsic properties not only shape activity and response properties of the neurons, but ion channel expression can vary 2-5 fold across preparations in the same identifiable cell, while the network continues to produce a stereotyped behavioral output (Schulz, Goaillard et al. 2006). Modeling studies have suggested that both intrinsic properties and synaptic strengths within a network can vary substantially while maintaining a stable output (Prinz, Bucher et al. 2004).

The leech heartbeat CPG is a small network responsible for blood flow within a closed circulatory system including two parallel heart tubes in the medicinal leech. The CPG is composed of 7 bilateral pairs of heart interneurons connected via both inhibitory chemical synapses and electrical junctions that give rise to a functionally stable, rhythmic output. Each of the 15 synapses within the core central pattern generator was measured for synaptic strength across a minimum of 7 individuals and the variability was calculated. Synaptic strengths within the leech heartbeat central pattern generator varied 3 to 14 fold between individuals (depending on the specific synapse). The balance between multiple inputs onto various postsynaptic targets was explored. Of the 5 comparisons made within the core CPG two showed a clear maintenance of synaptic strength ratios while the other three showed no such relationship.

We conclude that the leech heartbeat central pattern generator can withstand tremendous variability in synaptic strengths despite a relatively stable circuit output. The network clearly preserves the relationship of several different inputs despite tremendous variability.

Table of Contents

Table of Contents:
Introduction...1
Methods...9
Results...13
Discussion...26
Figure Legends...34
References...42

Figures:
Figure One...45
Figure Two...46
Figure Three...47
Figure Four...48
Figure Five...49
Figure Six...50
Figure Seven...51
Figure Eight...52
Figure Nine...53
Figure Ten...54
Figure Eleven...55

About this Master's Thesis

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School	Laney Graduate School
Department	Biological and Biomedical Sciences
Subfield / Discipline	Neuroscience
Degree	MS
Submission	Master's Thesis
Language	English
Research Field	Biology, Neuroscience
Parola chiave	Invertebrate Central Pattern Generator Synaptic Strength Variability
Committee Chair / Thesis Advisor	Calabrese, Ronald, Emory University
Committee Members	Wenner, Peter, Emory University Prinz, Astrid A, Emory University Hochman, Shawn, Emory University

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