Abstract
Rhythmic motor patterns, such as walking, are generated, in
part, by
rhythmically active neural networks called central pattern
generators (CPG's;
Marder and Calabrese, 1996). Typically, CPG's provide rhythmically
patterned
synaptic drive onto motor neurons in order to coordinate them, with
appropriate
phase differences, into a motor pattern appropriate for the
behavior. These
premotor patterns of drive contain both timing information and
patterns of
synaptic strengths. Invertebrate preparations, with their simple
and accessible
nervous systems, have been used to generate principles that
underlie how
premotor patterns of synaptic input interact with motor neurons to
produce
stereotyped motor outputs (Marder and Bucher, 2007). Here, I use
the leech
heartbeat CPG, a system in which patterns of synaptic drive onto
motor neurons
can be easily measured, to address how a CPG circuit coordinates
its motor
neurons to produce stereotyped motor patterns.
In the first of two studies, I show that, although the segmental
input
pattern is the primary determinant of motor neuron output, the
intrinsic
properties of the heart motor neurons play an important role in
determining how
they are coordinated by their segmental synaptic input pattern,
particularly when
receiving one of the two input patterns these motor neurons
receive.
In the second study, I show, in both modeling and in
follow-up
experiments in the living system, that the generation of one motor
pattern is a
consequence of the nearly synchronous premotor timing information
produced
by the leech heartbeat CPG. For the other motor pattern, I show
that premotor
timing information determines the range over which motor neurons
can fire
while synaptic strength profiles define the actual motor
progression.
These experiments provide a direct assessment of how motor
neuron
intrinsic properties interact with their premotor pattern of
synaptic drive to
produce rhythmic motor output. Furthermore, the data presented here
may
inform studies on motor pattern generation in other systems,
including studies
on recovery of locomotor control in patients with spinal cord
injury.
Table of Contents
TABLE OF CONTENTS
PAGE
CHAPTER
1:
GENERAL
INTRODUCTION
1
CHAPTER 2: CONTRIBUTION OF MOTOR NEURON INTRINSIC
PROPERTIES TO FICTIVE MOTOR PATTERN GENERATION
27
CHAPTER 3: Patterns of presynaptic activity and synaptic strength
interact to
produce
motor
output
93
CHAPTER
4:
GENERAL
DISCUSSION
167
APPENDIX
192
REFERENCE LIST
200
About this Dissertation
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