Doctor of Philosophy (PhD)
All animals face the same basic challenges in controling movement through their environment. The central nervous system must activate and effectively control muscle fibers capable of accomplishing the behavior. For multi functional muscles, an array of muscle fiber types must be selectively activated based on the behavioral task. In arthropods that have few motor neurons, it is not understod how the nervous system accomplishes this selective activation. If the selection cannot be achieved by recruitment of different motor neurons, then it most likely results from careful adjustment to the firing patterns of the motor neurons. I used the neuromuscular system of crab walking legs to pursue a detailed description of the relationship between neural firing patterns and the resultant muscle contraction, emphasizing how muscles with different walking use differ in physiological characteristics. Crabs use of a small number of identified motor neurons and their experimental tractability make them an attractive model system to investigate how nervous systems control behavioral movements. Experimentation was conducted on two brachyuran species Carcinus maenas and Libinia emarginata, the former an amphibious sideways walker and the latter an aquatic forward walker. Comparisons within and between the species showed that muscles that provide thrust in the animal’s most common walking direction had greater high frequency facilitation. Muscles that cycle more frequently during walking had shorter relaxation time constants across multiple behavioral criteria. These differences in pre- and postsynaptic kinetics reveal how the behavioral use of a muscle can constrain the array of physiological properties within the motor system. Computational models demonstrated that selective activation of postsynaptic muscle fibers can be accomplished by changing neural firing rates due to physiological differences described in previous chapters. Faster muscles were more sensitive to short-term changes in the excitatory firing pattern, while slower muscles were more sensitive to long-term changes. The ability to selectively recruit different muscle fiber types that are more sensitive to different aspects of the firing pattern of motor neurons allows the animal to have many functional motor units per muscle despite receiving excitatory innervation from as few as one motor neuron.
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Dewell, Richard Burkett, "Functional implications of the neuromuscular transform in decapod crustacean locomotion" (2009). LSU Doctoral Dissertations. 1462.