Graduation Term

2017

Degree Name

Master of Science (MS)

Department

School of Biological Sciences

Committee Chair

Wolfgang Stein

Abstract

The precise timing of action potentials generated in the nervous system is crucial for generating adequate behavior. Once generated, action potentials travel along axons towards the neurons or muscles they innervate. Axons are also responsible for preserving the temporal fidelity of the generated action potentials. One challenge axons face is that they can be of considerable length, and exposed to changes in internal and external conditions. Temperature fluctuations, for example, affect the ion channels that generate and propagate action potentials causing changes in action potential speed. It is unclear if, and how, the timing of action potentials can be preserved when action potential velocity changes. I used axons in the pyloric central pattern generator (CPG) of the crustacean stomatogastric nervous system to evaluate spike time arrival at different temperatures. CPGs are neural circuits that generate vital rhythmic behaviors including breathing and walking that must be robust to environmental challenges such as temperature fluctuations. To maintain functional behavior, the axons of CPG neurons must thus possess mechanisms to counterbalance detrimental temperature influences.

The pyloric CPG in the crustacean stomatogastric nervous system creates a well-defined triphasic rhythm generated by the lateral pyloric (LP), pyloric dilator (PD), and pyloric constrictor (PY) neurons. The pyloric CPG is robust to temperature fluctuations and produces a triphasic motor pattern that - similar to other rhythmic behaviors - requires a particular sequence of neuronal activities (’phasing’) for adequate functioning. While the CPG itself is temperature compensated, it is unknown how temperature affects the pyloric axons’ ability to maintain adequate timing of action potentials. Pyloric axons possess different neuronal identities, indicating that their response to temperature may be heterogenous. I hypothesize that changes in conduction velocities during temperature changes are coordinated such that the functionally adequate phasic activity of the pyloric neurons is preserved in the periphery.

To test this, I first measured the axonal diameters of LP, PD and PY using a voltage-sensitive dye technique. I found that LP’s axon is the largest and PY’s axon is the smallest (N=6 for all axons, One-Way-ANOVA). I then determined how temperature affects conduction velocity in the different sized axons. Across temperatures, the pyloric axons had different conduction velocities (LP > PD > PY), and conduction velocities increased equally (N=8, One-Way-ANOVA). Consequently, the effects of temperature on action potential travel times differed: at high temperatures, LP’s (largest axon) action potentials arrived much earlier than those of PY (smallest axon), disrupting functional behavior in the periphery. However, at lower temperatures the arrival times of LP's and PY’s action potentials did not disrupt their phase relationships. In conclusion, phase relationships are maintained at lower temperatures at the muscles; however, at higher temperatures phase relationships were lost.

Access Type

Thesis-Open Access

DOI

http://doi.org/10.30707/ETD2017.Cruz.M

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