Degree Type

Dissertation

Date of Award

1995

Degree Name

Doctor of Philosophy

Department

Zoology and Genetics

First Advisor

Charles D. Drewes

Second Advisor

Ralph A. Ackerman

Abstract

This study examines the effects of a variety of environmental stresses on the physiological of selected neural elements on the rapid escape reflex behavior in two freshwater oligochaetes, Lumbriculus variegatus and Branchiura sowerbyi. The stresses include Cd, anoxia, pH, ammonia/ammonium, temperature, and repeated mechanosensory stimuli. Neural parameters that were studied included: touch sensitivity, giant fiber excitability, and initiation of giant fiber-mediated muscle potentials. The rationale for this study is that impairment of these functions due to environmental stresses could reduce the effectiveness of rapid escape behavior. The primary method for assessing effects was non-invasive extracellular recordings of giant fiber spike activity and corresponding muscle potentials. In addition, the concentration of metal-binding proteins, as a result of Cd pre-treatment, was also assessed;Pre-treatment with 0.01 ppm CdCl2 rescued worms from a 100% lethal Cd pulse (1 ppm CdCl2 for 30 min). This protection was correlated to increases in metal-binding protein concentration; maximum induction occurred after 48 h and persisted for at least 14 days. Acute (5 min) or prolonged (7 day) exposure to anoxia had no apparent effect of neural parameters associated with the rapid escape behavior. Exposure to a wide range of pH (4-8) also had no apparent effect on the neural parameters. Worms did not survive exposure to pHs outside this range (pH 3 or 9). Exposure to 260 ppm NH4OH (pH 9.2) for 30 min reduced giant fiber conduction velocity by approximately 50%. Conduction velocity continued to be affected at least 7 days after the exposure ended. Changing the pH of the 260 ppm NH4OH solution alleviated the effects of the exposure. Conduction velocity was also reduced as temperature decreased below 20 ∘C. No acclimation of conduction velocity occurred between worms maintained at 10, 21, or 30 ∘C. Below 10 ∘C, efferent connection failed and a muscle potential could not be evoked despite being able to touch-evoke giant fiber spikes at temperatures as low as 2 ∘C. Touch-evoked spikes were easier to initiate when a second spike was preceded by an initial giant fiber spike (30% vs. 60%, respectively).

DOI

https://doi.org/10.31274/rtd-180813-12750

Publisher

Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/

Copyright Owner

David Evan Turnbull

Language

en

Proquest ID

AAI9540950

File Format

application/pdf

File Size

112 pages

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