Date of Award
Doctor of Philosophy
Zoology and Genetics
Charles D. Drewes
Ralph A. Ackerman
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).
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
David Evan Turnbull
Turnbull, David Evan, "Environmental determinants of escape reflex performance in a freshwater oligochaete " (1995). Retrospective Theses and Dissertations. 10990.