Degree Type


Date of Award


Degree Name

Doctor of Philosophy




This study represents the first anatomical, physiological, and behavioral studies of the giant nerve fiber systems and rapid withdrawals of aquatic oligochaetes. Behavioral analysis of escape in Branchiura sowerbyi, in combination with non-invasive electrophysiology, revealed a total response latency (i.e., time from stimulus onset to the onset of withdrawal) of 9 ms. Many factors contribute to the speed and efficacy of this response, including: (1) a sensitive mechanosensory system, (2) a short latency (2-3 ms) afferent-to-giant pathway, (3) a rapid intersegmental conduction (15-20 m/s) of lateral giant fiber (LGF) spikes, (4) an extremely short efferent latency (0.7-0.9 ms) from LGF spike to muscle potential, (5) a short coupling time (5-7 ms) from muscle potentials to the onset of movement, (6) a fast rate of segmental shortening, and (7) a complete (all-or-none) response mediated by a single LGF spike;Neuroanatomical studies demonstrated the neuronal cell body positions (two somata per segment), axonal geometry, and several ultrastructural features of the paired LGF system. Marked bilateral asymmetry (the left fiber four times larger than the right), segmentally-arranged axonal enlargements (blebs), intersegmental syncytia, and a myelin like sheath are among the most conspicuous specializations. Intracellular electrophysiology revealed that the small, right LGF, with a high input resistance (3.3 M[omega]) functions as the locus for spike initiation. The large, left fiber (R[subscript] IN = 1.5 M[omega]) functions primarily in rapid impulse conduction;Comparative studies of several aquatic species suggest that an arrangement of three dorsal giant fibers, functioning as two distinct pathways, has been evolutionarily conserved among oligochaetes. However, the LGF systems in posterior segments of aquatic species are larger and faster than the medial giant fiber systems. In addition, the all-or-none escape reflexes of tubificids contrast with the graded reflexes of earthworms. I hypothesize that fundamental differences in lifestyle, habitat, and predation pressures have led to this divergence.



Digital Repository @ Iowa State University,

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Mark Joseph Zoran



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142 pages

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Zoology Commons