A Hodgkin-Huxley model for conduction velocity in the medial giant fiber of the earthworm, Lumbricus terrestris
The speed of nerve impulse conduction, or conduction velocity, is crucial to the survival of animals. For example, rapid conduction velocity in the nerve pathways underlying escape behavior represents a distinct evolutionary advantage. Peripheral demyelinating diseases can lead to a loss of conduction velocity and subsequent serious symptoms and diseases, such as the fatigue and gait deficiencies commonly observed in multiple sclerosis patients. A better understanding of the biophysical mechanisms underlying conduction velocity may yield insights that could be valuable in the development of therapies for such diseases. Nerve cord gigantism and myelin sheath are the two basic mechanisms that increase the conduction speed of electrical nerve impulses. The giant fibers of the common earthworm Lumbricus terrestris are made up of many neurons electrically coupled by high fidelity gap junctions, permitting a unique perspective on the contribution of transmembrane ionic currents on conduction velocity. Furthermore, the previously noted taper in diameter of the oligochaete giant fibers along the longitudinal axis presents another unique opportunity to study the role of morphological properties on conduction velocity, even within a single fiber pathway. The role of these gap junctions and their interaction with axonal taper in predicting conduction velocity has not been studied closely in the annelid. Intracellular recording from individual giant fibers in earthworm is very challenging, and the genetic and pharmacological tools are not yet available to manipulate gap junction communication reliably. Because of these technical limitations, a combination of extracellular electrophysiology, histology, and computational modeling were used to explore the influence of, and interaction between, electrical coupling and axon diameter on conduction velocity. We observed that conduction velocity in the medial giant fiber (MGF) seems to be predicted by a nonlinear supra-additive interaction between axonal conductance and gap junction conductance. This suggests that both are critical considerations when studying nerve impulse conduction.
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