A Hodgkin-Huxley model for conduction velocity in the medial giant fiber of the earthworm, Lumbricus terrestris

Issue: 
2016
Institution: 
St. Olaf College, Northfield, Minnesota 55057

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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