THEORETICAL-STUDIES OF IMPULSE PROPAGATION IN SEROTONERGIC AXONS

Impulse propagation in small-diameter (1-3-mu-m) axons with inhomogene ous geometry was simulated. The fibres were represented by a series of 3-mu-m-long compartments. The cable equation was solved for each comp artment by a finite-difference approximation (Cooley and Dodge 1966). First-order differential equations governing temporal changes in membr ane potential or Hodgkin-Huxley (1952) conductance parameters were sol ved by numerical integration. It was assumed that varicosity and inter varicosity segments had the same specific cable constants and excitabi lity properties, and differed only in length and diameter. A single lo ng varicosity or a 'clump' of 3-mu-m-long varicosities changed the poi nt-to-point (axial) conduction velocity within as well as to either si de of the geometrically inhomogeneous regions. When 2-mu-m-diameter, 3 -mu-m-long varicosities were distributed over the 1-mu-m-diameter fibe r length as observed in serotonergic axons, mean axial conduction velo city was between that of uniform-diameter 1 and 2-mu-m fibers, and cha nged predictably with different cable parameters. Fibers with inexcita ble varicosity membranes also supported impulse propagation. These sim ulations provided a general theoretical basis for the slow (< 1 M/s) c onduction velocity attributed to small-diameter unmyelinated varicose axons.