Simulation of the diffusion of acetylcholine in the neuroeffector junctions of the sinus node

Traditionally, the diffusion of acetylcholine (ACh) from a neuron to cardiac muscle in a neuroeffector junction has been modeled as radial diffusion from a nerve ending into a spherical homogeneous medium. Various microscopic structures in the heart may or may not influence the spatial distribution of ACh within neuroeffector junctions. To determine the effect of microscopic anatomy on the diffusion of ACh in neuroeffector junctions, we simulated the diffusion of ACh in a two-dimensional inhomogeneous geometry that was based on micrographs of neuroeffector junctions in the sinus node. ACh was released at sites adjacent to a neuron. Simulations showed that the times of peak concentration after release and the peak concentrations per se were distributed symmetrically above and below and to the right and left of the neuron, but not radially about the neuron. We conclude that the diffusion of ACh in the neuroeffector junctions of the sinus node cannot be predicted well by a mathematical model that assumes radial diffusion in a spherical and homogeneous medium.