Geometry and mesh for a model of a neuron. The neuron geometry (A) is based on a artistic CAD rendering generated with the public domain version of the software Blender (http://www.blender.org). In order to conduct simulations in this geometry, the model was exported in the STL surface mesh format, imported into the open-source meshing package Gmsh , where the boundary was re-parametrized and the domain subsequently meshed with a volume mesh in 3D. The resulting mesh was then converted into a Comsol Multiphysics 3.5a model to serve as a geometry description for the URDME model. Assembly of active transport jump rate constants are conducted by URDME on the unstructured mesh shown in (B). For a mathematical background on how to obtain these constants on the unstructured mesh, see . URDME’s capability to use an unstructured mesh made up of tetrahedral and triangular elements is of vital importance in order to be able to resolve the complex geometry of the neuron.