The code provided is a computational model of a myelinated axon from the Peripheral Nervous System (PNS), focusing on the excitability and conduction properties of mammalian nerve fibers. The model aims to simulate how axons conduct nerve impulses and how they recover after action potentials, as described by McIntyre et al. in their 2002 study. ### Biological Basis #### Myelinated Axon Structure - **Nodal and Internodal Regions**: The axon is represented by alternating nodes (node of Ranvier) and internodal segments. The nodes are crucial for action potential generation and propagation. - **Paranodes and Internodes**: The paranodal regions (MYSA and FLUT) and internodal segments (STIN) correspond to distinct morphological regions of the axon. These regions physically insulate the axon, reducing ion leakage and enabling faster signal conduction. #### Morphological Parameters - **Fiber Diameter**: The model differentiates fibers based on their diameters, linking them to specific node and internodal dimensions. This affects conduction velocity and excitability. - **Nodal, Paranodal, and Internodal Lengths**: These lengths determine the spatial structure of the axon, influencing how impulses propagate. #### Electrical Properties - **Axonal Resistance and Capacitance**: The intracellular resistivity (`rhoa`) and membrane capacitance (`cm`) relate to the electrical passive properties of the axon. These parameters are crucial for determining the propagation of electrical impulses along the axon. - **Extracellular Mechanism**: The model includes extracellular mechanisms (xraxial, xg, and xc), significant for modeling ionic currents that influence membrane potentials during action potential propagation. #### Ion Channels and Action Potentials - **Axonal Ion Channels**: The insertion of `axnode` channels at nodal points corresponds to the concentration of voltage-gated sodium channels seen in biological nodes of Ranvier. These channels are essential for the rapid depolarization phase of action potentials. - **Passive Currents in Paranodal and Internodal Regions**: The insertion of passive (`pas`) channels models the myelin's insulation properties, allowing only minimal ion flow and supporting the action potential's saltatory conduction. #### Stimulation and Excitability - **Intracellular Stimulation**: The `IClamp` object simulates electrical stimulation of the axon, allowing the study of parameters influencing excitability, such as stimulus amplitude, duration, and delay. - **Recovery Cycle**: The model is designed to investigate the axon's recovery cycle after action potentials, focusing on afterpotentials that influence excitability and refractory periods. This model provides insights into the functional architecture of myelinated axons, contributing to our understanding of nerve impulse propagation and the factors affecting nerve excitability and conduction velocity.