The code provided is a computational model of a myelinated axon, using the Frankenhaeuser-Huxley framework with parameters derived from Reilly's work. This model attempts to simulate the electrical properties and behavior of nerve fibers, particularly focusing on the nodes of Ranvier, which are critical in saltatory conduction in myelinated neurons. ### Biological Basis of the Model #### Myelinated Axon Structure - **Nodes of Ranvier**: The model explicitly creates nodes of Ranvier, which are gaps in the myelin sheath covering the axon. These nodes are critical for the rapid propagation of action potentials via saltatory conduction. The code specifies 101 nodes of Ranvier, with geometric parameters like the gap width and internode length mirroring those seen in biological axons. - **Axon Diameter and Myelin**: The provided diameter includes the myelin sheath, emphasizing its insulating role, which speeds up electrical signal propagation. The specific axon diameter without myelin is calculated using a scaling factor (`SDD`), reflecting the structural relationship between the axon and its myelin. #### Electrical Properties - **Membrane Capacitance and Resistance**: The model considers specific membrane capacitance and cytoplasmic resistivity, essential factors in determining the axon's electrical properties. Myelinated axons have lower capacitance and higher resistance compared to unmyelinated ones due to the insulating properties of myelin, contributing to faster signal conduction. - **Ion Channels and Conductance**: Ion channel dynamics are critical in the propagation of action potentials. The model integrates Frankenhaeuser-Huxley parameters, including permeability rates for sodium (Na), potassium (K), and a third ion (potentially leakage currents). These control the ionic flow during action potential generation and propagation. #### Gating Variables - **Ion Concentrations**: The intracellular and extracellular concentrations of sodium and potassium are defined, which influence the resting potential and action potential characteristics. These concentrations are different than those in standard Hodgkin-Huxley models to reflect the specific physiology of myelinated axons. #### Temperature Effects - **Temperature**: The model includes a parameter for temperature (`celsius`), reflecting the physiological temperature at which these processes occur. Temperature can affect metabolic activity and the kinetics of ion channels, hence is incorporated to simulate biologically realistic conditions. ### Conclusions This model captures the essential biological features of myelinated axons, paying particular attention to the role of nodes of Ranvier in action potential propagation. By specifying parameters for ion concentrations, membrane properties, and axonal geometry, it attempts to reproduce the characteristic behavior of action potentials traveling down myelinated fibers. This setup reflects a detailed biological understanding of axonal conduction mechanisms seen in vertebrates, specifically modeled here with parameter data influenced by specific literature, such as works from Hines, Shrager, and McNeal.