The code provided is a computational model of action potential conduction in myelinated nerve fibers, with a specific focus on understanding the relative sensitivities of conduction parameters to variations in nodal and internodal characteristics. The model's biological basis is rooted in the structure and function of myelinated axons, which are specialized for rapid signal transmission in the nervous system. ### Biological Basis 1. **Myelinated Axons:** - Myelinated axons consist of alternating segments of nodes of Ranvier (exposed axonal membrane) and myelinated internodes (covered by myelin sheath). The myelin sheath is a fatty insulating layer that increases the speed of electrical conduction. 2. **Nodes of Ranvier:** - The nodes of Ranvier are critical for saltatory conduction, a process that allows for rapid action potential propagation. These nodes are characterized by a high concentration of voltage-gated sodium (Na\(^+\)) and potassium (K\(^+\)) channels. 3. **Internodal Regions:** - The internodal regions are covered by the myelin sheath, which serves to reduce membrane capacitance and increase resistance, thereby accelerating signal propagation by forcing the action potential to jump from node to node. 4. **Ionic Currents:** - The model includes Hodgkin-Huxley (HH) type conductances in the nodes, representing the gating dynamics of Na\(^+\) and K\(^+\) channels. Specifically, nodal sections have conductances for sodium (gnabar_hh) and potassium (gkbar_hh), while the internodal regions are modeled with passive properties (pas). 5. **Electrical Properties:** - Parameters such as axial resistance (Ra) and membrane capacitance (cm) are crucial for determining the speed and fidelity of action potential propagation. Ra affects the ease of current flow along the axon, and cm influences the time constant of membrane charging. 6. **Temperature and Resting Membrane Potential:** - The model simulates a physiological temperature of 18.5°C and a resting membrane potential around -65 mV, aligning with typical physiological conditions under which these fibers operate. ### Modeling Objective The primary aim of the code is to simulate how changes in nodal and internodal geometrical and biophysical parameters impact the conduction velocity and spike shape in myelinated fibers. By leveraging these simulations, the model can help unravel the complex interplay between anatomical structure and electrophysiological function in nerve fibers, with implications for understanding not only normal physiology but also pathological conditions where these parameters may be altered.