The code provided is a template in the NEURON simulation environment designed to model the structure and biophysical properties of a myelinated axon, specifically inspired by mammalian axons. This model, the MyelAxonMammal, reproduces the myelinated internodal and nodal regions found in real biological neurons, using a simplified representation of the alternating segments of myelinated axons. ### Biological Basis 1. **Axon Structure** - **Nodes of Ranvier (NODE):** These are the gaps between myelinated segments along an axon. In myelinated axons, nodes of Ranvier are critical for the rapid propagation of action potentials via saltatory conduction. In the code, the NODE sections are modeled to not contain active voltage-gated channels but instead utilize a passive mechanism. This choice simplifies the model, focusing on extracellular properties. - **Myelin Segments (MYSA, FLUT, STIN):** These regions represent different parts of the myelinated segment between nodes: - **MYSA** refers to the paranode region adjacent to the nodes. - **FLUT** models the flut of the myelin sheath, further along from the paranode. - **STIN** represents the internodal segment, which is extensively myelinated. - The myelin acts as an electrical insulator, enhancing the speed and efficiency of signal transmission through the axon. 2. **Electrical Properties** - The model uses passive electrical properties without inserting active ion channels into the segments, which is unusual compared to more detailed models of axonal conduction. This approach simplifies the biophysical computation while still allowing the examination of properties like axial resistance and capacitance. - **Extracellular mechanisms** are included to manage how the external environment influences electrical conductance in the model, aiming for realistic representations of electrical insulation provided by myelin. 3. **Sections and Connectivity** - The axon is divided into different sections corresponding to the biological realities of axonal myelination. These sections are aligned serially in a way that alternating myelinated and unmyelinated segments can simulate the rapid conduction seen in myelinated axons. - The script maintains clear biological fidelity in terms of geometry (lengths and diameters) and the interspersion of nodes and myelinated and unmyelinated segments. 4. **Biophysical Parameters** - The script includes parameters like axon diameter, fiber diameter, and axial resistance (`Ra`), which are necessary to simulate realistic electrical behaviors within the axon. The passive properties, i.e., specific membrane resistance `(g_pas)`, are set to high values to reflect the insulating properties of the myelin. ### Summary Overall, this NEURON code is structured to create a reduced complexity model of mammalian myelinated axons. It implements the fundamental structure of an axon with nodes and internodal segments, allowing the investigation of passive electrical properties and signal propagation effects in a myelinated neural fiber. Despite its simplification (i.e., lack of active channel dynamics), the model provides insights into the role of cellular geometry and insulation in neuronal signaling efficiency.