The code provided is a NEURON simulation template for modeling a myelinated axon of a mammal. This model focuses on the detailed representation of two distinct anatomical and functional regions of a myelinated axon: the **Nodes of Ranvier** (NODE) and the **internodes** (STIN), which includes the myelin sheaths provided by oligodendrocytes or Schwann cells. ### Biological Basis #### Nodes of Ranvier (NODE) - **Function**: The Nodes of Ranvier are gaps in the myelin sheath where the axon's membrane is exposed. They play a critical role in saltatory conduction, allowing for rapid signal transmission along the axon by enabling action potentials to "jump" from one node to the next. - **Modelling Key Aspects**: - **Axonal properties**: `nodeD`, `rhoa`, and other parameters define the diameter and axoplasmic resistance. - **Membrane Mechanisms**: The `axnode` mechanism is inserted, which likely simulates the ion channel dynamics typical of these sites (usually high concentrations of sodium channels). - **Extracellular Space**: The model considers the extracellular medium (`xg`, `xc`, `xraxial`), which impacts the electric fields and hence the potential changes during action potentials. #### Internodes (STIN) - **Function**: The internodal regions are the myelinated sections of the axon. Myelin, composed primarily of lipids, provides electrical insulation that increases conduction velocity. - **Modelling Key Aspects**: - **Myelin and Axonal Properties**: Parameters such as `fiberD`, `axonD`, `space_i` adjust the modeled geometry, encapsulating the thickness and structural properties of myelin. - **Passive Properties**: Passive electrical properties like membrane capacitance (`cm`) and specific membrane resistance (`g_pas`) are modified to reflect the presence of myelin. - **Membrane Mechanisms**: A passive (`pas`) conductance is included that simulates leak channels important for maintaining resting membrane potential. - **Extracellular Mechanisms**: The `extracellular` properties are adjusted considering the impact of the myelin (which adds layers of insulation), using `xg`, `xc`, and `xraxial`. ### Additional Biological Details - **Parameters Involving Myelin**: - `nl` (number of myelin layers), `mycm` (myelin capacitance), and `mygm` (myelin conductance) are used to represent the biophysical properties introduced by the multi-layered myelin sheath. - Myelin reduces axial resistance and capacitance by structurally separating the extracellular space from the axolemma (axon membrane), facilitating faster signal propagation. This template captures the essentials of a myelinated axon's structure and function, emphasizing how myelination influences neural signal transmission. It specifically represents mammalian models, likely reflecting the biological parameters and properties typical of mammals' myelinated nerve fibers.