# Biological Basis of the cFiberBuilder Code The provided code defines a computational model for simulating the electrical properties of C-fiber axons. These are small, unmyelinated nerve fibers that are involved in conveying information slowly, such as chronic pain, temperature, and itch sensations. The axonal model is implemented using the NEURON simulation environment, which allows for the construction and simulation of neuron models. ## Biological Component Representation ### Axonal Diameter and Segmentation - **Fiber Diameter (`fiberD`)**: Determines the diameter of the axon, impacting the axonal conduction velocity. Larger diameters generally increase conduction speed. - **Segment Number (`nsegments`)**: The axon is discretized into segments to simulate the propagation of electrical signals across its length. ### Membrane Potential and Passive Properties - **Resting Potential (`v`)**: Each model type sets the fiber's initial membrane potential (e.g., -60, -55, -70 mV), reflecting the resting state of the neuron. - **Passive Conductances (`g_pas`, `e_pas`)**: The passive leak channels simulate the ionic leakage across the cell membrane, contributing to the resting potential. ### Ion Channel Types The code models several ion channel types, with density and equilibrium potentials that mirror biological properties: - **Sodium Channels (`nahh`, `nattxs`, etc.)**: Responsible for depolarizing the neuron membrane, they play a crucial role in the initiation and propagation of action potentials. - **Potassium Channels (`borgkdr`, `kdrTiger`, `kf`, etc.)**: Provide repolarizing and hyperpolarizing currents, helping restore the resting potential after an action potential. - **Other Channels and Mechanisms**: - High and low voltage-activated calcium channels (`can`, `cat`), which can influence neurotransmitter release and other cellular processes. - Pumps and exchangers such as `NaKpumpSchild`, `CaPump`, `NaCaPump`, which maintain ionic gradients critical for neuronal excitability. ### Ion Concentrations - Set extracellular and intracellular concentrations of ions like Na⁺ and K⁺. The equilibrium potentials (`ek`, `ena`) are calculated using the Nernst equation, which is fundamental for understanding how ionic gradients drive membrane voltage changes. ### Temperature Effects - **Temperature (`temp`)**: Affects channel kinetics and overall neuron excitability, mimicking how real biological processes respond differently at various temperatures. ### Different Axonal Models The code includes multiple models representing different types of fibers based on specific literature or studies (Sundt, Tigerholm, Rattay, and Schild models). This diversity allows exploring different scenarios corresponding to various experimental or theoretical findings about C-fiber activity. ## Conclusion In summary, this code encapsulates a biologically-informed model of C-fiber axons, emphasizing the ion channel dynamics, passive electrical properties, and continuity of conduction through segmented membrane representation. These elements collectively contribute to understanding the biophysical basis of action potential propagation in unmyelinated nerve fibers and exploring how changes in these properties can affect neuronal function.