The provided code models the electrical properties of axons, specifically their response to external electrical stimulation. This is a common approach in computational neuroscience to understand how neurons, particularly axons, respond to electrical fields, which is relevant in contexts such as deep brain stimulation, nerve stimulation therapies, and understanding neural prosthetics. Below is a breakdown of the biological basis of the code: ### Biological Basis 1. **Axon Anatomy and Physiology:** - **Diameter and Depth:** The code considers the axon diameter and depth relative to the electrode as critical parameters. The diameter affects the axonal membrane properties and conduction velocity. The depth, calculated in terms of tissue penetration, influences the strength and distribution of the electric field experienced by the axon. - **Membrane and Axial Resistances:** The specific membrane resistance (Rm) and axial resistance (Ra) are used to characterize the electrical properties of the axonal membrane and the cytoplasm, respectively. These resistances influence how the current flows along the axon and across its membrane. 2. **Electrical Stimulation:** - **Depolarization Threshold (dVmt):** This is the critical voltage change (in mV) across the membrane required to trigger an action potential. An action potential is essential for nerve signal transmission, and reaching this threshold reflects the sensitivity of the axon to external electrical input. - **Anode and Cathode Positions:** These positions determine the spatial distribution of the external electrical field. Electrodes (anode and cathode) create electric fields that can modulate neural activity by depolarizing the axon membrane. - **Extracellular Resistance (re):** Represents the resistance encountered by electric currents in the extracellular space. This resistance affects how the extracellular electric field influences the axonal membrane potential. 3. **Electric Field and Membrane Potential:** - **Potential Due to Anode/Cathode (VA, VC):** The code calculates the potential difference on the membrane due to the external electric field created by the anode and cathode. This is a simplification of the electric field's effect in propagating along the axon. - **Ve and dVi:** These variables represent the external and intracellular electric potential differences, respectively. The interaction between these potentials helps determine the net effect on membrane depolarization. 4. **Model Output:** - **Axon Thresholds (Is):** The output of the model is the threshold current (Is) necessary to achieve the depolarization threshold given the axon’s depth and diameter. This is a measure of the sensitivity of an axon with specific geometric and resistance properties to an external electric field. ### Conclusion The code fundamentally models how external electrical stimulation influences axonal excitability based on various biophysical parameters. This relationship is vital for designing and understanding electrical stimulation therapies and technologies. The underlying biology emphasizes the electric properties of axons and how they are manipulated or utilized in therapeutic settings.