# Biological Basis of the Computational Neuroscience Model The provided code is part of a computational model simulating the extracellular stimulation of a myelinated axon, which is a type of nerve fiber with a myelin sheath that acts as an insulator, allowing for rapid signal conduction. The model specifically focuses on the responses of this axon to electrical stimulation applied via bipolar electrodes on the surface of a semi-infinite conductive medium. This has applications in understanding nerve stimulation and can be used for both scientific research and clinical applications related to neuromodulation. ## Key Biological Components ### Myelinated Axon - **Myelination**: The axon in the model is myelinated, meaning it is wrapped in myelin sheaths which facilitate fast and efficient transmission of action potentials. Myelination is crucial for high-speed neural communication and is a characteristic feature of vertebrate nervous systems. - **Internodes**: The model specifies internodes, sections of the axon that are covered by myelin, which correspond to areas where conduction speed is maximized due to minimized current leakage. ### Resting Membrane Potential - **v_init = -70 mV**: The initial voltage of the axon is set at -70 mV, which is close to the typical resting potential of neurons in mammals. This demonstrates the use of realistic biological parameters to start simulations. ### Extracellular Stimulation - **Extracellular Resistivity (RHOE)**: Set at 300 ohm cm, this parameter mimics the resistive properties of the extracellular fluid, crucial for determining how electrical current flows around neurons. ### Transfer Resistance - **Function rxm**: This function calculates the transfer resistance between electrode locations and points along the axon, considering distance limitations (like the neurite radius), which affect how electrical stimuli propagate through the tissue. ### Stimulation Protocol - **Waveforms**: Three types of waveforms are used for stimulation—pulse, square, and sine. These waveforms reflect different ways in which neurons might be electrically stimulated in actual biological scenarios. - **Bipolar Electrodes**: The model uses bipolar electrodes, applying anodal and cathodal currents, representing how electrical fields are used to influence neuronal activity differentially across space. ### Action Potential (AP) Detection - **APCount**: This mechanism checks if an action potential (spike) is elicited in response to the stimulation. An action potential signifies that the neuron has successfully processed an incoming stimulus to a degree that could propagate a signal in the nervous system. ## Implications and Applications This modeling framework provides valuable insights into how extracellular electrical stimulations can influence neuronal behavior, particularly axons. It captures essential aspects of biophysical interactions between external electrical fields and neural tissue. The model can be applicable in designing and optimizing protocols for electrical stimulation therapies used in clinical interventions like deep brain stimulation, peripheral nerve stimulation, and rehabilitation after nervous system injuries. It aids in understanding the dynamics of nerve excitability and the precise calibration of external stimuli necessary to achieve desired biological outcomes.