# Biological Basis of the Computational Model The provided computational model simulates the extracellular stimulation of a myelinated axon. In this context, a myelinated axon refers to a nerve fiber around which a myelin sheath is wrapped, contributing to efficient signal transmission along the neuron. ## Key Biological Concepts ### Axon and Myelin - **Myelinated Axon**: The model represents a myelinated axon, specifically a type found in certain neural pathways where rapid signal propagation is crucial. Myelin sheaths are fatty layers that insulate the axon, facilitating faster conduction of action potentials via saltatory conduction, where the electrical impulse "jumps" from one node of Ranvier to the next. - **Node of Ranvier**: These are small gaps in the myelin sheath of a myelinated axon. They are crucial for the rapid transmission of nerve impulses. The model involves computations suggesting spikes are initiated at these nodes. ### Electrical Properties - **Extracellular Stimulation**: The model simulates the effect of an external electric field applied parallel to the axon, which can induce action potentials (spikes) by depolarizing the membrane at the nodes of Ranvier. - **External Resistivity (RHOE)**: Set at 300 ohm cm, this parameter describes the resistivity of the extracellular medium, which affects the distribution of the electric field around the axon. - **Voltage and Membrane Dynamics**: The model is initialized with a membrane potential (`v_init`) of -70 mV, in accordance with typical neuronal resting potentials. The simulation incorporates various parameters to account for how the electric field externally influences the membrane potential at different points along the axon. ### Stimulation Protocol - **Stimulus Waveforms**: The model can simulate different waveform protocols like pulse and square wave forms. These are used to determine the minimal external stimulus intensity needed to evoke action potentials in the axon. - **Spike Detection**: An `APCount` mechanism is used to detect action potentials at a specified location along the axon, highlighting neural spike initiation and propagation in response to external stimulation. ## Simulation Specifics - **Extracellular Mechanism**: The model utilizes NEURON's extracellular mechanism, which helps simulate the effects of electrical fields on a neuron. Parameters like `xg` and `xc` are used, which in the context of the model act as surrogates for describing the insulating properties of myelin. - **Signal Propagation**: The axon is modeled to lie along the x-axis, and the stimulus is applied in the same direction. This setup emphasizes the biological scenario of axonal stimulation used in experimental and clinical electrophysiology. Overall, this computational model provides a detailed framework for studying how external electrical fields can induce and propagate action potentials in myelinated axons. Such models are invaluable for understanding the basic principles of neural signaling and for developing clinical applications like neural prosthetics or therapies that use electrical stimulation for rehabilitation.