# Biological Basis of the Model The code presented simulates extracellular electrical stimulation of neuron sections, an essential methodology used in computational neuroscience to study the effects of electric fields on neuronal activity. This type of stimulation is relevant for both scientific research into neural computation and for practical applications like deep brain stimulation and brain-machine interfaces. ## Key Biological Concepts ### 1. **Extracellular Stimulation** Extracellular stimulation involves applying electrical currents outside of neurons which can depolarize or hyperpolarize the cell membrane. This method is employed to study neuronal response to electrical inputs that mimic synaptic inputs or artificially control neuronal behavior. ### 2. **Transfer Resistance (rx)** The parameter `rx` represents the transfer resistance, crucial for understanding how effectively an external electrical stimulus can influence neuronal sections. It reflects the resistance encountered when current flows from an external electrode through the extracellular space to the neuronal membrane. ### 3. **Membrane Potential and Current** - **Membrane Potential (v)**: Although not directly manipulated in the code, the membrane potential is influenced by extracellular potentials. Changes in extracellular potential can alter the transmembrane potential difference. - **Membrane Current (im)**: The `im` variable represents the membrane current per area, which is essential for determining the effect of extracellular stimulation on the membrane potential and the resulting neuronal response. ### 4. **Stimulation Current (is)** `is` represents the input current from external electrodes. It acts as the driver for the extracellular potential (`ex`) impacting the neuron, allowing the study of how various current amplitudes affect neuronal responses. ### 5. **Extracellular Potential (ex) and Induced Potential (er)** - **Extracellular Potential (ex)**: This is the potential created in the extracellular space due to the applied stimulation current `is` and is calculated using transfer resistance `rx`. It acts as the primary influence on the neuron from the external field. - **Induced Potential (er)**: The induced potential reflects the interaction between applied stimulation and the neural section, taking into account the current density (`im`) and the geometry (area) of the neuronal section. This term helps estimate how extracellular fields influence current flow across the neuronal membrane. ## Summary The provided code models how extracellular electrical fields affect neurons by accounting for the transfer of electrical signals from external sources to neurons. The parameters and calculations used are based on essential biophysical principles governing how neurons interact with their electric environment, through variables such as transfer resistance and induced potentials affecting membrane currents. This type of modeling is important for understanding neuronal activation mechanisms and for developing interventions that modify neural activity, such as therapeutic stimulation devices.