# Biological Basis of the Computational Model The provided code simulates the activity of a granule cell, specifically modeling how this cell responds to synaptic inputs under the influence of transcranial direct current stimulation (tDCS). ## Granule Cells Granule cells are the most numerous type of neurons in the brain, primarily located in the cerebellum. They play a crucial role in processing inputs and contributing to motor coordination, learning, and cognitive functions. Their small, compact architecture and synaptic arrangements allow them to receive inputs from various sources and transmit information through parallel fibers. ## Synaptic Inputs The model includes the simulation of synaptic inputs using AMPA and NMDA receptors, key excitatory receptors found in many types of neurons, including granule cells: 1. **AMPA Receptors**: These receptors are responsible for fast synaptic transmission in the central nervous system. In the code, AMPA synapses are modeled with parameters like `tau_facil`, `tau_rec`, `tau_1`, and `U`, which represent synaptic dynamics such as facilitation, recovery, and neurotransmitter release probability, respectively. 2. **NMDA Receptors**: Known for their involvement in synaptic plasticity, such as learning and memory processes, NMDA receptors exhibit slower kinetics and are voltage-dependent. The NMDA synapses use similar parameters to capture their dynamic properties. ## External Electrical Fields (tDCS) Transcranial direct current stimulation (tDCS) is modeled to understand its modulation effects on granule cell activity. tDCS involves applying a weak electrical field to the brain to modulate neuronal excitability and synaptic activity: - The `extracellular` and `xtrau` mechanisms in the code are used to simulate the effect of the extracellular electric field on the neuron. - The function `setstim()` configures the electrical field parameters, potentially causing hyperpolarization or depolarization, affecting how synaptic inputs are integrated and modified in the cell. ## Simulation Setup The code sets up a simulation where synaptic inputs through AMPA and NMDA receptors are delivered to specific dendritic compartments. NetStim objects are used to generate synthetic activity patterns at different dendrites, indicative of neural activity under natural conditions. The synaptic response and the effect of tDCS on the granule cell's soma, or the cell body, are recorded over time. The membrane potential of the soma (`soma.v`) is recorded, which reflects how the cell integrates synaptic inputs under electrical modulation, revealing potential changes in neuronal output patterns. In summary, this model captures the dynamic interactions between synaptic inputs and external modulation by tDCS in cerebellar granule cells, providing insights into the cellular mechanisms underlying information processing and modulation in the brain.