The provided code is part of a computational neuroscience model that simulates synaptic interactions in an olfactory bulb microcircuit, focusing particularly on the communication between olfactory receptor neurons (ORNs), mitral cells, and granule cells. Here’s a breakdown of the biological basis: ### Biological Context - **Olfactory Bulb**: The olfactory bulb is a critical neural structure in the vertebrate brain involved in processing smell-related information. It contains several types of neurons, including mitral cells, which receive input from ORNs, and granule cells, which interact with mitral cells to modulate their output. - **Mitral Cells**: These neurons are the primary output neurons of the olfactory bulb and transmit olfactory information to other brain regions. They receive excitatory input from ORNs and interact reciprocally with inhibitory granule cells. - **Granule Cells**: Inhibitory interneurons that do not have axons. They form dendrodendritic synapses with mitral cells, mediating lateral inhibition, enhancing contrast, and helping in odor discrimination. ### Synaptic Dynamics - **Excitatory Synapses**: The code models AMPA and NMDA receptor-mediated excitatory synaptic inputs to mitral cells. These are typically glutamatergic synapses: - **AMPA Receptors**: Allow for fast synaptic transmission, as modeled by the rise and decay times (`tau1` and `tau2`). The conductance parameter `Gbar` is linked to receptor saturation to simulate physiological conditions. - **NMDA Receptors**: Involve more prolonged synaptic currents and are modulated by Mg²⁺ concentration (`MG_CONC`). The removal of Mg²⁺ block at depolarized potentials allows for calcium influx, crucial for synaptic plasticity. - **Inhibitory Synapses**: Focus on GABAergic transmission from granule cells to mitral cells. This involves GABA receptor-mediated synaptic currents, which help regulate the timing and extent of mitral cell firing, contributing to lateral inhibition and contrast enhancement in olfactory processing. ### Key Biophysical Parameters - **Receptor Saturation and Kinetics**: The code includes parameters (`RECEPTOR_SATURATION`, `RECEPTOR_SATN_CORRECTN`) to account for receptor saturation, affecting how synapses respond to neurotransmitter release over time. - **Mg²⁺ Block and NMDA Receptors**: The NMDA receptors are sensitive to magnesium block, and the `mitral_granule_NMDA_KMg_A` and `KMg_B` constants describe this voltage-dependent block, critical for their function. - **Plasticity**: The option to toggle short-term synaptic plasticity reflects ongoing changes in synaptic strength in response to activity, an essential feature for learning and memory, as seen in the olfactory system. ### Summary This code models critical aspects of synaptic communication in the olfactory bulb. It represents the interactions between mitral and granule cells, including excitatory and inhibitory synaptic currents modulated by AMPA, NMDA, and GABA receptors. The model incorporates critical biological details such as receptor kinetics, gating by ions (Mg²⁺), and synaptic plasticity, aiming to replicate the integrative and modulatory roles these neurons play in olfactory processing.