The code provided models the dynamics of synaptic transmission involving both AMPA and NMDA receptors, incorporating presynaptic short-term plasticity mechanisms. Here's a breakdown of the biological basis of the elements included in the model:
AMPA and NMDA Receptors: These are ionotropic glutamate receptors crucial for excitatory synaptic transmission in the brain. AMPA receptors are responsible for fast synaptic responses, while NMDA receptors are involved in slower responses due to their voltage-dependent Mg2+ block and their ability to allow Ca2+ influx, which is important for synaptic plasticity.
Dual-Exponential Conductance Profile: The model uses dual-exponential decay profiles (tau_r_* and tau_d_* parameters) to represent the kinetics of post-synaptic conductance changes. This captures the temporal dynamics of how quickly these receptors respond to neurotransmitter release and return to the baseline state after activation.
Short-term Plasticity Mechanisms: The model incorporates presynaptic short-term plasticity, referring to temporary increases (facilitation) or decreases (depression) in synaptic strength.
Facilitation (Fac) involves an increase in the probability of neurotransmitter release following prior activity. It is modeled by the parameter Fac, the relaxation time constant, reflecting how changes in calcium concentration affect vesicle release probability over tens to hundreds of milliseconds.
Depression (Dep) describes a reduction in release probability due to the depletion of readily-releasable vesicles or other mechanisms over a similar timescale. This is represented by the Dep parameter, also a relaxation time constant.
Utilization of Synaptic Efficacy (Use) is a parameter that models how much of the available resources are used in response to an action potential. The running value, u, is dynamically updated to reflect ongoing synaptic activity.
mg and mggate): NMDA receptors are sensitive to the membrane potential due to Mg2+ ions blocking the receptor channel when the cell is at resting potential. In the model, mggate is calculated based on voltage (v) and Mg2+ concentration, modeling voltage-dependent relief of Mg2+ block and allowing ion flow through NMDARs at depolarized potentials.Conductance and Current Calculations: The model distinguishes between AMPA and NMDA receptor-mediated currents (i_AMPA, i_NMDA) by calculating them from the product of conductance (g_*) and the driving force (difference between membrane potential v and reversal potential e).
Random Synaptic Activation: The use of a random number generator models the probabilistic nature of vesicle release, with release determined by the probability Pr derived from the facilitation and depression dynamics.
Overall, this model aims to capture key aspects of excitatory synaptic transmission and the dynamics of short-term synaptic plasticity, reflecting both the rapid signaling through AMPA receptors and the modulatory roles of NMDA receptors and the presynaptic machinery in synaptic strength regulation.