The code provided is a computational model designed to simulate the dynamics of hydrogen ion (H⁺) concentration in the synaptic cleft, a space between neurons where synaptic transmission occurs. The key aspects of the biological basis of this code are as follows: ### Biological Context 1. **Synaptic Cleft:** - The synaptic cleft is the narrow gap between the pre- and postsynaptic neurons where neurotransmitters are released to transmit signals from one neuron to another. - Regulation of hydrogen ion concentration in the synaptic cleft is crucial for maintaining the pH balance, which can influence synaptic transmission and neuronal excitability. 2. **Hydrogen Ion (H⁺) Concentration:** - The model focuses on the concentration of H⁺ ions, directly related to pH levels, which are a measure of acidity in the synaptic environment. - Proper pH levels are vital for optimal neurotransmitter function and receptor activity in the synaptic cleft. 3. **Buffering Systems:** - The parameter `b0` represents a buffering system that binds to free H⁺ ions to stabilize pH within the cleft. - `Kd` is the dissociation constant, a key variable that represents the affinity of the buffer for H⁺ ions. 4. **pH Homeostasis:** - The parameter `pH0` depicts the baseline pH, a reference point for maintaining steady-state conditions within the cleft. - Hydrogen ions in the synaptic cleft can affect the function of ion channels and neurotransmitter receptors, hence influencing synaptic efficacy and plasticity. 5. **Proton (H⁺) Dynamics:** - The code models the dynamic change in total proton concentration (`tot_prot`) and extracellular hydrogen ion concentration (`he`). - `tau` represents the time constant for proton dynamics, indicating the speed of achieving equilibrium in the synaptic cleft. ### Key Aspects from the Code - **Voltage Dependence:** - The `evaluate_fct(v)` procedure adjusts the rate of proton dynamics (`q`) based on membrane potential `v`, which implies an interaction between electrical activity and proton flux. - A threshold at `-30 mV` indicates that above this voltage, the proton dynamics are maintained at a certain rate (`q0`), potentially linking proton concentration changes to neuronal firing activity. - **Initial Conditions:** - The initial conditions compute baseline hydrogen concentration (`h0`) and total proton concentration incorporating buffering effects, indicating the starting pH conditions of the synaptic cleft. This model provides insights into the regulatory mechanisms affecting synaptic pH, crucial for understanding synaptic transmission fidelity and its modulation under different neuronal activities.