The provided code models the calcium dynamics near the cell membrane in a neuronal compartment. It focuses on the accumulation and decay of calcium ions (Ca²⁺) within a specified volume immediately adjacent to the membrane, which is represented as a thin shell of cytosolic space, characterized by a specific area and depth. The core biological aspects of this code are as follows: ### Calcium Dynamics 1. **Calcium Influx and Decay:** - The model describes calcium influx driven by calcium ion currents (`ica`) and its decay back to a resting concentration. - `ica` represents the transmembrane calcium current density, typically resulting from the opening of voltage-gated calcium channels. - The calcium influx leads to increased intracellular calcium concentration (`cai`). 2. **Cytosolic Calcium Buffering:** - Calcium ions are rapidly buffered by intracellular mechanisms or proteins; however, not included in the detailed way in the model, the code addresses the decay using a simple exponential mechanism. - This is represented as a decay to a resting baseline concentration (`cai0`), with `tau` (decay time constant) governing the rate of return to baseline. 3. **Compartment Volume:** - The code assumes the calcium accumulation happens in a small shell-like compartment defined by the `depth`, which, along with the area, defines the local extracellular volume where calcium concentration changes are considered. - This is critical in interpreting how a small volume can lead to significant changes in calcium concentration even for moderate currents. 4. **Biological Significance:** - Calcium ions play a crucial role as an intracellular signaling molecule, involved in neurotransmitter release, synaptic plasticity, and activation of calcium-dependent pathways. - Changes in intracellular calcium concentrations are fundamental for various neuronal functions and are tightly regulated due to their role in multiple signaling pathways. ### Key Model Parameters - **`cai0`:** Refers to the resting or baseline calcium concentration, which the model assumes the intracellular concentration will return to over time. - **`tau`:** The decay time constant signifies how quickly the system returns to the resting state after the influx, highlighting the capacity of the neuron to restore basal calcium levels after activity. Overall, this code segment models the local changes in intracellular calcium concentration influenced by transmembrane calcium currents and provides a simplified view of calcium buffering efficacy in the neuronal microenvironment.