The provided code models the decay of internal calcium concentration in a neuron, focusing on the biological processes that influence calcium dynamics. Calcium ions (Ca2+) play a crucial role in various cellular functions, including neurotransmitter release, gene expression, and activation of various signaling pathways. The model captures the dynamics of intracellular calcium concentration changes due to calcium currents and active pumping mechanisms.
Entry through Channels: Calcium enters the cell through voltage-gated calcium channels during neuronal activity, primarily at the synaptic terminals. The inward calcium current (ica) is a critical component managed in the code via the USEION ca statement, which reads the calcium current and concentration.
Calcium Concentration (cai): The code models intracellular calcium concentration, which becomes significant for triggering subsequent biological processes. The initial calcium concentration is set to cainf, representing equilibrium-like basal levels.
Calcium Pump Model: The model implements a simplified version of an ATPase calcium pump that clears calcium from the intracellular space. This pump is modeled using Michaelis-Menten kinetics, simplifying it to two parameters: kt (time constant of the pump) and kd (dissociation constant), which govern the pump's affinity and transport capacity. This simplification assumes high pump affinity and low transport capacity, reflecting physiological characteristics noted in literature (e.g., Blaustein 1988).
Decay/Buffering: The code simulates calcium decay as a form of buffering, removing calcium via mechanisms analogous to endogenous buffers or pumps that reduce free intracellular calcium. This is represented by a differential equation (ca' = drive_channel + (cainf-ca)/taur), where taur represents the rate at which calcium is cleared or buffered.
Depth (depth): Represents the depth of the submembrane shell where calcium dynamics are being modeled. This parameter is important for converting surface densities to volume concentrations.
FARADAY Constant: Employed to relate charge movements due to ionic currents to molar concentrations, reflecting the fundamental physical chemistry of ions.
This model seeks to replicate the biological processes governing intracellular calcium concentration changes resulting from calcium currents and removal via pumps and buffering systems. It captures essential aspects of calcium handling, providing a foundation for simulating neuronal activity and its modulation through calcium signaling. Through this model, one can study the impact of calcium dynamics on neuronal functions and their broader implications for neural circuit behavior.