The code provided models a type of potassium current known as the slow calcium-dependent potassium current, often referred to as the AHP (afterhyperpolarization) current. This current plays a crucial role in the regulation of neuronal excitability and the adaptation of firing rates in response to prolonged stimuli. Below, I detail the key biological components that are represented in the code:
iAHP)Potassium (K+) Ion: The iAHP model specifically pertains to potassium ion flow across the neuronal membrane. Potassium currents are fundamental in repolarizing the neuron after an action potential and restoring resting membrane potential.
Calcium (Ca2+) Dependence: The AHP current modeled here is calcium-dependent, denoting that the activity or opening of the channel is regulated by intracellular calcium concentration (cai). This aligns with biological observations where increased intracellular calcium, resulting from neuronal activity, leads to the activation of calcium-sensitive potassium channels.
Gating Variable (m): The model uses a gating variable m, which describes the proportion of potassium channels that are open at any given time. This reflects the biological concept of voltage-gated or receptor-gated ion channels that fluctuate between open and closed states.
Activation and Time Constants: The model describes m_inf (the steady-state activation level) and tau_m (the time constant for activation) which are computationally derived from calcium levels and voltage. This is a biological representation of how channel states depend on intracellular conditions and electrical signals.
Temperature Dependence: The model incorporates a temperature adjustment (tadj), reflecting how ionic channel behavior can vary significantly with temperature, which affects physiological reactions and kinetics.
Reference and Context of Observation: The code references the work by McCormick, Wang, & Huguenard (1993) in cerebral cortex modeling and notes its applicability to "bullfrog sympathetic ganglion cells." While the primary reference relates to mammalian neurons, the modeling of this potassium current has parallels in other species and cellular contexts.
Afterhyperpolarization and Neuronal Firing: AHP is key in neuron firing patterns, contributing to the refractory period following an action potential. The slow AHP current reduces neuronal excitability, modulating repetitive firing and excitatory post-synaptic potentials.
In summary, the provided code is constructed to model a slow calcium-dependent potassium current (iAHP), which is integral to neuronal firing adaptation and excitability regulation in response to calcium influx. This model, based on empirical studies from the cerebral cortex and other neuronal contexts, aligns with known physiological processes governing neuronal behavior.