The provided code models a component of neuronal electrical behavior known as the persistent sodium current (INaP). This current is primarily responsible for modulating subthreshold membrane potentials and plays a significant role in neuronal excitability and the propagation of action potentials.
Sodium Ions (Na+): In the context of neurons, sodium ions are crucial for generating action potentials. These ions flow into the cell through sodium channels when they open, leading to depolarization of the neuronal membrane.
Persistent Sodium Current (INaP): Unlike the transient sodium current responsible for the rapid depolarization phase of action potentials, the persistent sodium current is a non-inactivating, low-magnitude current that activates at subthreshold voltages. This contributes to the regulation of neuronal firing and excitability.
Activation Variable (minf): The model uses a function minf(v) to determine the fraction of sodium channels that are activated at a given membrane potential (v). This function is sigmoidal, reflecting the voltage-dependent dynamics of channel activation; it enables the channel to be more open as the membrane potential depolarizes.
Conductance (gbar): This parameter represents the maximum conductance of the persistent sodium channels. In a biological context, conductance is related to the density and properties of the sodium channels expressed on the neuronal membrane.
Reversal Potential (ena): The reversal potential for sodium ions (ena) is set to 45 mV, which is typical for sodium in many neuronal systems. This value is crucial because it determines the direction and driving force for sodium ions when the channels open.
Shifting Parameter (shm): This parameter appears to allow for shifts in the voltage sensitivity of channel activation, which might represent biological variability or experimental adjustments in the model to match observed data better.
The persistent sodium current modeled here contributes to several important neuronal behaviors:
Subthreshold Activities: INaP influences subthreshold membrane dynamics, affecting neuronal 'rheobase' (minimum current to evoke an action potential) and spike threshold.
Rhythmic Firing and Oscillations: The persistent sodium current plays a role in sustaining repetitive firing and contributing to rhythmic neuronal oscillations, which are important in processes like respiration and other rhythmic motor patterns.
Neuronal Excitability: By modulating membrane potentials closer to the threshold, INaP can increase neuronal responsiveness to synaptic inputs, affecting overall network excitability.
In summary, the model captures the essential features of the persistent sodium current, reflecting its role in modulating neuronal excitability and contributing to various forms of electrical signaling in neurons.