![]() The threshold variability dependent on d V/d t could be regulated by the intrinsic properties of membrane currents, especially Na + channel inactivation and K + channel activation. Particularly, spike threshold is dependent on the membrane potential changes, such as the rate of membrane potential depolarization (i.e., d V/d t) preceding spike initiation, which has been observed in many areas of the central nervous system. Many experiment recordings in vivo have shown that the spike threshold is not constant but dynamic, which varies with neuronal recent spiking activities and synaptic inputs. The spike threshold is a basic biophysical property for all spiking neurons, which functions as a high-pass filter and plays a crucial role in action potential initiation. This is a special membrane potential value that distinguishes suprathreshold depolarization from subthreshold. The action potential can be evoked when membrane depolarization reaches a threshold level. Neurons encode and propagate information by transforming various spatiotemporal patterns of synaptic input into sequences of action potentials or spikes, which are usually regarded as the principal carrier of information. Our study provides a fundamental description about how intrinsic biophysical properties contribute to the threshold dynamics in Type I and Type II neurons, which could decipher their significant functions in neural coding. ![]() These predictions are further attested in several other functionally equivalent cases of neural excitability. The Type II K+ current activates prior to spike initiation and there is a large net hyperpolarizing current at the perithresholds, which results in a depolarized threshold as well as a pronounced threshold dynamic. The outward K+ current in Type I neuron does not activate at the perithresholds, which makes its spike threshold insensitive to d V/d t. By analyzing subthreshold properties of membrane currents, we find the activation of hyperpolarizing current prior to spike initiation is a major factor that regulates the threshold dynamics. ![]() With phase plane analysis, we show that each threshold dynamic arises from the different separatrix and K+ current kinetics. It is observed that Type II spike threshold is more depolarized and more sensitive to d V/d t than Type I. Here, we use a biophysical model to investigate how spike threshold depends on d V/d t in two types of neuron. Although the dynamical and biophysical basis of their spike initiation has been established, the spike threshold dynamic for each cell type has not been well described. There are two basic classes of neural excitability, i.e., Type I and Type II, according to input-output properties. In many neurons, the threshold potential depends on the rate of membrane potential depolarization (d V/d t) preceding a spike. Dynamic spike threshold plays a critical role in neuronal input-output relations.
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