name | ion | type | speed | Rev. Pot | threshold |

I_{Na} Fast Sodium |
[Na] | act/inact | very fast | 45 mV | -50 mV |

I_{Nap} Persistent Sodium |
[Na] | act/inact(slow) | fast | 45 mV | -65 mV |

I_{K} Delayed rectifier |
[K] | act | fast | -100 mV | -40 mV |

I_{A} A-current |
[K] | act/inact | fast | -100 mV | -60 mV |

I_{AHP} Ca-dependent K |
[K] | act (Ca-dep) | moderate-slow | -100 mV | - |

I_{M} Slow potassium |
[K] | act | slow | -100 mV | -35 mV |

I_{K2} Slow potassium |
[K] | act/inact | slow | -100 mV | -40 mV |

I_{T} Transient Ca |
[Ca] | act/inact | slow | 150 mV | -60 mV |

I_{L} High thresh. Ca |
[Ca] | act | fast | 150 mV | -10 mV |

I_{h} Sag current |
[Ca]&[Na] | inact | slow | 0-40 mV | - |

I_{leak} Leak |
[Cl],[K], [Na] | passive | - | -60 mV | - |

Given the above table and the form for the kinetic parameters,
one can easily put together models for active
membranes. This can be regarded as a kind of mix and match affair
which results in a huge variety of models.
In spite of this, there
are virtually no differences between the fundamental models of
cardiac, smooth muscle, squid axon, thalamic relay cells, etc. Each
can be written as (2) where the *I*_{k} each satisfy

(4) |

The best known examples of these models are the Hodgkin-Huxley equations which have 3 currents, (i) passive leak, (ii) fast sodium, and (iii) delayed rectifier.

All of the currents mentioned in the table above have been found in cortical or thalamic neurons. These currents are responsible for the intrinsic firing properties of neurons which include three different types: (i) regular spiking neurons (ii) fast spike neurons (iii) bursting neurons.

Recall that the typical channel gate satisfies

The functions are generally of three different forms (see Figure 3)- 1.
- Exponential:
- 2.
- Linear-Exponential:
- 3.
- Logistic: