|INa Fast Sodium||[Na]||act/inact||very fast||45 mV||-50 mV|
|INap Persistent Sodium||[Na]||act/inact(slow)||fast||45 mV||-65 mV|
|IK Delayed rectifier||[K]||act||fast||-100 mV||-40 mV|
|IA A-current||[K]||act/inact||fast||-100 mV||-60 mV|
|IAHP Ca-dependent K||[K]||act (Ca-dep)||moderate-slow||-100 mV||-|
|IM Slow potassium||[K]||act||slow||-100 mV||-35 mV|
|IK2 Slow potassium||[K]||act/inact||slow||-100 mV||-40 mV|
|IT Transient Ca||[Ca]||act/inact||slow||150 mV||-60 mV|
|IL High thresh. Ca||[Ca]||act||fast||150 mV||-10 mV|
|Ih Sag current||[Ca]&[Na]||inact||slow||0-40 mV||-|
|Ileak 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 Ik each satisfy
Here instead of conductance, the permeability is used.
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)