Ion Channels

Also known as a Markov model,the schem defines the possible states (configurations) of the channel and the transitions between them. If the scheme falls into disjoint sub-schemes these are interpreted as independent serial gating complexes and the effective relative conductance os the product of the relative conductances of separate complexes. This enables Hodgkin Huxley styleto be economically represented as a collection of separate two-state complexes

Example

The conductance is specified relative to the maximum conductance for the channel. Different states can have different conductances, but other permeation properties are assumed to be the same for all open states.

Example

A fixed-rate transition, sometimes known as a time-dependent transition, is one where the probability of the transition occuring in a given time interal is always the same, independent of external factors such as the membrane potential.

Example

This is one of two ways of expressing a first order Boltzmann transition with one or two extra term(s) to make the rates saturate rather than become indefinitely large for extreme potentials. This form uses the mid-pint potential, vHalf at which forward and reverse rates are equal, and the timescale (reciprocol of the rate) at that point. The saturation timescale can be expressed either as tauMin, or separately for the forward and reverse rates as tauMinFwd and tauMinRev.

Example

This is one of two ways of expressing a first order Boltzmann transition with one or two extra term(s) to make the rates saturate rather than become indefinitely large for extreme potentials. This version requires the forward and reverse rates for a membrane potential of zero. Internally, these transitions are converted into exactly the same form as is used for the VHalfTransition.

Example

A one-way transition expressed in the form A x / (1 - exp(-x)) where x = (v - v0) / B. The parameters are the rate, A, the modpoint, v0, where x = 0, and the voltage scale, scale, which divides both the midpoint and v0. Note that this expression has numerous variants, so published parameters may not map directly onto the rate, midpoint and scale. Most often the signs of the rate and scale are reversed or the scale dependence of the numerator is wrapped into the rate, in a form sucha as alpha (v - v0) / (exp (beta (v - v0)) - 1).

Example

A one-way transition as used in the original HH model, giving the rate, beta, as beta = rate exp((v - midpoint)/ vscale)

Example

A one-way transition as used in the original HH model, giving the rate, beta, as beta = rate / (1 + exp(-(v - midpoint)/ vscale))

Example

KSComplexes provide an optional grouping of states within a channel. The effect is the same as specifying transitions between top-level that implicitly define separate gating complexes, with the exception that the complex also allows an instances attribute that can be used to specify that the channel has more tna one sequential instance of the same complex. This is primarily useful for the economical representation of Hodgkin-Huxley style channels that have multiple independent, but identically gated, two-state complexes.

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