Análisis dancístico de la danza Ayarachis Sobre el termino Ayarachi.

Quantitative analysis of the slow current was done using recordings of the voltage dependence of activation in four axons, and the time constant of activation and deactivation in three axons. One of these axons was recorded at room temperature; its time constants were increased to correspond to those recorded at 20°C, using an arbitrary Qio of 3, based on that measured for the fast current (Frankenhaeuser & Moore 1963). A model based on that of Dubois (19816), with a single open state and a single closed state, was used for the slow current. However, the expressions used to derive the voltage-dependent rate constants and Pj were the same as those used for the fast current, because these gave a better fit to the measurements than the simpler expressions used by Dubois. Because the model simulates a channel with only one open and one closed state, the current activates and deactivates with a single exponential time course in response to voltage steps.

2.3.1 Model o f the slow iC current

The model used was:

= (34)

% = a , ( l - i ) - |3 , ( s ) (35)

a , = 4 ( £ - B ) /( l- e x p ( ( S - E ) /C ) ) (36) P, = A (B -£ :) /( l- e x p ( ( £ - B ) /c ) ) (37) where

s is a parameter of slow current activation;

G/f^max is the slow conductance of the node;

CLs and pj are voltage-dependent rate constants;

A, B and C are constants, whose values are given in Table 1.

To facilitate the calculation of % and pj, two intermediate variables were first defined, 5oo and T^, as described in section 2.1.2 for the Na'^ current:

*- = “ 7 ( “ , + P j (38)

''. = !/(“ . + P.) (39)

where

is the steady-state value of 5 at a given potential;

Ti- is the time constant with which s changes after a step change in potential.

2.3.2 Analysing the slow iC current

The time constant was obtained at potentials positive to Ehoid by fitting the activation phase of currents in response to depolarising pulses with a single exponential:

^ ^start + h s act “ exp(~r/x , )) (40)

where

Istart is the total current at the start of a depolarising pulse;

act is the amount of slow current that activates during the pulse. The influence of the fast current was excluded from these measurements by using currents recorded in the presence of 4-aminopyridine (4-AP).

In one fibre, at potentials near and negative to E hold was obtained by fitting “tail” currents following depolarisations to around Ek with the single exponential function:

/ = (41)

where

Iks tail is the amount of slow current which deactivates during the tail;

I final is the steady-state current at the end of the tail, made up of leak

and the fraction of the slow K'*' conductance which remains active.

The fit region was chosen to exclude the deactivation of the fast current.

The steady-state slow conductance at potentials at and negative to the holding potential was estimated in the same fibre from Ifmai in Equation 41. The leak was estimated by fitting a straight line to the values of Ifmai at potentials negative to -115 mV (assuming the slow current to be completely deactivated at these potentials), and the remaining conductance at more positive potentials was assumed to be the slow conductance.

The steady-state value of the slow conductance at potentials positive to Ehoid

was calculated using the value of IksuiW (Equation 41) from tail currents aX Ehoid, after long depolarising pulses (at least 500 ms) to various potentials. The slow conductance active at Ehoid was added to these values, to obtain the total slow conductance at each potential.

In the other three fibres, where tail currents were recorded only at E hold, the total slow conductance was obtained by adding the fraction of the conductance active at E hold in the one fibre in which it was measured.

Measurements of the slow conductance in each fibre were normalised by fitting the Boltzmann function:

/ ( £ ) = /n a ï / ( l + e x p ( ( Ê ,- £ ) / À : ) ) (

42

and dividing by max, to obtain estimates of 5oo which were fitted with the Boltzmann function:

(43) where:

Es is the potential where slow current activation is half- maximal;

k is a slope factor, the voltage shift in mV for an e-fold change in 1^00*

The rate constants % and were calculated from the measured values o f and the fitted Boltzmann function for Soo as described for the m parameter o f the sodium current. The equations above for % and pj in terms of the constants A, B and C and potential E were fitted (using the logarithms of the rate constants) to derive the constants shown in Table 1 (page 148). Fits are shown in Figure 5.

lOOi W’ 0 -1 0 0 0 100 i-4 -1 0 0 0 100 E (mV) E (mV)

F ig u re 5 (See footnote on page 117.)

(left) Joo (filled symbols), measured from tail currents in isotonic KCl in four fibres (Fibre E2N02, diamonds; Fibres 92D18, C2916 and E2917, symbols as in Figure 4), and x, (open symbols), measured during depolarising pulses or from tail currents in isotonic KCl in three of these axons (Fibres 92D18, C2916 and E2N02). The smooth curves are calculated according to Equations 38 and 39, from the curves fitted to and p^ in the right panel.

(right) as (open symbols) and p, (filled symbols), estimated from the Boltzmann function fitted to and the measured x,; the smooth lines are fits to Equations 36 and 37, with the constants A, B and C given in Table 1.

2.4 Leak current

The leak current was modelled as a time-independent ohmic conductance with the following equation:

= (44)

where:

G^max is the leak conductance;