Biomedical Engineering Reference
In-Depth Information
As illustrated in Fig. 2, today's ionic cardiac cell models still largely follow the
concept of Hodgkin and Huxley. The kinetics of ionic currents that have been
identified in patch-clamp experiments are described in terms of first-order
'activation' and 'inactivation' gating processes, through Eqs. 4 and 5, and turned
into components of an electrical circuit, which underlies the set of model
equations. The main extension to the concept of Hodgkin and Huxley is
the inclusion of an additional set of equations governing the changes
in intracellular ion concentrations (sodium, potassium, calcium) and the
intracellular calcium uptake and release processes ('second-generation models';
see below).
Fig. 2.
Common layout of cardiac cell models. Data from patch-clamp experiments (
bottom
left
) provide quantitative information on each of the individual membrane current components
(
top left
). This information is turned into an electrical equivalent (
top right
; cf. Fig. 1). The
resulting equations, together with equations describing intracellular processes, e.g. the calcium
uptake and release by the sarcoplasmic reticulum (SR), are then compiled into a computer
model of the cardiac cell, an SA nodal cell in this case (
bottom right
). The
upper left diagram
shows all ionic currents that have been incorporated into SA nodal cell models: L-type calcium
current (
I
Ca,L
), T-type calcium current (
I
Ca,T
), hyperpolarization-activated current (
I
f
), rapid and
slow delayed rectifier potassium current (
I
Kr
and
I
Ks
, respectively), fast sodium current (
I
Na
),
sustained inward current (
I
st
), sustained and transient components of the 4-aminopyridine-
sensitive (4-AP-sensitive) outward current (
I
sus
and
I
to
, respectively), sodium-calcium exchange
current (
I
NaCa
), sodium-potassium pump current (
I
NaK
), sarcolemmal calcium pump current
(
I
p,Ca
), and calcium, potassium, and sodium background current (
I
b,Ca
,
I
b,K
and
I
b,Na
,
respectively).
