3.1 Physiology and Electrical Activity of Muscle and Nerve Cells
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61
U/mV Depolarisation
UNa+
Repolarisation
Post potential
Membran
threshold
Time
UK+
1 ms
Resting
potential (RP)
+60
+40
+20
0
-20
-40
-60
-80
Cell membrane
Potassium channel
intracellular
K+ 155 mmol 4 mmol
Na+ 12 mmol 145 mmol
Cl- 4 mmol 125 mmol
Ca2+<104 mmol 1.5 mmol
absolute
refractorial time
relative
K+
K+
K+
K+
Calcium channel Ca2+
(just for cardio myocyte)
Na+
+
-
+
-
Na+
Cl-
Cl-
Natrium channel
Chlorid channel
Na+
Na+
+
-
+
-
+
-
+
-
+
-
+
-
+
-
+
-
intracellurar
extracellular
extracellular
Ca2+
Ca-Plateau
(just for cardio myocyte)
Fig. 3.9: Potential course of an action potential and the Na+, Cl−- and K+ Ion currents through the
membrane wall of a nerve cell (mammal intra-/extracellular concentrations from [67]) and the sum
of the ionic currents as action potential (bottom left): due to the negative post potential, renewed
excitations of action potentials are only possible during the relative refractory period. In the case
of cardiac myocytes, there is still a Ca2+ ionic current, which is responsible for the Ca plateau. The
membrane wall (right) shows a temporary state during the post-potential phase with the corres-
ponding active channels.
no re-triggering is possible over a short period of time after the action potential
has been triggered.
2.
repolarisation: During this period, the Na+- channels close again, whereas the K+-
channels open with a delay.
3.
hyperpolarisation: Due to the concentration gradient, there is a flux of K+ ions
out of the cell and a lowering of the membrane potential. The delayed closure of
the K+- channels with respect to the Na+- channels leads to an overshoot called
hyperpolarisation. During this time, the potential threshold is increased for the
renewed triggering of an action potential (relative refractory period). Afterwards,
the equilibrium potential or resting potential of the cell is restored.
In the case of a muscle cell or cardiac muscle cell, the Ca2+-ionic currents must also be
taken into account, which, in contrast to the nerve cell, produce a pronounced plateau
in the potential curve of the action potential (cf. Figure 3.9).