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How is the plasma membrane a resistor?
low resistance for hydrophobic substances
high resistance for charged substances
What is the function of the Na-K pump?
continuously pumps sodium outside the cell and potassium inside the cell
this generates a concentration gradient
extracellular [Na+ ] is higher than intracellular [Na+ ]
extracellular [K+ ] is lower than intracellular [K+ ]
the difference between ion concentrations inside and outside the cell generates an electric potential
How is the electric potential measured?
Em = E[inside the cell] - E[outside the cell]
inside of the cell is always kept negative with reference to the outside
What happens when a positively charged molecule is allowed to pass through the membrane of the cell?
initially the concentration gradient and the electrical force will be to the same direction
The passage of few positive ions through the membrane creates a current that quickly changes the voltage of the cell to be positive, thus the direction of the electrical force will be flipped
What is the equilibrium potential?
the potential at which the concentration gradient is equal to the electrical force imposed by the movement of an ion across the membrane
each type of ion has it’s own value of equilibrium potential in the cell
it is calculated using the Nernst equation
What is the Nernst equation?
is used to calculate equilibrium potential (Ek ): Ek = RT/zF * log([ion concentration outside the cell]/[ion concentration inside of the cell])
Why is the resting potential of the cell always kept close to the equilibrium potential of K+ ions?
because the membrane is more permeable to K+ than other ions
What happens during mild hyperkalemia (high K+)?
abnormal heart rhythms
more than 7nM may cause cardiac arrest
What happens during hypokalaemia (low K+)?
muscle weakness
fatigue
How is the membrane potential calculated?
What does depolarization mean?
increasing cellular membrane potential towards positive values
What does repolarization mean?
means bringing membrane potential back to resting state after depolarisation
What does hyperpolarization mean?
decreasing cellular membrane potential towards negative values
What stimulates ion channels?
voltage change
ligand binding
temperature
pH
mechanical stimulation
calcium
What is the cellular phenomenon that is a cycle of depolarization followed by repolarization?
action potential
What are examples of the excitable cells?
neurons
skeletal myocytes
cardiac myocytes
smooth muscle cells
alpha cells
beta cells