Cellular communication - Lecture 9: Electrical properties of the plasma membrane

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?

E_m = 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?

1. initially the concentration gradient and the electrical force will be to the same direction

2. 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