Separation of opposite charges across the membrane
Potential: has the ability to do work
Membrane itself is not charged - Em refers to the difference in charge between the thin layers of ECF and ICF located next to the inside and outside of the membrane
Excitable cells (nerve and muscle)
Rapidly and transiently alter their membrane permeabilities in response to appropriate stimulation, resulting in fluctuations for Em
Resting membrane potential
Constant in non-excitable cells, membrane potential of excitable cells at rest
Unequal distribution of ions and their selective movement through the plasma membrane are responsible for the resting potential
Na+ is higher outside the cell - concentration gradient is inward
K+ is higher inside the cell - concentration gradient is outward
Greater expression of K+ leak channels
As K+ and Na+ are cations, the electrical gradient for both will always be towards the negatively charged side of the membrane
Plasma membrane is impermeable to the negatively charged intracellular proteins (A-)
Equilibrium potential
When the concentration gradient and electrical gradient of an ion balance each other out - no further net movement of the ion
Membrane potential at Ek = -90 mV
Membrane potential at ENa = +60 mV
Nernst equation
Enables the calculation of the equilibrium potential for specific ions
Concurrent K+ and Na+ effects on Em
At resting membrane potential:
Relatively large net diffusion of K+ outward as membrane more permeable to K+ → Ek = -90 mV
Relatively small net diffusion of Na+ inward - neutralizes some of the potential created by K+ alone
All cells are negative inside at rest: ~-70 mV
Goldman-Hodgkin-Katz (GHK) Equation
Used to calculate Em
Takes into account all of the permeable ions moving across a membrane (and the relative permeability of the membrane) to calculate the membrane equilibrium potential (Em)
PK+ = 1, PNa+ = 0.01
Plug in values for ion concentration → Em is -83 mV
Na+-K+ pump
Helps maintain Na+ and K+ concentration gradients - 3Na+ out for every 2K+ in
Generates hyperpolarizing current - inside more negatively charged
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