Membrane potentials Flashcards
How can glass capillaries achieve intracellular recordings
Microelectrode is filled with a conducting salt solution and a wire inserted into it to connect to an voltmeter that measures the difference in potential between the fine tip (<1um) and a ground
What does penetrating the neuronal membrane with the fine tip of an electrode reveal
A hyperpolarised resting membrane potential (-50 to -90mV, around -65mV)
Why is the plasma membrane called a lipid bilayer
It is made up of phospholipids with hydrophilic heads and hydrophobic tails that line up naturally to form a lipid bilayer when in an aqueous solution (Singer and Nicholson, 1972)
What does the fluid mosaic model describe
The structure of the plasma membrane as a mosaic of components, with cholesterol, proteins and carbohydrates floating in a sea of phospholipids
The phospholipids and embedded proteins can diffuse rapidly and laterally in the membrane
What are the 3 functions of the phospholipid bilayer
Impermeable, proteins allow communication and movement of molecules, insulator
3 functions of the phospholipid bilayer- impermeable
Impermeable to ions and organic molecules- can maintain the cell’s intracellular environment
3 functions of the phospholipid bilayer- proteins allow communicatino and movement of molecuels
The proteins that span from the intracellular to extracellular space allow the cell to communicate with its environment and allow selective movement of molecules across the membrane eg nutrients
3 functions of the phospholipid bilayer- insulator
The bilayer separates ionic charges in the intracellular and extracellular salt fluids, thus acting as an insulator between 2 conductors (capacitor), allowing charge to be stored on the neuronal membrane
What is the consequence of the asymmetric distribution of ions across the cell membrane with an impermeable membrane
Resting membrane potential of 0mV, as the negative and positive charges are balanced across the membrane (despite asymmetric distribution of ions)
Intracellular vs extracellular Na+ conc
Intracellular 18mM
Extracellular 145mM
Intracellular vs extracellular Cl- conc
Intracellular 5mM
Extracellular 115mM
Intracellular vs extracellular Ca2+ conc
Intracellular 100nM
Extracellular 2mM
Intracellular vs extracellular K+ conc
Intracellular 140mM
Extracellular 5mM
Intracellular vs extracelllar organic anions conc
Intracellular 75mM
Extracellular 15mM
What channels can mainly explain the hyperpolarised resting membrane potential
Leak channels in the membrane that are predominantly selective for K+ ions
What is the effect of K+ selective leak channels in the membrane on the movement of K+
K+ ions flow through these channels along their concentration gradient to from the inside to the outside of the cell, and accumulate on the outside surface of the membrane
What is the effect of K+ moving out of selective leak channels
As K+ is positively charged, there is a net accumulation of negative charges on the inner side of the membrane, and the membrane becomes hyperpolarised relative to the extracellular fluid
What is the effect on K+ of the membrane becoming hyperpolarised by movement of K+ out via leak channels
It generates an electrical force that attracts the K+ ions back into the cell
At first the diffusional gradient is stronger than the opposing electrical gradient and there continues to be a net efflux of K+
What happens as the membrane becomes progressively more hyperpolarised by the movement of K+ back into the cell
The system will eventually reach electrochemical equilibrium when the electrical gradient balances the diffusional gradient and there is no net flux of K+ across the membrane
What is the Nernst equation
Calculates the membrane potential at which equilibrium potential is reached by any single ion
How can we test the Nernst equation in real cells
By manipulating extracellular K+ concentration and examining the changes in membrane potential
The membrane potential at each conc should be predicted by the Nernst equation ie proportional to log ([K+]out (Bernstein, 1902)
What is the result of testing the Nernst equation in real cells by manupulating extracellular K+ concentration
Plotting extracellular K+ conc against RMP on a logarithmic scale reveals that it clearly deviates from the Nernst potential for K+ ions at low values of of [K+]out (Hodgkins and Horowicz, 1959)
Why is the RMP of cells not predicted by the Nernst equation for K+
The leak channels are weakly permeable to other ions, mainly Na+ ions moving into the cell (1/100 fold)
Other ions are moving across the membrane that are not taken into account by the Nernst equation for K+ alone
What can be used to calculate the correct electrochemical equilibrium potential for the leak channels (taking into account all ions) aka resting membrane potential
Goldman-Hodgkin-Katz equation- the average of Nernst potentials for each permeable ion weighted by their relative permeabilities