Resting Membrane Potential Flashcards
What are the principal players in the maintenance of steady-state trans-membrane gradients for Na+, K+, and Ca2+?
primary active ion pumps
Of particular importance to the bioelectric properties of cells are what tow gradients?
- the outwardly directed gradient for K+
- the inwardly directed gradient for Na+
*both of these are developed and maintained through the activity of the Na,K-ATPase
So let’s say you have permselective membrane for K+. You have 100 mM [K+] inside cell, 10 mM outside cell. You also have 10 mM Na+ inside cell, 100 mM outside cell. Describe the movement of K+.
K+ gradient causes K+ to leave the cell. This leaves a negative charge inside the cell (because of anions left in the cell that the membrane is not permeable for). This negative charge causes K+ to be pulled back into the cell. and gradually increases to a level that exactly balances the chemical force, resulting in an ‘equilibrium’ condition.
The equation that describes this balance of electrical and chemical forces is called the Nernst equation.
What equation describes the balance of electrical and chemical forces?
The Nernst Equation
E_k = -RT/(zF) * ln ( [K+]in/[K+]out )
E_k = -61.5 mV * log_10 ( [K+]in/[K+]out )
E_k = equilibrium potential difference (PD) which exactly opposes the chemical energy of the chemical gradient
[K+]in/[K+]out = the chemical gradient
z = valence of the ion question (+1 for K+)
R, gas constant; T, temperature, F, Faraday constant
Any manipulation that reduces the K+ gradient (i.e. by either decreasing intracellular [K+] or increasing extracellular [K+]), will ______ the equilibrium potential for K+.
decrease
If there is less energy in the chemical gradient, it will take _____ energy in an electrical gradient to ‘balance’ it.
less
What does the Goldman-Hodgkin-Katz Constant Field equation (Goldman equation) represent?
the more physiologically realistic case in which the membrane shows a finite permeability to the three major players (K+, Na+, Cl-)
Most channels display “gating” (the conformational change of the channel protein associated with “opening”). What are the five types of gating?
- voltage gated
- ligand gated
- second-messenger gated
- mechanically gated
- non-gated
Voltage gated channels
changes in membrane potential (typically, depolarization) induce channel opening (by means of conformational changes of an amino acid sequence in the channel protein, a ‘voltage sensor)
Ligand gated channels
the (non-covalent) binding of an external “ligand” (e.g. neurotransmitter) to a receptor site on the external face of the channel results in a conformational change of the channel protein that opens the channel
Second-messenger gated channel
modification of the cytoplasmic face of channel, via second messenger-mediated pathways, results in channel opening
- ex: non-covalent binding of a small molecule (similar to ligand gated)
- ex: covalent modification (e.g. phosphorylation)
Mechanically gated channel
physical deformation of the membrane (‘stretch’) results in channel opening
these channale proteins are usually tethered to cytoplasmic elements that ‘tug’ on the protein when stretched
Non-gated channel
changes in current flow through these channels is not due to conformational changes of the protein itself (but by other mechanisms…)
Channels generally show some degree of ____
selectivity
Most channels can have their behavior altered through the influence of a modifying agent. These can be drugs or toxins, but also include physiologically important molecules.
Toxins of interest:
- Tetrodotoxin (TTX)
- alpha-bungarutoxin
Where are these toxins often found and what is their influence?
Tetrodotoxin (TTX)
- basis of puffer fish poison
- blocks an important class of gated Na channels
alpha-bungarutoxin
- in venom of Banded Krait snake
- binds to and blocks the nicotinic acetylcholine (ligand)-gated channel of the neuromuscular junction
