Membranes 2 Flashcards
What is the Nernst equation used to solve for?
The Nernst Equation is used to solve for the voltage at which the ion in question is at equilibrium across a membrane.
- Eion = Vm = (-60/z)log([X]i/[X]o)
- Note: -60 is only at body temp. RT/F simplifies to -60 log at body temp.
Write out the nernst equation.
- What does z stand for?
- What might be
Eion = (-60/z)log([X]i/[X]o)
- z = valence on the ion that is being solved for
- Ex K+ = 1; Ca2+ = 2; Cl- = -1
- If you are solving for an ion with a negative valence (charge) then you have to change the formula to [X]out/[X]in as opposed to [X]i/[X]o.
Relate the Nernst equation, the Goldman equation, and Membrane potential.
The Nernst equation tells you at what voltage a particular ion is at equilibrium across a specific membrane.
-Note: The membrane potential is also a Voltage (Vm).
The Goldman equation tells you the Membrane potential (Vm) by averaging the relative permeabilities and concentrations of all the ions across the membrane.
-Note: the greater the ions relative permeability (Px) to the membrane the greater its influence will be on the membrane potential. Thus, the membrane potential voltage will be closest to the Ex (=Equilibrium voltage of ion x) ions that have the greatest permeability.
***This is the main concept behind membrane depolarization! The membrane potential is rapidly increased from ~ -60 mV to almost 0mV during the process of depolarization, which leads to an action potential. This happens not because the amount of Na+ changes inside or outside the cell. It is because the the relative permeability increases one thousand fold! From 0.05 to 50 PNa+. This rapidly shifts the membrane potential towards Sodium’s Equilibrium voltage.
Rank the equilibrium voltage (Ex) for each of the following ions from very negative to very positive
- K+
- Na+
- Cl-
- Ca2+
Ek = Very negative ENa = positive ECl = negative ECa = Very positive
What 3 factors are the driving factors behind the negative resting membrane potential?
- Negatively charged, impermeable proteins in the cell
- Na+/K+ pump. Maintains gradients despite leak channel
- Large K+ conductance (permeability). Keeps Vm near Ek
The large negatively charged proteins that are stuck in the cells help set up gradients by pulling positive charges, such as K+ into the cell. But the negative proteins also pull water into the cell. How do the cells ensure that the cell doesn’t lyse?
The Na/K ATPase pump. Pumps 3 Na+ OUT for every 2 K+ pumped IN. Thus, decreases ion concentration gradients in the cell causing some of the water to flow back out after the Na+ because of the change in osmolarity.
- Two major roles of Na/K ATPase pump are to set up gradients and remove water from the cell.
- Note: Not a big role in changing the electric potential
What are three mechanisms of opening gated ion channels within a membrane?
- Binding of secondary messengers to the protein forming the channel (Ex. = Phosphate)
- Neurotransmitter binding the protein (happens during tooth pain).
- Voltage change (Ex. = Na+ channels open in a positive feed back manner. As some Na+ channels open and begin to change the voltage, that causes more to open.
What factors go into ion channel selectivity?
Size and molecular interactions with cell wall. While sometimes smaller ions flow through channels more quickly, often larger ions that interact with the channel wall in a beneficial manner actually flow through faster.
Give four factors that effect the conductance of an ion channel.
- Size of the channel (larger is faster)
- Increase the number of channels open
- Increase the amount of time the channel is open
- The electrochemical gradient of the particular ion across the membrane.
Describe the mechanism of channel INACTIVATION.
-What is a common example?
Ion channel inactivation occurs via the Ball and chain model of inactivation:
- As the channel opens (Must happen before channel can be inactivated!), protein loops move into region open channel. Think of ball on a chain swinging into the open channel causing deactivation.
- Na+ channels are a common example. Following depolarization Na+ channels are almost immediately inactivated by this mechanism causing the absolute refractory period.
Describe the mechanism of a channel BLOCK.
-What is a very dangerous example? What is an example applicable to dentistry?
Channel Blocks occur following channel opening (NECESSARY first step).
- A protein/large molecule moves into the ion channel lodging itself there blocking the path of any ions that would normally pass through.
- Tetrodotoxin from puffer fish is a very dangerous blocker of Na+ channels.
- Lidocaine is a local anesthetic used in dentistry that also blocks Na+ channels. This stops the passing of pain signals by preventing the generation of action potentials.
What is a TRP channel? What role does it play that is relevant to dentistry?
TRP channels are Cation channels.
-TRP channels allow Cations out of cells following swelling of cell –> reduces swelling.
