L2 - Membrane Potentials (Chapter 5) Flashcards
What is meant by “membrane potential”? What are its units of measure in living cells?
- Charge distribution across a membrane – dependent on membrane transport
- Measured in millivolts
- When neg, there is a more negative charge on the inside of the cell then the outside
How is charge distributed throughout living cells?
For the most part, charges are balanced except at the membrane where charges build up
Is there a voltage throughout the entire cell, or is the voltage localized to one part of the cell? If so, which part?
- Voltage is localized at the membrane of the cell
- Opposites attract – anions and cations get as close together as they can, but are stopped by the membrane
- Why its called membrane potential and not cell potential
What is a “Donnan Equilibrium”?
- Equilibrium that can result in equal distribution of ions across a membrane with no energy input - imbalance of concentration of ions creating a chemical and electrical gradient
- Passive diffusion
In what way is it a special type of equilibrium? What happens with this type of equilibrium? (Donnan equilibrium)
Based on the starting condition and presence of non-penetrating ions, there can be an uneven distribution of molecules
Most of the membrane potential results from what process? What mechanism contributes to the remainder of the resting membrane potential?
- Passive diffusion - ~95% of resting membrane potential
- Active pumping - ~5% of resting membrane potential
Voltage (V)
- diff in electrical charge btwn 2 points – uneven distribution of charge
- In cells its the diff in charge across the membrane – ECF/ICF
- Measured in volts
Current (I)
– flow of charge across a membrane
- Vector quantity – has mag and direction
- Measured in amps
- Indirectly proportional to resistance, directly proportional to voltage
Membrane Current (Im)
current is flowing across the membrane from ICF to ECF and vice versa
- the flow of charge across the membrane
Axial Current (Ia)
current is flowing along the membrane
Inward Current (Ii)
- positively charged ions going into the cell
- EX: Na+, Ca+, and glutamate
- Depolarizes the cell membrane, excites the cell – can cause APs in excitable cells
Outward Current (Io)
- negatively charged ions going into the cell, or positively charged ions moving out of the cell
- EX: K+, Cl-, and GABA/glycine
- Chloride counts b/c it’s still making the ICF more negative
- Hyperpolarize the cell membrane – drops below membrane potential, inhibiting the cell from having an AP
Resistance (R)
– opp of membrane to current flow
- Measured in ohms
- Indirectly proportional to current (Decrease resistance = increase current)
- Influenced by gated channels – when they open, resistance decreases, and when they close, resistance increases
Membrane Resistance (Rm)
resistance across the membrane
Axial Resistance (Ra)
resistance along the membrane
What is the algebraic relationship between V, I and R?
- Ohm’s Law - V=IR or I=gV
- Increase in I or R would increase V, a decrease in I or R would decrease V
What does g stand for in the alternate equation for Ohm’s Law?
- conductance
- measured in siemens or mhos
- opposite of resistance
What is conductance (C)?
- storage of charge across a thin barrier/dielectric
- important for conduction speed
What is membrane capacitance?
- Cm
- referring to the capacitive property of phospholipid bilayer
- important for conduction speed
How do cell membranes act as capacitors?
- hydrophobic tails of phospholipids create a nonpolar region in the interior of the bilayer, preventing the passage of polar molecules (such as ions) through the membrane.
Sharp microelectrode
– impales the cell, making the cytoplasm of the cell continuous with the electrolyte solution inside of the electrode
- Recording and stimulating electrode
- intracellular recording
Patch clamping
– blunt, fire polished electrode is used
- Whole cell - suction is used to tear a hole in the membrane, and the blunt tip can then be inserted through the hole – similar to sharp electrode method
- On cell patch clamp - Form a seal on the membrane without breaking it, such that you have one or more ion channels within the patch you are recording from
- intracellular recording
Current clamping
manipulating the current in the cell by injecting current
Voltage clamping
– manipulate voltage
- Can study ionic currents, the increase or decrease as a result of voltage change
- Typically clamp the cell at the holding potential, and influence the cell to reach command potential
What is equilibrium potential?
- Membrane potential that exactly opposes net diffusion of an ion across the cell membrane
- Calculated for individual ions (Eion) – measures a single ion, diff from membrane potential which is measuring multiple ions
What else is equilibrium potential sometimes called?
Electromotive force – EMF
Nernst potential
Diffusion potential
How can you calculate equilibrium potential? What equation is used?
- Nernst equation
- R - gas constant
- T - absolute temp (kelvins)
- Z - ion valence
- F - faraday constant
If the Nernst equation is being used on an ion that is at body temp, what can it be simplified to?
Practice using sample equations from powerpoint:
Practice using sample equations from powerpoint:
What is reversal potential?
- Voltage at which the direction of current changes
- Current switches from inward to outward
- EX: at rest, K usually flows outside of the cell, making the membrane potential more negative until it reaches –94 mV or greater, which will cause the current to reverse and have K flow back into the cell
Have a basic knowledge of the concentration of diff ions:
- K is usually moving out of the cell
- Na is usually moving into the cell
- Ca is usually moving into the cell
- Cl is usually moving into the cell
