W2 L1 (Membrane potential) Flashcards
(68 cards)
1
Q
Membrane potential
A
The potential difference (voltage) is the distribution of positive and negative charges (ions) across the membrane with the inside being slightly negative. Also described as the separation of charges across a membrane.
2
Q
Millivolt (mV)
A
The units for measuring membrane potential. One millivolt = 1/1000 of a volt
3
Q
Ohm’s Law
A
V=IR
V=Voltage across the membrane
I=The flow of ions across the membrane
R=Resistance by the membrane
4
Q
Capacitor
A
A device used to store an electric charge, consisting of one or more pairs of conductors separated by an insulator.
5
Q
Explain how the membrane functions as a capacitor
A
Outside and inside the cell are conductors (the salt water) and in the middle is an insulator (the membrane). This separation of conductors by an insulator stores electric charge.
6
Q
Resting membrane potential
A
The state of an excitable membrane when there is not displaying an electric signal
7
Q
What leads to membrane potential?
A
The uneven distribution of ions is primarily responsible for the generation of the resting membrane potential. These ions are sodium, potassium, and anions; large negatively charged intercellular proteins
8
Q
Where are the concentrations of K and Na higher?
What maintains this gradient?
A
K is higher inside the cell, while Na is higher outside the cell. This gradient is maintained by the sodium-potassium pump.
9
Q
What is the range of normal resting potentials of most cells?
A
-5mV to -90mV
10
Q
What is the range of normal resting potentials for neurons?
A
-50mV to -75mV (-60mV is what we assume)
11
Q
Hyperpolarization
A
A decrease in Vm (ex. -60mV to -75mV)
12
Q
Depolarization
A
An increase in Vm (ex. -60mV to -50mV)
13
Q
What does Vm stand for?
A
Membrane voltage
14
Q
How do we record voltage
A
- Place a cell with a prong and another prong in a liquid-filled beaker
- Run the prongs through an amplifier and computer
- The amplifier will measure the difference between outside the cell and inside the cell
15
Q
Explain the formula for calculating Vm
A
Vm=(Q/C)
Vm=Membrane voltage
Q=Ionic charge
C=Membrane capitance
16
Q
What do selective channels do?
A
Separate specific ions
17
Q
Typical inside and outside concentrations (in mM) and permeability of K+
A
In: 125 Out: 5 Permeability: High
18
Q
Typical inside and outside concentrations (in mM) and permeability of Na+
A
In: 12 Out: 120 Permeability: Low
19
Q
Typical inside and outside concentrations (in mM) and permeability of Cl-
A
In: 10 Out: 125 Permeability: Very Low
20
Q
Typical inside and outside concentrations (in mM) and permeability of Ca 2+
A
In: 0.0002 Out: 2 Permeability: Ultra Low
21
Q
Typical inside and outside concentrations (in mM) and permeability of Large Anions (A-)
A
In: 130 Out: 0ish Permeability: Non existent (0)
22
Q
What determines resting permeability?
A
The number of open passive channels
23
Q
What are A-?
A
Large anions, amino acids, proteins, RNA, SO4, and PO4.
24
Q
How many times more permeable is K than Na?
A
50-75X
25
Equilibrium potential
The Vm where movement of an ion down its concentration gradient is equal to the force of an opposing electric potential force
26
Explain the basis for membrane resting potential
When the ions (charge) are separated across the membrane (capacitor) it creates the membrane potential
(voltage). This refers to the charge separation that occurs because K can flow out at rest, while the large
anions cannot. The natural loss of K due to diffusion to the ECF pulls some of the intracellular anions to the inside of the membrane, which
then attracts the K that has left (and other positive charges in the ECF) to the outside of the membrane.
The charge is now separated AT THE MEMBRANE and a membrane potential or voltage is created that
reflects the equilibrium potential for K (-90).
The small amount of Na entry does the same thing but in
reverse, and this pulls the membrane potential towards the equilibrium potential for Na (+60).
The net result is a membrane potential of somewhere between -50 and -75 (closer to Vk (-90) than Vna (+60)
because K is more permeable at rest than Na).
27
What is an electrochemical cell?
A device that converts chemical energy into electrical energy or vice versa when a chemical reaction is
occurring in the cell. It typically consists of two metal electrodes immersed into an aqueous solution
(electrolyte) with electrode reactions occurring at the electrode-solution surfaces. As an electrical
current passes, it must change from electrical current to ionic current and back to electrical current.
These changes of conduction mode are always accompanied by oxidation/reduction reactions and the
electrodes (loss or gain of electrons)
28
What is an anode?
The positive electrode where oxidation occurs in an electrochemical cell.
29
What is a cathode?
The negative electrode where reduction occurs in an electrochemical cell.
30
What are the only channels that set the Vm?
The only channels that affect the resting permeability and Vm are the passive channels.
31
Does the Na/K pump affect membrane potential?
Slightly, the inequality of displaced charge (2K in for 3Na out) leads to a slightly hyperpolarized membrane. An electrogenic pump often leads to a resting potential that is slightly more negative than it would be with simple diffusion.
32
In the Goldman equation, concentration gradient and permeability are the only 2 factors contributing to the resting membrane potential. However, does not the resting membrane potential also depend on charge separation?
The underlying cause of Vm is charge (ions) separation across the membrane capacitance. However,
this can only be achieved by selective ion channels allowing certain ions to move across the membrane.
The Goldman equation incorporates the influence of the permeability of specific ions (the selective ion
channels) and the concentration gradients for those ions (the driving force to move the ions across the
membrane) .
Thus, how many selective passive channels are open and the equilibrium potentials for those ions (determined by the gradient) sets how the charges are separated by the membrane and thus Vm itself.
33
Explain how a resting membrane potential is obtained between K and Anions.
1. K is pumped into the cell
2. K is highly concentrated in the cell and leaves
3. The K that has left draws the A- to near the membrane
4. Some K come back due to the electrical attraction and dynamic equilibrium is reached
34
What does the sign of the membrane potential mean?
By convention, the sign always designates the polarity of the excess charge on the inside of the membrane. A membrane potential of –90 mV means that the potential is of a magnitude of 90 mV, with the inside being negative relative to the outside; a +90 mV would have the same strength, but the inside would be more positive.
35
Explain how the resting potential for potassium (K) is reached
1. Potassium is actively pumped into the cell
2. Potassium passively leaves due to a concentration gradient
3. The membrane is impermeable to - Anions
4. K is drawn back towards the cell
5. An eq is reached where the electric force equals the concentration force, this doesn't mean there are equal amounts on both sides simply that there is no net movement
36
What is the resting potential of K+?
-90mV
37
What does Vion mean?
It means that the given substance is at equilibrium and that no net movement occurs.
38
What is the Nernst equation and what does it calculate?
Vion= (RT/FZ) x ln ([ion] out / [ion] in)
```
Vion= ions resting potential
R=Gas constant
T=Temp
F=Faradays constant
Z= Valence electrons
```
```
Z for:
K=+1
Na=+1
Ca=+2
Cl=-1
```
39
What is the resting potential of Na+?
+60mV
40
Explain how the resting potential for potassium (Na) is reached
1. Sodium is actively pumped out of the cell
2. Sodium passively enters due to a concentration gradient
3. Cl- is outside of the cell and with Na entering the cell become negative on the outside
4. Na then is attracted back out of the cell until the equilibrium is reached
41
Explain the relative concentrations of K, Na, Cl, A- and Ca inside and out of the cell
```
K higher inside
Na higher outside
Cl higher outside
Ca higher outside
A- higher inside
```
42
Why is the resting potential of -60mV closer to that of K (-90mv) than Na (+60mV)?
It is closer because there are more open passive K channels; but mainly because the concentration gradient for K is higher than that for Na so a higher electrical force (V) is required to counter-act the concentration gradient.
43
How does permeability of a given ion affect resting membrane potential?
The greater the permeability of the plasma membrane for a given ion, the greater is the tendency for that ion to drive the membrane potential toward the ion’s own equilibrium potential.
44
What is the resting potential of Cl-?
-70mV
45
What is the resting potential of Ca+2?
+120mV
46
Explain how chloride affect the resting membrane potential?
The membrane potential is in fact what affects the distribution of chloride ions. Most cells are permeable to chloride but don't actively transport it. This means that chloride passively distributes itself to achieve a state of equilibrium.
With no active forces acting on it, Cl- passively distributes itself to achieve an individual state of equilibrium. In this case, Cl- is driven out of the cell, establishing an inward concentration gradient that exactly counterbalances the outward electrical gradient (i.e., the resting membrane potential) produced by Na+ and K+ movement. Thus, the concentration difference for Cl- between the ECF and ICF is brought about passively by the presence of the membrane potential rather than maintained by an active pump
Therefore, in most cells Cl- does not influence resting membrane potential; instead, membrane potential passively influences the Cl- distribution.
47
In what ways do muscle and nerve cells use membrane potential?
To signal each other with action potentials. They rapidly alter membrane permeability to bring about fluctuations in membrane potential which serve as electrical signals.
48
Excitable tissue
Muscle or nervous tissue, they can produce electric signals when excited to communicate.
49
1. Polarization
Charges are separated across the membrane so it has potential. This polarization means that the membrane has a potential that is non-zero. At resting potential the membrane has a potential of -60mV.
50
2. Depolarization
A reduction in the amount of negative potential, fewer charges are separated than at resting potential. The quantitative value becomes more positive (towards 0).
51
3. Repolarization
An increase in the amount of negative potential, more charges are being separated. The quantitative value becomes more negative (away from 0)
52
4. Hyperpolarization
An increase in the negative potential past the resting potential. More charges are separated than at resting potential.
53
Types of electric signals
1. Graded potentials
| 2. Action potentials
54
Graded potentials
Short distance signals
55
Action potentials
Long distance signals
56
Types of triggering events
1. A change in the electrical field in the vicinity of an excitable membrane
2. An interaction of a chemical messenger with a surface receptor on a nerve or muscle cell membrane
3. A stimulus, such as sound waves stimulating specialized nerve cells in the ear
4. A spontaneous change of potential caused by inherent imbalances in the leak–pump cycle.
57
Leak channels
Channels that are open more than closed
58
Voltage-gated channel
Open and close in response to changes in membrane potential
59
Chemically-gated channel
Open due to the binding of a specific chemical or substance
60
Mechanically-gated channel
Respond to stretching or other mechanical deformation
61
Thermally-gated channel
Respond to changes in temperature (hot or cold)
62
Goldman Equation
Used in cell membrane physiology to determine the reversal potential across a cell's membrane, taking into account all of the ions that are permeant through that membrane.
Vm=(RT/F)x ln ((Pk [k]out + Pna [na]out + Pcl [cl]in)/(Pk [k]in + Pna [na] in + Pcl [cl] out))
* ** Chloride is flipped because it is negatively charged
* **Also calcium isn't included because calcium doesn't have any channels
63
What is Vm a combination of
[ ] gradients and permeabilities combined
64
How does the Na/K pump not create voltage
It creates gradients and these gradients are the reason for the voltage. Voltage is created due to specific ions being separated by specific channels. If there were no channels there would be no voltage.
65
What is voltage a result of?
It is a result of passive channels separating specific ions
66
Determine whether the [ ] gradient drives each substance into or out of the cell
K-Out
Na-In
Cl-In
Ca-In
67
Determine whether the electrical gradient drives each substance into or out of the cell
K-In
Na-In
Cl-Out
Ca-In
68
Determine whether the net drive for each substance is into or out of the cell
K-Out (Slightly)
Na-In (Lots)
Cl-In (Slightly)
Ca-In (Lots)
