FORM & FUNCTION (Membrane Potential)

Question 1

Membrane potential:

Answer 1

-electrical charge difference across the cell membrane
-measured in millivolts (mV)
-all cells have them
Ex. resting potential

Question 2

Excitable tissues:

Answer 2

-more negative RMP (resting membrane potential)
- (-70mv) to (-90mV)
Ex. neurons, muscles and glands

Question 3

Non-excitable tissues:

Answer 3

-less negative RMP
-epithelial cells (-53mV)
-RBC (-8.4mV)
-fibroblasts (-20 to -30mV)
-adipocytes (-58mV)

Question 4

Polarity inside vs. outside cell

Answer 4

-inside is more negatively charged relative to outside

Question 5

Magnitude of RMP:

Answer 5

-ranges from (-20mV) to (-100mV)

Question 6

Factors that contribute to RMP:

Answer 6

-unequal ionic distributions
-differences in membrane permeability to Na+ and K+ (role of leaky channels)
-active ion transport (Na+/K+ pump)

Question 7

Specialized cell types and RMP:

Answer 7

-only excitable cells (neuron, muscles, glands) can respond to changes in membrane potential to generate action potentials

Question 8

Unequal ionic distribution:

Answer 8

-more Na+ and Cl- outside the cell
-more K+ inside the cell
*different concentration gradients for Na+ and K+

Question 9

Differences in membrane permeability to Na+ and K+:

Answer 9

-cells contain many K+ leaky channels, (essentially permeable to K+)
-cells contain 100 more K+ leaky channels than Na+
-K+ movement to outside of cell
-Na+ movement to inside of cell
*more K+ leaving the cell than Na+ entering

Question 10

Active transport: Na+/K+ pump

Answer 10

-transport 3 Na+ to outside the cell and 2 K+ inside the cell
*generates a net negative charge inside in every cycle

Question 11

Changes in membrane potential:

Answer 11

-hyperpolarization
-depolarization

Question 12

Hyperpolarization:

Answer 12

-when MP becomes MORE negative than the RMP
>neuron is ‘super relaxed’

Question 13

Depolarization:

Answer 13

-when MP becomes LESS negative than the RMP
>neuron is ‘excited’

Question 14

Equilibrium potential (simple):

Answer 14

-MP when there is no net flow of ions
-concentration and electrochemical gradient balance each other out

Question 15

Equilibrium potential of ions:

Answer 15

-point at which the net flow of an ion across the membrane is zero
-point at which concentration gradient of an ion is EXACTLY BALANCED by the electrical potential difference across the membrane
*all ions want to reach their equilibrium potential
-Nernst Potential

Question 16

Goldman-Hodgkin-Katz (GHK) equation:

Answer 16

-goes beyond a single ion

Question 17

What are the variables in the Nernst equation? (factors for equilibrium potential)

Answer 17

-K+ concentration inside the cell
-K+ concentration outside the cell
-temperature
*for a particular ion

Question 18

Actual membrane potential (Vm):

Answer 18

-physiological value
-depends on concentration differences of MULTIPLE ions and their relative permeabilities across a cell membrane

Question 19

Equilibrium potential (Ex):

Answer 19

-constant value at a specific temperature
-depends solely on the concentration difference to ONE ion across the cell membrane

Question 20

Driving force for ion movement:

Answer 20

-difference between the actual membrane potential and the equilibrium potential for a specific ion
=Vm-Ex

Question 21

Variables for GHK Equation?

Answer 21

-concentration of multiple ions
-temperature
-permeability of ions

Question 22

Tiny movement of ions:

Answer 22

-is enough to generate electrical signals necessary for excitable cells to communicate

Question 23

During AP, permeability of Na:

Answer 23

-increases 500x because of the opening of voltage-gated sodium channel

Question 24

What specifically causes voltage-gated sodium channels to open?

Answer 24

-a depolarizing membrane potential to the “threshold”

Question 25

Major ion currents during AP:

Answer 25

-both Na and K channels are closed when at RMP
-when threshold is met=Na permeability increase significantly
>Na channels remain open until peak of AP, then they are closed and inactivated
-at peak of AP, K now has a stronger driving force (K+ moves out of cell and AP decreases)
>why get a slight hyperpolarization
>K channels remain open until RMP is reached again

Question 26

Threshold:

Answer 26

-(-75mV)

Question 27

Key processes of an AP:

Answer 27

1. Resting state
2. Depolarization
3. Peak of AP
4. Repolarization
5. Hyperpolarization
6. Returning to resting state

Question 28

Resting state:

Answer 28

-membrane is at resting potential
-voltage-gated Na and K channels are closed

Question 28

Depolarization:

Answer 28

-triggered when the membrane potential reaches threshold
-voltage-gated Na channels open=Na+ ions flood into the cell
-membrane potential becomes more positive

Question 29

Peak of AP:

Answer 29

-membrane becomes close to 100mV more depolarized compared to RMP
-voltage-gated Na channels start to close (inactivate)

Question 30

Repolarization:

Answer 30

-voltage-gated K channels open=allow K+ ions to exit the cell
-membrane potential returns towards resting state

Question 31

Hyperpolarization:

Answer 31

-some K+ channels remain open a bit longer
>causes the membrane to dip below the resting potential

Question 32

Return to resting state:

Answer 32

-Na and K channels reset to their original states
-Na/K pump works to restore ion balance across the membrane

Question 33

Na+ channels:

Answer 33

-have 3 states
1. Resting: activation gate closed, inactivation gate open
2. Activated state: activation gate open, inactivation gate open
3. Inactivated state: activation gate open, inactivation gate closed
*repolarization to RMP is required for VG Na+ channels to be reactivated

Question 34

Absolute refractory period:

Answer 34

-time during which a second AP is impossible to initiate, regardless of the stimulus strength
-all VG Na+ channels are inactivated

Question 35

Absolute refractory period ensures:

Answer 35

-ONE-way propagation of AP
-sets a limit on maximum firing frequency of the neuron

Question 36

Relative refractory period:

Answer 36

-follows the absolute refractory period
-a second AP can be initiated, but it requires a STRONGER stimulus than usual

Question 37

Relative refractory period allows:

Answer 37

-for the possibility of stimulus intensity coding through variations in firing frequency

Question 38

If have consistent stimulus:

Answer 38

-at threshold: 3ms between AP (1ms for absolute refractory and 2ms for relative refractory)
-stronger than threshold: 1ms between AP (only need to wait for absolute refractory to be done)