FORM & FUNCTION (Membrane Potential) Flashcards
Membrane potential:
-electrical charge difference across the cell membrane
-measured in millivolts (mV)
-all cells have them
Ex. resting potential
Excitable tissues:
-more negative RMP (resting membrane potential)
- (-70mv) to (-90mV)
Ex. neurons, muscles and glands
Non-excitable tissues:
-less negative RMP
-epithelial cells (-53mV)
-RBC (-8.4mV)
-fibroblasts (-20 to -30mV)
-adipocytes (-58mV)
Polarity inside vs. outside cell
-inside is more negatively charged relative to outside
Magnitude of RMP:
-ranges from (-20mV) to (-100mV)
Factors that contribute to RMP:
-unequal ionic distributions
-differences in membrane permeability to Na+ and K+ (role of leaky channels)
-active ion transport (Na+/K+ pump)
Specialized cell types and RMP:
-only excitable cells (neuron, muscles, glands) can respond to changes in membrane potential to generate action potentials
Unequal ionic distribution:
-more Na+ and Cl- outside the cell
-more K+ inside the cell
*different concentration gradients for Na+ and K+
Differences in membrane permeability to Na+ and K+:
-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
Active transport: Na+/K+ pump
-transport 3 Na+ to outside the cell and 2 K+ inside the cell
*generates a net negative charge inside in every cycle
Changes in membrane potential:
-hyperpolarization
-depolarization
Hyperpolarization:
-when MP becomes MORE negative than the RMP
>neuron is ‘super relaxed’
Depolarization:
-when MP becomes LESS negative than the RMP
>neuron is ‘excited’
Equilibrium potential (simple):
-MP when there is no net flow of ions
-concentration and electrochemical gradient balance each other out
Equilibrium potential of ions:
-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
Goldman-Hodgkin-Katz (GHK) equation:
-goes beyond a single ion
What are the variables in the Nernst equation? (factors for equilibrium potential)
-K+ concentration inside the cell
-K+ concentration outside the cell
-temperature
*for a particular ion
Actual membrane potential (Vm):
-physiological value
-depends on concentration differences of MULTIPLE ions and their relative permeabilities across a cell membrane
Equilibrium potential (Ex):
-constant value at a specific temperature
-depends solely on the concentration difference to ONE ion across the cell membrane
Driving force for ion movement:
-difference between the actual membrane potential and the equilibrium potential for a specific ion
=Vm-Ex
Variables for GHK Equation?
-concentration of multiple ions
-temperature
-permeability of ions
Tiny movement of ions:
-is enough to generate electrical signals necessary for excitable cells to communicate
During AP, permeability of Na:
-increases 500x because of the opening of voltage-gated sodium channel
What specifically causes voltage-gated sodium channels to open?
-a depolarizing membrane potential to the “threshold”
Major ion currents during AP:
-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
Threshold:
-(-75mV)
Key processes of an AP:
- Resting state
- Depolarization
- Peak of AP
- Repolarization
- Hyperpolarization
- Returning to resting state
Resting state:
-membrane is at resting potential
-voltage-gated Na and K channels are closed
Depolarization:
-triggered when the membrane potential reaches threshold
-voltage-gated Na channels open=Na+ ions flood into the cell
-membrane potential becomes more positive
Peak of AP:
-membrane becomes close to 100mV more depolarized compared to RMP
-voltage-gated Na channels start to close (inactivate)
Repolarization:
-voltage-gated K channels open=allow K+ ions to exit the cell
-membrane potential returns towards resting state
Hyperpolarization:
-some K+ channels remain open a bit longer
>causes the membrane to dip below the resting potential
Return to resting state:
-Na and K channels reset to their original states
-Na/K pump works to restore ion balance across the membrane
Na+ channels:
-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
Absolute refractory period:
-time during which a second AP is impossible to initiate, regardless of the stimulus strength
-all VG Na+ channels are inactivated
Absolute refractory period ensures:
-ONE-way propagation of AP
-sets a limit on maximum firing frequency of the neuron
Relative refractory period:
-follows the absolute refractory period
-a second AP can be initiated, but it requires a STRONGER stimulus than usual
Relative refractory period allows:
-for the possibility of stimulus intensity coding through variations in firing frequency
If have consistent stimulus:
-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)
