4A. Action Potentials Flashcards
Graded Potential–Spread by ____ current flow
Graded potentials die over ___ distances
Passive
short
Examples of graded potentials
- Postsynaptic potentials
- Receptor potentials
- End-plate potentials
- Pacemaker potentials
- Slow-wave potentials
Action potentials do not decrease in strength as they ____ from their site of ____ throughout the remainder of the cell membrane.
As long as that region of the cell membrane the AP is traversing contains ____ Na+ or ___channels which can be activated (that is they are not in the gate shut inactivated state)
- travel
- initiation
- voltage-gated
- Ca++
When the membrane reaches threshold potential:
- Voltage gated Na+ channels in _____ undergo conformational changes.
- Flow of Na+ ions into the ICF reverses the membrane potential from -70mV to ___
- Flow of K+ ions into the ECF restores the membrane potential to the ___ state
- membrane
- +30mV
- resting -70mV
Action Potentials
•Additional characteristics
–Na+ channels open during depolarization by ____.
–When the Na channels become inactive, the channels for open.
** This repolarizes the membrane.**
–the action potential develops at ___in the plasma membrane, it regenerates an identical action potential at the next ___in the membrane.
–So it travels along the plasma membrane undiminished.
- feedback
- K+
- one point
- point
Action Potentials: Ion Permeability Changes and Fluxes
Name this step.
Action Potentials: Ion Permeability Changes and Fluxes
Name this step.
Action Potentials: Ion Permeability Changes and Fluxes
Name this step.
Action Potentials: Ion Permeability Changes and Fluxes
Name this step.
Steps to an Action Potential
When membrane reaches threshold potential:
- 1.Initiation: Voltage-gated SODIUM (OR CALCIUM) channels in the membrane undergo conformational changes (S4 segment) and the channel rapidly OPENS
- 2.Upstroke: Extrordinarily rapid flow of sodium (or calcium) ions into the ICF rapidly reverses (< msecs) the membrane potential from -70 mV to +30 mV
- 3.Repolarization I: At the peak of the AP, VG channels rapidly INACTIVATE and
- Repolarization II: Voltage gated K+ channels also rapidly activate causing strong outward current of K+ that drives Vm negative to toward EK
- Afterhyperpolarization: This overshoot of the RMP past normal and more negative is due to the combined permeability of both the Kir (resting RMP) channels and the VGKCs (which repolarize after APs)
- VGICs provide the selective permeability for Na/K/Ca which underlie the different types of APs in excitable tissues
- Multiple types of VGICs exist for Na+, K+ and Ca++
- All derived from main ancestor and share basic structure
- VG Na channels – simplest
- VG Ca channels more diverse
- VG K channels REALLY diverse
Na+/Ca++ channels:
- very similar structurally
- Main alpha subunits shown all have accessory subunits as well
- 4 cassettes of 6 (S1-6) transmembrane regions which form the main channel
- Thus, 24 tm regions on a single peptide required
- P-regions form the actual ion pore
- S4 region (red) has a net + charge and is the main voltage sensor
- S4 is displaced when the membrane depolarizes and induces the conformational change required for channel opening
- 4 cassettes of 6 (S1-6) transmembrane regions which form the main channel
- Thus, 24 tm regions on a single peptide required
- P-regions form the actual ion pore
- S4 region (red) has a net + charge and is the main voltage sensor
•S4 is displaced when the membrane depolarizes and induces the conformational change required for channel opening
K+ channels:
- VERY, VERY diverse
- Not all are voltage-gated
- may have 2-8 tm regions
- These are:
- IKir – inward-rectifier (not VG)
- IKdr – delayed-rectifier (VG)
- A-type – transient (VG)
- Ca++-activated K+ channels (+/- VG)
- ATP – inhibited by ATP (VERY sig in diabetes)
Voltage-gated sodium channels:
- all are ____
- very high degree sequence homology
- by convention currents are _____ of movement of + CHARGES (eg Na+)
- by convention downward currents flow ___ the cell
- these inward currents are depolarizing and underlie initiation of APs
voltage-gated
drawn in the direction
INTO
Voltage-gated calcium channels:
- 3 functionally distinct types – __
- distinct activation and inactivation voltages and kinetics across different types
•3 major types that differ by:
- Vm range that activates S4-
- (gating Vm, as in high or low Vm,
- HVA, High-Vm Activated or Low VA) and
- open time – slow persistent or fast inactivating
- all VG
A plethora of potassium channels
- 6/7 TM: numerous diff types of
- VG (Kdr),
- non-VG and
- Ca++-activated K channels
- 2 TM: Kir – sets resting Vm (RMP)
- 2 TM: K(ATP) – site of oral hypoglycemic agents
Ion Channel Differentiation
Electrical interactions between ions and water create a hydrated radius around the ion
- Potassium is a diffuse point source (large radius, delocalized charge) and has less hydrated water, so it has smaller hydration radius that sodium (even though K+ is larger)
- Sodium is a conc point sources with more hydrated water, leading to a larger hydration radius than potassium
This radius can then be filtered
- Potassium channel does not strip hydration, so it accepts the smaller hydrated potassium and blocks the larger hydrated sodium
- Sodium channel does strip hydration, so it accepts the smaller sodium ion and blocks the larger potassium ion
