4A. Action Potentials

Question 1

Graded Potential–Spread by ____ current flow

Graded potentials die over ___ distances

Answer 1

Passive

short

Question 2

Examples of graded potentials

Answer 2

1. Postsynaptic potentials
2. Receptor potentials
3. End-plate potentials
4. Pacemaker potentials
5. Slow-wave potentials

Question 3

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)

Answer 3

• travel
• initiation
• voltage-gated
• Ca++

Question 4

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

Answer 4

• membrane
• +30mV
• resting -70mV

Question 5

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.

Answer 5

• • feedback
• K+
• one point
• point

Question 6

Action Potentials: Ion Permeability Changes and Fluxes

Name this step.

Answer 6

Question 7

Action Potentials: Ion Permeability Changes and Fluxes

Name this step.

Answer 7

Question 8

Action Potentials: Ion Permeability Changes and Fluxes

Name this step.

Answer 8

Question 9

Action Potentials: Ion Permeability Changes and Fluxes

Name this step.

Answer 9

Question 10

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

Answer 10

4. Repolarization II: Voltage gated K+ channels also rapidly activate causing strong outward current of K+ that drives Vm negative to toward EK
5. 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)

Question 11

Answer 11

• 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

Question 12

Answer 12

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

Question 13

Answer 13

• 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

Question 14

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)

Answer 14

Question 15

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

Answer 15

voltage-gated

drawn in the direction

INTO

Question 16

Voltage-gated calcium channels:

• 3 functionally distinct types – __
• distinct activation and inactivation voltages and kinetics across different types

•3 major types that differ by:

1. Vm range that activates S4-
   
   • (gating Vm, as in high or low Vm,
   • HVA, High-Vm Activated or Low VA) and
2. open time – slow persistent or fast inactivating

Answer 16

• all VG

Question 17

A plethora of potassium channels

Answer 17

• 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

Question 18

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

Answer 18

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