LECTURE 10: ACTION POTENTIALS Flashcards
Membrane Potential Generation = 6
- K+ efflux causes charge separation
- Negative inside
- Positive outside
- Charge separation produces voltage
- Open K+ channels, generate resting membrane potential.
- Efflux of K+ down its concentration gradient.
Membrane Potential Generation:
Em Equation
slide 5
in mV
Understanding Action Potentials = 3
- The action potential is a rapid reversal of membrane potential in response to a critical level of depolarization: i.e. Threshold
- Time-course < 5ms
- Longer in muscle
~10 ms skeletal
~250 ms cardiac
Ionic Basis of Action Potential:
Three states of Na+ channels
- Resting - CLOSED BUT CAPABLE OF OPENING
- Activated - RAPID OPENING TRIGGERED AT THRESHOLD
- OPEN (activated) - Inactivated - SLOW CLOSING TRIGGERED AT THRESHOLD
- CLOSED and not capable of opening (inactivated)
Ionic Basis of Action Potential
Two states of K+ channels
- Resting - closed
- Activated - delayed opening triggered at threshold.
- open
Ionic Basis of Action Potential = 2
- Voltage gated Na+ and K+ channels
- No direct involvement of pump or carriers, though indirect role for Na+/K+ ATPase pump
Action Potential Generation = 2
- Action potential determined by changes in Na+ and K+ conductance (gNa and gK)
- Opening and closing of voltage gated K+ channels is slower than for Na+ channels – gives rise to after hyperpolarization
Action Potential Function = 4
- Signal transmission over long distance
- Coding of stimulus features
- Trigger muscle contractions
- Neurotransmitter/hormone release
Frequency Coding: 3
- Action potential all or none
- Size independent of strength of input or distance travelled
- Frequency of firing increases with stimulus intensity
Passive Signal vs Action Potential
- Passive current flow
- Decays exponentially with distance
- Length constant () - Action potential
- All-or-nothing response
- Fixed amplitude
- Travels long distances (>1 m) without decay
Propagation of Action Potential
Propagation :
- Passive flow —>
- Depolarisation —>
- Opening Na+ channels
- REPEAT BACK TO PASSIVE FLOW
PROPAGATION OF ACTION POTENTIAL… NA+
Na+ enters the neuron
- Active area at peak of action potential
- Adjacent inactive area into which depolarisation is spreading; will soon reach threshold.
- Remainder of axon still at resting potential
Local current flow that depolarises adjacent inactive area from resting potential to threshold potential.
PROPAGATION OF ACTION POTENTIAL… K+
- Previous active area returned to resting potential; no longer active; in refractory peroid.
- Adjacent area that was brought to threshold by local current flow; now active at peak of action potential
- NEW ADJACENT inactive area into which depolarisation is spreading; will soon reach threshold.
- remainder of axon still at resting potential.
Unidirectional AP Propagation…WHAT IS REFACTORY PERIOD.
Refractory period:
Period in which new AP can’t be initiated Inactivation of Na+ channel
Explain the 3 stages of Refractory period.
- Resting
Activation gate CLOSED
Inactivation gate OPEN - Activated
Activation gate OPEN
Inactivation gate OPEN - Inactivated
Activation gate OPEN Inactivation gate CLOSED
Absolute vs Relative refractory period
- Inactivation of Na+ channels
- After hyperpolarization – requires stronger stimulus to reach threshold
FORWARDS VS BACKWARD CURRENT…
BACKWARD CURRENT
- flow does not reexcite previously active area because this area is in its refractory period.
FORWARD CURRENT flow excites new inactive area
Action Potential Propagation
Depolarization at distance depends on length constant ….
Large …. means the potential change spreads further
Action Potential Propagation EQUATION
SLIDE 18
UNDERSTANDING Saltatory Conduction = 3
1 * Role of myelin
2 * Improves passive current flow by insulation
3 * Action potential generation only at nodes of Ranvier (high density of Na+ channels)
WHAT IS CONDUCTION VELOCITY?
Compare speed of action potential (conduction velocity) between unmyelinated and myelinated axons.
Effect of axon diameter:
UNDERSTANDING Cardiac Action Potential
Action potentials can differ depending on properties of ion channels involved
- Phase 0, Na+ enters tge cell = depolarisation
- Phase 2, Ca++ enters the cell, Initiation of contraction.
- Phase 3, K+ exits the cell re-polarisation.
- Resting potential
Cardiac Pacemaker Potentials = 4
- Autorhythmic cells: pacemaker activity
- Funny Na+ channels: open when membrane potential becomes more negative (ie
hyperpolarization): net Na+ inward current. - Slow closing of K+ channels
- Transient (T-type) Ca2+ channels; long-lasting (L-type) Ca2+ channels
SUMMARY = 7
- Depolarisation (inside potential approached outside) of neurons opens voltage gated Na+ channels
- Sodium entry drives the membrane potential positive
- Sodium channels inactivate and voltage gates K+ channels
open restoring the resting potential - Inactivation of Na+ channels produces the refractory period and prevents retrograde propagation of AP.
- Large axon diameter or myelin increase conduction velocity by allowing graded potentials to spread further.
- Cardiac AP are sustained by Ca2+ entry
- Cardiac pacemaker cells have and unstable resting potential due to closure of K+ channels and Na+ leak
