Download Action Potential

4
Today’s lecture
Today’s Lecture
Restingpotential
Potential
Resting
Action potential
Potential
Action
Ionic basis
basis of
of the
theaction
actionpotential
potential
A review of the Resting Membrane Potential
Resting Membrane Potential
Est. Cytoplasm and CSF- Seperated by Plasma membrane
PRIMARILY PASSIVE POTASSIUM CHANNELS
Two Natural Factors influencing the DISTRIBUTION of Ions:
a. Diffusion
b. Electrostatic gradient
Potassium Permeable membrane
Cytoplasm
CSF
A review of the Resting Membrane Potential
Resting Membrane Potential
Two Natural Factors influencing the DISTRIBUTION of Ions:
a. Diffusion
b. Electrostatic gradient
Blue = +ive
Red = - ive
A review of the Resting Membrane Potential
Resting Membrane Potential
A review of the Resting Membrane Potential
Resting Membrane Potential
Four Key Ions are Unequally distributed
Na+, Cl-, K+, Poly-anions such as amino acids and
proteins.
Small concentrations
A review of the Resting Membrane Potential
Resting Membrane Potential
Two Channels allow for passage of ions between Cytoplasm and
CSF
a. Ion Channels (Passive)
Namely K+ since the others are quite scarce
b. Na-K ATPase pump (Active)
Components of an action potential
Review!
Small amount of ions actually involved (10-12 mol /cm2 of Plasma
membrane)
-> not enough to change the concentrations significantly
At the immediate boundary of the Plasma membrane
RMP NOT Unique to neurons
Thought experiment:
If we reversed the situation?
The voltage would be inverted!
Components of an action potential
Components of an Action Potential
1. Threshold: Minimum strength of current required
2. All or none phenomena:
- Either a complete action potential that propagates along the
axon or no response at all
- once generated, moves along the axon without a drop or gain
in amplitude
3. Always followed by a refactory period
Action Potential
Membrane Potential (mV)
end of rising phase
Na+ channels inactivate;
Na+ stops entering
50
0
-50
-65
rising phase
Na+ channels open;
Na+ enters
falling phase
K+ channels open;
K+ leaves
threshold
resting potential
undershoot
0
1
2
Time (ms)
Voltage Changes in neurons Hyperpolarization vs Depolarization
Voltage Changes in neurons:
Hyperpolarization vs Depolarization
Voltage-Gated Sodium Channel
The Voltage-Gated Sodium Channel
gating charge
+
3) channel opens
Na+
1) sensory receptor or
synaptic activity
Na+
Axon
2) depolarization
Action Potential Rising Phase
The action potential rising phase occurs because the neuronal
membrane contains voltage-gated Na+ channels that open in a
regenerative manner if depolarization passes the threshold level:
Na+ rushes in
POSITIVE
FEEDBACK
LOOP
voltage-gated Na+
channels open
BUT – It is a temporally isolated event
Voltage-Gated Sodium Channel
The Voltage-Gated Sodium Channel Has
Three States
closed
open
inactivated
+
+
+
+
+
+
Axon
End of Rising Phase
The action potential rising phase ends because the
voltage-gated Na+ channels inactivate
Na+ no
longer
enters
voltage-gated Na+
POSITIVE
channels inactivate
LOOP
ENDS
Action Potential
Voltage-Gated Sodium Channel
The Voltage-Gated Potassium Channel
gating charge
+
3) K+ channel
opens, K+ leaves
“Creaky” door (slow)
1) Na+ entry
(rising phase)
Na+
K+
Axon
2) depolarization
Action Potential
Action Potential
voltage-gated Na+ channels
refractory
closed
open
inactivated
closed
+
+
+
+
+
+
time
velocity
voltage
+
closed
+
closed
voltage-gated K+ channels
+
open
closed
Action Potential
Action Potential Summary
1. The membrane is depolarized past threshold, by a receptor
potential or synaptic potential.
2. Voltage-gated sodium channels open in a positive-feedback
loop, causing the rising phase.
3. The sodium channels inactivate, ending the rising phase and
initiating the refractory period.
4. Voltage-gated potassium channels open, causing the falling
phase and the undershoot (after-hyperpolarization).
5. The sodium channels close, ending the refractory period.
6. The potassium channels close, ending the undershoot and
restoring the membrane to its resting potential.