Transcript Chapter 04: The Action Potential

THE ACTION POTENTIAL
Stimulating electrode:
Introduces current that can
depolarize or
hyper-polarize
Recording electrode:
Records change in
Potential of the membrane
At a distance away
Resting Membrane Potential
• Membrane potential at which neuron membrane is at
rest, ie does not fire action potential
Ionic Equilibrium Potential
Membrane Potential (potential difference across the
plasma membrane) at which the net flow of an ion type =
zero
The number of ions moving into the cell = the number of
ions moving out of the cell for a particular species of ion
At Threshold Na influx equals K efflux
Voltage (mVolts) along Y axis
Time (msec)
• The Action Potential
•
Oscilloscope to visualize an AP
- Rising phase : rapid depolarization to reach the peak of 40mV
- Overshoot : part where inside neurons are more positive than
outside (> 0mV)
- Falling phase : rapid repolarization
- Undershoot : after-hyperpolarization
• The Generation of Multiple Action Potentials
•
Continuous depolarizing current injection can cause
multiple action potential generation
• The Generation of Multiple Action Potentials
•
Firing frequency reflects the magnitude of the
depolarizing current
- One way that stimulation intensity is encoded
•
There is a limit!
- Maximum firing frequency
~ 1000 Hz
- Absolute refractory period
: time required to initiate
the next AP once an AP is
initiated ~ 1 msec
- Relative refractory period :
for a few miliseconds after
the end of absolute
refractory period, current
needed to reach threshold
is above normal
The Action Potential, In Reality
• Key Properties of the Action Potential
•
Threshold
•
Rising phase
•
Overshoot
•
Falling phase
•
Undershoot
•
Absolute refractory period
- sodium channel deinactivation
•
Relative refractory period
- potassium channel closure
(hyperpolarization)
Action Potential Conduction
• Propagation
•
Depolarized to threshold
•
Sodium channels open
•
Influx of Na+
•
Positive charges coming in
depolarize the membrane
just ahead to threshold
•
Next population of sodium
channels open
Action Potential Conduction
• Propagation of the action potential
•
Orthodromic
- Action potential travels in one direction - down axon to the
axon terminal
•
Antidromic (experimental)
- Backward propagation is possible if the initiation of AP occurs
in the middle of axon
•
Cannot turn back on itself
- Refractory (inactivated sodium channels)
•
Typical conduction velocity: 10 m/sec
Six Characteristics of an Action Potential
#1 Triggered by depolarization
• a less negative membrane potential that occurs
transiently
• Understand depolarization, repolarization and
hyperpolarization
#2 Threshold
• Threshold depolarization needed to trigger the action
potential
• 10-20 mV depolarization must occur to trigger action
potential
#3 All or None
• Are all-or- none event
• Amplitude of AP is the same regardless of whether the
depolarizing event was weak (+20mV) or strong
(+40mV).
#4 No Change in Size
• Propagates without
decrement along axon
The shape (amplitude &
time) of the action
potential does not
change as it travels
along the axon
#5 Reverses Polarity
• At peak of action potential the membrane potential
reverses polarity
• Becomes positive inside as predicted by the Ena Called
OVERSHOOT
• Return to membrane potential to a more negative
potential than at rest
• Called UNDERSHOOT
#6 Refractory Period
• Absolute refractory period follows an action potential.
Lasts 1 msec
• During this time another action potential CANNOT be
fired even if there is a transient depolarization.
• Limits firing rate to 1000AP/sec