Action Potential
Brain and Behavior
01.28.2016
Information Transmission
within a Neuron
WHAT IS AN ION?
•
Atoms are made of:
 Protons  Positively charged particles
 Neutrons  Particles with No charge
 Electrons  Negatively charged particles
•
Ion = a charged molecule
 Charge may be positive or negative
WHAT IS AN ION?
•
Some important ions:
 Sodium (Na+)
 Potassium (K+)
 Chloride (Cl-)
 Calcium (Ca2+)
THE NEURON AT REST
•
Membrane of a neuron maintains an
electrical gradient
 Difference in electrical charge between
inside and outside of the cell
•
Neuron is surrounded by a membrane
 Protein molecules embedded in the
membrane form channels
Resting Potential of Neuron
• Polarization
 Difference in electrical charge between two locations
• Slightly negative electrical potential inside the membrane
compared to the outside (approx. -70 mV)
 Due to negatively charged proteins inside the cell
HOW IS THE RESTING POTENTIAL
MAINTAINED?
• Membrane is selectively permeable
 Some chemicals can pass freely across membrane
• Oxygen, carbon dioxide, urea, water
• NOT ions!  Can only cross when protein channels are open
 Ion channels when membrane is at rest:
• Sodium (Na+) channels  Closed
• Potassium (K+) channels  Mostly closed (K+ leaks out slowly)
HOW IS THE RESTING POTENTIAL
MAINTAINED?
• Sodium-Potassium Pump
 Made of proteins
 Repeatedly moves:
• 3 Na+ ions out of the cell
• 2 K+ ions into the cell
 Resulting distribution
• 10x more Na+ outside the membrane
• More K+ ions inside the membrane
Membrane potential
• Diffusion=concentration gradient
• Electrostatic pressure=electrical gradient
Sodium-potassium pump pushes 3 Na+ out for
every 2K+ in
IMPORTANCE OF THE RESTING MEMBRANE
POTENTIAL
•
Where would ions move if given the chance?
 Na+  Rush into the cell
•Electrical and Concentration gradients
“pulling it in”
 K+  Flow slowly out of the cell
•Electrical gradient “pulling it in”
•Concentration gradient “pushing it out”
IMPORTANCE OF THE RESTING MEMBRANE
POTENTIAL
• From resting potential (-70 mV), three options:
 Maintenance of the resting potential
 Hyperpolarization (“increased polarization”)
• Increase negative charge inside the neuron
(e.g., -85mV)
 Depolarization (“decreased polarization”)
• Decrease negative charge inside the neuron
(towards zero…ex: -65mV)
Electrical Potential of Neuron
• Membrane potential - difference in electrical charge
inside and outside cell: inside “-”, outside “+”
• Resting potential – membrane potential at rest -70mV
• Action potential – brief reversal
of membrane potential: inside
“+”, outside “-”, electrical
impulse
Electrical Potential of Neuron (cont.)
• Depolarization – “decreased polarization”, decrease “-” charge
inside neuron, membrane potential to 0
• Excitation threshold – level of depolarization to produce
action potential
• Repolarization – return to resting potential
• Hyperpolarization – “increased
polarization”, increase “-” charge
inside neuron, increase of
membrane potential
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https://www.youtube.com/watch?v=OZG8M_ldA1M
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Action Potential
External
stimulus
Repolarization
Hyperpolarization
Excitation threshold
reached Na+ in
Depolarization
K+ channels
close
K+ out
Action
potential
Na+ channels
close
https://www.yout
ube.com/watch?v
=U0NpTdge3aw
Action Potential
THE ACTION POTENTIAL
• Threshold of excitation (depolarization level)
 Stimulation beyond this level produces sudden, massive
depolarization of the membrane
 Result: Na+ channels open suddenly
• Causes rapid, massive influx of Na+ ions
 Results in more depolarization
• Reduces negative charge (to approx. +50 mV)
• This rapid depolarization = ACTION POTENTIAL
THE ALL-OR-NONE LAW
• Sub-threshold stimulation
 Produces a small amount of depolarization
• Proportional to the amount of current applied
• Stimulation above the threshold of excitation
 Stimulation that reaches the threshold produces the action
potential
 The neuron either “fires” or it doesn’t…
• …and all action potentials, once firing, are the same
REPOLARIZATION
•
Return to resting membrane potential (-70 mV)
•
Achieved by voltage-activated K+ channels…
• Shortly after Na+ channels open,
the K+ channels open
• K+ ions flow out of the membrane
 Due to concentration gradient
 Also, now due to electrical gradient
• Membrane potential decreases to slightly below normal resting
potential
 Temporary hyperpolarization results in
refractory period
POST ACTION POTENTIAL
• Membrane has returned to resting potential
• Post-action potential ion distribution:
 More Na+ ions in the cell
 Fewer K+ ions in the cell
• Sodium-Potassium Pump reactivates
 Restores original ion distribution
All-or-None Law Revisited
• All action potentials are equal:
– Amplitude (intensity)
– Velocity
• Strength of stimulus communicated in
FREQUENCY of action potentials
• All-or-none law applies to axon only
Refractory Period Revisited
• Hyperpolarization after action potential
– Additional APs are extremely difficult (if not
impossible) for a brief period
• Absolute refractory period – AP not possible
• Relative refractory period – AP is possible with
stronger than usual stimulus
Conduction of Action Potential
• All or none law
• Action potential always
remains the same
• Rate law
• Saltatory conduction
https://www.youtube.com/watch?v=ifD1YG07fB8
Role of Myelin
• Nodes of Ranvier
 Short (1 mm) unmyelinated gaps of axon
 Where membrane depolarization occurs
• Saltatory conduction
 APs “jump” down the axon from node to node
• Why have saltatory conduction?
 Conservation of energy
• Less work for sodium-potassium pumps
Multiple Sclerosis
• Loss of myelination in the central nervous system
• Myelinated axon develops sodium channels almost
exclusively at its nodes
• When myelination is lost, no new sodium channels
are formed
• Result: APs die out between nodes