Transcript The Neuron & Action Potential

Action Potential
How neurons send an electrical
message
How Neurons Communicate
• Action Potential is the electrical process
that neurons use to communicate with each
other
• Action Potentials are based on movements
of ions (charged particles) between the
outside and inside of the axon
• Action Potential is an All or Nothing
Process (like a gun firing)
Threshold:
Triggering Action Potential
Threshold is something in the environment (light, pressure,
heat, chemicals or another neuron) that tell the neuron to fire
and send an electrical signal (action potential)
How do you feel something that is intense?
More neurons fire, the intensity of their electric impulse
always stays the same.
Thresholds can tell an neuron to fire (excitory) or to
stop firing action potential (inhibitory)
Lou Gehrig’s Disease - too many inhibitory stimuli cause the
muscles to freeze up.
Parkinson’s Disease - too many excitatory stimuli cause the muscles
to move without control.
Steps to Action Potential
Step 1: Threshold is Reached
• Axon at Resting Potential - fluid inside the
axon is mostly negatively charged with
positive on the outside (polarized)
• An impulse is triggered in the neuron’s
dendrite when stimulated by pressure, heat,
light or a chemical messenger from another
neuron (stimulus threshold).
Resting Potential
• At rest, the inside of the cell is at -70 microvolts
• With inputs from dendrites inside becomes more positive
• If resting potential rises above threshold, an action potential
starts to travel from cell body down the axon
• Figure shows resting axon being approached by an AP
Step 2: Action Potential Begins
• When neuron fires, its axon membrane is
selectively permeable (has gates that open).
• Gates in the axon called ion channels open
allowing positive sodium ions to enter the axon
giving it a brief positive electrical charge the axon
(depolarized).
• The brief positive charge is action potential.
Depolarization Ahead of AP
• AP opens cell membrane to allow sodium (Na+) in
• Inside of cell rapidly becomes more positive than outside
• This depolarization travels down the axon as leading edge
of the AP
Step 3: Refractory Period
• As the next gates open allowing positive sodium ions in,
the previous gates close and begin to pump the positively
charged sodium ions out of the axon. (repolarized).
• This step is called the refractory period and the axon
cannot fire again until it returns to resting potential
(negative polarized state inside the axon).
• The entire process is like falling dominoes all the way
down the axon except these dominoes can set themselves
back up as soon as the fall over.
• Q: Why do you think the axon has to set itself back to a
resting state so quickly (3 milliseconds)?
• A: So the neuron can fire again and send another message
immediately after the last one.
Repolarization follows
• After depolarization potassium (K+) moves out restoring
the inside to a negative voltage
• This is called repolarization
• The rapid depolarization and repolarization produce a
pattern called a spike discharge
Action Potential Within a Neuron
DAILY
DOUBLE
How can a toilet represent Action
Potential?
• Full Toilet – Resting Potential
• Push Flush Lever – Threshold Stimulus
triggering Action Potential.
• Toilet Refilling/Can’t Flush –
Repolarization/Refractory Period
• Sewer Pipes – One-way
communication like action potential
only goes from dendrite end to axon
terminal end.