Download The Action Potential

THE ACTION
POTENTIAL
30S Biology
Crash course part 2
ELECTRICITY
 In
response to a stimulus, an electrical impulse
is fired down a neuron to the next.


1 strength, but frequency varies.
The nerve impulse is called the Action Potential
 The
body is electrically neutral (positive =
negative)
 Some parts are more positive or more negative
 Membranes are used to keep the negatives and
positives separate b/c they attract.

We keep them separate to build Action Potential
(potential energy)
ELECTRICITY
 Each
neuron is like a little
battery with the potential
to do work.
 When needed, the
electricity is allowed to
flow (in response to a
stimulus)

In a cell, we call this
difference between the
negative and positive a
Membrane Potential.
THE RESTING NEURON
 More
negative inside, positive outside
 The difference is known as the Resting
Membrane Potential

Always -70mV
 Na+
ions outside,
K+ ions inside, but
bigger negative
proteins inside.
 Overall negative
charge (-70mV)
 Neuron has a negative
membrane potential
- polarized.
SODIUM POTASSIUM PUMP
The organization of charges is done by a protein called
the sodium potassium pump.
 They are lots located in the axon membrane.

Pump in 2 potassium and lets out 3 sodium
 This creates the difference in charges called the
Electrochemical Gradient.

NATURE LIKES BALANCE

The neurons are always trying to restore balance


Ion channel proteins in the membrane help transport
positive and negative ions across to balance out the
charges.


It doesn’t like be depolarized.
There are a few different kinds
The movement of ions is key to all electrical events in
the neurons.
SENDING SIGNALS


Graded potential: small impulses that cause only a
few channel proteins to open. Causes small change in
membrane potential.
In order to send longer signals, a larger change is
needed called an axon potential.

We need to depolarize a resting neuron causing a large
change in the membrane potential.
ACTION POTENTIAL THRESHOLD
 At
resting state, all the
proteins are sitting closed
at
-70mV.
 THEN! A stimulus occurs!
 The
stimulus and
resulting change have to
be strong enough for the
true action potential to
kick in
Threshold = -55mV
 This is an all-or-nothing
event

To get to threshold, only the Na-K pumps open.
 Only the -55mV is reached, the other proteins open,
allowing many more Na+ ion to flow in.


This makes the whole thing very depolarized to the point
where it is positive (+40mV)
REPOLARIZATION

Once the charge has passed, repolarization begins.




Potassium channels open allowing potassium ions to flow
out which rebalances the charges.
Initially the repolarization goes to far, to -75mV at which
point it is hyperpolarized.
The Na-K channels then open once again to balance
everything – bring it back to rest.(-70mV)
When all of this is happening, the neuron cannot
respond to any other stimulus.
Called the Refractory Period
 It prevents signals from traveling in both direction at the
same time.

CONDUCTION VELOCITY

The threshold of the action potential does not change.
What does change is the frequency of the signals.
Weak stimulus triggers less frequent action potentials
 Stronger signals trigger more frequent action potentials.


Conduction Velocity: The speed of axon potential
Reflexes need fast AP
 Intestines and blood vessels have slow AP

MYELIN VS. NON-MYELIN NEURONS
 Myelin
axons
conduct signals
faster then nonmyelin axons.
 The myelin allows
the current to leap
from one gap to
the next (Nodes of
Ranvier).

Called Saltatory
Conduction.