The Nervous System
Action Potential Propagation
What is an action potential?
 An action potential is an electrical impulse or signal passed
from one neuron to another
 Electrical properties result from ionic concentration
differences across plasma membrane and permeability of
membrane
 Ions are any charged particles found in the cell
Important Ions
 Cations = Positive Ions
 Sodium = Na+1 charge
 Potassium = K+1 charge
 Calcium = Ca+2 charge
 Anions = Negative Ions
 Chlorine = Cl-1 charge
 Nerve cells are surrounded by a semi-permeable membrane
that allows some things to pass through and not others
Membrane Potential
 Caused when opposite charges are separated by a cell
membrane and want to move to balance each other out.
More cations on outside
of cell than inside of cell
so you have a membrane
potential
Electrode measures membrane
potential of a neuron
Axon
Phases of Membrane Potential
 1. Resting potential: –70mV (inside of axon more
negative than outside)
 2. Depolarization: 0 to +30 mV (inside of axon more
positive than outside)
 3. Repolarization: +30 back to –70mV (inside of axon
becoming more negative than outside)
 4. Hyperpolarization: -70 mV to –100mV (inside
axon way more negative than outside)
Phases of Membrane Potential
Hyperpolarization
Resting Potential
 When a neuron is not sending a signal, it is "at rest”. When a
neuron is at rest, the inside of the neuron is negative compared
to the outside. Although the concentrations of the different ions
attempt to balance out on both sides of the membrane, they
cannot because the cell membrane allows only some ions to
pass through channels (ion channels). The difference in the
voltage between the inside and outside of the neuron is
measured, you have the resting potential. The resting
membrane potential of a neuron is about -70 mV
(mV=millivolt) -this means that the inside of the neuron is 70
mV less than the outside.
Resting Potential
 At rest, there are relatively more sodium ions outside the
neuron and more potassium ions inside the neuron. The
sodium channels are closed, preventing sodium from
entering the cell.
Depolarization
 The action potential is an explosion of electrical activity
that is created by a depolarizing current. This means that
some event (a stimulus) causes the resting potential to
move toward 0 mV. This event (stimulus) happens when
a neurotransmitter binds to receptors on the dendrites.
This binding causes sodium channels to open, and
sodium begins to enter the cell.
When the
depolarization reaches about -55 mV a neuron will fire
an action potential.This is the threshold stimulus.
Depolarization
All or None Principle
 If the neuron does not reach this critical threshold
level, then no action potential will fire. Also, when
the threshold level is reached, an action potential of a
fixed sized will always fire...for any given neuron,
the size of the action potential is always the same.
There are no big or small action potentials in one
nerve cell - all action potentials are the same size.
Therefore, the neuron either does not reach the
threshold or a full action potential is fired this is the
"ALL OR NONE" principle.
Repolarization
 During repolarization, we see a return to the normal
resting potential state. The sodium channels begin to
close, and in an effort to balance the charges inside
and outside the cell, potassium moves out of the cell.
When potassium leaves the cell, the membrane
potential becomes more negative (returns to -70
mV).
Hyperpolarization
 During hyperpolarization, the potassium channels
continue to stay open, allowing more potassium to
leave the cell. This drops the electrical voltage down
to -100mV, hyperpolarizing the cell. Once potassium
channels close, a return to resting potential occurs.
Hyperpolarization
Action Potential Generated
 The movement of sodium ions into the cell depolarizes
adjacent sites on the axon, triggering the opening of
additional channels. The result is a chain reaction that spread
across the surface of the membrane like a line of falling
dominoes. The action potential continues to move down the
axon until it reaches the axon terminal and causes the
calcium channel to open. Calcium enters the terminal, binds
to the vesicles, and causes the vesicles to be released into the
synaptic cleft. The neurotransmitter moves through the
synaptic cleft and binds to the next neuron, continuing the
propagation of the action potential.
The Nerve Impulse
Copyright Pearson Prentice Hall
The Nerve Impulse
Copyright Pearson Prentice Hall
The Nerve Impulse
Copyright Pearson Prentice Hall
The Synapse
Synaptic cleft
Copyright Pearson Prentice Hall
Basic Steps to an Excitatory Action
Potential
1. Neuron is at resting
potential (-70 mV)
4. Membrane potential
becomes more positive
+30mV
2. Neurotransmitters bind to
receptors on dendrites
5. Na+ channels close
3. Causes Na+ to rush
into cell body
6. K+ rushes out of cell to
balance out differences in
membrane potential
Basic Steps to an Excitatory Action
Potential (continued)
7. Membrane potential
goes to -100mV but then
goes back to -70mV
8. Continues along length of
axon
9. Action Potential
reaches axon terminal
and Ca2+ channel open
11. Vesicles with NT inside
travel to end of axon
terminal
10. Ca2+ enters axon
terminal and binds to
vesicles
12. Vesicles release NTs into
synaptic cleft
13. NT’s travel through
synaptic cleft and bind to next
neuron