9.2 Electrochemical Impulse
• Nerves impulses are similar to
• Nerve impulses are slightly slower,
however they stay the same strength
through the entire length.
• Nerve impulses are electrochemical
messages made by the movement of
ions through the nerve cell
• Experimented by putting a tiny electrode
inside a squid’s large nerve cell:
– A fast change in electrical potential
difference across the membrane was
noted every time the nerve was excited.
– Resting membrane potential is about 70mV, but when the nerve was exited, it
changed to +40mV.
– This change in potential is called the
– This change did not last more than a
few milliseconds though before it
returned to it’s resting potential.
How an impulse is transmitted
• The nerve cell membrane is permeable to ions
which move across the membrane and set up an
electrical chemical potential.
• When a nerve becomes excited a rapid change
in the potential difference is detected.
• This potential difference is called the action
potential (+40 mV) and it travels along the
neuron from dendrite to axon.
• After the nerve impulse travels down the axon
the neuron returns to its original potential
difference called the resting potential (-70 mV).
How do neurons work?
• The neuron is polarized (more positive outside
• -The Na+-K+ pump uses ATP to pump 3 Na+
ions out of the neuron and 2 K+ ions in.
• -The neuron membrane is “leaky” to K+ ions so
some K+ ions diffuse out of the neuron.
• -Negative ions do not diffuse out of the neuron.
• The difference in
the outside and
the inside of the
• in most neurons
it is –70 mV.
• When the
neuron is at rest,
it is polarized
• -The dendrites receive an electrical
impulse resulting in a change in the
permeability of the membrane.
• -Na+ ions diffuse in to the neuron through
sodium channels and the polarity of the
membrane is reversed. This is called
depolarization and it moves as a wave
down the neuron.
So how do the nerve cell
membranes become charged?
– Due to the fact that neurons have a rich supply of positive
and negative ions both inside and outside the cell.
– The negatives don’t really do much to help with the process
as they are large and cannot move across the membrane, so
they just stay inside the cell.
– The electrochemical event is caused by an imbalance of
positive ions across the membrane.
– The highly concentrated potassium ions inside the nerve
cells have a tendency to diffuse outside the nerve cells.
– Also, the highly concentrated sodium ions outside the nerve
cell have a tendency to diffuse into the nerve cell.
– As potassium diffuses out of the neuron, sodium diffuses into
– The resting membrane is about 50 times more permeable to
potassium than it is to sodium, so more potassium ions
diffuse out of the nerve cell than sodium ions into the cell.
– This makes the resting membrane potential unequal, with
the inside of the cell being more negative and the outside
being more positive, making it a polarized membrane.
So what happens when the neuron
– When the nerve cell is excited, the membrane
becomes more permeable to sodium than potassium.
– It is believed that sodium gates in the membrane are
opened and the potassium gates close.
– There is a rapid flow of sodium ions into the cell,
creating a depolarization.
– Once the voltage inside the cell becomes positive, the
sodium gates slam shut and the inflow of sodium is
•Na+ channels open and Na+ ions flood into the
•K+ channels close at the same time and K+
ions can no longer leak out
•The interior of the neuron in that area becomes
positive relative to the outside of the neuron.
•This depolarization causes the electrical
potential to change from –70 mV to + 40 mV
– A sodium-potassium
pump that is in the
membrane restores the
resting membrane by
transporting sodium ions
out of the neuron while
moving potassium ions into
the neuron in a ratio of 3
Na+ to 2 K+ ions.
• This pump requires
energy from ATP and is
called the process of
Animation: The Nerve Impulse
• -Once the action potential has peaked a
refractory period occurs where the neuron
returns to its resting potential (polarized
• -The membrane becomes impermeable to Na+
ions and the Na+-K+ pump will pump the Na+
ions back out of the neuron.
• -During the refractory period the neuron cannot
transmit another nerve impulse (about 1 to 10
•The spike in voltage causes the K+ pumps to
open and K+ ions rush out
•The inside becomes negative again.
• Nerves conducting an
impulse cannot be
activated until the
condition of the resting
membrane is restored.
The period of
depolarization must be
completed and the nerve
must repolarize before the
next action potential is
• The time it takes for the
cell to repolarize is called
the refractory period.
The Refractory period
• So many K+ ions get out that the charge goes below
the resting potential. While the neuron is in this state
it cannot react to additional stimuli.
original resting potential
SODIUM CHANNELS OPEN
rapid depolarization (becomes positive in or negative on the
MORE SODIUM CHANNELS OPEN
excessive charge inside cell is rapidly lost
SODIUM CHANNELS CLOSE & POTASSIUM
temporarily drops below original resting potential before
POTASSIUM CHANNELS CLOSE
MOVEMENT OF THE ACTION
• In order for the impulse to move along the axon,
the impulse must move from one zone of
depolarization to adjacent regions.
• The positively charged ions that rush into the
nerve cell during depolarization, are then
attracted to the adjacent negative ions, which
are aligned along the inside of the nerve
• A similar attraction occurs along the outside of
the nerve membrane.
Why does the current move in one
• The electric current passes outward over the
membrane in all directions BUT the area to one side is
still in the refractory period and is not sensitive to the
• Therefore, the impulse moves from the dendrites
toward the axon.
• The positively charged sodium ions of the resting
membrane are attracted to the negative charge that has
accumulated along the outside of the membrane in the
area of action potential.
• The flow of the positively charged ions from the area of
the action potential toward the adjacent regions of the
resting membrane causes a depolarization in the nearby
• The electrical disturbance causes sodium channels to
open in the adjoining area and results in the movement
of the action potential.
What causes the inside of a neuron to become negatively charged?
Positively charged ions are lost from inside of the resting membrane
faster than they are added.
• A wave of
moves along the
and then enters
period as the
to close and
channels to open.
Why does the polarity of a
cell membrane reverse
during an action potential?
The action potential causes ion gates in the cell membrane
to open, allowing ions to flow across the membrane,
effectively reversing the polarity of the membrane.
When the nerve cell becomes excited, the membrane of
the cell becomes more permeable to sodium than to
The sodium gates of the membrane are opened and the
potassium gates are closed.
The highly concentrated sodium ions rush into the cell via
diffusion and charge attraction.
This rapid inflow of sodium causes the charge reversal.
9.2 Electrochemical Impulse
p.418 – 426
ANIMATIONS Neurons and Synapse Action
• Channel gating during an action potential
Propagation of the action potential.
Synaptic vesicle fusion and neurotransmitter release.
THRESHOLD LEVELS AND THE
• The threshold level is the minimum level of a
stimulus required to produce a response.
• The all-or-none response is the fact that a
nerve or muscle fibre responds completely or
not at all to a stimulus.
• If there is not enough potential to reach the
threshold level, no response is noted. If there is
more potential than the threshold level, there will
be a response.
• The more intense the stimulus, the greater the
frequency of the impulses.
Threshold Levels & the
• A potential stimulus must be above a
critical value (threshold level) to
produce a response.
Threshold Levels & the
• Increasing the intensity of the stimuli
above threshold will not produce an
• Intensity of impulse & speed of
transmission remain the same.
• Known as the all-or-none response.
– Neurons either fire maximally or not at all.
Neurons, Synapses, Action Potentials,
and Neurotransmission - The Mind
Threshold Levels & the
Differentiating Between Warm & Hot
• The more intense the stimulus, the
greater the frequency of impulses.
• Intense stimuli excite more neurons.
– Different neurons will have different
– This affects the number of impulses
reaching the brain.
• There are small spaces between neurons and
between a neuron and an effector. These
spaces are called synapses.
• Small vesicles that contain chemicals called
neurotransmitters are found in the end plates
• Impulses move along the axon and release
neurotransmitters from the end plate.
– Chemicals that are
produced within a
neuron, are released
by a stimulated
neuron, and cause an
effect on adjoining
– There are two types
1. Small molecule
released from the
and diffuse across the
depolarization of the
dendrites of the
• This does slow down
transmission, so the
greater number of
synapses, the slower
the speed of
• Spaces between two neurons or a neuron
& an effector is called a synapse.
• Synaptic vesicles containing
neurotransmitters (NTs) found in the end
plates of axons.
• Impulse down axon NTs released from
presynaptic neuron NTs diffuse across
synaptic cleft depolarizes postsynaptic
• Acetylcholine is an example of a
neurotransmitter that is found in the end
plates of many nerve cells. It can act as an
excitatory neurotransmitter on many
postsynaptic neurons by opening the sodium
• Once acetylcholine has done it’s job, and the
action potential has moved to the
postsynaptic neuron, we need
cholinesterase to break down the
acetylcholine so that the sodium channels
• Acetylcholine is known as an excitatory
• But not all neurotransmitters are excitatory
because we also have inhibitory
• Inhibitory neurotransmitters work by
opening the potassium channels, letting
out even more potassium from the
postsynaptic cell and then the neuron is
said to be hyperpolarized because it
becomes more negative in the cell.
• These inhibitory neurotransmitters help
prevent postsynaptic neurons from being
• Acetylcholine opens Na+ channels on
postsynaptic neuron, causing depolarization.
• Cholinesterase (from postsynaptic neuron)
• Inhibitory NTs make the postsynaptic
membrane more permeable to K+.
– Neuron becomes hyperpolarized.
Animation: Chemical Synapse (Quiz 1)
Close to Home Animation:
Close to Home Animation:
• Summation: Effect produced by the
accumulation of NTs from two or more
• The interaction of
what allows you to
throw a ball. As the
and contracts, the