Quick Membrane Review
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Tasers
• Interfere with the neurons ability to transfer
electrical impulses
• Over loads nervous system
• 25000-150000volts
• Taser
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Impulse Transmission
• Is an electrochemical reaction (the movement of
ions – mostly Na+ and K+)
• The actual impulse is caused by a movement of
ions into and out of the neuron
• An impulse requires an input of energy (ATP)
• An impulse is started by a stimulus - electric
shock, pH, pinch, light, sound, heat, etc.
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Every cell has a voltage, or membrane
potential, across its plasma membrane
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Voltage is electrical potential energy – a separation of opposite charges.
Charge attraction and concentration gradients lead to this “potential energy”
The cytoplasm is negatively charged and the extracellular fluid is positively
charged.
Voltage = membrane potential
A neuron in its resting state (not transmitting an electrical signal) usually has a
membrane potential of -70mV; the minus sign indicates the inside of the cell is
negative in charge with respect to the outside.
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Electrochemical Impulse or
Nerve Impulse or Action Potential
•The electrochemical impulse results from small changes in sodium and
potassium ions in the dendrites and then the axon of the neuron.
•These changes “move down” the axon.
The 3 Stages of Nerve Impulse Include:
1.Resting Potential (aka Polarization)
2.Action Potential (aka Depolarization)
3.Refractory Period (Repolarization)
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1. The Resting Membrane
• outside the neuron’s membrane is a
high concentration of sodium ions
causing it to be positively charged
compared to the inside
• inside the neuron is a high
concentration of potassium ions and
chloride ions (- ions)
• there is an electrical potential
difference between the inside and
outside of about - 70mV
• there are ion channels in the lipid
bilayer membrane that allow certain
ions to diffuse in and out (only certain
ion channels let sodium move and
different ones let potassium move).
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1. The Resting Membrane
• at resting membrane potential,
more potassium ions have the
potential to move out (because
there are more open potassium
ion channels), than sodium ions
have the potential to move in
(almost no sodium ion channels
open)
• also, another membrane protein
called the sodium potassium
pump moves 3 sodium out while
moving 2 potassium in (ATP must
be used for this to happen =
active transport)
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Resting Membrane Potential
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The Action Potential
• Nerve impulses only occur at the Nodes of Ranvier in myelinated neurons
• a nerve impulse is an action potential. An action potential begins when
the dendrites are stimulated with certain chemicals or an electric jolt.
• When the neuron receives a stimulus, the cell
membrane becomes more permeable to Na+ than K+
(Na+ channels open, and the K+ channels close).
• The highly concentrated Na+ ions rush into the cell
by diffusion and charge attraction.
• The rapid inflow of Na+ reverses the charges on both
sides of the membrane (approximately +40mV).
• This charge reversal is referred to as Depolarization
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Animation
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3.Refractory/Recovery
• voltage inside the cell becomes positive = the Na+ channels
slam closed
• K+ channels then open = K+ ions rush out.
• Restores + outside and – inside neuron
• Once action potential has passed, sodium is actively
pumped out of the cell while potassium is pulled back in
(Na+/K+ Pump)
• the membrane will go through a brief hyperpolarization
before returning to its RMP
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3.Refractory/Recovery
• Refractory period is the period of time the
nerve will not respond to a stimulus (needs
to return to resting potential)
– Period of time immediately after an
impulse when the neuron will not respond
to a stimulus – cannot conduct another
impulse until it has been repolarized
– Duration 1-10 milliseconds
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Schematic of an Action Potential
•Depolarization of one region in
a neuron stimulates
depolarization in
adjacent regions and allows
nerve impulses to move along
the membrane of the neuron.
•The region of the axon behind
the
location of the depolarization
undergoes repolarization and
refractory.
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Nerve impulse
http://youtu.be/x4PPZCLnVkA
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An Action Potential Can
Be Represented in Graphical Form
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An Action Potential Can
Be Represented in Graphical Form
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Step by Step
• Action potential and membrane animation
• Action Potential and Na/K Gates
• Action Potential
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Saltatory Conduction
• In myelinated axons, depolarization occurs only at the Nodes of
Ranvier.
•The flow of ions can only happen at those nodes so action potentials
have to “jump” from node to node
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All or None Response
•For an impulse to be
generated, the stimulus must
be strong enough to “jolt”
the membrane to allow
sodium ions to rush in.
•This level of intensity is
called the “threshold level”.
•Different neurons have
different threshold levels.
•If the stimulus is strong
enough to generate an action
potential, the impulse is
transmitted along the entire
neuron.
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All or None Response cont…
•However, if the stimulus is much stronger than the
threshold, the neuron will not fire any stronger (the
graph stays the same)
•Thus, neurons fire maximally or not at all. This is the
all-or-none response.
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Strength & Type of Stimulus
How do you distinguish between different stimuli? For
instance, how is it that your body can tell the difference
between the touch from a feather or a rock?
There are 2 ways:
1. A stronger stimulus causes
more frequent impulses along
a neuron.
2. A stronger stimulus
causes more neurons to
be stimulated
because the threshold
of many neurons are reached.
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Concept Review:
Action Potential
Animation
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• Draw on Board Example of
• Graph of action potential
• What is happening on the membrane
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Check Your Understanding
1. What is a polarized membrane?
2. What causes the inside of a neuron to become negatively charged?
3. Why does the polarity of a cell membrane reverse during an
action potential?
4. Why do nerve impulses move faster along myelinated nerve fibers?
5. Some people report that they have a high pain tolerance. Explain t
terms of threshold levels.
6. What is the “all or none response”?
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