Nervous System Part II: How a Neuron Works Teacher Pages needs coding
1
Nervous System: Part II
How A Neuron Works
Essential Knowledge Statement
3.E.2 Continued
2
Animals have nervous systems
that detect external and internal
signals, transmit and integrate
information, and produce
responses
This is the essential knowledge statement from
the curriculum framework. Detect---process--response
2
Today we will focus specifically on how a
neuron works. How does a neuron cause the
transmission of information?
3
Identify the Numbered Structures
Lets begin by reviewing the components: 1 =
dendrites 2 = nucleus 3= axon 4= axon hillock 5
= cell body
Explain that today’s lesson will talk about events
that occur INSIDE the neuron’s membrane and
OUTSIDE the neuron’s membrane. (Show them
on the diagram where these areas are located.)
4
Resting Potential
Scroll over the bottom of the picture to activate
the animation’s controls and press play.
Questions on following slides.
5
5
Nervous System Part II: How a Neuron Works Teacher Pages needs coding
Have students write out a description of a resting
potential on a note card or piece of paper. They
will likely want you to show the video a couple
of times. When they have their description, ask
them to trade with someone. Play the video one
last time asking the card holders to correct any
misinformation that is written. Return the card to
the owner.
Describe a Resting Potential:
• What is the charge inside the neuron at rest?
• Why is the cell negative inside and positive
outside? (be specific)
6
6
Resting potential = -70 mV Negatively charged
proteins inside the cell are too large to move
through resulting in an overall negative charge.
Potassium moves out through the channels
resulting in more positive external environment.
Na/K pumps move sodium out and potassium in.
Source of Charge Differences:
7
7
Scroll over the bottom of the picture to activate
the animation’s controls and press play.
8
8
Action Potential
• Action potentials propagate impulses along
neurons.
9
– Membranes of neurons are polarized by the
establishment of electrical potentials across the
membranes.
– In response to a stimulus, Na+ and K+ gated
channels sequentially open and cause the
membrane to become locally depolarized.
– Na+/K+ pumps, powered by ATP, work to maintain
membrane potential.
9
So, what is “polarized” anyway? Magnets have
two poles & opposites attract while likes repel.
Water has two polar H—O bonds due to the
electronegativity differences between the
elements H & O. Water is a polar molecule since
it is 3-D with a slightly negative end (where the
unshared electron pairs are located) and a
slightly positive end (where the H’s are located).
Polarity just means a “separation” of charge.
One side of the neuronal membrane is more
negative than the other side.
Nervous System Part II: How a Neuron Works Teacher Pages needs coding
Label the graph of the action potential as we
through the next several slides.
Provide students with a blank graph to label as
you go through the action potential steps.
There’s more detail about the process in these
teacher notes than students need to know, but it’s
here just in case a student “corners you”
regarding the elegant details of the process!
10
10
Generation of Action Potentials:
A Closer Look
• An action potential can be considered
as a series of stages
• At resting potential
1. Most voltage-gated sodium (Na+)
channels are closed; most of the
voltage-gated potassium (K+) channels
are also closed
11
Key
Na+
K+
Membrane potential
(mV)
+50
0
Threshold
−50
12
−100
OUTSIDE OF CELL
Sodium
channel
Potassium
channel
1
Resting potential
Time
INSIDE OF CELL
Inactivation loop
1
Resting state
• When an action potential is generated
13
 Threshold & Resting State: The role of
voltage-gated ion channels in the generation of
an action potential. Ask students to read the
graph and estimate the resting potential (-70
mV).
2. Voltage-gated Na+ channels open first and
Na+ flows into the cell
3. During the rising phase, the threshold is
crossed, and the membrane potential
increases to and past zero
 Voltage-gated Na+ channels can exist in any
of three distinct states: deactivated (closed),
activated (open), or inactivated (closed). Before
an action potential occurs, the axonal membrane
is at its normal resting potential, and Na+
channels are in their deactivated state, blocked
on the extracellular side by their activation gates.
In response to an electrical current (in this case,
an action potential), the activation gates open,
allowing positively-charged Na+ ions to flow into
the neuron through the channels, and causing the
voltage across the neuronal membrane to
increase. Because the voltage across the
membrane is initially negative, as its voltage
increases to and past zero, it is said to
depolarize.
 This increase in voltage constitutes the rising
phase of an action potential.
Nervous System Part II: How a Neuron Works Teacher Pages needs coding
Key
Na+
K+
Membrane potential
(mV)
+50
0
−50
14
2
Depolarization
OUTSIDE OF CELL
1
−100
Sodium
channel
Threshold
2
Potassium
channel
Resting potential
Time
INSIDE OF CELL
Inactivation loop
1
Resting state
• When an action potential is generated
2. Voltage-gated Na+ channels open first and
Na+ flows into the cell
3. During the rising phase, the threshold is
crossed, and the membrane potential
increases to and past zero
4. During the falling phase, voltage-gated
Na+ channels become inactivated; voltagegated K+ channels open, and K+ flows out
of the cell
15
 Voltage-gated Na+ channels can exist in any
of three distinct states: deactivated (closed),
activated (open), or inactivated (closed). Before
an action potential occurs, the axonal membrane
is at its normal resting potential, and Na+
channels are in their deactivated state, blocked
on the extracellular side by their activation gates.
In response to an electrical current (in this case,
an action potential), the activation gates open,
allowing positively-charged Na+ ions to flow into
the neuron through the channels, and causing the
voltage across the neuronal membrane to
increase. Because the voltage across the
membrane is initially negative, as its voltage
increases to and past zero, it is said to
depolarize.
 This increase in voltage constitutes the rising
phase of an action potential.
Key
Na+
K+
+50
Rising phase of the action potential
Membrane potential
(mV)
3
16
−50
2
Depolarization
OUTSIDE OF CELL
INSIDE OF CELL
Inactivation loop
1
Resting state
−100
Sodium
channel
Potassium
channel
Action
potential
3
0
Threshold
2
1
Resting potential
Time
Nervous System Part II: How a Neuron Works Teacher Pages needs coding
Key
Na+
K+
3
+50
Membrane potential
(mV)
Rising phase of the action potential
2
Depolarization
OUTSIDE OF CELL
3
Threshold
2
4
1
−100
Sodium
channel
Falling phase of the action potential
0
−50
17
4
Action
potential
Potassium
channel
Resting potential
Time
With its inactivation gate closed, the channel is
said to be inactivated. With the Na+ channel no
longer contributing to the membrane potential,
the potential decreases back to its resting
potential as the neuron depolarizes and
subsequently hyperpolarizes itself. This
decrease in voltage constitutes the falling
phase of the action potential.
INSIDE OF CELL
Inactivation loop
1
 At the peak of the action potential, when
enough Na+ has entered the neuron and the
membrane's potential has become high
enough, the Na+ channels inactivate
themselves by closing their inactivation gates.
Closure of the inactivation gate causes
Na+ flow through the channel to stop, which in
turn causes the membrane potential to stop
rising.
Resting state
5. During the undershoot, membrane permeability to
K+ is at first higher than at rest, then voltage-gated
K+ channels close and resting potential is restored
18
Key
Na+
K+
3
+50
Membrane potential
(mV)
Rising phase of the action potential
2
Depolarization
OUTSIDE OF CELL
−100
Sodium
channel
Potassium
channel
Falling phase of the action potential
3
0
−50
19
4
Action
potential
Threshold
2
4
1
5
1
Resting potential
Time
INSIDE OF CELL
Inactivation loop
1
5
Resting state
Undershoot
Figure 48.11a
+50
20
Membrane potential
(mV)
Action
potential
3
0
2
−50
4
Threshold
1
Resting potential
−100
Time
5
1
 The raised voltage opened many more
potassium channels than usual, and some of
these do not close right away when the
membrane returns to its normal resting
voltage. Hence, there is an undershoot or
hyperpolarization that persists until the
membrane potassium permeability returns to
its usual value.
Nervous System Part II: How a Neuron Works Teacher Pages needs coding
Refractory Period
21
• During the
refractory period
after an action
potential, a second
action potential
cannot be initiated
• The refractory
period is a result
of a temporary
inactivation of the
Na+ channels
The absolute refractory period is considered to
be a time during which no stimulus, no matter
how great, can cause an action potential to be
generated. This occurs after each nerve
depolarization/action potential generation
process. A very crude example (thus use with
caution!) for explaining the refractory period to
students is to compare it to flushing a toilet. One
must wait for the tank to fill before one may reflush a toilet no matter how impatient you are!
(Apologies.)
Conduction of Action Potentials
22
23
• At the site where the
action potential is
generated, usually the
axon hillock, an electrical
current depolarizes the
neighboring region of the
axon membrane
• Action potentials travel in
only one direction:
toward the synaptic
terminals
• Inactivated Na+
channels behind
the zone of
depolarization
prevent the
action potential
from traveling
backwards
Axon
Action
potential
1
24
Na+
Plasma
membrane
Cytosol
Figure 48.12 Conduction of an action potential.
Nervous System Part II: How a Neuron Works Teacher Pages needs coding
Axon
Plasma
membrane
Action
potential
1
K+
2
25
Cytosol
Na+
Figure 48.12 Conduction of an action potential.
Action
potential
Na+
K+
Axon
Plasma
membrane
Action
potential
1
K+
2
26
Cytosol
Na+
Figure 48.12 Conduction of an action potential.
Action
potential
Na+
K+
K+
Action
potential
3
Na+
K+
Sequence the following in order of occurren
• Depolarization
• Resting state
• Repolarization
• Hyperpolarization
27
Ask them to sequence these events (in their head
or on paper)
Sequenced in order of occurrence
• Resting state
• Depolarization
• Hyperpolarization
• Repolarization
• Resting state
28
Once you show the sequence then ask students to
describe the events using the graphic on the next
/
+50
29
Membrane potential
(mV)
?
• Resting
state 3
0
• Depolarization
2
• Hyperpolarization
−50
?
• Repolarization
1
• Resting state
?
−100
Time
4
5
1
What is represented by each question mark?
Describe what is occurring at 1,2,3,4,5 using the
words: Resting state, depolarization,
repolarization, and hyperpolarization
Nervous System Part II: How a Neuron Works Teacher Pages needs coding
30
A(n) ___ in Na+ permeability and/or
a(n) ___ in K+ permeability across a
neuron’s plasma membrane could shif
membrane potential from −70 mV to
−80 mV.
Answer: c
a. increase; increase
b. increase; decrease
c. decrease; increase
d. decrease; decrease
Adding a poison that specifically disables the Na+/K+ pum
to a culture of neurons will cause
31
32
a. the resting membrane potential to drop to 0 mV.
b. the inside of the neuron to become more negative
relative to the outside.
c. the inside of the neuron to become positively
charged relative to the outside.
d. sodium to diffuse out of the cell and potassium to
diffuse into the cell.
Answer: a
Name three specific
adaptions of the neuron
+
membrane that allow it to Responses should include voltage-gated K
specialize in conduction channels, sodium/potassium pumps, voltagegated Na+ channels.
32
Evolutionary Adaptations
of Axon Structure
33
• The speed of an action potential increases
with the axon’s diameter
• In vertebrates, axons are insulated by a
myelin sheath, which causes an action
potential’s speed to increase
• Myelin sheaths are made by glia—
oligodendrocytes in the CNS and Schwann
cells in the PNS
Emphasize that students need to be able to write
about these evolutionary adaptations when
answering free-response questions.
Nervous System Part II: How a Neuron Works Teacher Pages needs coding
Node of Ranvier
Layers of myelin
Axon
Schwann
cell
Axon
Myelin sheath
Nodes of
Ranvier
Schwann
cell
Nucleus of
Schwann cell
Figure 48.13 Schwann cells and the myelin
sheath. Point out the bottom right photograph is
a cross section of the Schwann cell.
34
0.1 µm
35
Can you explain why impulses travel faster in
myelinated sheaths?
36
Next time we will explore what
happens when the impulse
reaches the end of the axon.
36
Created by:
37
Debra Richards
Coordinator of Secondary Science Programs
Bryan ISD
Bryan, TX