UNIT B: Human Body Systems
Chapter 8: Human Organization
Chapter 9: Digestive System
Chapter 10: Circulatory System and
Lymphatic System
Chapter 11: Respiratory System
Chapter 12: Nervous System:
Section 12.2
Chapter 13: Urinary System
Chapter 14: Reproductive System
UNIT B Chapter 12: Nervous System
Chapter 12: Nervous System
In this chapter, you will learn about the
structure and function of the nervous
system.
How might a researcher study the
effects of frequent head trauma?
Sport-Related Head Trauma and Brain
Function. Neurosurgeon Dr. Robert Cantu has
studied the brains of many deceased athletes,
including hockey and football players. He has
found that these players often suffered from
chronic traumatic encephalopathy (CTE), a
degenerative brain disease caused by repeated
blunt impact to the head.
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How might one determine which part
of the brain has been affected by
repeated blunt impacts?
Given the available information about
CTE, what steps do you feel should be
taken to prevent its occurrence (if any)?
UNIT B Chapter 12: Nervous System
Section 12.2
12.2 Transmission of Nerve Impulses
The nervous system uses the nerve impulse to convey
information.
The nerve impulse can be studied using excised axons and a
voltmeter called an oscilloscope.
• Voltage: measured in millivolts (mV); a measure of the
electrical potential difference between two points
• In a neuron, the two points are in the inside (axoplasm) and
the outside of the axons
• On a voltmeter, voltage is displayed as a trace (pattern) over
time
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UNIT B Chapter 12: Nervous System
Section 12.2
Resting Potential
In an axon that is not conducting an impulse, the voltmeter
records a potential difference across an axon membrane
equal to -70mV.
• This reading, known as the resting potential, shows
that the inside of the axon is negative compared to the
outside (there is polarity across the axonal membrane)
• The resting potential is the potential difference across
the membrane in a resting neuron
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UNIT B Chapter 12: Nervous System
Section 12.2
The polarity of the resting axonal membrane is due to a difference in
ion distribution on each side.
• The concentration of Na+ is greater outside the axon than inside
• The concentration of K+ is greater inside the axon than outside
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Figure 12.4 Action Potential.
UNIT B Chapter 12: Nervous System
Section 12.2
• This unequal distribution is maintained by carrier proteins
called sodium-potassium pumps, which actively transport
Na+ out of the axon and K+ into the axon
o The pumps are always working because the membrane
is permeable to Na+ and K+
o The membrane is more permeable to K+, therefore there
are always more positive ions outside the membrane
than inside
o Negatively charged organic ions on the inside of the
axon also contribute to the polarity across a resting
axonal membrane
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UNIT B Chapter 12: Nervous System
Action Potential
An action potential is a rapid change in polarity across an
axonal membrane as the nerve impulse occurs.
• An all-or-none phenomenon: if a stimulus causes the
membrane to depolarize to a certain level (threshold), an
action potential occurs
• The strength of an action potential does not change, but an
intense stimulus can cause an axon to fire (start an action
potential) more often
• Requires two gated channel proteins in the membrane:
o One channel protein allows Na+ to pass into the axon
o One channel protein allows K+ to pass out of the axon
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Section 12.2
UNIT B Chapter 12: Nervous System
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Figure 12.4 Action Potential. c. The changes in the transmembrane
potential of the axon are a result of sodium ions flowing into the axon and
potassium ions flowing out. An action potential lasts only a few seconds.
Section 12.2
UNIT B Chapter 12: Nervous System
Action Potential: Sequence of Events
Sodium Gates Open
(Depolarization)
• When an action potential begins,
sodium channel gates open, and
Na+ flows down its concentration
gradient into the axon
• As Na+ moves inside the axon, the
membrane potential changes from
-70 mV to +35 mV
• This is called depolarization
because the charge inside the axon
changes from negative to positive
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Section 12.2
UNIT B Chapter 12: Nervous System
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Section 12.2
Figure 12.4 Action Potential. a. The action potential begins as the sodium gates
(purple) open and Na+ ions move into the axon through facilitated diffusion. This is
depolarization as the membrane potential jumps from −70 to +35 millivolts.
UNIT B Chapter 12: Nervous System
Action Potential: Sequence of Events
Potassium Gates Open
(Repolarization)
• The potassium channel gates open,
and K+ flows down its
concentration gradient out of the
axon
• As K+ flows out of the axon, the
action potential becomes more
negative again (repolarization)
o During this time, it briefly
becomes slightly more negative
that its original resting potential
(hyperpolarization)
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Section 12.2
UNIT B Chapter 12: Nervous System
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Figure 12.4 Action Potential. b. The repolarization of a neuron occurs as the
potassium gates (orange) open and K+ ions move out of the axon through
facilitated diffusion.
Section 12.2
UNIT B Chapter 12: Nervous System
Section 12.2
Conduction of an Action Potential
Action potentials in nonmyelinated axons
• The action potential travels down an axon one small section
at a time
• When an action potential has moved on, the previous
section undergoes a refractory period, during which the
sodium gates are unable to open
• The action potential cannot move backward; it always
moves down an axon
• When the refractory period is over, the sodium-potassium
pump has restored the ion distribution by pumping Na+ out
of the axon and K+ into the axon
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UNIT B Chapter 12: Nervous System
Section 12.2
Action potentials in myelinated axons
• The gated ion channels that produce an action potential are
concentrated at the nodes of Ranvier
• Ion exchange only occurs at these nodes, therefore the
action potential travels faster than in nonmyelinated axons
• The action potential “jumps” from node to node (saltatory
conduction)
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UNIT B Chapter 12: Nervous System
Section 12.2
Transmission Across a Synapse
Every axon branches into endings that have a small swelling called
an axon terminal
• Each terminal lies close to the dendrite or cell body of another
neuron or a muscle cell
• This region of close proximity is called a synapse or chemical
synapse
o Membrane of the first neuron: presynaptic membrane
o Membrane of the second neuron: postsynaptic membrane
• Two neurons at a synapse do not physically touch each other;
they are separated by a tiny gap called the synaptic cleft
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UNIT B Chapter 12: Nervous System
Section 12.2
An action potential cannot cross a synapse.
• Communication between two neurons at a chemical synapse is
carried out by neurotransmitters (chemicals stored in the
synaptic vesicles in axon terminals)
Figure 12.5 Structure
and function of a
synapse. Transmission
across a synapse from
one neuron to another
occurs when an action
potential causes a
neurotransmitter to be
released at the
presynaptic membrane.
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UNIT B Chapter 12: Nervous System
When an action potential arrives
at an axon terminal:
• Gated channels for Ca2+ open,
and Ca2+ enters the terminal
• Ca2+ interacts with contractile
proteins, which contract and
pull the synaptic vesicles to
the presynaptic membrane
• Rise in Ca2+ stimulates
synaptic vesicles to merge
with the presynaptic
membrane, resulting in
exocytosis
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Section 12.2
UNIT B Chapter 12: Nervous System
• Neurotransmitter
molecules are released
into the synaptic cleft
and diffuse across the
synapse to the
postsynaptic membrane,
where they bind to
specific receptors
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Section 12.2
UNIT B Chapter 12: Nervous System
• Depending on the
neurotransmitter, the
postsynaptic neuron
can either be excited
(causing an action
potential) or inhibited
(stopping an action
potential)
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Section 12.2
UNIT B Chapter 12: Nervous System
Section 12.2
Synaptic Integration
A neuron may receive many
excitatory and inhibitory signals
since its dendrites and cell body
can have synapses with many
other neurons.
• Excitatory signals: cause a
depolarizing effect
• Inhibitory signals: cause a
hyperpolarizing effect
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Figure 12.6 Synaptic integration.
UNIT B Chapter 12: Nervous System
Synaptic integration is the
summing up of the excitatory
and inhibitory signals in a
postsynaptic neuron.
• If the combined signals cause
the membrane potential to
rise above threshold, an
action potential will occur
Figure 12.6 Synaptic integration.
a. Inhibitory signals and excitatory signals
are summed up in the dendrites and cell
body of the postsynaptic neuron. Only if the
combined signals cause the membrane
potential to rise above threshold does an
action potential occur. b. In this example,
threshold was not reached.
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Section 12.2
UNIT B Chapter 12: Nervous System
Section 12.2
Neurotransmitters
Once a neurotransmitter has been released into a synaptic
cleft and has initiated a response, it is removed from the cleft.
• This prevents continuous stimulation (or inhibition) of
postsynaptic membranes
o In some synapses, the postsynaptic membrane contains
enzymes that break down the neurotransmitter
o In other synapses, the presynaptic membrane reabsorbs
the neurotransmitter for repackaging in synaptic
vesicles
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UNIT B Chapter 12: Nervous System
Neurotransmitters
Many drugs that affect the nervous system act by interfering or
enhancing the action of neurotransmitters. Drugs can:
• Enhance or block the release of neurotransmitter
• Mimic the neurotransmitter
• Block the receptor for the neurotransmitter
• Interfere with the removal of the neurotransmitter
Example:
• Sarin gas is a chemical weapon that inhibits
acetylcholinesterase (AChE), an enzyme that is responsible
for the breakdown of acetylcholine (ACh)
o Leads to prolonged ACh activity (convulsive spasms)
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Section 12.2
UNIT B Chapter 12: Nervous System
Section 12.2
Check Your Progress
1. Describe the activity of the sodium-potassium pump
present in neurons.
2. Explain how the changes in Na+ and K+ ion
concentrations that occur during an action potential are
associated with depolarization and repolarization.
3. Define refractory period, saltatory conduction, and
synaptic integration.
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UNIT B Chapter 12: Nervous System
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Section 12.2
UNIT B Chapter 12: Nervous System
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Section 12.2