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The Brain
• Overview: Command and Control Center
• The human brain contains an estimated 100
billion nerve cells, or neurons
• Each neuron may communicate with thousands
of other neurons
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• Functional magnetic resonance imaging is a
technology that can reconstruct a threedimensional map of brain activity
Figure 48.1
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• The results of brain imaging and other research
methods reveal that groups of neurons function
in specialized circuits dedicated to different
tasks
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Nervous systems
• Nervous systems consist of circuits of neurons
and supporting cells
• All animals except sponges have some type of
nervous system
• What distinguishes the nervous systems of
different animal groups is how the neurons are
organized into circuits
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Organization of Nervous Systems
• The simplest animals with nervous systems,
the cnidarians
– Have neurons arranged in nerve nets
Nerve net
(a) Hydra (cnidarian)
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• Sea stars have a nerve net in each arm
– Connected by radial nerves to a central nerve
ring
Radial
nerve
Nerve
ring
(b) Sea star (echinoderm)
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• In relatively simple cephalized animals, such as
flatworms
– A central nervous system (CNS) is evident
Eyespot
Brain
Nerve
cord
Transverse
nerve
(c) Planarian (flatworm)
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• Annelids (worms) and arthropods (crustacenas, insects)h
ave segmentally arranged clusters of neurons called
ganglia
• These ganglia connect to the CNS and make up a
peripheral nervous system (PNS)
Brain
Brain
Ventral
nerve cord
Ventral
nerve
cord
Segmental
ganglia
Segmental
ganglion
(d) Leech (annelid)
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(e) Insect (arthropod)
• Nervous systems in molluscs correlate with the animals’
lifestyles.
• Sessile molluscs have simple systems whereas more
active, predatory molluscs have more sophisticated
systems.
Anterior
nerve ring
Ganglia
Brain
Longitudinal
nerve cords
Ganglia
(f) Chiton (mollusc)
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(g) Squid (mollusc)
• In vertebrates the central nervous system
consists of a brain and dorsal spinal cord
• The PNS connects to the CNS
Brain
Spinal
cord
(dorsal
nerve
cord)
Figure 48.2h
Sensory
ganglion
(h) Salamander (chordate)
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Information Processing
• Nervous systems process information in three
stages
– Sensory input, integration, and motor output
Sensory input
Integration
Sensor
Motor output
Effector
Figure 48.3
Peripheral nervous
system (PNS)
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Central nervous
system (CNS)
• Sensory neurons transmit information from
sensors that detect external stimuli and internal
conditions
• Sensory information is sent to the CNS where
interneurons integrate the information
• Motor output leaves the CNS via motor
neurons which communicate with effector cells
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• The three stages of information processing
– Are illustrated in the knee-jerk reflex
2 Sensors detect 3 Sensory neurons
4 The sensory neurons communicate with
motor neurons that supply the quadriceps. The
a sudden stretch in convey the information
to
the
spinal
cord.
motor neurons convey signals to the quadriceps,
the quadriceps.
Cell body of causing it to contract and jerking the lower leg forward.
sensory neuron
Gray matter
in dorsal
5 Sensory neurons
root ganglion
from
the quadriceps
Quadriceps
also communicate
muscle
White
with interneurons
matter
in the spinal cord.
Hamstring
muscle
Spinal cord
(cross section)
The
1 reflex is
initiated by tapping
the tendon connected
to the quadriceps
Figure 48.4 (extensor) muscle.
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Sensory neuron
Motor neuron
Interneuron
The interneurons
6
inhibit motor neurons
that supply the
hamstring (flexor)
muscle. This inhibition
prevents the hamstring
from contracting,
which would resist
the action of
the quadriceps.
Neuron Structure
• Most of a neuron’s organelles are located in the cell body.
Neurons have dendrites which are highly branched
extensions that receive signals from other neurons.
Dendrites
Cell body
Nucleus
Synapse
Signal
Axon direction
Axon hillock
Presynaptic cell
Postsynaptic cell
Myelin sheath
Figure 48.5
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Synaptic
terminals
• The axon is a long extension of a dendrite that
transmits signals to other cells at synapses.
• The axon may be covered with a myelin
sheath, which facilitates signal transmission.
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Supporting Cells (Glia)
• Glia are supporting cells that are essential for
the structural integrity of the nervous system
and for the normal functioning of neurons.
• In the CNS, astrocytes provide structural
support for neurons and regulate the
extracellular concentrations of ions and
neurotransmitters
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50 µm
• Astrocytes
Figure 48.7
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• Oligodendrocytes (in the CNS) and Schwann
cells (in the PNS) are glia that form the myelin
sheaths around the axons of many vertebrate
neurons
Node of Ranvier
Layers of myelin
Axon
Schwann
cell
Axon
Myelin sheath
Nodes of
Ranvier
Schwann
cell
Nucleus of
Schwann cell
Figure 48.8
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0.1 µm
• The nervous system transmits information
using a combination of electrical and chemical
signals.
• Signals are transmitted along neurons
electrically and signals are transmitted between
neurons at synapses chemically.
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How a neuron delivers a signal
Dendrites
Cell Body
Axon
Electrical
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Synapse
Chemical
Membrane potentials
• Ion pumps and ion channels maintain the
resting potential of a neuron
• Across its plasma membrane, every cell has a
voltage called a membrane potential
• The inside of a cell is negative relative to the
outside
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Basic Electrical Concepts
• Like charges repel one another.
• Opposite charges attract one another.
• Because of the attractive force between positive and
negative charges, energy must be expended and work
must be done to separate them.
• Conversely, if positive and negative charges are allowed to
come together, energy is liberated, and this energy can be
used to perform work.
• Thus, when positive and negative charges are separated,
they have the potential to perform work.
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Basic Electrical Concepts
• Potential is measured as voltage (1mV = 0.001V).
• Voltage is measured between two points (potential).
• Current is measured as the amount of charge moving
between two points.
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• The membrane potential of a cell can be
measured
APPLICATION Electrophysiologists use intracellular recording to measure the membrane potential of
neurons and other cells.
TECHNIQUE
A microelectrode is made from a glass capillary tube filled with an electrically conductive
salt solution. One end of the tube tapers to an extremely fine tip (diameter < 1 µm). While looking through a
microscope, the experimenter uses a micropositioner to insert the tip of the microelectrode into a cell. A
voltage recorder (usually an oscilloscope or a computer-based system) measures the voltage between the
microelectrode tip inside the cell and a reference electrode placed in the solution outside the cell.
Microelectrode
–70 mV
Voltage
recorder
Figure 48.9
Reference
electrode
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The Resting Potential
• The resting potential is the membrane
potential of a neuron that is not transmitting
signals.
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• In all neurons, the resting potential depends on
the ionic gradients that exist across the plasma
membrane
EXTRACELLULAR
FLUID
CYTOSOL
[Na+]
15 mM
–
+
[Na+]
150 mM
[K+]
150 mM
–
+
[K+]
5 mM
–
+
[Cl–]
10 mM
–
[Cl–]
+ 120 mM
[A–]
100 mM
–
+
Plasma
membrane
Figure 48.10
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Understanding resting potential
• The concentration of Na+ is higher in the
extracellular fluid than inside the cell, while the
opposite is true for K+.
• The inside of the cell contains negatively
charged molecules such as proteins and amino
acids and negatively charged ions such as
phosphate and sulfate.
• The cell membrane has different permeabilities
for different ions and permeability for K+ is
higher than for Na+.
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• Because K+ ions are much more concentrated
inside the nerve cell than outside, they diffuse
out of the cell. Negatively charged ions cannot
follow as there are no channels for them to flow
through.
• The loss of positively charged K+ ions means
there is net negative charge inside the cell.
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• The net flow of K+ ions will continue and the
negative charge will increase until the
difference in charge between the inside and
outside of the cell (which attracts K+ ions back
into the cell) balances the effect of the
concentration gradient for K+, which is causing
K+ ions to flow out.
• If it was only the flow of K+ ions that
established the cell’s resting potential then a
resting potential of -85mV would be
established.
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• However, there is a trickle of Na+ ions that
flows into the cell down its concentration
gradient in the opposite direction to the flow of
K+ ions.
• This flow of Na+ is counteracted by active
pumping of Na+ out of the cell, but even so the
resting potential of a neuron is typically
apprximately -70mV, rather than -85mV.
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Excitable cells
• All cells have a membrane potential, but only
certain cells such as neurons and muscle cells
can change their membrane potentials in
response to a stimulus.
• Such cells are called excitable cells.
• Special ion channels called gated channels
allow neurons to change their membrane
potential.
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Gated Ion Channels
• Gated ion channels open or close in direct
response to membrane stretch or the binding of
a specific ligand or in response to a change in
the membrane potential.
• How the resting potential changes, depends on
the gated channels that open.
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Gated Ion Channels
• Opening K+ channels will increase the flow of
K+ out of the cell and hyperpolarize the cell
making the potential more negative.
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Hyperpolarization
Stimuli
Membrane potential (mV)
+50
0
–50
Threshold
Resting
potential
Hyperpolarizations
–100
0 1 2 3 4 5
Time (msec)
Figure 48.12a
(a) Graded hyperpolarizations
produced by two stimuli that
increase membrane permeability
to K+. The larger stimulus produces
a larger hyperpolarization.
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Gated Ion Channels
• Opening Na+ channels, in contrast, will allow
Na+ to flow into the cell and will depolarize the
cell reducing the negative charge and even
making it positive.
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Depolarization
Stimuli
Membrane potential (mV)
+50
0
–50
Threshold
Resting
potential
Depolarizations
–100
0 1 2 3 4 5
Time (msec)
Figure 48.12b
(b) Graded depolarizations produced
by two stimuli that increase
membrane permeability to Na+.
The larger stimulus produces a
larger depolarization.
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• Hyperpolarization and depolarization are both
called graded potentials because the
magnitude of the change in membrane
potential varies with the strength of the
stimulus
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Production of Action Potentials
• In most neurons, however, depolarizations are
graded only up to a certain membrane voltage,
called the threshold.
• A stimulus strong enough to produce a
depolarization that reaches the threshold
triggers a different type of response, called an
action potential.
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Stronger depolarizing stimulus
+50
Membrane potential (mV)
Action
potential
0
–50
Threshold
Resting
potential
–100
0 1 2 3 4 5 6
Time (msec)
Figure 48.12c
(c) Action potential triggered by a
depolarization that reaches the
threshold.
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Action potential
• An action potential is a brief all-or-none
depolarization of a neuron’s plasma membrane
• It is the type of signal that carries information
along axons.
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Production of an action potential
• Both voltage-gated Na+ channels and voltagegated K+ channels are involved in the
production of an action potential
• When a stimulus depolarizes the membrane
Na+ channels open, allowing Na+ to diffuse into
the cell
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• After the Na+ have opened they close and K+
channels open.
• K+ flows out of the cell rapidly reversing the
polarity of the cell and briefly undershooting the
cell’s resting potential briefly before the resting
potential is restored.
• During the undershoot a second action
potential cannot be initiated and this time when
the cell is insensitive to stimulation is called the
refractory period.
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Conduction of Action Potentials
• An action potential can be used to transmit a
signal because the action potential can “travel”
long distances by regenerating itself along the
length of the axon.
• At the site where the action potential is
generated, the electrical current depolarizes
the neighboring region of the axon membrane.
• Rather like tipping one in a line of standing
dominoes the effect of an action potential is
transmitted along the length of an axon.
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Axon
Action
potential
–
–
+
+
+
+
+
+
+
–
–
–
–
–
–
+
+
–
–
–
–
–
–
–
–
+
+
+
+
+
+
+
Na+
Action
potential
K+
+
+
–
–
–
+
2
–
–
+
+
+
+
+
+
–
–
–
–
–
–
–
–
+
+
+
+
Na+
–
+
+
+
–
–
1
K+
Action
potential
K+
Figure 48.14
3
–
–
–
+
+
+
+
+
+
+
–
–
–
–
+
+
+
+
–
–
–
–
–
+
–
–
–
–
+
+
+
+
Na+
K+
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An action potential is generated
as Na+ flows inward across the
membrane at one location.
The depolarization of the action
potential spreads to the neighboring
region of the membrane, re-initiating
the action potential there. To the left
of this region, the membrane is
repolarizing as K+ flows outward.
The depolarization-repolarization process is
repeated in the next region of the
membrane. In this way, local currents
of ions across the plasma membrane
cause the action potential to be propagated
along the length of the axon.
Direction of transmission
• The direction of flow is one directional because
of the fact that there is a refractory period after
a section of membrane depolarizes when it
cannot be restimulated.
• This prevents the signal being sent back in the
opposite direction along the axon.
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Conduction Speed
• In many instances it is important that action
potentials be transmitted quickly.
• The speed of transmission of an axon potential
increases with the diameter of an axon
because thicker axons offer proportionally less
resistance to the flow of current.
• Thus, many invertebrates (including squid,
lobsters and cockroaches) have evolved giant
axons that enable them to react quickly to
stimuli.
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• Vertebrates, however, have come up with a
different solution to increasing the speed of
transmission.
• In vertebrates axons are myelinated, insulated
with a layer of membranes deposited by glial or
Schwann cells.
• Myelin is a poor conductor and prevents the
electrical signal being dissipated outside the
neuron so it is more effectively and quickly
transmitted along it.
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Multiple Sclerosis
• People who suffer from multiple sclerosis have
neurons in which the myelin sheaths gradually
deteriorate.
• This disrupts nerve signal transmission and
leads to progressive loss of body function.
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• Neurons have myelinated and unmyelinated
sections
• Gated ion channels are concentrated in the
unmyelinated sections, called the nodes of
Ranvier.
• Action potentials jump between unmyelinated
sections of neuron in a process called saltatory
conduction with depolarization skipping the
myelinated sections in between, which speeds
up transmission.
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Schwann cell
Depolarized region
(node of Ranvier)
Myelin
sheath
––
–
Cell body
+
++
+
++
–––
––
–
+
+
Axon
+
++
––
–
Figure 48.15
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Synaptic transmission
• Neurons communicate with other cells at
synapses, which are junctions between
neurons or between neurons and sensory
receptors or muscle cells.
• The transmitting cell is the presynaptic cell and
the receiving cell is the postsynaptic cell.
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• In an electrical synapse electrical ion currents
flows directly from one cell to another via gap
junctions.
• However, the vast majority of synapses are
chemical synapses.
• In a chemical synapse the electrical signal is
converted into a chemical signal that travels
across the synapse and is converted back into
an electrical signal in the postsynaptic cell.
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• In a chemical synapse, a presynaptic neuron when
stimulated by an action potential releases chemical
neurotransmitters, which are stored in the synaptic
terminal.
Postsynaptic
neuron
5 µm
Synaptic
terminal
of presynaptic
neurons
Figure 48.16
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• When an action potential reaches a synaptic
terminal it depolarizes the membrane and triggers
an influx of Ca++ ions.
• The influx of Ca++ ions causes neurotransmitter
molecules to be released into the synaptic cleft.
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Postsynaptic cell
Presynaptic
cell
Synaptic vesicles
containing
neurotransmitter
5
Presynaptic
membrane
Na+
K+
Neurotransmitter
Postsynaptic
membrane
Ligandgated
ion channel
Voltage-gated
Ca2+ channel
1 Ca2+
4
2
Synaptic cleft
Figure 48.17
3
Ligand-gated
ion channels
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Postsynaptic
membrane
6
A Chemical Synapse (an example)
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Direct Synaptic Transmission
• The neurotransmitter molecules binds to ligandgated ion channels and the binding causes the ion
channels to open, generating a postsynaptic
potential.
• Neurotransmitter molecules are quickly removed
or broken down to terminate the synaptic
response.
• Postsynaptic potentials fall into two categories
– Excitatory postsynaptic potentials (EPSPs)
– Inhibitory postsynaptic potentials (IPSPs)
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Summation of Postsynaptic Potentials
• Unlike action potentials postsynaptic potentials
are graded because they are influenced by the
amount of neurotransmitter released and do
not regenerate themselves.
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• Since most neurons have many synapses on
their dendrites and cell body a single EPSP is
usually too small to trigger an action potential
in a postsynaptic neuron
Terminal branch of
presynaptic neuron
Membrane potential (mV)
Postsynaptic
E1
neuron
0
Threshold of axon of
postsynaptic neuron
Resting
potential
–70
Figure 48.18a
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E1
E1
(a) Subthreshold, no
summation
• If two EPSPs are produced in rapid succession an
effect called temporal summation occurs in which
the effects added together produce an action
potential.
E
1
Axon
hillock
Action
potential
E1 E1
(b) Temporal summation
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• In spatial summation EPSPs produced nearly
simultaneously by different synapses on the
same postsynaptic neuron add together
E1
E2
Action
potential
E1 + E2
(c) Spatial summation
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• Through summation an IPSP can counter the
effect of an EPSP
E1
I
E1
Figure 48.18d
I
E1 + I
(d) Spatial summation
of EPSP and IPSP
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• The input from inhibitory and excitatory is
integrated in a part of the neuron called the
axon hillock.
• Acting as the neuron’s integrating center the
axon hillock weighs the inputs and if they are
sufficient to reach the threshold an action
potential is generated and travels along the
axon.
• If the threshold is not reached, no action
potential is produced.
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Neurotransmitters
• There are a large number of different
neurotransmitters and same neurotransmitter
can produce different effects in different types
of cells.
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Table 48.1
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Acetylcholine
• Acetylcholine
– Is one of the most common neurotransmitters
in both vertebrates and invertebrates
– In the CNS acetylcholine can be inhibitory or
excitatory.
– Acetylcholine is also released at synapses
between nerves and muscle cells and induces
muscle contraction.
• Nicotine’s physiological and psychological effects
are caused by its binding to acetylcholine
receptors
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Biogenic Amines
• Biogenic amines are derived from amino acids
and include epinephrine, norepinephrine,
dopamine, and serotonin. They are active in
both the CNS and PNS.
• Dopamine and serotonin affect sleep, mood,
attention and learning.
• Parkinson’s Disease is associated with a lack
of dopamine production.
• Prozac enhances the effect of serotonin by
slowing its uptake after release.
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Amino Acids and Peptides
• Various amino acids and peptides are active as
neurotransmitters in the brain.
• These include the neuropeptides called
endorphins, which act as natural analgesics
reducing the perception of pain.
• Opiates such as heroin and morphine bind to
endorphin receptors in the brain.
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• Concept 48.5: The vertebrate nervous system
is regionally specialized
• In all vertebrates, the nervous system shows a
high degree of cephalization and distinct CNS
and PNS components
Central nervous
system (CNS)
Brain
Spinal cord
Peripheral nervous
system (PNS)
Cranial
nerves
Ganglia
outside
CNS
Spinal
nerves
Figure 48.19
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• The brain provides the integrative power that
underlies the complex behavior of vertebrates
• The spinal cord integrates simple responses to
certain kinds of stimuli and conveys information
to and from the brain
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The Peripheral Nervous System
• The PNS transmits information to and from the
CNS and plays a large role in regulating a
vertebrate’s movement and internal
environment
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• The PNS can be divided into two functional
components
– The somatic nervous system and the
autonomic nervous system
Peripheral
nervous system
Somatic
nervous
system
Autonomic
nervous
system
Sympathetic
division
Figure 48.21
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Parasympathetic
division
Enteric
division
• The somatic nervous system carries signals to
skeletal muscles
• The autonomic nervous system regulates the
internal environment, in an involuntary manner
• The autonomic nervous system is divided into
the sympathetic, parasympathetic, and enteric
divisions.
• The sympathetic and parasympathetic divisions
have antagonistic effects on target organs.
–
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• The sympathetic division correlates with the
“fight-or-flight” response: arousal and energy
generation. Heart beats faster, bronchi dilate,
digestion is inhibited, epinephrine (adrenalin) is
released.
• The parasympathetic division promotes a
return to self-maintenance functions. Promotes
calming.
• The enteric division controls the activity of the
digestive tract, pancreas, and gallbladder
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Parasympathetic division
Sympathetic division
Action on target organs:
Constricts pupil
Location of
of eye
preganglionic neurons:
brainstem and sacral
Stimulates salivary
segments of spinal cord
gland secretion
Neurotransmitter
Constricts
released by
bronchi in lungs
preganglionic neurons:
acetylcholine
Slows heart
Cervical
Location of
Stimulates activity
postganglionic neurons: of stomach and
in ganglia close to or
intestines
within target organs
Stimulates activity
of pancreas
Thoracic
Stimulates
Neurotransmitter
gallbladder
released by
postganglionic neurons:
acetylcholine
Promotes emptying
Lumbar
Action on target organs:
Dilates pupil
Location of
of eye
preganglionic neurons:
Inhibits salivary
thoracic and lumbar
gland secretion
segments of spinal cord
Sympathetic
Relaxes bronchi
ganglia
Neurotransmitter
in lungs
released by
preganglionic neurons:
Accelerates heart
acetylcholine
Inhibits activity of
stomach and intestines Location of
postganglionic neurons:
Inhibits activity
some in ganglia close to
of pancreas
target organs; others in
a chain of ganglia near
Stimulates glucose
spinal cord
release from liver;
inhibits gallbladder
Neurotransmitter
Stimulates
released by
adrenal medulla
postganglionic neurons:
norepinephrine
Inhibits emptying
of bladder
of bladder
Figure 48.22
Promotes erection
of genitalia
Synapse
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Sacral
Promotes ejaculation and
vaginal contractions