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Chapter 38
The Nervous System
Lecture Outlines by Gregory Ahearn,
University of North Florida
Copyright © 2011 Pearson Education Inc.
Chapter 38 At a Glance
 38.1 What Are the Structures and Functions of
Nerve Cells?
 38.2 How Do Neurons Produce and Transmit
Information?
 38.3 How Do Nervous Systems Process
Information?
 38.4 How Are Nervous Systems Organized?
 38.5 What Are the Structures and Functions of
the Human Nervous System?
Biology: Life on Earth, 9e
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38.1 What Are the Structures and Functions of
Nerve Cells?
 The nervous system has two principal cell
types:
– Neurons, often called nerve cells, which receive,
process, and transmit information
– Glia, which assist neuronal function by:
–Providing nutrients
–Regulating the composition of the extracellular
fluid in the brain and spinal cord
–Modulating communication between neurons
–Speeding up the movement of electrical
signals within neurons
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38.1 What Are the Structures and Functions of
Nerve Cells?
 The functions of a neuron are localized in
separate parts of the cell
– A neuron must perform four functions:
–Receive information from the environment
–Process the information and produce electrical
signals
–Conduct electrical signals over distances to a
junction where it meets another cell
–Transmit information to other neurons,
muscles, or glands
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38.1 What Are the Structures and Functions of
Nerve Cells?
 The functions of a neuron are localized in
separate parts of the cell (continued)
– Typical neurons have four distinct parts that
carry out these four functions:
–Dendrites
–A cell body
–An axon
–Synaptic terminals
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38.1 What Are the Structures and Functions of
Nerve Cells?
 Dendrites respond to stimuli
– Dendrites are branched tendrils protruding from the cell
body that perform the “receive information” function
– Their branches provide a large surface area for
receiving signals, either from the environment or from
other neurons
– Dendrites of sensory neurons respond to specific
stimuli, such as pressure, odor, light, body
temperature, blood pH, or the position of a joint
– Dendrites of neurons in the brain and spinal cord
usually respond to chemicals, called
neurotransmitters, that are released by other
neurons
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38.1 What Are the Structures and Functions of
Nerve Cells?
 The cell body processes signals from the dendrites
– Electrical signals travel down the dendrites and converge
on the neuron’s cell body, which integrates incoming
information, performing the “process information” function
– If incoming signals are positive enough, a large, rapid
electrical signal called an action potential is
produced
– The cell body also contains other organelles such as the
nucleus, endoplasmic reticulum, and Golgi apparatus
that are typical of other cells, synthesizing complex
molecules and coordinating the cell’s metabolism
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38.1 What Are the Structures and Functions of
Nerve Cells?
 The axon conducts action potentials along long
distances
– In a typical neuron, a long, thin strand called an
axon extends outward from the cell body and
conducts action potentials from the cell body to
synaptic terminals at the axon’s end
–Single axons may stretch from our spinal cord
to our toes, a distance of about 3 feet
–Axons are typically bundled together into
nerves, much like wires are bundled within an
electrical cable
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38.1 What Are the Structures and Functions of
Nerve Cells?
 At synapses, signals are transmitted from one
cell to another
– The site where a neuron communicates with
another cell is called a synapse
– A typical synapse consists of:
–The synaptic terminal, which is a swelling at
the end of an axon of the “sending” neuron
–A dendrite or cell body of a “receiving” neuron,
muscle, or gland cell
–A small gap separating the two cells
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38.1 What Are the Structures and Functions of
Nerve Cells?
 At synapses, signals are transmitted from one
cell to another (continued)
– Most synaptic terminals contain
neurotransmitters that are released in response
to an action potential reaching the terminal
– The plasma membrane of the receiving neuron
bears receptors that bind the neurotransmitters
and stimulate a response in this cell
– Therefore, at a synapse, the output of the first
cell becomes the input to the second cell
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A Neuron
1 Synaptic terminals:
Transmit signals from
other neurons
2 Dendrites:
Receive signals
from other neurons
3 Cell body:
Integrates signals;
coordinates the
neuron’s metabolic
activities
neurotransmitters
dendrite synaptic
terminal
receptors
synapse
4 An action
potential starts here
5 Axon: Conducts
the action potential
6 Synaptic terminals:
Transmit signals to
other neurons
7 Dendrites
(of other neurons):
Receive signals
Fig. 38-1
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38.2 How Do Neurons Produce and Transmit
Information?
 Information is carried within a neuron by
electrical signals and is transmitted between
neurons by neurotransmitters released from one
neuron and received by a second neuron
– An unstimulated, inactive neuron maintains a
constant electrical voltage difference, or
potential, across its plasma membrane, called a
resting potential
–The voltage inside the cell is always negative
and ranges from about –40 to –90 millivolts
(mV)
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38.2 How Do Neurons Produce and Transmit
Information?
 If the membrane potential becomes less
negative, it reaches a level called threshold
and triggers an action potential
– During an action potential, the membrane
potential rises rapidly to +50 mV inside the cell,
then returns to resting potential
– The action potential signal flows down the axon
to the synaptic terminals with no change in
voltage from the cell body to the synaptic
terminals
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Electrical Events During an Action Potential
4
action potential
threshold
resting
potential
3
1
2
less
more negative
negative
time
(milliseconds)
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5
Fig. 38-2
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38.2 How Do Neurons Produce and Transmit
Information?
 Myelin speeds up the conduction of action potentials
– The thicker an axon, the faster the action potential moves
– In addition to variable thickness, neurons can increase
the rate of action potential conduction by covering
portions of the axon with a fatty insulation called myelin
– Myelin is formed by glial cells that wrap themselves
around the axon, leaving naked nodes inbetween the
segments of myelin
– In myelinated neurons, action potentials “jump” rapidly
from node to node, traveling at a rate of 10 to 330 feet
per second
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A Myelinated Axon
An action potential jumps
from node to node, greatly
speeding up conduction
down the axon
Schwann cell
node
myelin
myelin
sheath
axon
axon
Fig. 38-3
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38.2 How Do Neurons Produce and Transmit
Information?
 Neurons use chemicals to communicate with
one another at synapses
– Although there are electrical synapses where
electrical activity can pass directly from neuron to
neuron through gap junctions connecting the
insides of the cells, as occurs in heart tissue,
most synapses use chemicals to transmit
information from one neuron to another
– A synapse is where the synaptic terminal of one
neuron meets the dendrite of another
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38.2 How Do Neurons Produce and Transmit
Information?
 Neurons use chemicals to communicate with one
another at synapses (continued)
– The two neurons do not actually touch at a synapse
– A tiny gap, the synaptic cleft, separates the first, or
presynaptic neuron, from the second, or
postsynaptic neuron
– The presynaptic neuron sends neurotransmitter
chemicals across the gap to the postsynaptic neuron
– There are many types of neurotransmitters
– A synaptic terminal contains scores of vesicles, each
full of neurotransmitter molecules
– When an action potential is initiated, it travels down an
axon until it reaches its synaptic terminal
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Table 38-1
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38.2 How Do Neurons Produce and Transmit
Information?
 Neurons use chemicals to communicate with one
another at synapses (continued)
– The inside of the terminal becomes positively charged
and triggers a cascade of changes that cause some of
the vesicles to release neurotransmitters into the
synapatic cleft
– The outer surface of the plasma membrane of the
postsynaptic neuron is packed with receptor proteins that
are specialized to bind the neurotransmitter released by
the presynaptic neuron
– The neurotransmitter molecules diffuse across the gap
and bind to these receptors
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Author Animation: The Synapse
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The Structure and Function of the Synapse
1 An action potential
is initiated
2 The action potential
reaches the synaptic
terminal of the
presynaptic neuron
synaptic
vesicle
3 The positive charge of
the action potential causes
the synaptic vesicles to
release neurotransmitters
4 Neurotransmitters
bind to receptors on the
postsynaptic neuron
synaptic terminal
of presynaptic
neuron
dendrite of
postsynaptic
neuron
neurotransmitters
synaptic cleft
6 Neurotransmitters
are taken back into
the synaptic terminal,
are degraded, or
diffuse out of the
synaptic cleft
neurotransmitter
5 Neurotransmitter
binding causes ion
channels to open, and
ions flow in or out
receptor
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ions
Fig. 38-4
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38.2 How Do Neurons Produce and Transmit
Information?
 Synapses produce excitatory or inhibitory
postsynaptic potentials
– At most synapses, the binding of
neurotransmitter molecules to receptors on a
postsynaptic neuron opens ion channels in the
neuron’s plasma membrane
– Depending on which channels are associated
with the receptors, ions such as Na+, K+, Ca2+, or
Cl– may move through these channels causing a
small, brief change in voltage, called a
postsynaptic potential or PSP
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Author Animation: Electrical Signals in Neurons
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38.2 How Do Neurons Produce and Transmit
Information?
 Synapses produce excitatory or inhibitory postsynaptic
potentials (continued)
– If the postsynaptic neuron becomes more negative, its
resting potential moves farther away from threshold,
reducing the likelihood of firing an action potential
– This change in voltage is called an inhibitory
postsynaptic potential (IPSP)
– If the postsynaptic neuron becomes less negative, then
its resting potential will move closer to threshold, and it
will be more likely to fire an action potential
– This voltage change is called an excitatory
postsynaptic potential (EPSP)
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38.2 How Do Neurons Produce and Transmit
Information?
 Neurotransmitter action is usually brief
– Some neurotransmitters—like acetylcholine, the
transmitter that stimulates skeletal muscle
cells—are rapidly broken down by enzymes in
the synaptic cleft
– Many other neurotransmitters are transported
back into the presynaptic neuron
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38.2 How Do Neurons Produce and Transmit
Information?
 Summation of postsynaptic potentials determines the
activity of a neuron
– The dendrites and cell body of a single neuron often
receive EPSPs and IPSPs from the synaptic terminals of
thousands of presynaptic neurons
– The voltages of all the PSPs that reach the postsynaptic
cell body at about the same time are added up, a
process called integration
– If the excitatory and inhibitory postsynaptic potentials,
when added together, raise the electrical potential inside
the neuron above threshold, the postsynaptic cell will
produce an action potential
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38.3 How Do Nervous Systems Process
Information?
 A nervous system must be able to perform four
operations:
– Determine the type of stimulus
– Determine and signal the intensity of a stimulus
– Integrate information from many sources
– Initiate and direct appropriate responses
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38.3 How Do Nervous Systems Process
Information?
 The nature of a stimulus is determined by connections
between the senses and the brain
– All nervous systems interpret what a stimulus is by
monitoring which neurons are firing action potentials
– For example, the brain interprets action potentials that
occur in the axons of the eye and travel to the visual
areas of the brain as the sensation of light
– Therefore, you distinguish the sound of music from the
taste of coffee, or the bitterness of coffee from the
sweetness of sugar, because these different stimuli result
in action potentials in different axons that connect to
different areas of the brain
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38.3 How Do Nervous Systems Process
Information?
 The intensity of a stimulus is coded by the
frequency of action potentials
– Because all action potentials are the same size
and duration, no information about the strength
or intensity of a stimulus can be encoded in a
single action potential
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38.3 How Do Nervous Systems Process
Information?
 The intensity of a stimulus is coded by the frequency of action
potentials (continued)
– Intensity is coded in two other ways:
– First, the intensity can be signaled by the frequency of action
potentials in a single neuron—the more intense the stimulus, the
faster the neuron fires action potentials
– Second, most nervous systems have many neurons that can
respond to the same input
– Stronger stimuli excite more of these neurons, whereas
weaker stimuli excite fewer neurons that fire at the same time
– A gentle touch may cause a single touch receptor in the skin
to fire action potentials very slowly; a hard poke may cause
several touch receptors to fire, some very rapidly
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Signaling Stimulus Intensity
sensory neuron 1
sensory
neuron 1
sensory
neuron 2
Sensory neuron 1
fires slowly;
sensory neuron 2
is silent
sensory neuron 2
(a) Gentle touch
sensory neuron 1
Sensory neurons
1 and 2 both fire
sensory
neuron 1
(b) Hard poke
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sensory
neuron 2
sensory neuron 2
time
Fig. 38-5
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38.3 How Do Nervous Systems Process
Information?
 The nervous system processes information from
many sources
– The brain is continually bombarded by sensory
stimuli from both inside and outside the body
– The brain must evaluate these inputs, determine
which ones are important, and decide how to
respond
– A large number of neurons may funnel their
signals to fewer neurons
–For example, many sensory neurons may
converge onto a small number of brain cells
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38.3 How Do Nervous Systems Process
Information?
 The nervous system processes information from
many sources (continued)
– Some of the brain cells act as “decision-making”
cells, adding up the postsynaptic potentials that
result from the synaptic activity of the sensory
neurons
– Depending on their relative strength (and other
internal factors, such as hormones or metabolic
activity), they produce appropriate outputs
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38.3 How Do Nervous Systems Process
Information?
 The nervous system produces outputs to
muscles and glands
– Action potentials from the decision-making
neurons may travel to other parts of the brain, to
the spinal cord, or to the sympathetic and
parasympathetic nervous system
– Ultimately, the output of the nervous system will
stimulate activity in the muscles or glands that
produce behaviors
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38.3 How Do Nervous Systems Process
Information?
 The nervous system produces outputs to muscles and
glands (continued)
– The same principles of connectivity and intensity coding
for sensory inputs are used for the brain’s outputs
– Which muscles or glands are activated is determined by
their connections to the brain or spinal cord
– For example, the neurons that activate biceps
muscles are different than those that activate muscles
of the face
– How hard a muscle contracts is determined by how
many neurons connect to it and how fast those
neurons fire action potentials
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38.4 How Are Nervous Systems Organized?
 Most behaviors are controlled by pathways composed
of four elements:
– Sensory neurons respond to a stimulus, either internal
or external to the body
– Interneurons receive signals from sensory neurons,
hormones, or neurons that store memories; based on this
input, interneurons often activate motor neurons
– Motor neurons receive information from sensory
neurons or interneurons and activate muscles or glands
– Effectors, usually muscles or glands, perform the
response directed by the nervous system
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38.4 How Are Nervous Systems Organized?
 Simple behaviors, such as reflexes, may be
controlled by activity in as few as two or three
neurons—a sensory neuron, a motor neuron,
and an interneuron in between, usually
stimulating a single muscle
– In humans, simple reflexes such as the knee-jerk
or pain-withdrawal reflexes are produced by
neurons in the spinal cord
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38.4 How Are Nervous Systems Organized?
 Complex behaviors are organized by
interconnected neural pathways in which
several types of sensory input converge on a
set of interneurons
– By integrating the postsynaptic potentials from
multiple sources, the interneurons “decide” what
to do and stimulate motor neurons to direct the
appropriate activity in muscles and glands
– Hundreds, or even millions of neurons, mostly in
the brain, may be required to perform complex
actions such as playing the piano
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38.4 How Are Nervous Systems Organized?
 Complex nervous systems are centralized
– In the animal kingdom, there are only two
nervous system designs:
–A diffuse nervous system, such as that of
cnidarians (Hydra, jellyfish, and their relatives)
–A centralized nervous system, found in more
complex organisms
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38.4 How Are Nervous Systems Organized?
 Complex nervous systems are centralized
(continued)
– Radially symmetrical cnidarians have no “front
end,” so there has been no evolutionary
pressure to concentrate the senses in one place
– Cnidarian nervous systems are composed of a
network of neurons, often called a nerve net,
woven through the animal’s tissues, with a
cluster of neurons, called a ganglion, but
nothing like a real brain
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Nervous System Organization
ring of ganglia
diffuse network
of neurons
(a) Hydra
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Fig. 38-6a
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38.4 How Are Nervous Systems Organized?
 Complex nervous systems are centralized
(continued)
– Almost all other animals are bilaterally
symmetrical, with definite head and tail ends
–Because the head is usually the first part of
the body to encounter food, danger, and
potential mates, it is advantageous to have
sense organs concentrated there
–Sizable ganglia evolved that integrate the
information gathered by the senses and direct
appropriate actions
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38.4 How Are Nervous Systems Organized?
 Complex nervous systems are centralized
(continued)
– Over evolutionary time, the major sense organs
of animals with complex nervous systems
became localized in the head, and the ganglia
became centralized into a brain
– This process, called cephalization, reached a
peak in the vertebrates where all nervous system
cell bodies are localized in the brain and spinal
cord
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Nervous System Organization
brain
nerve cords
(b) Flatworm
cerebral
ganglia
(brain)
(c) Octopus
Fig. 38-6b, c
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The nervous system of all mammals, including humans,
can be divided into two parts:
– The central nervous system (CNS), consisting of the
brain and spinal cord
– The peripheral nervous system (PNS), consisting of
neurons that lie outside the CNS and the axons that
connect these neurons with the CNS
– The cell bodies of neurons of the PNS are often
located in ganglia alongside the spinal cord or in
ganglia near target organs, such as ganglia in the
head and neck that control the salivary glands
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Organization and Functions of the Vertebrate
Nervous System
Fig. 38-7
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The peripheral nervous system links the central
nervous system with the rest of the body
– Nerves of the PNS:
– Connect the brain and spinal cord with muscles,
glands, sensory organs, and the digestive, respiratory,
urinary, reproductive, and circulatory systems
– Contain axons of sensory neurons, bringing sensory
information to the central nervous system from all
parts of the body
– These nerves also contain the axons of motor neurons
that carry signals from the central nervous system to
the glands and muscles
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The motor portion of the peripheral nervous
system consists of two parts:
– The somatic nervous system
– The autonomic nervous system
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The somatic nervous system controls voluntary
movement
– Motor neurons of the somatic nervous system
form synapses with skeletal muscles and control
voluntary movement
–Lifting a cup of coffee or adjusting your iPod
are both controlled by the somatic nervous
system
– The cell bodies of somatic motor neurons are
located in the spinal cord, and their axons go
directly to the muscles they control
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The autonomic nervous system controls
involuntary actions
– Motor neurons of the autonomic nervous
system innervate the heart, smooth muscles,
and glands, and produce involuntary actions
–It is controlled by the hypothalamus, medulla,
and pons—parts of the brain
–It consists of two divisions that innervate the
same organs, but with opposing actions:
–The sympathetic division
–The parasympathetic division
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The neurons of the sympathetic division release
the neurotransmitter norepinephrine onto their
target organs, preparing the body for stressful or
energetic actions
– During these activities, it directs some of the
blood supply from the digestive tract to the
muscles of the arms and legs
– The heart rate accelerates, the pupils of the eyes
open wider, and the air passages in the lungs
expand
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The neurons of the parasympathetic division
release acetylcholine onto their target organs
– The parasympathetic division controls
maintenance activities that can be carried out at
leisure, often called “rest and digest”
– Under parasympathetic control, the digestive
tract becomes active, the heart rate slows, and
air passages in the lungs constrict, because the
body requires less blood flow and less oxygen
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The Autonomous Nervous System
PARASYMPATHETIC
DIVISION
SYMPATHETIC
DIVISION
eye
constricts pupil
stimulates
salivation
and tears
dilates pupil
salivary and
lacrimal glands
inhibits
salivation
and tearing
cranial
cranial
constricts
airways
cervical
lungs
relaxes
airways
reduces
heartbeat
cervical
heart
stimulates
pancreas to
release insulin
and digestive
enzymes
thoracic
increases
heartbeat
stimulates glucose
production and
release
liver
stimulates
secretion of
epinephrine and
norepinephrine
from adrenal
medulla
pancreas
kidney
stomach
stimulates
digestion
spleen
lumbar
dilates blood
vessels in gut
kidney
small
intestine
thoracic
inhibits
digestion
lumbar
large
intestine
rectum
urinary
bladder
sacral
sacral
relaxes
bladder
stimulates bladder
to contract
sympathetic
ganglia
uterus
stimulates
sexual arousal
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external
genitalia
stimulates
orgasm
Fig. 38-8
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The central nervous system consists of the spinal cord
and brain
– The CNS receives and processes sensory information,
generates thoughts, and directs responses
– The brain and spinal cord are protected from physical
damage in three ways:
– The skull surrounds the brain, and a chain of
vertebrae protect the spinal cord
– The triple connective tissue layer of meninges lies
between the bone and spinal cord
– Between the mininges layers is the cerebrospinal fluid
that cushions the brain and spinal cord, and nourishes
the cells of the CNS
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The central nervous system consists of the
spinal cord and brain
– The brain is also protected from potentially
damaging chemicals by the blood–brain
barrier, a capillary system that is far less
permeable than capillaries in the rest of the body
and selectively transports needed materials into
the brain while keeping many dangerous
substances out
– Generally, the blood–brain barrier keeps watersoluble substances from diffusing from the blood
into the brain, but many lipid-soluble substances
can still diffuse across the capillary walls
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The spinal cord controls many reflexes and
conducts information to and from the brain
– The spinal cord extends from the base of the
brain to the lower back
– Nerves carrying axons of sensory neurons
emerge from the dorsal part of the spinal cord,
and nerves carrying axons of motor neurons
emerge from the ventral part
– These nerves merge to form the spinal nerves
that innervate most of the body
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The spinal cord (continued)
– Because of their resemblance to the roots of a tree that
merge into a single trunk, these branches are called the
dorsal and ventral roots of the spinal nerves
– Swellings on each dorsal root, called the dorsal root
ganglia, contain the cell bodies of sensory neurons
– In the center of the spinal cord is a butterfly-shaped area
of gray matter
– This consists of the cell bodies of motor neurons that
control voluntary muscles and the autonomic nervous
system, plus interneurons that communicate with the
brain and other parts of the spinal cord
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The spinal cord (continued)
– The gray matter is surrounded by white matter,
which contains myelin-coated axons of neurons
that extend up or down the spinal cord
–These axons carry sensory signals from
internal organs, muscles, and the skin up to
the brain
–Axons also extend downward from the brain,
carrying signals that direct the motor portions
of the peripheral nervous system
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The spinal cord (continued)
– If the spinal cord is severed, body parts that are
innervated by motor and sensory neurons
located below the injury are paralyzed and feel
numb, though the motor and sensory neurons,
the spinal nerves, and the muscles remain intact
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The Spinal Cord
white matter
contains
myelinated
axons
gray matter
contains the cell
bodies of motor
neurons and
interneurons
dorsal root
contains the
axons of
sensory
neurons
dorsal root
ganglion
contains the
cell bodies of
sensory neurons
spinal
nerve
ventral root
contains the axons
of motor neurons
Fig. 38-9
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The neuronal circuits for many reflexes reside in
the spinal cord
– The simplest type of behavior is the reflex, a
largely involuntary movement of a body part in
response to a stimulus
– In vertebrates, many reflexes are produced by
the spinal cord and peripheral neurons, and do
not use the brain
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The pain-withdrawal reflex involves neurons of
both the central and peripheral nervous systems
– If you lean your hand on a tack, the tissue
damage activates pain sensory neurons
– Action potentials in the axons of these pain
sensory neurons travel up the spinal nerve and
enter the spinal cord through a dorsal root
– Within the gray matter of the cord, the pain
sensory neuron stimulates an interneuron, which
stimulates a motor neuron
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The pain-withdrawal reflex (continued)
– Action potentials in the axon of the motor neuron
leave the spinal cord through a ventral root and
travel in a spinal nerve to a skeletal muscle
– The action potential stimulates the muscle, which
contracts, so you withdraw your hand away from
the tack
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Author Animation: Reflex Arcs
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The Pain-withdrawal Reflex
2 The signal is
transmitted by the
pain sensory neuron
to the spinal cord
1 A painful
stimulus activates
a pain sensory
neuron
dorsal root
interneuron
sensory
neuron
spinal
cord
motor
neuron
stimulus
5 The effector
muscle causes a
withdrawal response
ventral
root
3 The signal is
transmitted to an
interneuron and then
to a motor neuron
4 The motor
neuron stimulates
the effector muscle
Fig. 38-10
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 Many spinal cord interneurons also have axons that
extend up to the brain
– Action potentials in these axons inform the brain about
pricked hands and may trigger more complex behaviors,
such as shrieks and learning about the dangers of
thumbtacks
– The brain, in turn, sends action potentials down axons in
the spinal cord white matter to interneurons and motor
neurons in the gray matter, which modify spinal reflexes
– With enough training, or motivation, you can suppress
the pain-withdrawal reflex
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 Some complex actions are coordinated within the
spinal cord
– The wiring for some fairly complex activities also resides
within the spinal cord
– All the neurons needed for basic movements of
walking and running are contained in the spinal cord
– The advantage of the semi-independent arrangement
between brain and spinal cord probably increases speed
and coordination, because messages do not have to
travel up to the brain and back down again just to swing
a leg forward while walking
– The brain’s role in these semi-automatic behaviors is to
initiate, guide, and modify spinal motor neuron activity
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The brain consists of many parts that perform specific
functions
– All vertebrate brains consist of three major parts:
– The hindbrain
– The midbrain
– The forebrain
– Scientists believe that in the earliest vertebrates, these
three anatomical divisions were also functional divisions:
– The hindbrain governed breathing and heart rate
– The midbrain controlled vision
– The forebrain dealt with the sense of smell
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A Comparison of Vertebrate Brains
thalamus
optic lobe
cerebellum
cerebrum
medulla
midbrain
forebrain
midbrain
hindbrain
cerebrum
cerebellum
(a) Embryonic vertebrate brain
midbrain
cerebellum
cerebrum
(d) Horse brain
(b) Shark brain
cerebrum
midbrain
cerebrum
cerebellum
midbrain
(inside
(c) Goose brain
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(e) Human brain
cerebellum
Fig. 38-11
Copyright © 2011 Pearson Education Inc.
38.5 What Are the Structures and Functions of the
Human Nervous System?
 The brain consists of many parts that perform
specific functions (continued)
– In nonmammalian vertebrates, the three
divisions remain prominent
– However, in mammals—particularly humans—
the brain regions are significantly modified
– Some have been reduced in size; others,
especially the forebrain, are greatly enlarged
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The hindbrain consists of the medulla, pons, and
cerebellum
– In both structure and function, the medulla is very much
like an enlarged extension of the spinal cord
– Like the spinal cord, the medulla has neuron cell
bodies at its center, surrounded by a layer of myelincovered axons
– It controls several automatic functions, such as
breathing, heart rate, blood pressure, and swallowing
– Certain neurons in the pons, located just above the
medulla, appear to influence transitions between sleep
and wakefulness; other pons neurons affect the rate and
pattern of breathing
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The cerebellum is crucial for coordinating
movements of the body
– It receives information both from command
centers in the forebrain that control movement
and from position sensors in muscles and joints
– By comparing information from these two
sources, the cerebellum guides smooth,
accurate motions and body position
– The cerebellum is also involved in motor learning
as a result of practice of a repeated activity
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The midbrain contains clusters of neurons that
contribute to movement, arousal, and emotion
– The midbrain is quite small in humans, and
contains an auditory relay center and clusters of
neurons that control reflex movements of the
eyes
–For example, if you are sitting in class and
someone races through the door, centers in
your midbrain are alerted and direct your gaze
to the new, and potentially interesting or
threatening, visual stimulus
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The midbrain (continued)
– The midbrain also contains neurons that produce
the transmitter dopamine
– One cluster of neurons, called the substantia
nigra, helps control movement
– Another cluster is an essential part of the
“reward circuit” that is responsible for
pleasurable sensations and, unfortunately,
addiction
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The midbrain (continued)
– Finally, the midbrain contains a portion of the reticular
formation that consists of dozens of interconnected
clusters of neurons in the medulla, pons, and midbrain,
which neurons send axons to the forebrain
– These neurons receive input from virtually every sense,
from every part of the body, and from many areas of the
brain as well
– The reticular formation plays a role in sleep and
wakefulness, emotion, muscle tone, and some
movements and reflexes
– It filters sensory inputs before they reach the
conscious regions of the brain
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The midbrain (continued)
– Activities of the reticular formation allow you to
read and concentrate in the presence of a variety
of distracting stimuli, such as music or smells
– An example is the mother who wakens upon
hearing the faint cry of her infant, but sleeps
through loud traffic noise outside her window
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The forebrain includes the thalamus, hypothalamus,
and cerebrum
– The thalamus is a complex relay station that channels all
sensory information, except olfaction, from all parts of the
body to the cerebral cortex
– Signals traveling from the spinal cord, cerebellum,
medulla, pons, and reticular formation also pass
through the thalamus
– The hypothalamus contains many clusters of neurons
that release hormones into the blood or control the
release of hormones from the pituitary gland
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The forebrain includes the thalamus,
hypothalamus, and cerebrum (continued)
– Other regions of the hypothalamus direct the
activities of the autonomic nervous system
– The hypothalamus, through its hormone
production and neural connections, maintains
homeostasis by influencing body temperature,
food intake, water balance, heart rate, blood
pressure, the menstrual cycle, and circadian
rhythms
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The cerebrum consists of two cerebral
hemispheres
– Each hemisphere is composed of an outer
cerebral cortex, several clusters of neurons
beneath the cortex near the thalamus, and
bundles of axons that both interconnect the two
hemispheres and connect the hemispheres with
the midbrain and hindbrain
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 Structures in the interior of the cerebrum
– Clusters of neurons in the amygdala produce sensations
of pleasure, fear, or sexual arousal when stimulated
– The hippocampus plays an important role in the
formation of long-term memory, particularly of places,
and is thus required for learning
– All vertebrates have a hippocampus part of their brain
– Birds, such as jays and nutcrackers, remember
where they store seeds during the winter and have
a larger hippocampus than other birds
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The basal ganglia consist of structures deep within the
cerebrum, as well as substantia nigra in the midbrain
– These structures are important in the overall control of
movement
– The basal ganglia appear to be essential to the decision
to initiate a particular movement and to suppress other
movements
– In Parkinson’s disease, the substantia nigra
degenerates, and affected people have a hard time
starting a movement
– In Huntington’s disease, basal ganglia in the cerebrum
degenerate, and affected people make involuntary,
undirected, flailing movements
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The Human Brain
meninges
FOREBRAIN
(within dashed
blue line)
cerebral
cortex
skull
corpus
callosum
cerebral cortex
(gray matter)
hypothalamus
thalamus
pituitary
gland
myelinated axons
(white matter)
corpus
callosum
thalamus
basal
ganglia
MIDBRAIN
cerebellum
pons
HINDBRAIN
hypothalamus
medulla
spinal cord
(a) A lateral section of the human brain
hippocampus
substantia nigra
(b) A cross-section of the brain
Fig. 38-12
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The limbic system is a diverse group of structures
– These structures include the hypothalamus, the
amygdala, and the hippocampus, as well as nearby
regions of the cerebral cortex, located in a ring between
the thalamus and cerebral cortex
– The limbic system helps to produce emotions and
emotional behaviors, including fear, rage, calm,
hunger, thirst, pleasure, and sexual responses
– Other brain regions are also involved in emotions,
including other parts of the cerebral cortex, the
midbrain, the hindbrain, and even the spinal cord
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The Limbic System
limbic region
of cortex
cerebral cortex
corpus callosum
thalamus
olfactory
bulb
hypothalamus
Biology: Life on Earth, 9e
amygdala
hippocampus
Fig. 38-13
Copyright © 2011 Pearson Education Inc.
38.5 What Are the Structures and Functions of the
Human Nervous System?
 The cerebral cortex
– The cerebral cortex is the thin outer layer of each
cerebral hemisphere, with billions of neurons packed in a
highly organized way into a sheet just a few millimeters
thick
– The cortex is folded into convolutions, which are
raised, wrinkled ridges that increase its surface area
to over two square yards
– Neurons in the cortex receive sensory information,
process it, direct voluntary movements, create
memories, and allow us to be creative and even
envision the future
– The cortexes in each hemisphere communicate with
each other through a large band of axons, the corpus
callosum
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The cerebral cortex is divided into four
anatomical regions: frontal, parietal, occipital,
and temporal
– It can also be divided into functional areas:
–Primary areas are regions where signals
originating in sensory organs such as the eyes
and ears are received and converted into
subjective impressions
–Nearby association areas interpret the sounds
as speech or music, and the visual stimuli as
recognizable objects or words
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The Cerebral Cortex
Frontal
Lobe
primary
motor area
premotor
area
leg
trunk
arm
higher
intellectual
functions
primary
sensory area
Parietal
Lobe
hand
sensory
association
area
face
speech
motor area
memory
tongue
primary
auditory
auditory association
area
area: language
comprehension
Occipital
Lobe
visual
association
area
primary
visual
area
Temporal
Lobe
Fig. 38-14
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The cerebral cortex (continued)
– Primary sensory areas in the parietal lobe interpret
sensations of touch that originate in all parts of the body
– In an adjacent region of the frontal lobe, primary motor
areas command movements in corresponding areas
of the body by stimulating the motor neurons in the
spinal cord that innervate muscles, allowing you to
walk
– Like the primary sensory area, the primary motor area
has adjacent association areas, including the motor
association area which directs the motor area to
produce movements
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The cerebral cortex (continued)
– Behind the bones of the forehead lies the
association areas of the frontal lobe
–They are important in complex reasoning
functions such as short-term memory, decision
making, predicting the consequences of
actions, controlling aggression, planning for
the future, and working for delayed rewards
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The cerebral cortex (continued)
– Damage to the cortex from trauma, stroke, or a tumor
results in specific deficits such as problems with speech,
difficulty reading, or the inability to sense or move
specific parts of the body
– Most brain cells of adults cannot be replaced, so if a
brain region is destroyed, these deficits may be
permanent
– Training sometimes allows undamaged regions of the
cortex to take over and restore some of the lost functions
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Author Animation: The Human Brain
Biology: Life on Earth, 9e
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 How do neuroscientists learn about the
functions of brain regions?
– The functions of different parts of the brain were
discovered by examining the behaviors and
abilities of people who suffered brain injuries,
often in accidents or wars
– In 1848, Phineas Gage was setting an explosive
charge to clear rocks from a railroad line under
construction when the gunpowder triggered
prematurely
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 How do neuroscientists learn about the
functions of brain regions? (continued)
– The blast blew a 13-pound steel rod through
Gage’s skull, severely damaging both of his
frontal lobes
– Gage survived his wounds, but his personality
changed radically
– Before the accident, Gage was conscientious,
industrious, and well-liked; after his recovery, he
became impetuous, profane, and incapable of
working toward a goal
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A Revealing Accident
Fig. 38-15
Biology: Life on Earth, 9e
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 Studies of other people with brain injuries have
revealed that many parts of the brain are highly
specialized
– One patient with damage to the left frontal lobe was
unable to name fruits and vegetables, although he could
name everything else
– Other victims of brain damage are unable to recognize
faces, suggesting that the brain has regions specialized
to recognize categories of things
 Modern neuroscience has powerful techniques for
visualizing brain structure and activity, offering insights
into the functioning of the human brain
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The “left brain” and the “right brain” are specialized for
different functions
– The structural brain symmetry does not extend to brain
function
– In the 1950s, Roger Sperry of the California Institute
of Technology studied people whose hemispheres had
been separated by cutting the corpus callosum to
prevent the spread of epilepsy from one hemisphere
to the other
– Severing the corpus callosum prevented the two
hemispheres from communicating with each other
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 The “left brain” and the “right brain” are specialized for different
functions (continued)
– Sperry made use of the fact that axons from the eyes, which are not
severed by the surgery, follow a pathway that causes the left half of
each visual field to be “seen” by the high hemisphere and the right
half to be seen by the left hemisphere
– Sperry used a device that projected different images onto the left and
right visual fields and thus sent different signals to each hemisphere
– When Sperry projected an image of a nude figure onto only the
left visual field, the subjects would blush and smile, but would
claim to have seen nothing
– The same figure projected onto the right visual field was readily
described verbally
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Specialization of the Cerebral Hemispheres
Left
HEART
LEFT HEMISPHERE
1. Controls right side of body
2. Input from right visual field,
right ear, left nostril
3. Centers for language, speech,
reading, mathematics, logic
Right
RIGHT HEMISPHERE
1. Controls left side of body
2. Input from left visual field,
left ear, right nostril
3. Centers for spatial perception,
music, creativity, recognition
of faces and emotions
retina
optic nerve
optic
chiasma
corpus
callosum
visual cortex
Biology: Life on Earth, 9e
Fig. 38-16
Copyright © 2011 Pearson Education Inc.
38.5 What Are the Structures and Functions of the
Human Nervous System?
 The “left brain” and the “right brain” are specialized for
different functions (continued)
– These and later experiments revealed that, in righthanded people, the left hemisphere is usually dominant
in speech, reading, writing, language, comprehension,
mathematical ability, and logical problem solving
– The right side of the brain is superior to the left in musical
skills, artistic ability, recognizing faces, spatial
visualization, and the ability to recognize and express
emotions
– Recent experiments indicate that the left–right dichotomy
is not as rigid as was once believed
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38.5 What Are the Structures and Functions of the
Human Nervous System?
 Learning and memory involve biochemical and
structural changes in specific parts of the brain
– Learning has two phases: working memory and longterm memory
– Remembering a telephone number long enough to
dial it is working memory; if the number is called often
enough, it becomes permanently remembered in longterm memory
– The frontal and parietal lobes of the cerebral cortex, and
some of the basal ganglia deep in the cerebrum, are the
primary sites of working memory
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38.5 What Are the Structures and Functions of the
Human Nervous System?
– Most working memory probably requires the repeated activity of a
particular neural circuit in the brain, and as long as the circuit is
active, the memory stays
– In contrast, long-term memory seems to be structural and the result
of persistent changes in the expression of certain genes
– It may require the formation of new, long-lasting synaptic
connections between specific neurons, or the long-term
strengthening of existing, but weak, synapses
– For many memories, converting working memory into long-term
memory seems to involve the hippocampus, which is believed to
process new memories and transfer them to the frontal and temporal
lobes of the cerebral cortex for permanent storage
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