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Chapter 49
Nervous Systems
Overview: Command and Control Center
• The human brain contains about 100 billion
neurons, organized into circuits more complex
than the most powerful supercomputers
• A recent advance in brain exploration involves a
method for expressing combinations of colored
proteins in brain cells, a technique called
“brainbow”
• This may allow researchers to develop detailed
maps of information transfer between regions of
the brain
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Figure 49.1
• Each single-celled organism can respond to
stimuli in its environment
• Animals are multicellular and most groups
respond to stimuli using systems of neurons
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Concept 49.1: Nervous systems consist of
circuits of neurons and supporting cells
• The simplest animals with nervous systems, the
cnidarians, have neurons arranged in nerve nets
• A nerve net is a series of interconnected nerve
cells
• More complex animals have nerves
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• Nerves are bundles that consist of the axons of
multiple nerve cells
• Sea stars have a nerve net in each arm
connected by radial nerves to a central nerve
ring
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Figure 49.2
Nerve net
(a) Hydra (cnidarian)
Radialnerve
Nervering
(b) Sea star(echinoderm)
Eyespot
Brain
Nervecords
Transversenerve
Brain
Ventralnerve cord
Segmentalganglia
(c) Planarian(flatworm)
(d) Leech (annelid)
(h) Salamander(vertebrate)
(e) Insect (arthropod) (f) Chiton (mollusc) (g) Squid (mollusc)
Brain
Brain
Brain
Ventralnerve cord
Segmentalganglia
Anteriornerve ring
Longitudinalnerve cords
Ganglia
Ganglia
Spinalcord(dorsalnervecord)
Sensoryganglia
Figure 49.2a
Nerve net
(a) Hydra (cnidarian)
Radialnerve
Nervering
(b) Sea star (echinoderm)
• Bilaterally symmetrical animals exhibit cephalization, the clustering of sensory organs at the front end of the body
• Relatively simple cephalized animals, such as flatworms, have a central nervous system (CNS)
• The CNS consists of a brain and longitudinal nerve cords
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Figure 49.2b
Eyespot
Brain
Nervecords
Transversenerve
Brain
Ventralnerve cord
Segmentalganglia
(c) Planarian (flatworm) (d) Leech (annelid)
• Annelids and arthropods have segmentally
arranged clusters of neurons called ganglia
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Figure 49.2c
(e) Insect (arthropod) (f) Chiton (mollusc)
Brain
Ventralnerve cord
Segmentalganglia
Anteriornerve ring
Longitudinalnerve cords
Ganglia
• Nervous system organization usually correlates
with lifestyle
• Sessile molluscs (for example, clams and
chitons) have simple systems, whereas more
complex molluscs (for example, octopuses and
squids) have more sophisticated systems
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Figure 49.2d
(h) Salamander (vertebrate)(g) Squid (mollusc)
Brain
Brain
Ganglia
Spinalcord(dorsalnervecord)
Sensoryganglia
• In vertebrates
– The CNS is composed of the brain and spinal
cord
– The peripheral nervous system (PNS) is
composed of nerves and ganglia
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Organization of the Vertebrate Nervous
System
• The spinal cord conveys information from and
to the brain
• The spinal cord also produces reflexes
independently of the brain
• A reflex is the body’s automatic response to a
stimulus
– For example, a doctor uses a mallet to trigger
a knee-jerk reflex
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Quadricepsmuscle
Cell body ofsensory neuron indorsal rootganglion
Gray matter
White matter
Hamstringmuscle
Spinal cord(cross section)
Sensory neuron
Motor neuron
Interneuron
Figure 49.3
• Invertebrates usually have a ventral nerve cord
while vertebrates have a dorsal spinal cord
• The spinal cord and brain develop from the
embryonic nerve cord
• The nerve cord gives rise to the central canal
and ventricles of the brain
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Figure 49.4Central nervoussystem (CNS)
Brain
Spinal cord
Peripheral nervoussystem (PNS)
Cranial nerves
Ganglia outsideCNS
Spinal nerves
Figure 49.5
Gray matter
Whitematter
Ventricles
• The central canal of the spinal cord and the
ventricles of the brain are hollow and filled with
cerebrospinal fluid
• The cerebrospinal fluid is filtered from blood and
functions to cushion the brain and spinal cord as
well as to provide nutrients and remove wastes
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• The brain and spinal cord contain
– Gray matter, which consists of neuron cell
bodies, dendrites, and unmyelinated axons
– White matter, which consists of bundles of
myelinated axons
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Glia
• Glia have numerous functions to nourish,
support, and regulate neurons
– Embryonic radial glia form tracks along which
newly formed neurons migrate
– Astrocytes induce cells lining capillaries in the
CNS to form tight junctions, resulting in a
blood-brain barrier and restricting the entry of
most substances into the brain
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Figure 49.6CNS PNS
VENTRICLE
Cilia
Neuron Astrocyte
Oligodendrocyte
Capillary Ependymal cell
LM
50
m
Schwann cell
Microglial cell
Figure 49.6a
CNS PNS
VENTRICLE
Cilia
Neuron Astrocyte
Oligodendrocyte
Capillary Ependymal cell
Schwann cell
Microglial cell
The Peripheral Nervous System
• The PNS transmits information to and from the CNS and regulates movement and the internal environment
• In the PNS, afferent neurons transmit information to the CNS and efferent neurons transmit information away from the CNS
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• The PNS has two efferent components: the
motor system and the autonomic nervous system
• The motor system carries signals to skeletal
muscles and is voluntary
• The autonomic nervous system regulates
smooth and cardiac muscles and is generally
involuntary
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Efferent neuronsAfferent neurons
Central NervousSystem
(information processing)
Peripheral NervousSystem
Sensoryreceptors
Internaland external
stimuli
Autonomicnervous system
Motorsystem
Control ofskeletal muscle
Sympatheticdivision
Parasympatheticdivision
Entericdivision
Control of smooth muscles,cardiac muscles, glands
Figure 49.7
• The autonomic nervous system has
sympathetic, parasympathetic, and enteric
divisions
• The sympathetic division regulates arousal
and energy generation (“fight-or-flight”
response)
• The parasympathetic division has
antagonistic effects on target organs and
promotes calming and a return to “rest and
digest” functions
• The enteric division controls activity of the
digestive tract, pancreas, and gallbladder
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Parasympathetic division
Action on target organs:
Constricts pupilof eye
Stimulates salivarygland secretion
Constrictsbronchi in lungs
Slows heart
Stimulates activityof stomach and
intestines
Stimulates activityof pancreas
Stimulatesgallbladder
Promotes emptyingof bladder
Promotes erectionof genitalia
Cervical
Thoracic
Lumbar
Synapse
Sacral
Sympatheticganglia
Sympathetic division
Action on target organs:
Dilates pupil of eye
Accelerates heart
Inhibits salivarygland secretion
Relaxes bronchiin lungs
Inhibits activity ofstomach and intestines
Inhibits activityof pancreas
Stimulates glucoserelease from liver;inhibits gallbladder
Stimulatesadrenal medulla
Inhibits emptyingof bladder
Promotes ejaculationand vaginal contractions
Figure 49.8
Concept 49.2: The vertebrate brain is
regionally specialized
• Specific brain structures are particularly
specialized for diverse functions
• These structures arise during embryonic
development
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Figure 49.9a
Embryonic brain regions Brain structures in child and adult
Forebrain
Midbrain
Hindbrain
Telencephalon
Diencephalon
Mesencephalon
Metencephalon
Myelencephalon
Cerebrum (includes cerebral cortex, whitematter, basal nuclei)
Diencephalon (thalamus, hypothalamus,epithalamus)
Midbrain (part of brainstem)
Pons (part of brainstem), cerebellum
Medulla oblongata (part of brainstem)
Midbrain
Forebrain
Hindbrain
Telencephalon
Diencephalon
Mesencephalon
Metencephalon
Myelencephalon
Spinal cord
Cerebrum Diencephalon
Midbrain
Pons
Medullaoblongata
Cerebellum
Spinal cord
ChildEmbryo at 5 weeksEmbryo at 1 month
Figure 49.9b
Figure 49.9c
Adult brain viewed from the rear
Cerebellum
Basal nucleiCerebrum
Left cerebralhemisphere
Right cerebralhemisphere
Cerebral cortex
Corpus callosum
Figure 49.9d
Diencephalon
Thalamus
Pineal gland
Hypothalamus
Pituitary gland
Spinal cord
Brainstem
Midbrain
Pons
Medullaoblongata
Arousal and Sleep
• The brainstem and cerebrum control arousal and sleep
• The core of the brainstem has a diffuse network of neurons called the reticular formation
• This regulates the amount and type of information that reaches the cerebral cortex and affects alertness
• The hormone melatonin is released by the pineal gland and plays a role in bird and mammal sleep cycles
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Figure 49.10
Eye
Reticular formation
Input from touch,pain, and temperaturereceptors
Input from nervesof ears
• Sleep is essential and may play a role in the
consolidation of learning and memory
• Dolphins sleep with one brain hemisphere at a
time and are therefore able to swim while
“asleep”
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Figure 49.11
Low-frequency waves characteristic of sleep
High-frequency waves characteristic of wakefulness
Key
Location Time: 0 hours Time: 1 hour
Lefthemisphere
Righthemisphere
Biological Clock Regulation
• Cycles of sleep and wakefulness are examples
of circadian rhythms, daily cycles of biological
activity
• Mammalian circadian rhythms rely on a
biological clock, molecular mechanism that
directs periodic gene expression
• Biological clocks are typically synchronized to
light and dark cycles
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• In mammals, circadian rhythms are coordinated
by a group of neurons in the hypothalamus
called the suprachiasmatic nucleus (SCN)
• The SCN acts as a pacemaker, synchronizing
the biological clock
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Figure 49.12
Wild-type hamster
Wild-type hamster withSCN from hamster
hamster
hamster with SCNfrom wild-type hamster
RESULTS
Beforeprocedures
After surgeryand transplant
Cir
cad
ian
cycle
pe
rio
d (
ho
urs
)
24
23
22
21
20
19
Emotions
• Generation and experience of emotions involve
many brain structures including the amygdala,
hippocampus, and parts of the thalamus
• These structures are grouped as the limbic
system
• The limbic system also functions in motivation,
olfaction, behavior, and memory
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Figure 49.13
Hypothalamus
Thalamus
Olfactorybulb
Amygdala Hippocampus
• Generation and experience of emotion also require interaction between the limbic system and sensory areas of the cerebrum
• The structure most important to the storage of emotion in the memory is the amygdala, a mass of nuclei near the base of the cerebrum
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Figure 49.14
Nucleus accumbens Amygdala
Happy music Sad music
Concept 49.3: The cerebral cortex controls
voluntary movement and cognitive functions
• The cerebrum, the largest structure in the
human brain, is essential for awareness,
language, cognition, memory, and
consciousness
• Four regions, or lobes (frontal, temporal,
occipital, and parietal), are landmarks for
particular functions
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Figure 49.15
Motor cortex(control ofskeletal muscles)
Frontal lobe
Prefrontal cortex(decision making,planning)
Broca’s area(forming speech)
Temporal lobe
Auditory cortex (hearing)
Wernicke’s area(comprehending language)
Somatosensory cortex(sense of touch)
Parietal lobe
Sensory associationcortex (integration ofsensory information)
Visual associationcortex (combiningimages and objectrecognition)
Occipital lobe
Cerebellum
Visual cortex(processing visualstimuli and patternrecognition)
Language and Speech
• Studies of brain activity have mapped areas
responsible for language and speech
• Broca’s area in the frontal lobe is active when
speech is generated
• Wernicke’s area in the temporal lobe is active
when speech is heard
• These areas belong to a larger network of
regions involved in language
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Figure 49.16
Hearingwords
Speakingwords
Seeingwords
Generatingwords
Max
Min
Lateralization of Cortical Function
• The two hemispheres make distinct contributions
to brain function
• The left hemisphere is more adept at language,
math, logic, and processing of serial sequences
• The right hemisphere is stronger at pattern
recognition, nonverbal thinking, and emotional
processing
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• The differences in hemisphere function are
called lateralization
• Lateralization is partly linked to handedness
• The two hemispheres work together by
communicating through the fibers of the corpus
callosum
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Information Processing
• The cerebral cortex receives input from sensory
organs and somatosensory receptors
• Somatosensory receptors provide information
about touch, pain, pressure, temperature, and
the position of muscles and limbs
• The thalamus directs different types of input to
distinct locations
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• Adjacent areas process features in the sensory
input and integrate information from different
sensory areas
• Integrated sensory information passes to the
prefrontal cortex, which helps plan actions and
movements
• In the somatosensory cortex and motor cortex,
neurons are arranged according to the part of
the body that generates input or receives
commands
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Figure 49.17
Frontal lobe Parietal lobe
Primarymotor cortex
Primarysomatosensorycortex
GenitaliaToes
Abdominalorgans
Tongue
Jaw
Hip
Kn
ee
Tongue
Pharynx
Head
Neck
Tru
nk
Hip
Leg
Frontal Lobe Function
• Frontal lobe damage may impair decision
making and emotional responses but leave
intellect and memory intact
• The frontal lobes have a substantial effect on
“executive functions”
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Figure 49.UN01
Phineas Gage
Evolution of Cognition in Vertebrates
• Previous ideas that a highly convoluted
neocortex is required for advanced cognition
may be incorrect
• The anatomical basis for sophisticated
information processing in birds (without a highly
convoluted neocortex) appears to be the
clustering of nuclei in the top or outer portion of
the brain (pallium)
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Human brain
Avian brain
Thalamus
Midbrain
Hindbrain Cerebellum
Avian brainto scale
Thalamus
Midbrain
Hindbrain
Cerebellum
Cerebrum (includingcerebral cortex)
Cerebrum(including pallium)
Figure 49.18
Concept 49.4 Changes in synaptic
connections underlie memory and learning
• Two processes dominate embryonic
development of the nervous system
– Neurons compete for growth-supporting factors
in order to survive
– Only half the synapses that form during embryo
development survive into adulthood
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Neural Plasticity
• Neural plasticity describes the ability of the
nervous system to be modified after birth
• Changes can strengthen or weaken signaling at
a synapse
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Figure 49.19
N2
N1
N2
N1
(a) Synapses are strengthened or weakened in response toactivity.
(b) If two synapses are often active at the same time, thestrength of the postsynaptic response may increase atboth synapses.
Neural
plasticity
Memory and Learning
• The formation of memories is an example of
neural plasticity
• Short-term memory is accessed via the
hippocampus
• The hippocampus also plays a role in forming
long-term memory, which is stored in the
cerebral cortex
• Some consolidation of memory is thought to
occur during sleep
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Long-Term Potentiation
• In the vertebrate brain, a form of learning called
long-term potentiation (LTP) involves an
increase in the strength of synaptic transmission
• LTP involves glutamate receptors
• If the presynaptic and postsynaptic neurons are
stimulated at the same time, the set of receptors
present on the postsynaptic membranes
changes
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Figure 49.20PRESYNAPTIC
NEURON
GlutamateMg2
Ca2
Na
NMDAreceptor(closed)
StoredAMPAreceptor
NMDA receptor (open)
POSTSYNAPTICNEURON
(a) Synapse prior to long-term potentiation (LTP)
(b) Establishing LTP
(c) Synapse exhibiting LTP
Depolarization
Actionpotential
2
1
3
1
2
3
4
Figure 49.20a
PRESYNAPTICNEURON
GlutamateMg2
Ca2
Na
NMDAreceptor(closed)
StoredAMPAreceptor
NMDA receptor (open)
POSTSYNAPTICNEURON
(a) Synapse prior to long-term potentiation (LTP)
Figure 49.20b
(b) Establishing LTP
1
23
AMPAreceptor
NMDA receptor
Mg2
Ca2
Na
Figure 49.20c
(c) Synapse exhibiting LTP
Depolarization
Actionpotential
AMPAreceptor
NMDA receptor
13
42
Stem Cells in the Brain
• The adult human brain contains neural stem
cells
• In mice, stem cells in the brain can give rise to
neurons that mature and become incorporated
into the adult nervous system
• Such neurons play an essential role in learning
and memory
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Figure 49.21
Concept 49.5: Nervous system disorders can
be explained in molecular terms
• Disorders of the nervous system include
schizophrenia, depression, drug addiction,
Alzheimer’s disease, and Parkinson’s disease
• Genetic and environmental factors contribute to
diseases of the nervous system
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Figure 49.22
Genes shared with relatives ofperson with schizophrenia
12.5% (3rd-degree relative)
25% (2nd-degree relative)
50% (1st-degree relative)
100%
50
40
30
20
10
0
Relationship to person with schizophrenia
Ris
k o
f d
eve
lop
ing
sc
hiz
op
hre
nia
(%
)
Ind
ivid
ua
l,g
en
era
lp
op
ula
tio
n
Fir
st
co
usin
Un
cle
/au
nt
Ne
ph
ew
/n
iec
e
Fra
tern
al
twin
Ide
nti
ca
ltw
in
Gra
nd
ch
ild
Ha
lf s
iblin
g
Pa
ren
t
Fu
ll s
iblin
g
Ch
ild
Schizophrenia
• About 1% of the world’s population suffers from
schizophrenia
• Schizophrenia is characterized by hallucinations,
delusions, and other symptoms
• Available treatments focus on brain pathways
that use dopamine as a neurotransmitter
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Depression
• Two broad forms of depressive illness are
known: major depressive disorder and bipolar
disorder
• In major depressive disorder, patients have a
persistent lack of interest or pleasure in most
activities
• Bipolar disorder is characterized by manic
(high-mood) and depressive (low-mood) phases
• Treatments for these types of depression include
drugs such as Prozac
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Drug Addiction and the Brain’s Reward
System
• The brain’s reward system rewards motivation
with pleasure
• Some drugs are addictive because they
increase activity of the brain’s reward system
• These drugs include cocaine, amphetamine,
heroin, alcohol, and tobacco
• Drug addiction is characterized by compulsive
consumption and an inability to control intake
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• Addictive drugs enhance the activity of the
dopamine pathway
• Drug addiction leads to long-lasting changes in
the reward circuitry that cause craving for the
drug
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Figure 49.23Nicotinestimulatesdopamine-releasingVTA neuron.
Inhibitory neuron
Dopamine-releasingVTA neuron
Cerebralneuron ofrewardpathway
Opium and heroindecrease activityof inhibitoryneuron.
Cocaine andamphetaminesblock removalof dopaminefrom synapticcleft.
Rewardsystemresponse
Alzheimer’s Disease
• Alzheimer’s disease is a mental deterioration characterized by confusion and memory loss
• Alzheimer’s disease is caused by the formation of neurofibrillary tangles and amyloid plaques in the brain
• There is no cure for this disease though some drugs are effective at relieving symptoms
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Figure 49.24
Amyloid plaque Neurofibrillary tangle 20 m
Parkinson’s Disease
• Parkinson’s disease is a motor disorder
caused by death of dopamine-secreting
neurons in the midbrain
• It is characterized by muscle tremors, flexed
posture, and a shuffling gait
• There is no cure, although drugs and various
other approaches are used to manage
symptoms
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