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Chapter 48
• The Nervous System
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
– 3-D map of brain activity
Figure 48.1
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• Brain imaging reveals groups of neurons
function in specialized circuits dedicated to
different tasks
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• All animals except sponges have some type of
nervous system
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Organization of Nervous Systems
• Simplest animals  nerve nets
Nerve net
Figure 48.2a
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(a) Hydra (cnidarian)
• Sea stars nerve net connected by radial nerves
to a central nerve ring
Radial
nerve
Nerve
ring
Figure 48.2b
(b) Sea star (echinoderm)
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• Simple cephalized animals, e.g. flatworms
– central nervous system (CNS)
Eyespot
Brain
Nerve
cord
Transverse
nerve
Figure 48.2c
(c) Planarian (flatworm)
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• Annelids and arthropods
– Segmentally arranged clusters of neurons
called ganglia
• Ganglia connect to the CNS
– and make up a peripheral nervous system
(PNS)
Brain
Ventral
nerve
cord
Segmental
ganglion
Figure 48.2d, e
Brain
Ventral
nerve cord
Segmental
ganglia
(d) Leech (annelid)
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(e) Insect (arthropod)
• Molluscs
• Sessile molluscs simple systems
• Complex molluscs  sophisticated
Anterior
nerve ring
Ganglia
Brain
Longitudinal
nerve cords
Figure 48.2f, g
(f) Chiton (mollusc)
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Ganglia
(g) Squid (mollusc)
• Vertebrates
– CNS  brain and dorsal spinal cord
– The PNS connects to the CNS
Brain
Spinal
cord
(dorsal
Sensory
ganglion
nerve
cord)
Figure 48.2h
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(h) Salamander (chordate)
Information Processing
• 3 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 information is sent to the CNS
– Where interneurons integrate the information
• Motor output leaves the CNS via motor
neurons
– Communicate with effector cells
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• 3 stages in the knee-jerk reflex
2 Sensors detect
a sudden stretch in
the quadriceps.
3 Sensory neurons
convey the information
to the spinal cord.
Cell body of
sensory neuron
in dorsal
root ganglion
4 The sensory neurons communicate with
motor neurons that supply the quadriceps. The
motor neurons convey signals to the quadriceps,
causing it to contract and jerking the lower leg forward.
Gray matter
5 Sensory neurons
from the quadriceps
also communicate
with interneurons
in the spinal cord.
Quadriceps
muscle
White
matter
Hamstring
muscle
6
Spinal cord
(cross section)
Sensory neuron
Motor neuron
Figure 48.4
1 The reflex is
initiated by tapping
the tendon connected
to the quadriceps
(extensor) muscle.
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Interneuron
The interneurons
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
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
• Dendrites
– Receive signals from other neurons
• Axon
– Transmits signals to other cells at synapses
– May be covered with myelin sheath
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Dendrites
Axon
Cell
body
Figure 48.6a–c (a) Sensory neuron
(b) Interneurons
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(c) Motor neuron
Supporting Cells (Glia)
• Structural integrity & normal functioning of
neurons
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• In the CNS, astrocytes
50 µm
– structural support for neurons
Figure 48.7
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• Oligodendrocytes (in the CNS) and Schwann
cells (in the PNS)
– glia that form the myelin sheaths around the
axons
Node of Ranvier
Layers of myelin
Axon
Schwann
cell
Schwann
cell
Axon
Nodes of
Myelin sheath Ranvier
Nucleus of
Schwann cell
Figure 48.8
0.1 µm
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• Ion pumps and channels maintain the resting
potential of a neuron
• Every cell has a voltage
– Called a membrane potential
• The inside of a cell is negative (-)
– Relative to the outside
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• Measurement
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
• Membrane potential of a neuron that is not
transmitting signals
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• Depends on ionic gradients
EXTRACELLULAR
FLUID
CYTOSOL
[Na+]
15 mM
[K+]
150 mM
[Cl–]
10 mM
[A–]
100 mM
–
+
[Na+]
150 mM
–
+
[K+]
5 mM
–
+
–
+
–
+
[Cl–]
120 mM
Plasma
membrane
Figure 48.10
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• Conc. of Na+ is higher in the extracellular fluid
than in the cytosol
• K+ higher in cytosol
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Gated Ion Channels
• Open or close in response to a change in the
membrane potential
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Action potentials
• Signals conducted by axon
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• hyperpolarization
– Increase in the magnitude of the membrane
Stimuli
potential
Membrane potential (mV)
+50
0
–50
Threshold
Resting
potential
Hyperpolarizations
–100
0 1 2 3 4 5
Time (msec)
(a) Graded hyperpolarizations
produced by two stimuli that
increase membrane permeability
to K+. The larger stimulus produces
Figure 48.12a a larger hyperpolarization.
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• depolarization
– Reduction in the magnitude of the membrane
Stimuli
potential
Membrane potential (mV)
+50
0
–50
Threshold
RestingDepolarizations
potential
–100
0 1 2 3 4 5
Time (msec)
(b) Graded depolarizations produced
by two stimuli that increase
membrane permeability to Na+.
The larger stimulus produces a
Figure 48.12b larger depolarization.
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Production of Action Potentials
• A stimulus strong enough to produce a
depolarization that reaches the threshold
 action potential
Stronger depolarizing stimulus
Membrane potential (mV)
+50
Action
potential
0
–50
Threshold
Resting
potential
–100
0 1 2 3 4 5 6
Time (msec)
Figure 48.12c
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(c) Action potential triggered by a
depolarization that reaches the
threshold.
• Action potential
– Brief all-or-none depolarization of a neuron’s
plasma membrane
– Signal that carries information along axons
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generation of an action potential
Na+
– –
– –
+ +
+ +
Na+
+ +
– –
Na+
+ +
+ +
K+
3. Rising phase of
action potential
– –
Depolarization opens the activation
gates on most Na+ channels, while the
K+ channels’ activation gates remain
closed. Na+ influx
makes the inside of
3 Rising phase of the action potential
the membrane positive with respect
to the outside.
4
+50
+ +
+ +
K+
–
A stimulus opens the
+
activation gates on some Na channels. Na+
influx through those channels depolarizes the
membrane. If the depolarization reaches the
Depolarization
threshold,
it triggers an action potential.
– –
2. Depolarization
2
– –
– –
0
Membrane potential
(mV)
+ +
The inactivation gates on
most Na+ channels close,
blocking Na+ influx. The
activation gates on most
K+ channels open,
permitting K+ efflux
which again makes
the inside of the cell
negative.
Action
potential
Na+
+ +
3
2
–50
4
–100
5
1
1
Resting potential
5. Undershoot
Time
Threshold
Na+
Extracellular fluid
Na+
Potassium
channel
Cytosol
Na+
+ +
– –
– –
+ +
K
+
– –
–
Sodium
channel
+ +
Activation
gates
Plasma
+ + membrane
+ + + + + +
igure 48.13
4.
Falling
+ + phase of
Acton
– – Potential
+K
Na+
1. Resting
1
state
Na+
K+
Inactivation
gate
The activation gates on the Na+ and K+ channels
are closed, and the membrane’s resting potential is maintained.
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5
Undershoot
Both gates of the Na+ channels
are closed, but the activation gates on some K+
channels are still open. As these gates close on
most K+ channels, and the inactivation gates
open on Na+ channels, the membrane returns to
its resting state.
Conduction of Action Potentials
• Travels long distances by regenerating itself
along the axon
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• At the site where the action potential is
generated an electrical current depolarizes the
neighboring region of the axon membrane
Axon
Action
potential
–
–
+
+
+
+
+
+
+
+
+Na +
–
–
–
–
–
–
–
–
–
–
–
–
–
+
+
+
+
+
+
+
–
K+
+
–
–
+
+
–
–
K+
+
Action
potential
–
+
–
+
Na+
+
–
+
–
K+
Figure 48.14
+
+
–
–
–
–
+
+
+
–
–
+
–
–
+
+
K+
+
–
–
+
–
+
–
–
+
–
–
+
+
+
+
–
Action
potential
–
+
Na++
+
–
–
–
+
–
+
+
–
+
+
–
–
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1
An action potential is generated
as Na+ flows inward across the
membrane at one location.
2
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.
3
The depolarization-repolarization process
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.
is
Conduction Speed
• Increases with the diameter of an axon
• Myelinated axons also faster
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• Action potentials in myelinated axons
– Jump between the nodes of Ranvier in a
process called saltatory conduction
Schwann cell
Depolarized region
(node of Ranvier)
Myelin
sheath
––
–
Cell body
+
++
+
++
–––
––
–
+
+
Axon
+
++
––
–
Figure 48.15
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• Neurons communicate with other cells at
synapses
• Electrical synapse
– current flows directly from one cell to another
via a gap junction
• Most synapses are chemical synapses
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• Presynaptic neuron releases 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 action potential reaches a terminal
release of neurotransmitters into the synaptic
cleft
Postsynaptic cell
Presynaptic
cell
Synaptic vesicles
containing
neurotransmitter
5
Presynaptic
membrane
Ligandgated
ion channel
Ca2+ channel
Ca2+
4
2
3
Synaptic cleft
Figure 48.17
Neurotransmitter
Postsynaptic
membrane
Voltage-gated
1
Na+
K+
Ligand-gated
ion channels
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Postsynaptic
membrane
6
Neurotransmitters
• The same neurotransmitter can produce
different effects in different types of cells
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• Major neurotransmitters
Table 48.1
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Acetylcholine
• One of the most common neurotransmitters
• Can be inhibitory or excitatory
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Biogenic Amines
• Include epinephrine, norepinephrine,
dopamine, and serotonin
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Gases
• e.g. nitric oxide and carbon monoxide
– local regulators in the PNS
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Specialization and regionalization
• High degree of cephalization and distinct CNS
and PNS components in verts.
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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• Brain  integrative power
• Spinal cord integrates simple responses to
certain kinds of stimuli and conveys information
to and from the brain
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• Spinal cord and 4 ventricles of the brain
– Are hollow, (derived from the dorsal embryonic
nerve cord)
Gray matter
White
matter
Ventricles
Figure 48.20
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The Peripheral Nervous System
• Transmits information to and from the CNS
– regulates movement and internal environment
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PNS
Peripheral
nervous system
Somatic
Autonomic
nervous
system
nervous
system
Sympathetic
division
Figure 48.21
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Parasympathetic
division
Enteric
division
• Somatic nervous system
– signals to skeletal muscles
• Autonomic nervous system
– Regulates internal environment, involuntary
– Divided into the sympathetic, parasympathetic,
and enteric divisions
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Sympathetic and Parasympathetic divisions
• Antagonistic
Parasympathetic division
Sympathetic division
Action on target organs:
Location of
preganglionic neurons:
brainstem and sacral
segments of spinal cord
Neurotransmitter
released by
preganglionic neurons:
acetylcholine
Action on target organs:
Dilates pupil
of eye
Constricts pupil
of eye
Inhibits salivary
gland secretion
Stimulates salivary
gland secretion
Constricts
bronchi in lungs
Sympathetic
ganglia
Cervical
Accelerates heart
Slows heart
Location of
postganglionic neurons:
in ganglia close to or
within target organs
Stimulates activity
of stomach and
intestines
Stimulates
gallbladder
Thoracic
Inhibits activity
of pancreas
Stimulates glucose
release from liver;
inhibits gallbladder
Promotes emptying
of bladder
Figure 48.22
Location of
postganglionic neurons:
some in ganglia close to
target organs; others in
a chain of ganglia near
spinal cord
Lumbar
Stimulates
adrenal medulla
Promotes erection
of genitalia
Neurotransmitter
released by
preganglionic neurons:
acetylcholine
Inhibits activity of
stomach and intestines
Stimulates activity
of pancreas
Neurotransmitter
released by
postganglionic neurons:
acetylcholine
Relaxes bronchi
in lungs
Location of
preganglionic neurons:
thoracic and lumbar
segments of spinal cord
Inhibits emptying
of bladder
Sacral
Synapse
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Promotes ejaculation and
vaginal contractions
Neurotransmitter
released by
postganglionic neurons:
norepinephrine
• Sympathetic division
– “fight-or-flight” response
• Parasympathetic division
– return to self-maintenance functions
• Enteric division
– Activity of the digestive tract
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Embryonic Development of the Brain
– 3 embryonic regions: the forebrain, the
midbrain, and the hindbrain
Embryonic brain regions
Forebrain
Midbrain
Hindbrain
Midbrain
Hindbrain
Forebrain
Figure 48.23a
(a) Embryo at one month
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• By 5th week of development
– 5 brain regions have formed
Embryonic brain regions
Telencephalon
Diencephalon
Mesencephalon
Metencephalon
Myelencephalon
Mesencephalon
Metencephalon
Diencephalon
Myelencephalon
Spinal cord
Telencephalon
Figure 48.23b
(b) Embryo at five weeks
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• As development continues
– most profound change occurs in forebrain,
cerebrum
Brain structures present in adult
Cerebrum (cerebral hemispheres; includes cerebral
cortex, white matter, basal nuclei)
Diencnephalo (thalamus, hypothalamus, epithalamus)
Midbrain (part of brainstem)
Pons (part of brainstem), cerebellum
Medulla oblongata (part of brainstem)
Diencephalon:
Cerebral hemisphere
Hypothalamus
Thalamus
Pineal gland
(part of epithalamus)
Brainstem:
Midbrain
Pons
Pituitary
gland
Medulla
oblongata
Spinal cord
Cerebellum
Central canal
Figure 48.23c
(c) Adult
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The Brainstem
• Medulla oblongata, the pons, and the midbrain
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• The medulla oblongata
–
visceral functions
• The pons
– visceral functions
• The midbrain
– integration
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Arousal and Sleep
• reticular formation
Eye
Input from ears
Reticular formation
Figure 48.24
Input from touch,
pain, and temperature
receptors
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The Cerebellum
• Coordination of motor, perceptual, and
cognitive functions
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Diencephalon
• Epithalamus, thalamus, and hypothalamus
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• Thalamus
– sensory input and information output
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• Hypothalamus
– Homeostasis
– Basic survival behaviors such as feeding,
fighting, fleeing, and reproducing
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Hypothalamus & Circadian Rhythms
• Such as the sleep/wake cycle
• Biological clock
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• Biological clocks require external cues for
synchronized with environmental cycles
EXPERIMENT
In the northern flying squirrel (Glaucomys sabrinus), activity normally begins with the onset of darkness and ends
at dawn, which suggests that light is an important external cue for the squirrel. To test this idea, researchers monitored the activity of captive
squirrels for 23 days under two sets of conditions: (a) a regular cycle of 12 hours of light and 12 hours of darkness and (b) constant darkness.
The squirrels were given free access to an exercise wheel and a rest cage. A recorder automatically noted when the wheel was rotating and
when it was still.
(a) 12 hr light-12 hr dark cycle
Light
Dark
Light
Dark
1
Days of experiment
RESULTS
When the squirrels
were exposed to a regular light/dark
cycle, their wheel-turning activity
(indicated by the dark bars) occurred
at roughly the same time every day.
However, when they were kept in
constant darkness, their activity phase
began about 21 minutes later each day.
(b) Constant darkness
5
10
15
20
Figure 48.25
12
16
20
24
4
Time of day (hr)
8
12
12
16
20
24
Time of day (hr)
CONCLUSION
The northern flying squirrel’s internal clock can run in constant darkness, but it does so on
its own cycle, which lasts about 24 hours and 21 minutes. External (light) cues keep the clock running on a 24-hour cycle.
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4
8
12
The Cerebrum
• Develops from the embryonic telencephalon
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• Right and left cerebral hemispheres
Left cerebral
hemisphere
Right cerebral
hemisphere
Corpus
callosum
Neocortex
Figure 48.26
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Basal
nuclei
• In humans, the largest and most complex part
of the brain
– Is the cerebral cortex,  sensory information
is analyzed, motor commands issued, and
language generated
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• Corpus callosum
– Communication between the right and left
cerebral cortices
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• Cerebral cortex controls voluntary movement
and cognitive functions
• 4 lobes for ea. side
– Frontal, parietal, temporal, and occipital
Frontal lobe
Parietal lobe
Speech
Frontal
association
area
Taste
Speech
Smell
Somatosensory
association
area
Reading
Hearing
Auditory
association
area
Visual
association
area
Vision
Figure 48.27
Temporal lobe
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Occipital lobe
Frontal lobe
Parietal lobe
Genitalia
Toes
Lips
Jaw
Tongue
Tongue
Pharynx
Primary
motor cortex
Figure 48.28
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Abdominal
organs
Primary
somatosensory
cortex
• The left hemisphere
– language, math, logical operations, and the
processing of serial sequences
• The right hemisphere
– pattern recognition, nonverbal thinking, and
emotional processing
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• Portions of the frontal lobe, (Broca’s area and
Wernicke’s area)
language
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Emotions
• Limbic system
Thalamus
Hypothalamus
Prefrontal cortex
Olfactory
bulb
Amygdala
Figure 48.30
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Hippocampus
• limbic system includes 3 parts
– The amygdala, hippocampus, and olfactory
bulb
• mediate primary emotions
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Memory and Learning
• The frontal lobes
– short-term memory
– Interact with the hippocampus and amygdala
to consolidate long-term memory
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Neural Stem Cells
• cells that can differentiate into mature neurons
Figure 48.34
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Diseases and Disorders of the Nervous System
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Schizophrenia
• Hallucinations, delusions, blunted emotions,
and many other symptoms
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Depression
• Bipolar disorder and major depression
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• Bipolar disorder
– Manic (high-mood) and depressive (low-mood)
phases
• Major depression
– Persistent low mood
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Alzheimer’s Disease
• deterioration characterized by confusion,
memory loss, and other symptoms
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• AD is caused by the formation of
– Tangles and senile plaques in the brain
Senile plaque
Neurofibrillary tangle
Figure 48.35
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20 m
Parkinson’s Disease
• Motor disorder characterized by difficulty in
initiating movements, slowness of movement,
and rigidity
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