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Nervous System

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Chapter 48/49
Nervous Systems
The human brain contains an estimated 100
billion nerve cells, or neurons
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Each neuron may communicate with thousands of
other neurons
Complex information processing network is at work
Different neurons do different
things
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Functional magnetic resonance imaging (fMRI)
can reconstruct a three-dimensional map of brain activity
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The results of brain imaging and other research
methods revealed that groups of neurons function
in specialized circuits dedicated to different tasks
Nervous systems consist of circuits
of neurons and supporting cells
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All animals except sponges have some type of
nervous system
What distinguishes the nervous systems of
different animal groups is… how the neurons
are organized into circuits
Organization of Nervous
Systems
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The simplest animals with nervous systems,
the cnidarians
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They have neurons arranged in nerve nets
Nerve net
(a) Hydra (cnidarian)
Sea Stars
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Sea stars have a nerve net in each arm
connected by radial nerves to a central nerve
ring
Radial
nerve
Nerve
ring
(b) Sea star (echinoderm)
Flatworms
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In relatively simple cephalized animals, such
as flatworms a central nervous system (CNS)
is evident
Eyespot
Brain
Nerve
cord
Transverse
nerve
(c) Planarian (flatworm)
Annelids and arthropods
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Have segmentally arranged clusters of neurons
called ganglia
These ganglia connect to the CNS and make
up a peripheral nervous system (PNS)
Brain
Brain
Ventral
nerve
cord
Segmental
ganglion
Ventral
nerve cord
Segmental
ganglia
(d) Leech (annelid)
(e) Insect (arthropod)
Molluscs
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Nervous systems in molluscs correlate with
the animals’ lifestyles
Sessile molluscs have simple systems
More complex molluscs have more
sophisticated systems
Anterior
nerve ring
Ganglia
Brain
Longitudinal
nerve cords
(f) Chiton (mollusc)
Ganglia
(g) Squid (mollusc)
Vertebrates
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The central nervous system consists of a
brain and dorsal spinal cord
The PNS connects to the CNS via nerves
Brain
Spinal
cord
(dorsal
nerve
cord)
Sensory
ganglion
(h) Salamander (chordate)
Information Processing
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Nervous systems process information in three stages:
Sensory input, integration, and motor output
Sensory input
Integration
Sensor
Motor output
Effector
Peripheral nervous
system (PNS)
Central nervous
system (CNS)
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Sensory neurons transmit information from
sensors (receptors) that detect external stimuli
and internal conditions
Sensory information is sent to the CNS
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Where interneurons integrate the information
Motor output leaves the CNS via motor
neurons
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Which communicate with effector cells
Cells of the Nervous System
Neuron Structure
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Cell body
Dendrites
Axons
Myelin Sheath
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Schwann cells
Synaptic Terminals
Synapse
Neurotransmitters
Dendrites
Cell body
Nucleus
Synapse
Signal
Axon direction
Axon hillock
Presynaptic cell
Postsynaptic cell
Myelin sheath
Synaptic
terminals
Types of neurons
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Sensory Neurons
Interneurons
Motor Neurons
Supporting Cells (Glia)
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Glia are supporting cells
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They are essential for the structural integrity of the
nervous system and for the normal functioning of
neurons
Node of Ranvier
Layers of myelin
Axon
Schwann
cell
Axon
Myelin sheath
Nodes of
Ranvier
Schwann
cell
Nucleus of
Schwann cell
0.1 Вµm
Ion pumps and ion channels
maintain the resting potential of a
neuron
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Across its plasma membrane, every cell has a
voltage called a membrane potential
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Resting Membrane Potential: membrane potential
of a neuron that is not transmitting signals
The inside of a cell is negative relative to the
outside
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The concentration of Na+ is higher in the
extracellular fluid than in the cytosol
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While the opposite is true for K+
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A neuron that is not transmitting signals
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Contains many open K+ channels and fewer open
Na+ channels in its plasma membrane
The diffusion of K+ and Na+ through these
channels
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Leads to a separation of charges across the
membrane, producing the resting potential
Gated Ion Channels
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Gated ion channels open or close
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In response to membrane stretch or the binding of
a specific ligand
In response to a change in the membrane
potential
Action Potential
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Action potentials are the signals conducted by
nerve fibers
If a cell has gated ion channels
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Its membrane potential may change in response to
stimuli that open or close those channels
Hyperpolarization
Stimuli
Membrane potential (mV)
Stimuli may trigger an
increase in the
magnitude of the
membrane potential
+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
a larger hyperpolarization.
Depolarization
stimuli may trigger
a reduction in the
magnitude of the
membrane
potential
+50
Membrane potential (mV)
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Stimuli
0
–50
Threshold
Resting Depolarizations
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
larger depolarization.
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Hyperpolarization and depolarization
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Are both called graded potentials because the
magnitude of the change in membrane potential
varies with the strength of the stimulus
Production of Action
Potentials
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In most neurons,
depolarizations are graded
only up to a certain
membrane voltage, called
the threshold
A stimulus strong enough to
produce a depolarization
that reaches the threshold
triggers a different type of
response, called an action
potential
Stronger depolarizing stimulus
+50
Membrane potential (mV)
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Action
potential
0
–50
Threshold
Resting
potential
–100
0 1 2 3 4 5 6
Time (msec)
(c) Action potential triggered by a
depolarization that reaches the
threshold.
Action Potential
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An action potential
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Is a brief all-or-none depolarization of a neuron’s
plasma membrane
Is the type of signal that carries information along
axons
Step 1…Sodium gates
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Both voltage-gated Na+ channels and voltagegated K+ channels
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Are involved in the production of an action potential
When a stimulus depolarizes the membrane
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Na+ channels open, allowing Na+ to diffuse into the
cell
Step 2…Potassium gates
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As the action potential subsides
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K+ channels open, and K+ flows out of the cell
A refractory period follows the action potential
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During which a second action potential cannot be
initiated
Generation of an action potential
Na+
Na+
– –
– –
– –
– –
+ +
+ +
+ +
+ +
K+
Rising phase of the 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
the membrane positive with respect
to the outside.
Na+
+ +
+ +
– –
– –
+50
+ +
– –
K+
– –
–50
Na+
+ + + + + + + +
– –
3
+ +
– –
– –
2
4
Falling phase of the action potential
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.
Threshold
5
1
1
Resting potential
Na+
Potassium
channel
+ +
Activation
gates
+ +
Na+
+ +
+ +
– –
– –
+ +
+ +
– –
– –
+ +
K+
Plasma membrane
– – – – – – – –
Cytosol
– –
Sodium
channel
1
+ +
Time
Depolarization A stimulus opens the
activation gates on some Na+ channels. Na+
influx through those channels depolarizes the
membrane. If the depolarization reaches the
threshold, it triggers an action potential.
Extracellular fluid
– –
Action
potential
0
–100
2
+ +
4
Na+
+ +
+ +
K+
Membrane potential
(mV)
3
Na+
Na+
– –
K+
– –
Inactivation
gate
Resting state
The activation gates on the Na+ and K+ channels
are closed, and the membrane’s resting potential is maintained.
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
Axon
Action
potential
–
+
+
+
+
+
+
+
–
–
–
–
–
–
+
+
–
–
–
–
–
–
–
–
+
+
+
+
+
+
–
+
Na+
Action
potential
K+
+
+
–
–
–
+
–
+
1
2
–
+
+
+
+
+
–
–
–
–
Na+
–
+
+
–
–
–
–
+
–
–
+
+
+
+
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.
K+
Action
potential
K+
+
+
+
–
–
–
–
–
+
–
–
–
–
+
+
+
+
+
–
+
K+
An action potential is generated
as Na+ flows inward across the
membrane at one location.
3
–
–
–
+
+
+
+
+
+
–
–
Na+
–
An electrical
current
depolarizes
the
neighboring
region of
the axon
membrane
The depolarization-repolarization process is
repeated in the next region of the
membrane. In this way, local currents
of ions across the plasma membrane
cause the action potential to be propagated
along the length of the axon.
Conduction Speed
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The speed of an action potential
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Increases with the diameter of an axon
In vertebrates, axons are myelinated
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Also causing the speed of an action potential to
increase
Saltatory Conduction
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Action potentials in myelinated axons
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Jump between the nodes of Ranvier in a process
called saltatory conduction
Schwann cell
Depolarized region
(node of Ranvier)
Myelin
sheath
––
–
Cell body
+
++
+
++
–––
––
–
+
+
+
++
Axon
––
–
Synapse
Electrical synapse
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Electrical current flows
directly from one cell to
another via a gap junction
Chemical synapse Postsynaptic
neuron
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A presynaptic neuron
releases chemical
neurotransmitters, which
are stored in the synaptic
terminal
Synaptic
terminal
of presynaptic
neurons
5 Вµm
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Neurotransmitter release
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When an action potential reaches a terminal
the final result is the release of
neurotransmitters into the synaptic cleft
Postsynaptic cell
Presynaptic
cell
Synaptic vesicles
containing
neurotransmitter
5
Presynaptic
membrane
Na+
K+
Neurotransmitter
Postsynaptic
membrane
Ligandgated
ion channel
Voltage-gated
Ca2+ channel
1 Ca2+
4
2
Synaptic cleft
3
Ligand-gated
ion channels
Postsynaptic
membrane
6
Synaptic Transmission
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Neurotransmitters bind to ligand-gated ion
channels
Binding causes the ion channels to open,
generating a postsynaptic potential and
generation of action potential
Fate of neurotransmitter in the cleft
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Diffuses out of the synaptic cleft
Taken up by surrounding cells and degraded by
enzymes
Degraded in the cleft
Neurotransmitters
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Different neurons may release different
neurotransmitters
The same neurotransmitter can produce
different effects in different types of cells
Neurotransmitters
Neurotransmitters
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Acetylcholine
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Biogenic amines
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Include epinephrine, norepinephrine, dopamine, and
serotonin
Are active in the CNS and PNS
Amino acids and peptides
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Is one of the most common neurotransmitters in both
vertebrates and invertebrates
Can be inhibitory or excitatory
Are active in the brain
Gases such as nitric oxide and carbon monoxide
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Are local regulators in the PNS
Vertebrate Nervous System
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Regionally specialized
In all vertebrates, the
nervous system shows a
high degree of
cephalization and distinct
CNS and PNS
components
Central nervous
system (CNS)
Brain
Spinal cord
Peripheral nervous
system (PNS)
Cranial
nerves
Ganglia
outside
CNS
Spinal
nerves
Central Nervous System
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The brain provides the integrative power
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That underlies the complex behavior of vertebrates
The spinal cord integrates simple responses to
certain kinds of stimuli
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And conveys information to and from the brain
Peripheral Nervous System
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The PNS transmits information to and from
the CNS
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The cranial nerves originate in the brain
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And plays a large role in regulating a vertebrate’s
movement and internal environment
And terminate mostly in organs of the head and
upper body
The spinal nerves originate in the spinal cord
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And extend to parts of the body below the head
PNS components
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The somatic nervous system
The autonomic nervous system
Peripheral
nervous system
Somatic
nervous
system
Autonomic
nervous
system
Sympathetic
division
Parasympathetic
division
Enteric
division
Somatic and Autonomic
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The somatic nervous system
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Carries signals to skeletal muscles
The autonomic nervous system
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Regulates the internal environment, in an
involuntary manner
Is divided into the sympathetic, parasympathetic,
and enteric divisions
Sympathetic and
parasympathetic divisions
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They have antagonistic effects on target organs
Sympathetic division
Parasympathetic 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
Inhibits activity of
stomach and intestines
Thoraci
c
Inhibits activity
of pancreas
Stimulates activity
of pancreas
Neurotransmitter
released by
postganglionic neurons:
acetylcholine
Stimulates
gallbladder
Stimulates glucose
release from liver;
inhibits gallbladder
Lumbar
Stimulates
adrenal medulla
Promotes emptying
of bladder
Promotes erection
of genitalia
Relaxes bronchi
in lungs
Inhibits emptying
of bladder
Synapse
Sacral
Promotes ejaculation and
vaginal contractions
Location of
preganglionic neurons:
thoracic and lumbar
segments of spinal cord
Neurotransmitter
released by
preganglionic neurons:
acetylcholine
Location of
postganglionic neurons:
some in ganglia close to
target organs; others in
a chain of ganglia near
spinal cord
Neurotransmitter
released by
postganglionic neurons:
norepinephrine
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The sympathetic division
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The parasympathetic division
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Correlates with the “fight-or-flight” response
Promotes a return to self-maintenance functions
The enteric division
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Controls the activity of the digestive tract, pancreas,
and gallbladder
The Brainstem
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The brainstem consists of three parts: the medulla
oblongata, the pons, and the midbrain
Brainstem
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The medulla oblongata
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The pons
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Also participates in visceral functions
The midbrain
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Contains centers that control several visceral
functions
Contains centers for the receipt and integration of
several types of sensory information
All three areas are center of reflex actions
Arousal and Sleep
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A diffuse network of
neurons called the
reticular formation is
present in the core of the
brainstem
A part of the reticular
formation, the reticular
activating system (RAS)
regulates sleep and
arousal
Eye
Reticular formation
Input from touch,
pain, and temperature
receptors
Input from ears
The Cerebellum
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Important for
coordination and error
checking during motor,
perceptual, and
cognitive functions
Also involved in learning
and remembering motor
skills
The Diencephalon
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The embryonic diencephalon develops into
three adult brain regions
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The epithalamus, thalamus, and hypothalamus
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The epithalamus
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The thalamus
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Includes the pineal gland and the choroid plexus
Is the main input center for sensory information
going to the cerebrum and the main output center
for motor information leaving the cerebrum
The hypothalamus regulates
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Homeostasis
Basic survival behaviors such as feeding, fighting,
fleeing, and reproducing
Circadian Rhythms
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The hypothalamus also regulates circadian
rhythms (Such as the sleep/wake cycle)
Animals usually have a biological clock
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Which is a pair of suprachiasmatic nuclei (SCN)
found in the hypothalamus
The Cerebrum
Cerebral Hemispheres
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The cerebrum has right and left cerebral
hemispheres that each consist of cerebral
cortex overlying white matter and basal nuclei
Left cerebral
hemisphere
Right cerebral
hemisphere
Corpus
callosum
Cortex
Basal
nuclei
Basal Nuclei
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The basal nuclei
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Are important centers for planning and learning
movement sequences
Cerebral Cortex
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In mammals
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The cerebral cortex has a convoluted surface
called the neocortex
In humans it’s the largest and most complex
part of the brain: where sensory information
is analyzed, motor commands are issued,
and language is generated
A thick band of axons, the corpus callosum
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Provides communication between the right and
left cerebral cortices
Cerebral Cortex
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The cerebral cortex controls voluntary
movement and cognitive functions
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
Temporal lobe
Occipital lobe
Information Processing in the
Cerebral Cortex
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Each of the lobes contains primary sensory
areas and association areas
Specific types of sensory input
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Enter the primary sensory areas
Adjacent association areas
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Process particular features in the sensory input and
integrate information from different sensory areas
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In the somatosensory cortex and motor
cortex neurons are distributed according
to the part of the body that generates
sensory input or receives motor input
Frontal lobe
Parietal lobe
Toes
Lips
Jaw
Tongue
Tongue
Pharynx
Primary
motor cortex
Figure 48.28
Genitalia
Abdominal
organs
Primary
somatosensory
cortex
Lateralization of Cortical
Function
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During brain development, in a process called
lateralization
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The left hemisphere
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Competing functions segregate and displace each other in
the cortex of the left and right cerebral hemispheres
Becomes more adept at language, math, logical
operations, and the processing of serial sequences
The right hemisphere
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Is stronger at pattern recognition, nonverbal thinking, and
emotional processing
Language and Speech
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Studies of brain activity have mapped specific
areas of the brain responsible for language
and speech
Max
Hearing
words
PET scans
showing
brain activity
levels
Seeing
words
Min
Speaking
words
Generating
words
Language and Speech
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Portions of the frontal lobe, Broca’s area and
Wernicke’s area are essential for the
generation and understanding of language
Broca’s area damage: comprehend speech
but have impaired ability to mechanically form
speech
Wernicke’s area damage: no language
comprehension, can’t understand either
spoken or written language. Speak in rapid
nonsensical manner
Emotions
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The limbic system is a ring of structures
around the brainstem
Thalamus
Hypothalamus
Prefrontal cortex
Olfactory
bulb
Amygdala
Hippocampus
Limbic system
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This limbic system includes three parts of the cerebral
cortex: The amygdala, hippocampus, and olfactory
bulb. Also some parts of the thalamus and
hypothalamus
These structures interact with the neocortex to
mediate primary emotions and attach emotional
“feelings” to survival-related functions (reproduction,
aggression, feeding)
Structures of the limbic system form in early
development and provide a foundation for emotional
memory, associating emotions with particular events
or experiences
Memory and Learning
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The frontal lobes
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Are a site of short-term memory
Interact with the hippocampus and amygdala to
consolidate long-term memory
Many sensory and motor association areas of
the cerebral cortex are involved in storing and
retrieving words and images
CNS injuries and diseases
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Unlike the PNS, the mammalian CNS
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Cannot repair itself when damaged or diseased
Research on nerve cell development and stem
cells
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Hot area of research
May one day make it possible for physicians to
repair or replace damaged neurons
Neural Stem Cells
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The adult human brain
contains stem cells that
can differentiate into
mature neurons
The induction of stem cell
differentiation and the
transplantation of cultured
stem cells
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Are potential methods for
replacing neurons lost to
trauma or disease
Diseases and Disorders of the
Nervous System
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Mental illnesses and neurological disorders take a
huge toll on society
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patient’s loss of a productive life
high cost of long-term health care
The direct costs of mental health services in the United States in 1996 totaled
$69.0 billion. This figure represents 7.3 percent of total health spending. An
additional $17.7 billion was spent on Alzheimer’s disease The indirect costs of
mental illness were estimated in 1990 at $78.6 billion
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Causes (genetic & environmental) are not often
clear at present
Treatments that exist are not great and usually
amount to control of symptoms and not cure for the
underlying abnormality
Schizophrenia
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About 1% of the world’s population suffers
from schizophrenia
There are several forms each characterized
by an inability to distinguish reality
Symptoms
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Hallucinations, delusions, blunted emotions, and
many other symptoms
Treatments have focused on brain pathways
that use dopamine as a neurotransmitter
Depression
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Two broad forms of depressive illness:
bipolar disorder and major depression
Bipolar disorder is characterized by
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In major depression
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Manic (high-mood) and depressive (low-mood)
phases
Patients have a persistent low mood
Treatments for these types of depression
include a variety of drugs such as Prozac and
lithium
Alzheimer’s Disease
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Alzheimer’s disease (AD)
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Is a mental deterioration characterized by confusion,
memory loss, and other symptoms
AD is caused by the formation of neurofibrillary
tangles and senile plaques (amyloid protein) in the
brain
Senile plaque Neurofibrillary tangle
20 пЃ­m
Parkinson’s Disease
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Parkinson’s disease is a motor disorder
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Caused by the death of dopamine-secreting
neurons in midbrain nucleus (substantia nigra)
Characterized by difficulty in initiating movements,
slowness of movement, and rigidity
Multiple sclerosis
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Caused by immune destruction of myelin
sheaths
Leads to improper nerve impulse conduction
and associated loss of function
Nerve toxins
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Tetanus
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Botulism
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Blocks inhibitory synapses leading to spastic
paralysis
Prevents release of Acetylcholine leading to
flaccid paralysis
Curare
пЃ¬
Prevents binding of Acetylcholine to receptors
leading to flaccid paralysis
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