Transcript Ch 48 Nervous System

Chapter 48
Nervous Systems
Figure 48.1
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• Nervous systems consist of circuits of neurons
and supporting cells
• All animals except sponges
– Have some type of nervous system
• What distinguishes the nervous systems of
different animal groups
– Is how the neurons are organized into circuits
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Organization of Nervous Systems
The simplest animals with
nervous systems, the cnidarians
(neurons arranged in nerve nets)
Sea stars have a nerve net in each
arm (Connected by radial nerves to a
central nerve ring)
Radial
nerve
Nerve
ring
Nerve net
Figure 48.2a
(a) Hydra (cnidarian)
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Figure 48.2b
(b) Sea star (echinoderm)
In cephalized animals,
such as flatworms (A CNS
is evident)
In vertebrates: The central nervous
system consists of a brain and
dorsal spinal cord (PNS connects
to the CNS)
Eyespot
Brain
Brain
Nerve
cord
Transverse
nerve
Spinal
cord
(dorsal
nerve
cord)
Figure 48.2h
Figure 48.2c
(c) Planarian (flatworm)
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Sensory
ganglion
(h) Salamander (chordate)
Information Processing
• Nervous systems process information in three
stages
– Sensory input, integration, and motor output
Sensory input
Integration
Sensor
Motor output
Effector
Figure 48.3
Peripheral nervous
system (PNS)
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Central nervous
system (CNS)
• Sensory neurons transmit information from
sensors
– That detect external stimuli and internal
conditions
• Sensory information is sent to the CNS
– Where interneurons integrate the information
• Motor output leaves the CNS via motor
neurons
– Which communicate with effector cells
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Neuron Structure
• Most of a neuron’s organelles
– Are located in the cell body
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
• Oligodendrocytes (in the CNS) and Schwann
cells (in the PNS)
– Are glia (supporting cells) that form the myelin
sheaths around the axons of many vertebrate
neurons
Node of Ranvier
Layers of myelin
Axon
Schwann
cell
Axon
Myelin sheath
Nodes of
Ranvier
Schwann
cell
Nucleus of
Schwann cell
Figure 48.8
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0.1 µm
• Ion pumps and ion channels maintain the
resting potential of a neuron
• Across its plasma membrane, every cell has a
voltage
– Called a membrane potential
• The inside of a cell is negative
– Relative to the outside
• The resting potential
– Is the membrane potential of a neuron that is
not transmitting signals
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• In all neurons, the resting potential
–
Depends on the ionic gradients that exist across the plasma membrane
–
The concentration of Na+ is higher in the extracellular fluid than in the cytosol
–
While the opposite is true for K+
EXTRACELLULAR
FLUID
CYTOSOL
[Na+]
15 mM
–
+
[Na+]
150 mM
[K+]
150 mM
–
+
[K+]
5 mM
–
+
10 mM
–
[Cl–]
+ 120 mM
[A–]
100 mM
–
+
[Cl–]
Plasma
membrane
Figure 48.10
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• Impulse begins when a
neuron is stimulated by
another neuron or by the
environment
• Electrical impulse moves
in one direction:
Dendrites → Cell Body → Axon
• Synapse: gap between 2
neurons
• Neurotransmitters send
the signal to the following
neuron
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Action Potential
• A stimulus strong enough to produce a
depolarization that reaches the threshold
– Triggers a different type of response, called an
Stronger depolarizing stimulus
action potential
Membrane potential (mV)
+50
Action
potential
0
–50
Threshold
Resting
potential
–100
Figure 48.12c
0 1 2 3 4 5 6
Time (msec)
(c) Action potential triggered by a
depolarization that reaches the
threshold.
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•
Depolarization: reduction in magnitude of membrane potential
(inside becomes less negative)
•
Hyperpolarization: increase in the magnitude of the membrane
potential (inside becomes more negative)
•
When a stimulus depolarizes the membrane
–
•
As the action potential subsides
–
•
Na+ channels open, allowing Na+ to diffuse into the cell
K+ channels open, and K+ flows out of the cell
A refractory period follows the action potential
–
During which a second action potential cannot be initiated
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• The 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
Threshold
5
1
1
Resting potential
Na+
Potassium
channel
+ +
Activation
gates
+ +
+ +
– –
– –
+ +
+ +
– –
– –
+ +
K+
– – – – – – – –
Cytosol
– –
Sodium
channel
1
Na+
+ +
Plasma membrane
Figure 48.13
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.
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.
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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
• At the site where the action potential is generated, usually
the axon hillock
– An electrical current depolarizes the neighboring
region of the axon membrane
Axon
VIDEO
Action
potential
– –
+ ++
Na
+ +
– –
K+
+ +
– –
– –
+ +
K+
Figure 48.14
+
–
–
+
+
–
–
+
+
+
+
+
+
+
–
–
+
–
–
+
–
–
+
–
–
+
–
–
+
–
–
+
+
–
–
+
+
–
–
+
Action
potential
–
+
Na
–
+
+
+ +
–
–
K+
+ +
– –
– –
+ +
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 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
•
The speed of an action potential
–
•
Increases with the diameter of an axon
In vertebrates, axons are myelinated
–
Also causing the speed of an action potential to increase
Schwann cell
Depolarized region
(node of Ranvier)
Myelin
sheath
––
–
Cell body
Figure 48.15
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+
++
+
++
–––
––
–
+
+
+
++
Axon
––
–
• Neurons communicate with other cells at
synapses
• In an electrical synapse
– Electrical current flows directly from one cell to
another via a gap junction
• The vast majority of synapses
– Are chemical synapses
VIDEO
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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
3
Synaptic cleft
Figure 48.17
Ligand-gated
ion channels
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Postsynaptic
membrane
6
Major neurotransmitters
• Can produce different effects in different types of
cells
Table 48.1
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• The vertebrate nervous system is regionally
specialized
• In all vertebrates, the nervous system
– Shows a high degree of cephalization and
distinct CNS and PNS components
Central nervous
system (CNS)
Brain
Spinal cord
Peripheral nervous
system (PNS)
Cranial
nerves
Ganglia
outside
CNS
Spinal
nerves
Figure 48.19
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• The central canal of the spinal cord and the
four ventricles of the brain
– Are hollow, since they are derived from the
dorsal embryonic nerve cord
Gray matter
White
matter
Ventricles
Figure 48.20
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The Peripheral Nervous System
• The PNS transmits information to and from the
CNS
– And plays a large role in regulating a
vertebrate’s movement and internal
environment
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• The cranial nerves originate in the brain
– And terminate mostly in organs of the head
and upper body
• The spinal nerves originate in the spinal cord
– And extend to parts of the body below the
head
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• The PNS can be divided into two functional
components
– The somatic nervous system and the
autonomic nervous system
Peripheral
nervous system
Somatic
nervous
system
Autonomic
nervous
system
Sympathetic
division
Figure 48.21
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Parasympathetic
division
Enteric
division
• The somatic nervous system
– Carries signals to skeletal muscles
• The autonomic nervous system
– Regulates the internal environment, in an
involuntary manner
– Is divided into the sympathetic,
parasympathetic, and enteric divisions
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• The sympathetic and parasympathetic divisions
– Have antagonistic effects on target organs
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
Synapse
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Sacral
Promotes ejaculation and
vaginal contractions
Neurotransmitter
released by
postganglionic neurons:
norepinephrine
• The sympathetic division
– Correlates with the “fight-or-flight” response
• The parasympathetic division
– Promotes a return to self-maintenance
functions
• The enteric division
– Controls the activity of the digestive tract,
pancreas, and gallbladder
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The Brainstem
• The brainstem consists of three parts
– The medulla oblongata, the pons, and the
midbrain
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• The medulla oblongata
– Contains centers that control several visceral
functions
• The pons
– Also participates in visceral functions
• The midbrain
– Contains centers for the receipt and integration
of several types of sensory information
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The Cerebellum
• The cerebellum
– Is important for coordination and error
checking during motor, perceptual, and
cognitive functions, learning and remembering
motor skills
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The Diencephalon
• The embryonic diencephalon develops into
three adult brain regions
– The epithalamus, thalamus, and hypothalamus
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• The epithalamus
– Includes the pineal gland and the choroid
plexus
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• The thalamus
– Is the main input center for sensory information
going to the cerebrum and the main output
center for motor information leaving the
cerebrum
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• The hypothalamus regulates
– Homeostasis
– Basic survival behaviors such as feeding,
fighting, fleeing, and reproducing
– sleep/wake cycle
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The Cerebrum
• The cerebrum
– Develops from the embryonic telencephalon
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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
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, where sensory
information is analyzed, motor commands are
issued, and language is generated
• A thick band of axons, the corpus callosum
– Provides communication between the right and
left cerebral cortices
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• The cerebral cortex controls voluntary
movement and cognitive functions
• Each side of the cerebral cortex has four lobes
– 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
• The left hemisphere
– Becomes more adept at language, math,
logical operations, and the processing of serial
sequences
• The right hemisphere
– Is stronger at pattern recognition, nonverbal
thinking, and emotional processing
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• CNS injuries and diseases are the focus of
much research
• Unlike the PNS, the mammalian CNS
– Cannot repair itself when damaged or
assaulted by disease
• Current research on nerve cell development
and stem cells
– May one day make it possible for physicians to
repair or replace damaged neurons
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Schizophrenia
• About 1% of the world’s population
– Suffers from schizophrenia
• Schizophrenia is characterized by
– Hallucinations, delusions, blunted emotions,
and many other symptoms
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Depression
• Two broad forms of depressive illness are
known
– Bipolar disorder and major depression
• Bipolar disorder is characterized by
– Manic (high-mood) and depressive (low-mood)
phases
• In major depression
– Patients have a persistent low mood
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Alzheimer’s Disease
• Alzheimer’s disease (AD)
– Is a mental deterioration characterized by
confusion, memory loss, and other symptoms
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Parkinson’s Disease
• Parkinson’s disease is a motor disorder
– Caused by the death of dopamine-secreting
neurons in the substantia nigra
– Characterized by difficulty in initiating
movements, slowness of movement, and
rigidity
– No cure
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