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D-Anatomy and
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Chapter 10
Lecture
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Nervous System I
10.1: Introduction
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Dendrites
• Cell types in neural tissue:
• Neurons
• Neuroglial cells (also known as
neuroglia, glia, and glial)
Cell body
Nuclei of
neuroglia
Axon
2
© Ed Reschke
Divisions of the
Nervous System
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Brain
• Central Nervous System (CNS)
• Brain
• Spinal cord
• Peripheral Nervous System (PNS)
• Cranial nerves
• Spinal nerves
Cranial
nerves
Spinal
cord
Spinal
nerves
3
(a)
Divisions Nervous System
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Central Nervous System
(brain and spinal cord)
Brain
Peripheral Nervous System
(cranial and spinal nerves)
Cranial
nerves
Sensory division
Spinal
cord
Sensory receptors
Spinal
nerves
Motor division
Somatic
Nervous
System
Skeletal muscle
Autonomic
Nervous
System
Smooth muscle
Cardiac muscle
Glands
4
(a)
(b)
10.2: General Functions of
the Nervous System
• The three general functions of the nervous system:
• Receiving stimuli = sensory function
• Deciding about stimuli = integrative function
• Reacting to stimuli = motor function
5
Functions of Nervous System
• Sensory Function
• Sensory receptors gather information
• Information is carried to the CNS
• Motor Function
• Decisions are acted upon
• Impulses are carried to
effectors
• Integrative Function
• Sensory information used to create:
• Sensations
• Memory
• Thoughts
• Decisions
6
10.3: Description of Cells of
the Nervous System
• Neurons vary in size and shape
• They may differ in length and size of their axons and dendrites
• Neurons share certain features:
• Dendrites
• A cell body
• An axon
7
Neuron Structure
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Chromatophilic
substance
(Nissl bodies)
Dendrites
Cell body
Nucleus
Nucleolus
Neurofibrils
Axonal
hillock
Impulse
Axon
Synaptic knob of
axon terminal
Nodes of Ranvier
Myelin (cut)
Axon
Nucleus of
Schwann cell
Schwann
cell
Portion of a
collateral
8
Myelination of Axons
• White Matter
• Contains myelinated axons
• Considered fiber tracts
• Gray Matter
• Contains unmyelinated structures
• Cell bodies, dendrites
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Dendrite
Unmyelinated
region of axon
Myelinated region of axon
Node of Ranvier
Axon
Neuron
cell body
Neuron
nucleus
(a)
Enveloping
Schwann cell
Schwann
cell nucleus
Longitudinal
groove
Unmyelinated
axon
(c)
9
10.4: Classification of Neurons
and Neuroglia
• Neurons vary in function
• They can be sensory, motor, or integrative neurons
• Neurons vary in size and shape, and in the number of axons and dendrites
that they may have
• Due to structural differences, neurons can be classified into three (3) major
groups:
• Bipolar neurons
• Unipolar neurons
• Multipolar neurons
10
Classification of Neurons:
Structural Differences
• Bipolar neurons
• Two processes
• Eyes, ears, nose
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Dendrites
• Unipolar neurons
• One process
• Ganglia of PNS
• Sensory
Peripheral
process
Axon
• Multipolar neurons
• 99% of neurons
• Many processes
• Most neurons of CNS
Direction
of impulse
Central
process
Axon
Axon
11
(a) Multipolar
(b) Bipolar
(c) Unipolar
Classification of Neurons:
Functional Differences
• Sensory Neurons
• Afferent
• Carry impulse to CNS
• Most are unipolar
• Some are bipolar
• Interneurons
• Link neurons
• Aka association
neurons or internuncial
neurons
• Multipolar
• Located in CNS
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Central nervous system
Peripheral nervous system
Cell body
Dendrites
Sensory
receptor
Cell body
Axon
(central process)
Axon
(peripheral process)
Sensory (afferent) neuron
Interneurons
• Motor Neurons
• Multipolar
• Carry impulses away from CNS
• Carry impulses to effectors
Motor (efferent) neuron
Axon
Effector
(muscle or gland)
Axon
Axon
terminal
12
Types of Neuroglial Cells
in the PNS
1) Schwann Cells
• Produce myelin found on peripheral myelinated neurons
• Speed up neurotransmission
2) Satellite Cells
• Support clusters of neuron cell bodies (ganglia)
13
Types of Neuroglial Cells
in the CNS
1) Microglia
• Phagocytic cell
3) Oligodendrocytes
• Myelinating cell
2) Astrocytes
• Scar tissue
• Mop up excess ions, etc.
• aid in metabolism of certain
substances
• form blood-brain barrier
4) Ependyma or ependymal
• Ciliated
• Line central canal of spinal cord
• Line ventricles of brain
• help regulate composition of CSF
14
Types of Neuroglial Cells
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Fluid-filled cavity
of the brain or
spinal cord
Neuron
Ependymal
cell
Oligodendrocyte
Astrocyte
Microglial cell
Axon
Myelin
sheath (cut)
Capillary
Node of
Ranvier
15
Regeneration of A Nerve Axon
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Skeletal
muscle fiber
Motor neuron
cell body
Changes
over time
Site of injury
Schwann cells
Axon
(a)
Distal portion of
axon degenerates
(b)
Proximal end of injured axon
regenerates into tube of sheath cells
(c)
Schwann cells
degenerate
(d)
Schwann cells
proliferate
(e)
Former connection
reestablished
16
20
10.5: The Synapse
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• Nerve impulses pass from
neuron to neuron at synapses,
moving from a pre-synaptic
neuron to a post-synaptic
neuron.
Synaptic
cleft
Impulse
Dendrites
Axon of
presynaptic
neuron
Axon of
postsynaptic
neuron
Axon of
presynaptic
neuron
Impulse
Cell body of
postsynaptic
neuron
Impulse
17
Synaptic Transmission
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Direction of
nerve impulse
• Neurotransmitters are
released when impulse
reaches synaptic knob
• As nerve impulse reaches
the synaptic knob, Ca2+
channels open & allow Ca2+
to move into the synaptic
knob
• this stimulates the synaptic
vesicles to fuse with the
membrane allowing
exocytosis of the
neurotransmitter
Axon
Ca+2
Synaptic knob
Synaptic
vesicles
Presynaptic neuron
Ca+2
Cell body or dendrite
of postsynaptic neuron
Mitochondrion
Ca+2
Synaptic
vesicle
Vesicle releasing
neurotransmitter
Axon
membrane
Neurotransmitter
Synaptic cleft
Polarized
membrane
Depolarized
membrane
(a)
18
10.6: Cell Membrane Potential
• A cell membrane is usually electrically charged, or polarized, so that the
inside of the membrane is negatively charged with respect to the outside of
the membrane (which is then positively charged).
• This is as a result of unequal distribution of ions on the inside and the
outside of the membrane.
19
Distribution of Ions
• Potassium (K+) ions are the major intracellular positive ions (cations).
• Sodium (Na+) ions are the major extracellular positive ions (cations).
• This distribution is largely created by the Sodium/Potassium Pump (Na+/K+
pump).
• This pump actively transports sodium ions out of the cell and potassium
ions into the cell.
• also due to the presence of ion-selective channels
20
Resting Potential
• Resting Membrane Potential
(RMP):
• It is a polarized membrane
• Inside of cell is negative relative
to the outside of the cell
• Due to distribution of ions inside
vs. outside
• Na+/K+ pump restores
• 70 mV difference from inside to
outside of cell
• RMP = -70 mV
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High Na+
Low Na+
Impermeant
anions
High K+
Axon
Cell body
Low K+
Axon terminal
(a)
+
–
+
–
+
–
+
–
–
+
+
–
+
–
+ –
–
+
–
+
–
+
–
+
–
+
+
–
–
+
–
+
–70 mV
(b)
+
–
High Na+
Na+
Low K+
+
+
–
–
Low Na+
Pump
K+
High K+
+ –
–
+
+
–
+
–
+
–
–
+
–
+
–
+
–
+
+
–
–
+
+
–
–
+
–70 mV
(c)
25
21
+
–
–
+
Local Potential Changes
• Caused by various stimuli:
• Temperature changes
• Light
• Pressure
Gatelike mechanism
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Protein
Cell
membrane
Fatty acid
tail
(a) Channel closed
Phosphate
head
• Environmental changes affect the membrane potential by
opening a gated ion channel
(b) Channel open
• Channels are 1) chemically gated, 2) voltage gated, or 3)
mechanically gated
22
Local Potential Changes
• If membrane potential becomes more negative, it has hyperpolarized
• If membrane potential becomes less negative, it has depolarized
• Graded (or proportional) to intensity of stimulation, meaning the
greater the stimulation, the greater the depolarization
• if the depolarization is great enough, reach threshold potential
• Reaching threshold potential results in a nerve impulse, starting an
action potential
23
Local Potential Changes
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Na+
Na+
–62 mV
Neurotransmitter
(a)
Chemically-gated
Na+ channel
Presynaptic
neuron
Voltage-gated
Na+ channel
Trigger zone
Na+
Na+
Na+
Na+
Na+
–55 mV
24
(b)
Action Potentials
• At rest, the membrane is
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Na+
polarized (RMP = -70)
• Threshold stimulus
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
reached (-55)
Na+
Na+
Na+
Na+
–0
–70
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
(a)
• Sodium channels open
and membrane
depolarizes (toward 0)
K+
Na+
Na+
K+
Na+
K+
K+
K+
K+
K+
–0
K+
Na+ channels open
K+ channels closed
Threshold
stimulus
K+
K+
Na+
K+
Na+
K+
Na+
K+
K+
K+
K+
–70
• Potassium leaves
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Region of depolarization
(b)
cytoplasm and
membrane repolarizes
(+30)
K+
• Brief period of
hyperpolarization (-90)
Na+
K+
K+
K+
Na+
Na+
Na+
K+
K+
K+
K+
K+
K+
Na+
Na+
Na+
K+
K+
K+
K+
K+
–0
–70
K+
K+
Region of repolarization
(c)
K+ channels open
Na+ channels closed
K+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
25
Action Potentials
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+40
Action potential
Membrane potential (millivolts)
+20
0
–20
Resting potential
reestablished
–40
Resting
potential
–60
–80
Hyperpolarization
0
1
2
3
4
Milliseconds
5
6
7
8
26
Action Potentials
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Region of
action potential
+
+
+
+
+
+
+
+
+
+
+
–
–
–
–
–
–
–
–
–
+
+
–
–
–
–
–
–
–
–
–
+
+
+
+
+
+
+
+
+
+
+
+
+
–
–
–
–
–
–
+
–
–
–
–
–
–
+
+
+
+
+
+
+
+
(a)
+
–
+
–
+
–
+
Direction of nerve impulse
–
–
–
+
+
+
+
+
+
+
+
+
+
+
+
(b)
+
–
–
–
–
–
–
–
+
+
–
–
–
–
–
–
–
–
–
+
+
–
–
+
(c)
+
+
+
+
+
+
+
+
+
27
Animation:
The Nerve Impulse
Please note that due to differing
Please
note
that duesome
to differing
operating
systems,
animations will
operating
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until the some
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Most animations
will
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the latest version
of the Flash
Player,
the
latest
version of
which
is available
at the Flash Player,
which
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28
Animation:
Action Potential Propagation
in Myelinated Neurons
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29
Animation:
Action Potential Propagation
in Unmyelinated Neurons
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30
All-or-None Response
• If a neuron responds at all, it responds completely
• A nerve impulse is conducted whenever a stimulus of threshold intensity or
above is applied to an axon
• All impulses carried on an axon are the same strength
31
Refractory Period
• Absolute Refractory Period
• Time when threshold stimulus does not start another action
potential
• Relative Refractory Period
• Time when stronger threshold stimulus can start another action
potential
32
Impulse Conduction
33
10.7: Synaptic Transmission
• This is where released neurotransmitters cross the synaptic cleft and
react with specific molecules called receptors in the postsynaptic neuron
membrane.
• Effects of neurotransmitters vary.
• Some neurotransmitters may open ion channels and others may close
ion channels.
34
Animation:
Chemical Synapse
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Please
note
that duesome
to differing
operating
systems,
animations will
operating
animations
not appearsystems,
until the some
presentation
is will
not
appear
until the presentation
is Show
viewed
in Presentation
Mode (Slide
viewed
in Presentation
Mode
(Slide
Show
view). You
may see blank
slides
in the
view).
Youor
may
see Sorter”
blank slides
inAll
the
“Normal”
“Slide
views.
“Normal”
orwill
“Slide
Sorter”
Allin
animations
appear
afterviews.
viewing
animations
appear
viewing
Presentationwill
Mode
andafter
playing
each in
Presentation
Mode
and playing
animation. Most
animations
willeach
require
animation.
Most animations
will
require
the latest version
of the Flash
Player,
the
latest
version of
which
is available
at the Flash Player,
which
is available at
http://get.adobe.com/flashplayer.
http://get.adobe.com/flashplayer.
35
Neurotransmitter Release
36
Fate of Neurotransmitters
• Enzyme degradation
• Reuptake
Synaptic Potentials
• EPSP
• Excitatory postsynaptic potential
• Graded
• Depolarizes membrane of postsynaptic neuron by opening Na+ channels
• Action potential of postsynaptic neuron becomes more likely
• IPSP
• Inhibitory postsynaptic potential
• Graded
• Hyperpolarizes membrane of postsynaptic neuron by opening K+ channels
• Action potential of postsynaptic neuron becomes less likely
38
Summation of
EPSPs and IPSPs
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• EPSPs and IPSPs are added
together in a process called
summation
• More EPSPs lead to greater
probability of an action potential
Neuron
cell body
Nucleus
Presynaptic
knob
Presynaptic
axon
38
39
Neurotransmitters
40
Neuropeptides
• Neurons in the brain or spinal cord synthesize neuropeptides.
• These neuropeptides act as neurotransmitters or neuromodulators.
•Alter the neuron’s response to the neurotransmitter or block
neurotransmitter’s release
• Examples include:
• Enkephalins
• Beta endorphin
• Substance P
41
10.8: Impulse Processing
• Way the nervous system processes nerve impulses and acts upon them
• Neuronal Pools
• Convergence
• Divergence
42
Neuronal Pools
• Groups of interneurons that make synaptic connections with each
other
• Interneurons work together to perform a common function
• Each pool receives input from other neurons
• Each pool generates output to other neurons
• May be excitatory or inhibitory effect
• If excitatory but not to threshold, makes the neuron more
responsive to further stimulation – called facilitation
43
Convergence
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• Neuron receives input from several
neurons
• Incoming impulses represent information
from different types of sensory receptors
1
• Allows nervous system to collect,
process, and respond to information
2
• Makes it possible for a neuron to sum
impulses from different sources
3
(a)
45
44
Divergence
• One neuron sends impulses to
several neurons
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• Can amplify an impulse
• Impulse from a single neuron in
CNS may be amplified to activate
enough motor units needed for
muscle contraction
4
6
5
46
45
(b)