12
Neural Tissue
PowerPoint® Lecture Presentations prepared by
Jason LaPres
Lone Star College—North Harris
© 2012 Pearson Education, Inc.
Functions of the Nervous System
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Divisions of the Nervous System
1. Central nervous system (CNS)
 Brain
 Spinal cord
2. Peripheral nervous system (PNS)
 Cranial nerves; 12 pairs
 Spinal nerves; 31 pairs
 Ganglia
 Sensory receptors (pick up internal and external
impulses)
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Functions of the CNS
 Integrate, process and coordinate:
 Sensory data from inside and outside body
 Motor commands control activities of peripheral organs (e.g.,
skeletal muscles)
 Higher functions of brain intelligence, memory, learning,
emotion, etc
Functions of PNS
 Deliver sensory information to the CNS
 Carry motor commands to peripheral tissues and systems
• Spinal nerves —carry impulses to and from the spinal
cord
• Cranial nerves —carry impulses to and from the brain
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• Cells in the nervous system
1. Neurons
• Cells that send and receive signals via
electric impulses (action potentials)
2. Neuroglia (glial cells)
• Cells that support and protect neurons
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Figure 12-3 A Structural Classification of Neurons
Anaxonic neuron
Bipolar neuron
Unipolar neuron
Dendrites
Dendritic
branches
Initial
segment
Axon
Multipolar neuron
Dendrites
Cell
body
Dendrite
Cell body
Cell
body
Axon
Cell
body
Synaptic
terminals
Axon
Synaptic
terminals
In brain and sense organs
All cell processes look alike
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Axon
In special sense
Sensory neurons
Organs (sight, smell
Single process leaves
& hearing)
one axon and one dendrite the cell body,
splits, giving rise to fused
dendrites & axon
Synaptic
terminals
Most common, include
all skeletal muscle motor
neurons
Many dendrites, a cell body
and an axon
• Structure of a Multipolar neuron
1. Cell body (soma)
2. Short, branched dendrites
3. Long, single axon
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1. Cell Body
• Large nucleus
• Perikaryon (cytoplasm)
• Cytoskeleton: neurofilaments & neurotubules
• Mitochondria (produce energy)
• RER and ribosomes (produce neurotransmitters)
compartmentalized as Nissl bodies which stain gray (gray
matter)
• No centrioles: no mitosis
• Axon hillock: thickened region that attaches to the axon- action
potential generated here and propagated down the axolemma
of axon.
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2. Dendrites
• Highly branched, many fine processes
• Receive information from other neurons
• Conduct impulses towards the cell body
• 80–90% of neuron surface area
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3. Axon (nerve fiber)
• Long extension that carries electrical signal (action
potential) to target cell
• Conduct impulses away from the cell body
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Structures of the Axon
• Axoplasm: Cytoplasm of axon, contains neurofibrils,
neurotubules, enzymes, organelles
• Axolemma: Specialized cell membrane that covers the
axoplasm
• Initial segment: Attaches to axon hillock of the cell body
• Collaterals: side branches
• Synaptic terminals - Contain vesicles filled with neurotransmitters
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Figure 12-1b The Anatomy of a Multipolar Neuron
Dendritic branches
Nissl bodies (RER
and free ribosomes)
Mitochondrion
Axon hillock
Initial segment
of axon
Golgi apparatus
Neurofilament
Nucleus
Axolemma
Axon
Synaptic
terminals
Nucleolus
Dendrite
PRESYNAPTIC CELL
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See Figure 12–2
An understanding of neuron
function requires knowing its
structural components.
POSTSYNAPTIC
CELL
The synapse is the area where a neuron communicates
with another cell.
Involves
1. Presynaptic cell
2. Synaptic cleft
3. Postsynaptic cell
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1. Presynaptic cell
― will always be a neuron
― sends message
― contains synaptic terminal, an expanded area of
axon that contains synaptic vesicles containing
neurotransmitters
• Chemical messengers released at presynaptic
membrane
• Affect neurotransmitter receptors of postsynaptic
membrane
• Broken down by enzymes when function is complete
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2. Synaptic cleft
― small space between presynaptic and postsynaptic
cell
― filled with extracellular fluid through which
neurotransmitters diffuse
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3. Postsynaptic cell
― Cell that receives message; plasma membrane has
receptors for neurotransmitters
― can be a neuron or another type of cell
 Neuromuscular junction: Synapse between
neuron and muscle
 Neuroglandular junction: synapse between
neuron and gland
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The Structure of a Typical Synapse
Presynaptic
membrane
Postsynaptic
membrane
Synaptic
vesicles
Synaptic
cleft
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Three Functional Classifications of Neurons
1. Sensory neurons
•
Afferent neurons of PNS
2. Motor neurons
•
Efferent neurons of PNS
3. Interneurons
•
Association neurons
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1. Sensory Neurons (Afferent Neurons)
• Visceral sensory neurons: Monitor internal
environment and the status of other organ systems
• Somatic sensory neurons: Monitor the external
environment and our position in it.
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Structures of Sensory Neurons
• Mostly unipolar
• Afferent fibers (containing axons) extend from
sensory receptors to CNS
• Cell bodies grouped in peripheral sensory ganglia
(collection of neuron cell bodies in PNS)
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Sensory receptors are either cells monitored by sensory neurons
or processes of specialized sensory neurons
Three Types of Sensory Receptors
I.
II.
Interoceptors

Monitor internal systems (digestive, respiratory,
cardiovascular, urinary, reproductive)

Internal senses (taste, deep pressure, pain)
Exteroceptors

External senses (touch, temperature, pressure)

Distance senses (sight, smell, hearing)
III. Proprioceptors
•
Monitor position and movement (skeletal muscles and
joints)
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2. Motor Neurons (Efferent Neurons)
• Carry instructions from CNS to peripheral effectors via
efferent fibers (axons)
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Motor Neurons
•
Two major efferent systems
1. Somatic nervous system (SNS)
- Skeletal muscles
2. Autonomic (visceral) nervous system (ANS)
- Smooth muscle, cardiac muscle, glands
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3. Interneurons (Association neurons)
• (~20 billion, most common type of neuron)
• Located in CNS between sensory and motor neurons
• Function in distribution of sensory information and
coordination of motor activity
• Also function involved in higher functions in brain
• Memory, planning, learning, perception, emotion
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12-3 Neuroglia
Neuroglia
• Half the volume of the nervous system
• Four types of neuroglia in CNS and two types in PNS
• Support, insulate, nourish & protect neurons
• Actively divide (brain tumor= gliomas)
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Four Types of Neuroglia in the CNS
1. Ependymal cells
2. Astrocytes
3. Oligodendrocytes
4. Microglia
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Neuroglia in the CNS
1. Ependymal Cells
 Assist in producing, circulating and monitoring
cerebrospinal fluid (CSF)
 Line central canal of spinal cord and ventricles of brain
forming an epithelium called ependyma
 Ciliated cells with highly branched processes
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2.
Astrocytes
• Most numerous
• Star shaped cells with large cell bodies with many
processes
1) Maintain blood–brain barrier (isolates CNS)
 Form barrier between capillaries and neurons
 Control the chemical environment of
the brain
2) Create three-dimensional framework for CNS
3) Repair damaged neural tissue
4) Guide neuron development
5) Control interstitial environment
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3. Oligodendrocytes
• Contain long extensions that myelinate (insulate) CNS
axons
• Myelination increases speed of action potentials
• Many oligodendrocytes cooperate to form a myelin
sheath along the length of the axon
• Makes nerves appear white
Internodes: Myelinated segments of axon
Nodes (also called nodes of Ranvier): Gaps
between internodes
•
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Myelination
• White matter
• Regions of CNS with many myelinated nerves
• Gray matter
• Unmyelinated areas of CNS and cell bodies
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4. Microglia
• Least numerous, smallest, spiderlike cells
• Migrate through neural tissue
• Immune cells: Clean up cellular debris, waste
products, and pathogens
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Figure 12-5b Neuroglia in the CNS
Myelinated
axons
Internode
Myelin
(cut)
White
matter
Axon
Oligodendrocyte
Astrocyte
Axolemma
Node
Unmyelinated
axon
Basement
membrane
Capillary
A diagrammatic view of neural tissue in the CNS, showing relationships between
neuroglia and neurons
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Figure 12-5b Neuroglia in the CNS
CENTRAL CANAL
Ependymal
cells
Gray
matter
Neurons
Microglial
cell
A diagrammatic view of neural tissue in the CNS, showing relationships between
neuroglia and neurons
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Figure 12-4 An Introduction to Neuroglia
Neuroglia
are found in
Central Nervous System
contains
Ependymal cells
Line ventricles
(brain) and central
canal (spinal cord);
assist in
producing,
circulating, and
monitoring
cerebrospinal fluid
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Astrocytes
Maintain blood–brain
barrier; provide structural
support; regulate ion,
nutrient, and dissolvedgas concentrations;
absorb and recycle
neurotransmitters; form
scar tissue after injury
Oligodendrocytes
Myelinate CNS
axons; increase
Speed of
conduction of AP;
White matter
Microglia
Remove cell
debris,
wastes, and
pathogens
by
phagocytosis
Two types of Neuroglia of the Peripheral Nervous
System
1. Satellite cells
• Surround cell bodies of neuron in ganglia
• Regulate environment around neuron
2. Schwann cells
• Form myelin sheath around peripheral axons
• One Schwann cell sheaths one segment of axon
• Many Schwann cells sheath entire axon
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Figure 12-4 An Introduction to Neuroglia
Neuroglia
are found in
Peripheral Nervous System
contains
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Satellite cells
Schwann cells
Surround neuron
cell bodies in
ganglia; regulate O2,
CO2, nutrient, and
neurotransmitter
levels around
neurons in ganglia
Surround all axons
in PNS; responsible
for myelination of
peripheral axons;
participate in repair
process after injury
Figure 12-6a
Schwann Cells and Peripheral Axons
Axon hillock
Nucleus
Axon
Myelinated
internode
Initial
segment
(unmyelinated)
Nodesof Ranvier
Schwann
cell nucleus
Axon
Neurilemma
Axon
Myelin
covering
internode
Axolemma
A myelinated axon, showing the
organization of Schwann cells along
the length of the axon. Also shown
are stages in the formation of a
myelin sheath by a single Schwann
cell along a portion of a single axon.
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Dendrite
• Neuroglia: Neural Responses to Injuries in
PNS
• Schwann cells play an important role in repairing
damaged neurons in the PNS
• In the process of Wallerian degeneration, axon and
myelin degenerate distal to injury
 If axon makes normal synaptic contacts, normal
function may be regained
 If axon stops growing or wanders off, normal
function may not return
 Process controlled by nerve growth factor (NGF)
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The process of repair of damaged PNS nerves, or Wallerian degeneration
Site of injury
Step 1: Distal to the
injury site, the axon and
myelin degenerate and
fragment.
Step 2: The Schwann
cells do not degenerate;
instead, they proliferate
along the path of the
original axon. Over this
period, macrophages
move into the area and
remove the degenerating
debris distal to the injury
site.
Step 3: As the neuron
recovers, its axon grows
into the site of injury and
then distally, along the
path created by the
Schwann cells.
Step 4: As the axon
elongates, the Schwann
cells wrap around it. If the
axon reestablishes its
normal synaptic contacts,
normal function may be
regained. However, if it stops
growing or wanders off in
some new direction, normal
function will not return.
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Axon Myelin Proximal stump Distal stump
Macrophage
Cord of proliferating Schwann cells
Nerve Regeneration in CNS is limited as
•Astrocytes
― Do not produce NGF
― Secrete growth-inhibitory molecules that block
growth
― Produce scar tissue that obstructs axon regrowth
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Neurons and Neuroglia
• Neurons:
• Perform all communication, information
processing, and control functions of the nervous
system
• Neuroglia :
• Maintain physical and biochemical structure of
neural tissue
• Essential to survival and function of neurons
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12-4 Transmembrane Potential
• Transmembrane potential is the potential difference
measured across a plasma membrane and expressed in
millivolts
• Results from the uneven distribution of positive and
negative ions across the plasma membrane
• Changes in response to membrane permeability
• Important for the function of cells; changes in
transmembrane potential can cause muscle contraction,
gland secretion, or transfer of information
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Ions pass through the membrane through Channel
Proteins
 Passive Channels (Leak Channels)
 Are always open
 Permeability changes with conditions
 Active Channels (Gated Channels)
 Open and close in response to stimuli
 At resting potential, most gated channels are closed
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Three Classes of Gated Channels
1. Chemically gated channels
2. Voltage-gated channels
3. Mechanically gated channels
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1. Chemically Gated Channels
Chemically gated channel
 Open in presence of
specific chemicals
 Found on neuron cell
body and dendrites
Resting state
Presence of ACh
ACh
Channel closed
Binding
site
Gated
channel
Channel open
A chemically gated Na+ channel that
opens in response to the presence of
ACh at a binding site.
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2. Voltage Gated Channels
Voltage-gated channel
 Respond to changes in transmembrane potential
–70 mV
 Characteristic of excitable membrane (membrane
that can generate action potential)
 Found in whole length of neural axons, skeletal
muscle sarcolemma, cardiac muscle
Channel closed
–60 mV
Voltage-gated sodium channels
Voltage-gated potassium channels
Channel open
Voltage-gated calcium channels
+30 mV
Channel inactivated
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Inactivation
gate
3. Mechanically gated Channels
Mechanically gated channel
 Respond to membrane
distortion
Channel closed
 Found in sensory receptors
(touch, pressure, vibration)
Applied
pressure
Channel open
Pressure
removed
Channel closed
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Resting Membrane Potential
• Inside membrane is slightly negative (-70 mV) due to
slight excess of negatively charged ions and proteins
• The extracellular fluid (ECF) and intracellular fluid
(cytosol) differ greatly in ionic composition
• Concentration of Na+ ions highest outside cell
• Concentration of K+ ions highest inside cell
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Active Forces across the Membrane
Sodium–Potassium Exchange Pump
• Carries 3 Na+ out of cell and 2 K+ in to cell against their
concentration gradients
• Requires ATP
• Balances passive forces of diffusion through leak
channels
• Very important for maintaining resting potential of cell
at –70 mV
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Figure 12-9 The Resting Potential is the Transmembrane Potential of an Undisturbed Cell
EXTRACELLULAR FLUID
–70
–30
Cl–
0
+30
mV
3 Na+
Na+ leak
channel
K+
leak
channel
Sodium–
potassium
exchange
pump
Plasma
membrane
CYTOSOL
Protein
2 K+
Protein
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Protein
•
Five Main Membrane Processes in Neural Activities
1. Resting potential: The transmembrane potential of resting cell
2. Graded potential: Temporary, localized change in resting
potential caused by stimulus
•
The effect decrease with distance from the stimulus
3. Action potential: If graded potential is large enough, it produces
an action potential, an electrical stimulus that propagates along
surface of axon to one or more synapse
•
Does not diminish as it moves from source
4. Synaptic activity: release of neurotransmitters at presynaptic
membrane
•
Produces graded potentials in postsynaptic membrane
5. Information processing
•
Response (integration of stimuli) of postsynaptic cell
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Figure 12-8 An Overview of Neural Activities
1
2
4
3
3
Graded
potential
Resting stimulus
potential produces
may
produce
5
Action potential
triggers
Information
processing
Presynaptic neuron
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Postsynaptic cell
Graded Potentials (local potentials) are changes in
transmembrane potential that cannot spread far from site of
stimulation
• Due to opening of chemically-gated channels
• Occur on dendrites or cell bodies and spreads to
trigger zone/
• Allow passage of a relatively small amount of ion
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Figure 12-8 An Overview of Neural Activities
1
2
4
3
3
Graded
potential
Resting stimulus
potential produces
may
produce
5
Action potential
triggers
Information
processing
Presynaptic neuron
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Postsynaptic cell
Characteristics of Graded Potentials
1. Graded: vary in magnitude with stimulus strength
• Intense or prolonged stimulus  more ion gates
open  voltage changes more
2. Decremental: Decreases in intensity with distance
travelled
• Local potentials cannot have long-distance effects
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3. Signal can be either excitatory (depolarizing) or inhibitory
(hyperpolarizing)
• Excitatory:
― Depolarization is a shift in transmembrane potential
toward a more positive potential
― makes the membrane less negative than the resting
potential.
― Caused by opening of chemically-gated sodium
channels
― The membrane is then more sensitive and it makes the
neuron easier to excite
• Inhibitory:
― Hyperpolarization makes the membrane potential more
negative than the resting potential.
― Caused by opening of chemically-gated potassium
channels
―The membrane is then less sensitive and it makes the
neuron harder to excite.
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Figure 12-13 Depolarization, Repolarization, and Hyperpolarization
Chemical
stimulus
applied
Chemical
stimulus
removed
Transmembrane
potential (mV)
Repolarization
Resting potential
Depolarization
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Chemical Chemical
stimulus stimulus
applied removed
Hyperpolarization
Return to
resting potential
4. Reversible: When chemical stimulus is removed,
transmembrane potential returns to resting levels, a
process called repolarization.
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Graded potentials: Localized
changes in the transmembrane
potential
Initial
segment
The events in the propagation
of a graded potential
Extracellular
Fluid
Cytoplasm
A neuron plasma membrane
at normal resting potential
A chemical stimulus opens the
chemically gated sodium channels,
producing a depolarization.
Local
current
Local current
Movement of positive charges causes a local current.
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12-5 Action Potential
Action Potentials
• Propagated changes in transmembrane potential that
affect an entire excitable membrane
• Electrical events known as nerve impulses
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• Initiating Action Potential
• The transmembrane potential at which an action
potential begins is called the threshold
• Resting potential for axon = ~ -70 mV
• Threshold for an axon = ~ -60--55 mV
• Need a graded depolarization of axon hillock large
enough (10 to 15 mV) to change resting potential (–70
mV) to threshold level of voltage-gated sodium
channels (–60 to –55 mV)
• All-or-none principle
• If a stimulus exceeds threshold amount, the action
potential is the same, no matter how large the stimulus
• Action potential is either triggered, or not
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Figure 12-14 Generation of an Action Potential
Resting
Potential
–70 mV
The axolemma contains both voltagegated sodium channels and voltagegated potassium channels that are
closed when the membrane is at the
resting potential.
KEY
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=
Sodium ion
=
Potassium ion
Figure 12-14 Generation of an Action Potential
Depolarization to Threshold
–60 mV
Local
current
KEY
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=
Sodium ion
=
Potassium ion
Figure 12-14 Generation of an Action Potential
Activation of Sodium
Channels and Rapid
Depolarization
+10 mV
KEY
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=
Sodium ion
=
Potassium ion
Figure 12-14 Generation of an Action Potential
Inactivation of Sodium
Channels and Activation
of Potassium Channels
+30 mV
KEY
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=
Sodium ion
=
Potassium ion
Figure 12-14 Generation of an Action Potential
Closing of Potassium
Channels
–90 mV
KEY
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=
Sodium ion
=
Potassium ion
Figure 12-14 Generation of an Action Potential
Sodium channels close, voltagegated potassium channels open
Transmembrane potential (mV)
DEPOLARIZATION
REPOLARIZATION
Resting
potential
Voltage-gated sodium
ion channels open
Threshold
All channels closed
Graded potential
causes threshold
ABSOLUTE REFRACTORY PERIOD
Cannot respond
Time (msec)
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RELATIVE REFRACTORY PERIOD
Responds only to a larger
than normal stimulus
Steps in the Generation of Action Potentials: Summary
1. Graded potentials at axon hillock
2. If threshold is reached, depolarization occurs
caused by activation of voltage-gated Na channels
3. Repolarization by inactivation of voltage-gated Na
channels and activation of voltage-gated K channels
4. Hyperpolarization caused by voltage-gated K
channels staying open longer than needed to reach
RMP.
5. Return to normal permeability by closing of voltagegated K channels
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Refractory Period is the brief time period after an action
potential has been initiated during which an axon is either
Absolute Refractory Period
• Sodium channels open or inactivated
• incapable of generating another action potential
Relative Refractory Period
• Membrane potential almost normal
• greater than normal amount of stimulation is needed
to generate action potential
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Over time, the Sodium–Potassium Exchange
Pump returns intracellular and extracellular ion
concentrations to prestimulation levels
• Requires energy (1 ATP for each 2 K+/3 Na+ exchange)
• Without ATP, neurons stop functioning
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Propagation of Action Potentials
•
Propagation moves action potentials generated in
axon hillock along entire length of axon
•
Two methods of propagating action potentials
1. Continuous propagation (unmyelinated
axons)
2. Saltatory propagation (myelinated axons)
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Figure 12-15 Continuous Propagation of an Action
Potential along an Unmyelinated Axon
As an action potential develops at the initial segment , the
transmembrane potential at this site depolarizes to +30 mV.
Action
potential
Extracellular Fluid
+30 mV
–70 mV
–70 mV
Na+
Cell membrane
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Cytosol
As the sodium ions entering at  spread away from the
open voltage-gated channels, a graded depolarization
quickly brings the membrane in segment  to threshold.
Graded depolarization
–60 mV
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–70 mV
An action potential now occurs in segment  while
segment  beings repolarization.
Repolarization
(refractory)
+30 mV
Na+
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–70 mV
As the sodium ions entering at segment  spread laterally,
a graded depolarization quickly brings the membrane in
segment  to threshold, and the cycle is repeated.
–60 mV
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Saltatory Propagation
• Action potential along myelinated axon
• Faster and uses less energy than continuous
propagation
• Myelin insulates axon, prevents continuous
propagation
• Local current “jumps” from node to node
• Depolarization occurs only at nodes
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Figure 12-16 Saltatory Propagation along a Myelinated
Axon
An action potential
has occurred at the
initial segment .
Extracellular Fluid
–70 mV
–70 mV
+30 mV
Na+
Myelinated
internode
Myelinated
internode
Plasma membrane
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Myelinated
internode
Cytosol
A local current
produces a graded
depolarization that
brings the axolemma
at the next node to
threshold.
–60 mV
Local
current
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–70 mV
An action potential
develops at node . Repolarization
(refractory)
+30 mV
Na+
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–70 mV
A local current
produces a graded
depolarization that
brings the axolemma
at node  to
threshold.
–60 mV
Local
current
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Continuous and saltatory propagation of an action potential
An action potential here at time 0
triggers an action potential here at time 1
which triggers an action potential here at time 2
which triggers an action potential here at time 3
and so on along the axon, in a series of tiny steps.
Continuous propagation
An action potential here at time 0
triggers an action potential here at time 1
which triggers an action potential here at time 2
which triggers an action potential here at time 3
and so on along the axon, skipping the
segments in between.
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Saltatory propagation
Characteristics of action potentials
1. All-or-none law
• voltage gates at the trigger zone either open if
threshold is reached or don’t
2. Nondecremental
• does not get weaker with distance
3. Irreversible
• once started goes to completion and cannot be stopped
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Table 12-3 A Comparison of Graded Potentials and Action Potentials
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12-6 Axon Diameter and Speed
• Both axon diameter and myelination affect
propagation speed
• Axon Diameter and Propagation Speed
• Ion movement is related to cytoplasm concentration
• Axon diameter affects action potential speed
• The larger the diameter, the lower the resistance
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Information
• “Information” travels within the nervous system
• As propagated electrical signals (action potentials)
• The most important information (vision, balance,
motor commands)
• Is carried by large-diameter, myelinated axons Type
A fibers
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12-7 Synapses
 Communication occurs between neurons or between
neurons and other cells at synapses.
 Action potentials (nerve impulses) are transmitted from
presynaptic neuron to postsynaptic neuron (or other
postsynaptic cell) across a synapse
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Two Types of Synapses
1. Electrical synapses
•
Direct physical contact between cells
2. Chemical synapses
•
Signal transmitted across a gap by chemical
neurotransmitters
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Electrical Synapses
• Occur when cells are touching
• Gap junctions (connexons) allow ions to pass between cells
• Produce continuous local current and action potential
propagation
• Extremely rare
• Are found in areas of brain, eye, some ganglia
Chemical Synapses
• Found in most synapses between neurons and all synapses
between neurons and other cells
• Cells not in direct contact
• Action potential may or may not be propagated to postsynaptic
cell, depending on:
• Amount of neurotransmitter released
• Sensitivity of postsynaptic cell
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Two Classes of Neurotransmitters
1. Excitatory neurotransmitters
•
Cause depolarization of postsynaptic membranes
•
Promote action potentials
2. Inhibitory neurotransmitters
•
Cause hyperpolarization of postsynaptic membranes
•
Suppress action potentials
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The Effect of a Neurotransmitter on a postsynaptic
membrane
• Depends on the type of receptor for the
neurotransmitter
• For example, acetylcholine (ACh)
• Causes depolarization and promotes action potentials
in skeletal muscle
• But inhibits cardiac neuromuscular junctions
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Cholinergic Synapses
•
Any synapse that releases ACh
1. All neuromuscular junctions with skeletal muscle
fibers
2. Many synapses in CNS
3. All neuron-to-neuron synapses in PNS
4. All neuromuscular and neuroglandular junctions of
ANS parasympathetic division
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Events at a Cholinergic Synapse
1. Action potential arrives, depolarizes synaptic terminal
2. Calcium ions enter synaptic terminal, trigger
exocytosis of ACh
3. ACh binds to receptors, depolarizes postsynaptic
membrane
4. ACh removed by AChE
•
AChE breaks ACh into acetate and choline
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Table 12-4 Synaptic Activity
Mitochondrion
Acetylcholine
Synaptic
vesicle
SYNAPTIC
TERMINAL
Choline
Acetylcholinesterase
Acetate
POSTSYNAPTIC
MEMBRANE
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SYNAPTIC
CLEFT
(AChE)
ACh
receptor
Synaptic Fatigue
• Occurs when neurotransmitter cannot recycle fast
enough to meet demands of intense stimuli
• Synapse inactive until ACh is replenished
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12-8 Neurotransmitters and Neuromodulators
• Over 100 substances known as neurotransmitters and
neuromodulators
• Vary from hormones to amino acids
• Can be excitatory or inhibitory
• Can have
 Direct effect on membrane potential
 Indirect effect on membrane potential
 Lipid soluble gases that exert their effects inside the
cell
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Postsynaptic Potentials: Graded potentials developed in a
postsynaptic cell in response to neurotransmitters
•
Two Types of Postsynaptic Potentials
1. Excitatory postsynaptic potential (EPSP)
•
Graded depolarization of postsynaptic membrane
•
A neuron becomes facilitated as EPSPs accumulate
•
Raises transmembrane potential closer to threshold until a
small stimulus can trigger action potential
2. Inhibitory postsynaptic potential (IPSP)
•
Graded hyperpolarization of postsynaptic membrane
•
A neuron that receives many IPSPs is inhibited from
producing an action potential
•
Because the stimulation needed to reach threshold is
increased
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Summation
•
To trigger an action potential
•
One EPSP is not enough
•
EPSPs (and IPSPs) combine through summation
1. Temporal summation
2. Spatial summation
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Temporal Summation.
First stimulus arrives
FIRST
STIMULUS
Initial
segment
Second stimulus arrives and is
added to the first stimulus
Action potential is generated
SECOND
STIMULUS
ACTION
POTENTIAL
PROPAGATION
Threshold
reached
Temporal Summation. Temporal summation occurs on a membrane that receives
two depolarizing stimuli from the same source in rapid succession.
 The effects of the second stimulus are added to those of the first.
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Spatial Summation.
Two stimuli arrive simultaneously
Action potential is generated
TWO
SIMULTANEOUS
STIMULI
Threshold
reached
Spatial Summation. Occurs when sources of stimulation
arrive simultaneously, but at different locations.

effects are cumulative
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ACTION
POTENTIAL
PROPAGATION
Figure 12-21a Presynaptic Inhibition and Presynaptic Facilitation
Action
potential
arrives
GABA
release
Inactivation of
calcium channels
Ca2+
2. Less calcium
enters
1. Action
potential
arrives
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3. Less
neurotransmitter
released
Presynaptic inhibition
4. Reduced
effect on
postsynaptic
membrane
Figure 12-21b Presynaptic Inhibition and Presynaptic Facilitation
Action
potential
arrives
Serotonin
release
Activation of
calcium channels
Ca2+
Ca2+
2. More calcium
enters
1. Action
potential
arrives
3. More
neurotransmitter
released
Presynaptic facilitation
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4. Increased
effect on
postsynaptic
membrane
Many Drugs affect nervous system by stimulating
receptors that respond to neurotransmitters
• Can have complex effects on perception, motor
control, and emotional states
Information processing by the brain is complex
and involves interacting groups of neurons
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Multiple Sclerosis (MS):
• Autoimmune disorder causing destruction of myelin
sheaths around axons in optic nerve, brain, spinal cord
accompanied by destruction of oligodentrocytes
• More common in females
• Symptoms include partial loss of vision and problems with
speech, balance and general motor coordination
• Remissions & relapses result in progressive, cumulative loss
of function
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Anesthetics
• Agent that causes a local or general loss of sensation
including feelings of pain
• Lidocaine: local anesthetic that blocks Na+ channels
from opening; nerve impulses that will be interpreted
as pain are not generated
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