_ch_11_Neural Tissue_lecture_presentation.ppt

© 2011 Pearson Education, Inc.
PowerPoint® Lecture Presentations prepared by
Alexander G. Cheroske
Mesa Community College at Red Mountain
11
Neural Tissue

Section 1: Nervous System Components
• Learning Outcomes
• 11.1 Sketch and label the structure of a typical
neuron, and describe the functions of each
component.
• 11.2 Classify and describe neurons on the basis of
their structure and function.
• 11.3 Describe the locations and functions of
neuroglia in the CNS.
• 11.4 Describe the locations and functions of
Schwann cells and satellite cells.

Figure 11 Section 1
The major components and functions of the nervous system
Central Nervous System
The central nervous system
(CNS) consists of the brain and
spinal cord and is responsible
for integrating, processing, and
coordinating sensory data and
motor commands.
Information processing
includes the integration and
distribution of information in
the CNS.
Peripheral Nervous
System
The peripheral
nervous system
(PNS) includes all the
neural tissue outside
the CNS.
The motor division of the
PNS carries motor commands
from the CNS to peripheral
tissues and systems.
includes
The autonomic
nervous system
(ANS) provides
automatic regulation
of smooth muscle,
cardiac muscle,
glands, and adipose
tissue.
The somatic
nervous
system
(SNS)
controls
skeletal
muscle
contractions.
The sensory division of the PNS
brings information to the CNS
from receptors in peripheral
tissues and organs.
Somatic sensory
receptors provide
position, touch,
pressure, pain, and
temperature sensations.
Special sensory
receptors provide
sensations of
smell, taste,
vision, balance,
and hearing.
Visceral sensory receptors
monitor internal organs.
Receptors are sensory structures that detect
changes in the internal or external
environment.
Skeletal
muscle
Effectors are target organs whose
activities change in response to neural
commands.
• Smooth muscle
• Cardiac muscle
• Glands
• Adipose tissue

Module 11.1: Neurons
• Neuron components
• Dendrites
• Highly branched, bearing spines 0.5–1 µm long
(dendritic spines)
• CNS neurons receive most information here
• Neuron receives stimuli from environment or other
neurons at dendrites
• Cell body
• Contains nucleus
• Organelles contained within perikaryon (peri, around +
karyon, nucleus)
• Cytoskeleton comprised of neurofilaments and
neurofibrils (extend into dendrites and axon)

• Neuron components (continued)
• Axon
• Carries information toward other cells
• Transport of materials (enzymes and lysosomes) using
neurotubules (= axoplasmic transport)
• Occurs in both directions
• Back toward cell body = retrograde flow
• Components
• Axon hillock (base or initial segment)
• Axolemma (plasma membrane of axon)
• Axoplasm (cytoplasm of axon with organelles, structural
components, and transported materials)
• Collateral branches (communicate with other cells)

• Neuron components (continued)
• Telodendria (telo-, end + dendron, tree)
• Axonal extensions at end of axon trunk
• Terminate at synaptic terminals
• Where neuron communicates with other cells
Animation: Neurophysiology: Neuron Structure

Figure 11.1 1
A diagrammatic view of a representative neuron
Dendrites
Dendritic spines of dendrites
Axon
Axon hillock Axoplasm
Axolemma
Nissl bodies
(clusters of
RER and free
ribosomes)
Mitochondrion
Nucleus
Nucleolus
Cell Body
Perikaryon Neurofilament
Telodendria
Synaptic terminals

• Synapse
• Specialized site of communication between
neuron and another cell
• Components
• Presynaptic cell (before synaptic cleft)
• Usually a neuron
• May have synaptic knob
• Has synaptic vesicles that contain
neurotransmitters (chemical messengers
synthesized in cell body)
• Presynaptic membrane (where neurotransmitters are
released)

• Synapse (continued)
• Components (continued)
• Postsynaptic cell (after synaptic cleft)
• Can be a neuron or other type of cell
• Postsynaptic membrane (bears receptors for
neurotransmitters
• Synaptic cleft (narrow space between cells)

Figure 11.1 2
Synaptic cleft
Synaptic knob
Synaptic vesicles
A representative synapse
Telodendrion
of presynaptic cell
Cytoplasm of
postsynaptic cell
Postsynaptic
membrane
Presynaptic
membrane
Endoplasmic
reticulum
Mitochondrion

Figure 11.1 3
Synapses with another neuron
Neuromuscular junctions
The type of synapses
Neuron 1
Neuroglandular synapses
Neuron
Neuron
Telodendria
Synaptic terminals
Neuroglandular
synapses
Neuron 2
Axolemma
Dendrites
Gland
cells
Skeletal
muscle
fibers
Neuromuscular
junctions
Collateral
branch
Collateral
branch
Synapses with
another neuron

• Most CNS neurons lack centrioles and cannot
divide
• Neurons lost to injury or disease are seldom
replaced
• Some neural stem cells exist but mostly inactive
• Exceptions:
• Olfactory epithelium (smell)
• Retina of eye
• Hippocampus (area of brain for memory storage)

Module 11.1 Review
a. Name the structural components of a typical
neuron.
b. Describe a synapse.
c. Why is a CNS neuron not usually replaced
after it is injured?

Module 11.2: Classification of neurons
• Four major anatomical classes of neurons
1. Anaxonic neurons
• All cell processes look alike (dendrites vs. axons)
• Located in brain and special sense organs
• Functions are poorly understood
2. Bipolar neurons
• Two distinct processes
1. One with branching dendritic processes
2. One axon
• Rare, but occur in special sense organs
• Small (30 µm in length)

• Four major anatomical classes of neurons (continued)
3. Unipolar neurons
• Dendrites and axon are continuous (fused)
• Cell body lies off to one side
• Initial segment where dendrites converge
• Most sensory neurons of peripheral nervous system
• May extend 1 meter or more
4. Multipolar neurons
• Two or more dendrites and one axon
• Most common neurons in CNS
• Can be as long as unipolar (voluntary motor neurons)

Figure 11.2 1 – 4
An anaxonic neuron
The four major anatomical classes of neurons
A bipolar neuron
A unipolar neuron A multipolar neuron
Dendrites
Initial
segment
Axon
Axon
Synaptic
terminals
Cell body
Dendrites
Axon
Synaptic
terminals
Dendrites
Dendritic
process
Axon
Synaptic
terminals

• Three major functional classes
1. Sensory neurons (~10 million in body)
2. Interneurons (~20 billion in body)
3. Motor neurons (~500,000 in body)

• Functional relationships of neurons
• Sensory receptors (relay stimuli to sensory
neurons)
• Interoceptors (intero-, inside)
• Monitor sensations inside body from various systems
• Proprioceptors
• Monitor body position and movement of joints and
muscles
• Exteroceptors (extero, outside)
• Monitor sensations from external environment

• Functional relationships of neurons (continued)
• Afferent nerve fibers (axons from receptor to CNS)
• Sensory ganglia
• Contain cell bodies of unipolar sensory neurons
• Somatic sensory neurons (outside world)
• Visceral sensory neurons (internal conditions)
• Central Nervous System
• Interneurons
• Usually between sensory and motor neurons
• Also responsible for higher functions (memory, etc.)

• Functional relationships of neurons (continued)
• Central Nervous System and Peripheral
Nervous System
• Motor neurons (originate in CNS and transmit
impulses to effectors through PNS)
• Somatic motor neurons (skeletal muscles)
• Visceral motor neurons (smooth and cardiac muscle,
glands, and adipose tissue)
• Synapse with 2nd set of neurons at autonomic
ganglia

Module 11.2 Review
a. Classify neurons according to their structure.
b. Classify neurons according to their function.
c. Are unipolar neurons in a tissue sample of the
PNS more likely to function as sensory
neurons or motor neurons?

Module 11.3: CNS neuroglia
• Neuroglia (or glial cells)
• Cells that support and protect neurons
• Are abundant and diverse
• ~Half the volume of nervous system

• CNS neuroglia
1. Ependymal cells
• Form epithelia (ependyma) lining fluid-filled passageway in
brain and spinal cord
• Fluid = cerebrospinal fluid (CSF)
• Also surrounds brain and spinal cord
• Assist in producing, circulating, and monitoring CSF
2. Microglia
• Embryologically related to monocytes and macrophages
• Migrate into CNS
• Persist as mobile phagocytic cells
• Remove cellular debris, waste products, and pathogens

• CNS neuroglia
3. Astrocytes
• Maintain blood–brain barrer
• Isolates CNS from chemicals and hormones in the blood
• Provide structural support
• Regulate ion, nutrient, and dissolved gas
concentrations in interstitial fluid
• Absorb and recycle neurotransmitters
• Form scar tissue after CNS injury

• CNS neuroglia
4. Oligodendrocytes (oligo-, few)
• Provide CNS framework by stabilizing axons
• Produce myelin
• Coats axons and increases speed of neural impulse transmission
• Cell process wraps axon with layers of myelin and plasma
membrane creating myelin sheath
• One oligodendrocyte wraps axonal segments of many neurons
• Myelin sheath is incomplete
• Myelin-wrapped areas = internodes
• Gaps between internodes = nodes

• CNS neuroglia
4. Oligodendrocytes (continued)
• Axons that have myelin sheath = myelinated
• Appear white due to lipid content
• Constitute white matter of the CNS
• Axons that lack myelin sheath = unmyelinated
• Contribute to gray matter of the CNS
• Along with neuron cell bodies and dendrites

Figure 11.3
Section of
spinal cord
Ependymal cell
Microglial cell
Neurons
Gray matter White matter
Myelinated
axons
Astrocytes Oligodendrocyte
Capillary
Myelin
(cut) Nodes
Unmyelinated axon
Myelin sheath in internode

Module 11.3 Review
a. Identify the neuroglia of the central nervous
system.
b. Which glial cell protects the CNS from chemicals
and hormones circulating in the blood?
c. Which type of neuroglia would occur in increased
numbers in the brain tissue of a person with a
CNS infection?

Module 11.4: PNS neuroglia
• PNS neuroglia
• Schwann cells
• Form sheath around peripheral axons
• Outer surface of Schwann cell is called neurilemma
• Cover both myelinated and unmyelinated axons
• A single cell myelinates an axon
• A single cell can wrap many unmyelinated neurons
• Satellite cells
• Surround neuron cell bodies in ganglia
• Regulate intercellular environment (much like
astrocytes)

Figure 11.4 1
Nucleus
Internode
(myelinated)
Node
Axon hillock
Initial
segment
(unmyelinated)
Schwann cell
Neurilemma
Axon
A Schwann cell
Myelin covering
internode
Axolemma
Cell body
Dendrite

Figure 11.4 2
Myelin covering
internode
Schwann
cell nucleus
Axon
Neurilemma
The steps in the myelination
of an axon in the PNS

Figure 11.4 3
Axons
Schwann
cell nucleus
Node
Internode
(unmyelinated)
Satellite cells
A single Schwann cell forming the internode
of many unmyelinated axons
Neurilemma
Axons
Schwann
cell
Schwann
cell nucleus

• Repair of damaged nerves in PNS
1. Axon and myelin degenerate distal to injury
2. Schwann cells proliferate along original axon path
• Macrophages move in and remove cellular debris
3. Axon grows along original path created by Schwann cells
4. Schwann cells wrap around elongating axon
• If axon makes normal synaptic contacts, normal function may
be regained
• If axon stops growing or wanders off, normal function may not
return
• Repair that does not restore full function = Wallerian
degeneration

Figure 11.4 4
The process of repair of damaged PNS nerves, or Wallerian degeneration
Axon Myelin Proximal stump Distal stump
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.
Macrophage
Cord of proliferating Schwann cells
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.
Site of injury

• Only limited repair can occur in CNS due to:
1. Many more axons involved
2. Astrocytes produce scar tissue that can
prevent axon growth
3. Astrocytes release chemicals that block axon
regrowth

Module 11.4 Review
a. Identify the neuroglia of the peripheral
nervous system.
b. Describe the neurilemma.
c. In which part of the nervous system does
Wallerian degeneration occur?

Section 2: Neurophysiology
• 11.5 Explain how the resting potential is created and
maintained.
• 11.6 Describe the functions of gated channels with respect to
the permeability of the plasma membrane.
• 11.7 Describe graded potentials.
• 11.8 Describe the events involved in the generation and
propagation of an action potential.
• 11.9 Describe continuous propagation and saltatory
propagation, and discuss the factors that affect the
speed with which action potentials are propagated.

• 11.10 Describe the general structure of synapses
in the CNS and PNS, and discuss the events
that occur at a chemical synapse.
• 11.11 Discuss the significance of postsynaptic
potentials, including the roles of excitatory
postsynaptic potentials and inhibitory
postsynaptic potentials.
• 11.12 Discuss the interactions that make the
processing of information in neural tissue
possible.

• Neurophysiology
• Transmembrane potential
• An unequal distribution of charge across a cell membrane
• Inside membrane is slightly negative
• Due to slight excess of negatively charged ions and proteins
• Outside membrane is slightly positive
• Due to slight excess of positively charged ions
• Results from differences in membrane permeability to various
ions and active transport
• Is characteristic of all cells
Animation: Transmembrane Potentials

Figure 11 Section 2 1
Extracellular fluid
The unequal distribution of charges inside and outside the
plasma membrane, which produces a transmembrane potential
Plasma membrane
Protein Cytosol
Protein
Protein

• Neural activity and transmembrane potential
• Changes in transmembrane potential can cause muscle
contraction, gland secretion, or transfer of information
• Resting potential
• Transmembrane potential of a cell at rest
• All neural activities begin with a change from resting potential
• Graded potential
• Temporary, localized change in resting potential due to typical
stimulus
• Decreases with distance from stimulus

• Neural activity and transmembrane potential (continued)
• Action potential
• Electrical event involving one location of axonal membrane
• Can be triggered by sufficiently large graded potential
• Is propagated along axon surface toward synaptic terminals
• Synaptic activity
• Typically involves release of neurotransmitters (like ACh) by
presynaptic cell
• Compounds bind to receptors on postsynaptic cell, changing its
permeability producing a graded potential

• Neural activity and transmembrane potential
(continued)
• Information processing
• Integration of stimuli at individual cell level
• Response of postsynaptic cell to stimulated
receptors and other stimuli

Figure 11 Section 2 2
An overview of the role of the transmembrane
potential in neural activity
Resting
potential
Graded
potential
Presynaptic neuron
stimulus
produces
may
produce
Action potential
Postsynaptic cell
triggers
Information
processing

Module 11.5: Resting potential
• Extracellular fluid (ECF) has high concentrations of Na+
and Cl–
• Cytosol has high concentrations of K+ and negatively
charged proteins (Pr–)
• These proteins cannot cross plasma membrane
• Neuron resting potential is usually near 0.07 volts (V)
or –70 millivolts (mV) (slightly negative inside)
• Charged ions cannot freely cross plasma membrane
• Can move across membrane only through membrane
channels or active transport mechanisms

• Leak channels
• Always open
• Size, shape, and structure determine which ions
will pass
• Potassium ions diffuse out of cell through K+ leak
channels
• Sodium ions diffuse into cell through Na+ leak
channels
• Primarily causes the transmembrane potential
Animation: Neurophysiology: Ion Movement

Figure 11.5 1
Plasma
membrane
Passive leak channels, which are primarily
responsible for the transmembrane potential

• Active transport
• Sodium–potassium exchange pump
• Ejects 3 Na+ for 2 K+ recovered from ECF
• Maintains stable resting potential
Animation: Neurophysiology: Sodium Potassium
Exchange Pump

Figure 11.5 2
Potassium ions
can diffuse out of
the cell through
potassium leak
channels.
The unit of measurement
of potential difference is
the volt (V), and the
transmembrane
potential of a neuron is
usually near 0.07 V. Such a
value is usually expressed
as –70 mV (or –70
millivolts—thousandths of
a volt) with the minus sign
indicating that the interior
is negatively charged.
Plasma
membrane
The cytosol contains an
abundance of negatively
charged proteins,
whereas the extracellular
fluid contains relatively
few. These proteins
cannot cross the plasma
membrane.
CYTOSOL
Protein
Protein Protein
EXTRACELLULAR FLUID
The sodium–potassium exchange
pump ejects 3 Na+ for every 2 K+
recovered from the extracellular
fluid. At a transmembrane
potential of –70 mV, the rate of
Na+ entry versus K+ loss is 3:2,
and the exchange pump
maintains a stable resting
potential.
Sodium ions can
diffuse into the
cell through
sodium leak
channels.
Sodium–
potassium
exchange
pump
An overview of the events responsible for the normal resting potential of a neuron

• Electrochemical gradients
• Chemical gradient
• Concentration gradient for an ion across plasma membrane
• Electrical gradient
• Attraction between opposite charges or repulsion between like
charges (+/+ or –/–)
• Equilibrium potential
• When electrical and chemical gradients are equal and opposite,
resulting in no net movement across membrane
• In most cells, the gradients for Na+ and K+ are most important

• Potassium ion gradients
• At normal resting potential, the electrical and
chemical gradients are in opposition, but not equal
• The net electrochemical gradient for K+ is out of the
cell
• If the plasma membrane were freely permeable to
potassium ions, K+ would continue to leave the cell
until an equilibrium potential of –90 mV

Figure 11.5 3
Protein
Protein
Resting
potential
Equilibrium
potential
The dynamics of potassium ion gradients
Potassium Ion Gradients
At normal resting potential, an electrical gradient
opposes the chemical gradient for potassium ions (K+).
The net electrochemical gradient tends to force
potassium ions out of the cell.
Potassium
electrical
gradient
Potassium
electrical
gradient
Potassium
chemical
gradient
Potassium
chemical
gradient
If the plasma membrane were freely permeable to
potassium ions, the outflow of K+ would continue until
the equilibrium potential (–90 mV) was reached.
Net potassium
electrochemical
gradient

• Sodium ion gradients
• At normal resting potential, both the chemical
and electrical gradients cause Na+ to move into
the cell
• If the plasma membrane were freely permeable
to sodium, Na+ would continue to enter the cell
until an equilibrium potential of +66 mV was
reached
A&P Flix: Resting Membrane Potential

Figure 11.5 3
Protein Protein
Resting
potential
Equilibrium
potential
The dynamics of sodium ion gradients
Sodium Ion Gradients
At the normal resting potential, chemical and electrical
gradients combine to drive sodium ions (Na+) into the
cell.
If the plasma membrane were freely permeable to
sodium ions, the influx of Na+ would continue until
the equilibrium potential (+66 mV) was reached.
Sodium
chemical
gradient
Sodium
electrical
gradient
Net sodium
electrochemical
gradient
Sodium
chemical
gradient
Sodium
electrical
gradient

Module 11.5 Review
a. Define resting potential.
b. What effect would decreasing the
concentration of extracellular potassium ions
have on the transmembrane potential of a
neuron?
c. What happens at the sodium–potassium
exchange pump?

Module 11.6: Gated channels
• Gated channels
• Resting potential remains stable until the cell is
disturbed or stimulated
• Changes in transmembrane potential primarily
occur due to gated channels opening or closing
in response to stimuli
• Three different gated channel classes
1. Chemically gated channels
2. Voltage-gated channels
3. Mechanically gated channels

Figure 11.6 1

• Chemically gated channels
• Open when they bind specific chemicals
• Example: neuromuscular junction receptors that
bind ACh
• Most abundant on dendrites and cell body of
neurons
• Where most synaptic communication occurs

Figure 11.6 1
The function of chemically gated channels
Extracellular fluid
Plasma
membrane
Cytosol
Gated
channel
(closed)
Resting state Arrival of ACh
Binding
site
Gated channel opens
ACh
ACh

• Voltage-gated channels
• Characteristic of excitable membranes (capable of
generating and conducting an action potential)
• Open or close in response to changes in transmembrane
potential
• Most important for neurons
• Voltage-gated potassium channels
• Voltage-gated calcium channels
• Voltage-gated sodium channels
• Have two gates that function independently
• Activation gates (open on stimulation)
• Inactivation gates (close to stop sodium entry)

Figure 11.6 2
Channel inactivated
Channel closed Channel open
Inactivation
gate
Activation
gate
The function of voltage-gated channels

• Mechanically gated channels
• Open in response to physical distortion of
membrane
• Important in sensory receptors
• Examples: touch, pressure, vibration

Figure 11.6 3
Pressure
removed
Applied
pressure
The function of mechanically gated channels
Channel closed
Channel closed Channel open

Module 11.6 Review
a. Define gated channels.
b. Identify the three types of gated channels, and
state the conditions under which each operates.
c. What effect would a chemical that blocks voltage-
gated sodium channels in neuron plasma
membranes have on a neuron’s ability to conduct
an action potential?

Module 11.7: Graded potentials
• Graded potentials
• Also known as local potentials
• Changes in transmembrane potential that cannot
spread far from stimulation site
• Example: effects of chemically gated sodium
channels

• Graded potentials produced by chemically gated Na+
channels
• At resting potential, chemically gated sodium channels are
closed
• Binding of chemical, opens channels allowing sodium influx
• Positively charged ions entering the cell cause depolarization
(shift from resting potential to more positive)
• Intracellular Na+ spread out, attracted to negative charges
lining membrane (= local current)
• Extracellular Na+ moves to replace

Figure 11.7 1 – 3
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.
Movement of positive charges causes a local current.
Local
current
Local current
Initial
segment

• Degree of depolarization decreases with
distance from stimulation site
• Ions enter at one location
• Spread occurs in all directions
• Change in transmembrane potential proportional
to stimulus intensity
• Greater stimulus = more open channels = more
ion flow

Figure 11.7 4
Transmembrane
potential
The effect of distance from the stimulation site
on the degree of depolarization

• Graded potentials produced by chemically gated
Na+ channels (continued)
• With removal of chemical stimulus, membrane
returns to resting potential
• Na+ pumped out of cell
• = Repolarizition

• Effects of chemically gated potassium
channels
• Some chemicals open K+ channels
• Potassium ions leave cytoplasm
• Results in more negative transmembrane
potential
• = Hyperpolarization

Figure 11.7 5
Transmembrane
potential (mV)
A chemical stimulus
opens chemically
gated sodium ion
channels.
Removal of the
chemical stimulus
leads to repolarization.
A different chemical stimulus opens
chemically gated potassium channels,
causing hyperpolarization.
Repolarization
Depolarization
Resting potential
Hyperpolarization
Chemical
stimulus
removed
Return to
resting potential
Time
The changes in transmembrane potential over time when different
chemical stimuli are applied to the axon hillock

Figure 11.7 6

Module 11.7 Review
a. Define graded potential.
b. Describe depolarization, repolarization, and
hyperpolarization.
c. What factors account for the local currents
associated with graded potentials?

Module 11.8: Action potential generation
• Action potential generation
• Information transfer in neurons
• Reception of information as graded potentials on dendrites and
cell bodies
• At synaptic terminals, graded potentials cause release of
neurotransmitters
• Distance between cell body and synaptic terminals can be large
• Graded potentials only travel short distances
• Action potentials can travel longer distances
• Are propagated changes in transmembrane potential that affect
entire excitable membrane
Animation: Neurophysiology: Action Potential

• Channel types and transmembrane potential
• Leak channels are responsible mainly for
resting potential
• Chemically gated channels often produce
graded potentials
• Voltage-gated channels produce action
potentials

• Prior to action potential generation
• Transmembrane potentials are at resting levels
• Sodium channels are closed but capable of
opening
• Activation gate closed
• Inactivation gate open
• Potassium channels are closed but capable of
opening
• Single gate closed
A&P Flix: Generation of an Action Potential

• Steps of action potential generation and propagation
1. Depolarization to threshold
• Graded depolarization large enough to open voltage-gated
sodium channels
• = Threshold
• Approximate transmembrane potential of –60 mV
2. Activation of Na+ channels and rapid depolarization
• Sodium ions rush into cell through open channels
• Causes rapid depolarization
• From –60 mV to a positive value

• Steps of action potential generation and propagation
(continued)
3. Inactivation of Na+ channels and activation of K+ channels
• At ~+30 mV, sodium inactivation gates close
• = Sodium channel inactivation
• Voltage-gated potassium channels open
• Potassium ions leave the cell
• Begins repolarization
4. Potassium channels close
• As membrane reaches resting potential (–70 mV)
• K+ ions continue to leave cell until all channels are closed
• Produces brief hyperpolarization

Figure 11.8 2
During the relative refractory period, the
membrane can respond only to a
larger-than-normal stimulus.
During the absolute refractory period,
the membrane cannot respond to further
stimulation.
Potassium channels close, and both sodium
and potassium channels return to their
normal states.
Sodium channels close, voltage-gated
potassium channels open, and potassium
ions move out of the cell. Repolarization
begins.
Voltage-gated sodium channels open and
sodium ions move into the cell. The
transmembrane potential rises to +30 mV.
A graded depolarization brings an area of
excitable membrane to threshold (–60 mV).
The changes in the
transmembrane potential
at one location during
the generation of an
action potential
DEPOLARIZATION REPOLARIZATION
Threshold
Resting
potential
Transmembrane
potential
(mV)
ABSOLUTE
REFRACTORY
PERIOD
RELATIVE
REFRACTORY
PERIOD
Time (msec)

• Graded and action potential analogy: gun firing
• Graded potential
• Pulling trigger of gun
• Enough pressure will cause gun to fire
• Action potential
• Firing of gun
• Enough pressure on trigger will cause gun to fire same
way every time
• Stimulus triggers action potential or not at all
• = All-or-none principle

Module 11.8 Review
a. Define action potential.
b. List the events involved in the generation and
propagation of an action potential.
c. Compare the absolute refractory period with
the relative refractory period.

Module 11.9: Action potential propagation
• Action potential propagation
• A generated action potential does not itself move along the
axon
• Once generated at the initial segment, the action potential is
regenerated at each adjacent axonal segment
• = Propagation (not conduction)
• Two types of action potential propagation
1. Continuous propagation
2. Saltatory propagation
Animation: Neurophysiology: Continuous and
Saltatory Propagation

• Continuous propagation
• Occurs along unmyelinated axons
• Appears to move in a series of tiny steps
• Each step takes ~1 msec
• = Propagation speed of ~1 m/s

• Steps of continuous propagation
1. Action potential develops at initial segment
• Transmembrane potential = +30 mV
2. Entering sodium spreads away from voltage-gated channels
to depolarize adjacent segment to threshold
3. Action potential occurs in adjacent segment while initial
segment begins repolarizing
4. Sodium enters new segment, spreads, and causes
depolarization of next adjacent axonal segment
• Action potential can only move forward because last axonal
segment is in absolute refractory period

Animation: Neurophysiology: Positive Potential
A&P Flix: Propagation of an Action Potential

Figure 11.9 1
Initial
segment
Axon
hillock
As the sodium ions entering at 1
spread away from the open
voltage-gated channels, a graded
depolarization quickly brings the
membrane in segment 2 to
threshold.
An action potential now occurs
in segment 2 while segment 1
begins repolarization.
As the sodium ions entering at
Segment 2 spread laterally, a
graded depolarization quickly
brings the membrane in
Segment 3 to threshold. The
action potential can only move
forward, not backward, because
the membrane at segment 1
is in the absolute refractory
period of repolarization.
Repolarization
(refractory)
Graded depolarization
Cell membrane Cytosol
Action
potential
Extracellular fluid
Step 4:
Step 3:
Step 2:
The events that occur in continuous propagation
Step 1:
As an action potential develops
at the initial segment 1 , the
transmembrane potential at this
site depolarizes to +30 mV.

• Saltatory propagation (saltere, leaping)
• Occurs in myelinated axons
• Only exposed nodes can respond to depolarizing
stimulus
• Internodes covered with myelin prevent ion flow
across membrane
• Prevents continuous propagation
• Much faster than continuous propagation
• Speed varies with axon diameter
• Larger axon = faster current

• Steps of saltatory propagation
1. Action potential occurs at initial segment
2. Local current produces graded depolarization
to threshold at next node
3. Action potential develops at node
4. Local current flow produces graded
depolarization to threshold at next node

Figure 11.9 2
Step 4:
Step 3:
Step 2:
The events that occur in saltatory propagation
Step 1:
An action potential
has occurred at the
initial segment 1 .
A local current
produces a graded
depolarization that
brings the axolemma
at the next node to
threshold.
An action potential
develops at node 2 .
A local current
produces a graded
depolarization that
brings the 3
axolemma at node
to threshold.
Local
current
Repolarization
(refractory)
Local
current
Cell membrane Cytosol
Extracellular fluid
Myelinated
internode
Myelinated
internode
Myelinated
internode

Module 11.9 Review
a. Define continuous propagation and saltatory
propagation.
b. What is the relationship between myelin and
the propagation speed of action potentials?

Module 11.10: Synaptic events
• Synaptic events
• Transmission of a message or “nerve impulse” within a
neuron
• = Action potential generation and propagation
• Transfer of a message between cells (from a neuron to
another neuron or effector cell)
• = Must be relayed across a synapse
• Types of synapses
1. Chemical synapses
2. Electrical synapses
Animation: Neurophysiology: Synapse

• Chemical synapses
• Rely on neurotransmitter release
• Those that release acetylcholine (ACh) are
cholinergic synapses
• Most abundant synapse type
• Most of those between neurons
• All synapses between neurons and other cells

• Events at a cholinergic synapse
1. Depolarization of synaptic knob by arriving
action potential
2. Opening of voltage-gated calcium channels
• Influx of Ca2+ causes exocytosis of ACh from
synaptic vesicles
• Ca2+ quickly removed to end release of ACh

• Events at a cholinergic synapse (continued)
3. ACh diffuses across synaptic cleft and binds to
chemically gated Na+ channels
• Na+ diffuses into postsynaptic cell and depolarizes
membrane
• More ACh bound = larger depolarization
4. Acetylcholinesterase (AChE, an enzyme) breaks
down ACh
• Makes effects on postsynaptic cell temporary
• Occurs within 20 msec

Figure 11.10 2
The events that occur at a cholinergic synapse
Mitochondrion
Acetylcholine
Synaptic
vesicle
SYNAPTIC
KNOB
SYNAPTIC
CLEFT
POSTSYNAPTIC
MEMBRANE
Choline
Acetate
Acetylcholinesterase
(AChE)
ACh
receptor
CoA
Acetyl-CoA
Events Occurring at Synapse
An arriving action potential
depolarizes the synaptic knob.
Calcium ions enter the
cytoplasm, and after a brief
delay, ACh is released through
the exocytosis of synaptic
vesicles.
ACh binds to sodium channel
receptors on the postsynaptic
membrane, producing a
graded depolarization.
Depolarization ends as ACh is
broken down into acetate and
choline by AChE.
The synaptic knob reabsorbs
choline from the synaptic cleft
and uses it to synthesize new
molecules of ACh.
1
2
3
4
5

• Chemical synapse physiology
• Synaptic fatigue
• After extended stimulation, the recycling of neurotransmitter
unable to keep up with demand
• Synapse weakens until neurotransmitter can be replenished
• Synaptic delay
• Release and binding of neurotransmitters takes
~0.2–0.5 msec
• With many neurons, cumulative delay may be significant
• Rapid reflexes involve few synapses

• Electrical synapses
• Presynaptic and postsynaptic membranes are locked
together by gap junctions
• Changes in transmembrane potential are transferred directly
between cells through local current flow
• Occur in CNS and PNS but extremely rare
• Some areas of brain, eye, ciliary ganglia of PNS
• Less adaptable and complex compared to chemical
synapses
• Example: changes in chemical environment or multiple
neurotransmitter affecting postsynaptic cell response

Figure 11.10 3
The structure of an electrical synapse
Presynaptic
neuron
Postsynaptic neuron
Gap junctions connecting
presynaptic and
postsynaptic neurons

Module 11.10 Review
a. Describe the parts of a chemical synapse.
b. Contrast an electrical synapse with a
chemical synapse.
c. What is synaptic fatigue, and how is it
reversed and eliminated?

Module 11.11: Information processing within a
neuron
• Information processing within a neuron
• Postsynaptic potentials
• Graded potentials in postsynaptic cell in response
to a neurotransmitter
• Two types
1. Excitatory postsynaptic potential (EPSP)
• Graded depolarization caused by neurotransmitter
arrival
• Shifts transmembrane potential closer to threshold
(= facilitated)
• More facilitation, the less additional stimulus needed
to reach threshold

neuron
• Postsynaptic potentials (continued)
• Two types (continued)
2. Inhibitory postsynaptic potential (IPSP)
• Graded hyperpolarization
• Example: opening of chemically gated K+ channels
• Shifts transmembrane potential farther from threshold
(= inhibited)
• More inhibition, larger-than-usual stimulus needed
to reach threshold

Figure 11.11 1
Postsynaptic potentials, graded potentials that develop in the postsynaptic membrane in response to
a neurostransmitter
An excitatory postsynaptic potential,
or EPSP, a graded depolarization
An inhibitory postsynaptic potential,
or IPSP, a graded hyperpolarization
Summation: the integration of
the effects of graded potentials
on a segment of the plasma
membrane
Time 1:
Depolarizing
stimulus
applied
Stimulus
removed
Time 3:
Depolarizing
stimulus
applied
Stimuli
removed
Time
Time 2:
Hyperpolarizing
stimulus applied
Time 3:
Hyperpolarizing
stimulus applied
Stimulus
removed
Resting potential Resting potential
EPSP
IPSP
EPSP
IPSP

neuron
• Integration of information at postsynaptic cell
• Single postsynaptic cell may receive information from
thousands of synapses
• Some excitatory, some inhibitory
• Net effect at axon hillock determines cell response
• Axon hillock is closest to initial segment where action
potential is generated
• Threshold at axon hillock is lowest of cell body
• Is the simplest information processing in the nervous system
• Allows neurons to respond to changes in oxygen, nutrients,
or abnormal chemicals

Figure 11.11 2
Axon
hillock
Initial
segment
Glial cell
processes
Dendrite Synaptic
knobs
The axon hillock, the site at which a single neuron
integrates the excitatory and inhibitory stimuli it
receives across thousands of synapses

neuron
• Summation
• Integration of graded potential effects on plasma
membrane segment
• May be combining opposite stimulations (EPSP +
IPSP) or similar stimulations (EPSP + EPSP or
IPSP + IPSP)
• Individual EPSP or IPSP has small effect on
transmembrane potential (~0.5 mV)
• Summation of EPSPs can lead to action potential
• Threshold commonly ~10 mV

neuron
• Two types of summation
1. Temporal summation (tempus, time)
• A single synapse stimulated repeatedly
• Example: before effects of one EPSP can dissipate,
another arrives
• = More ACh release = more postsynaptic cell
depolarization
• Possibly to threshold at initial segment

Figure 11.11 3
Temporal summation, in which a single synapse is active repeatedly
Temporal Summation
Initial
segment
Threshold
reached
ACTION
POTENTIAL
PROPAGATION
FIRST
STIMULUS
SECOND
STIMULUS

neuron
• Two types of summation (continued)
2. Spatial summation
• Involves multiple synapses activated simultaneously
• Example: EPSPs at multiple sites allowing Na+
channels to open
• May lead to action potential at initial segment
• Degree of depolarization dependent on
1. Number of synapses are active at a particular moment
2. Distance from initial segment

Figure 11.11 3
Spatial Summation
Spatial summation, in which multiple synapses are active simultaneously
TWO
SIMULTANEOUS
STIMULI
ACTION
POTENTIAL
PROPAGATION
Threshold
reached

Module 11.11 Review
a. Define excitatory postsynaptic potential (EPSP)
and inhibitory postsynaptic potential (IPSP).
b. Compare temporal summation with spatial
summation.
c. If a single EPSP depolarizes the initial segment
from a resting potential of –70 mV to –65 mV,
and threshold is at –60 mV, will an action
potential be generated?

Module 11.12: Higher-level information
processing
• Higher-level information processing
• Involves regulatory neurons
• Facilitate or inhibit presynaptic neurons by:
• Affecting cell body membrane
• Altering sensitivity of synaptic knobs

Figure 11.12 1
The positions of regulatory neurons,
which facilitate or inhibit the activities
of presynaptic neurons
Regulatory
neurons
Presynaptic
neuron
Postsynaptic
neuron

processing
• Involves different neurotransmitters
• More than 100 exist and work in different ways
• May have direct or indirect effects on ion
channels
• Indirect effects usually involve G proteins
• Trigger formation or release of second messengers
to alter postsynaptic cell activity

Figure 11.12 2

processing
• In nervous system, complex information is
translated to action potentials
• Solely on frequency of action potentials
• Example: muscle contraction changes in
response to increasing action potential
frequency

Figure 11.12 3
How the rate of action potentials arriving at a neuromuscular junction determines the nature of the resulting
muscle contraction
Time
Maximum tension (in tetanus)
Muscle
tension
KEY
Arrival
of action
potential
=
Twitch contractions
Muscle
tension
Frequency of action potentials
(per second)
Incomplete tetanus Tetanus

Figure 11.12 4

Module 11.12 Review
a. Describe the role of regulatory neurons.
b. What determines the frequency of action
potential generation?
c. The greater the degree of sustained
depolarization at the axon hillock, the
__________ (higher or lower) the frequency
of generation of action potentials.

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