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Chapter 11 Part C
Fundamentals
of the Nervous
System and
Nervous
Tissue
© Annie Leibovitz/Contact Press Images
© 2016 Pearson Education, Inc.
PowerPoint® Lecture Slides
prepared by
Karen Dunbar Kareiva
Ivy Tech Community College
11.7 The Synapse
• Nervous system works because information
flows from neuron to neuron
• Neurons are functionally connected by
synapses, junctions that mediate information
transfer
– From one neuron to another neuron
– Or from one neuron to an effector cell
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11.7 The Synapse
• Presynaptic neuron: neuron conducting
impulses toward synapse (sends information)
• Postsynaptic neuron: neuron transmitting
electrical signal away from synapse (receives
information)
– In PNS may be a neuron, muscle cell, or gland
cell
• Most function as both
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Figure 11.15 Synapses.
Axon of
presynaptic
neuron
Synapses
Cell body
(soma) of
postsynaptic
neuron
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11.7 The Synapse
• Synaptic connections
– Axodendritic: between axon terminals of one
neuron and dendrites of others
– Axosomatic: between axon terminals of one
neuron and soma (cell body) of others
– Less common connections:
• Axoaxonal (axon to axon)
• Dendrodendritic (dendrite to dendrite)
• Somatodendritic (dendrite to soma)
– Two main types of synapses:
• Chemical synapse
• Electrical synapse
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Figure 11.16 Axodendritic, axosomatic, and axoaxonal synapses.
Axodendritic
synapses
Dendrites
Axosomatic
synapses
Cell body
Axoaxonal
synapses
Axon
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Chemical Synapses
• Most common type of synapse
• Specialized for release and reception of chemical
neurotransmitters
• Typically composed of two parts
– Axon terminal of presynaptic neuron: contains
synaptic vesicles filled with neurotransmitter
– Receptor region on postsynaptic neuron’s
membrane: receives neurotransmitter
• Usually on dendrite or cell body
– Two parts separated by fluid-filled synaptic cleft
• Electrical impulse changed to chemical across
synapse, then back into electrical
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Chemical Synapses (cont.)
• Transmission across synaptic cleft
– Synaptic cleft prevents nerve impulses from
directly passing from one neuron to next
– Chemical event (as opposed to an electrical one)
– Depends on release, diffusion, and receptor
binding of neurotransmitters
– Ensures unidirectional communication between
neurons
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Chemical Synapses (cont.)
• Information transfer across chemical
synapses
– Six steps are involved:
1.
2.
AP arrives at axon terminal of presynaptic neuron
Voltage-gated Ca2+ channels open, and Ca2+
enters axon terminal
– Ca2+ flows down electrochemical gradient from ECF to
inside of axon terminal
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Slide 2
Focus Figure 11.3 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon terminal.
Mitochondrion
Ca2+
Ca2+
Ca2+
Ca2+
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
Postsynaptic
neuron
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Slide 3
Focus Figure 11.3 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon terminal.
2 Voltage-gated Ca2+ channels
Mitochondrion
open and Ca2+ enters the axon
terminal.
Ca2+
Ca2+
Ca2+
Ca2+
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
Postsynaptic
neuron
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Chemical Synapses (cont.)
3. Ca2+ entry causes synaptic vesicles to
release neurotransmitter
• Ca2+ causes synaptotagmin protein to react with
SNARE proteins that control fusion of synaptic
vesicles with axon membrane
• Fusion results in exocytosis of neurotransmitter into
synaptic cleft
• The higher the impulse frequency, the more vesicles
exocytose, leading to a greater effect on the
postsynaptic cell
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Slide 4
Focus Figure 11.3 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon terminal.
2 Voltage-gated Ca2+ channels
Mitochondrion
open and Ca2+ enters the axon
terminal.
Ca2+
Ca2+
Ca2+
Ca2+
3 Ca2+ entry causes
synaptic vesicles to
release neurotransmitter
by exocytosis.
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
Postsynaptic
neuron
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Chemical Synapses (cont.)
4. Neurotransmitter diffuses across the
synaptic cleft and binds to specific
receptors on the postsynaptic membrane
• Often chemically gated ion channels
5. Binding of neurotransmitter opens ion
channels, creating graded potentials
• Binding causes receptor protein to change shape,
which causes ion channels to open
– Causes a graded potential in postsynaptic cell
» Can be an excitatory or inhibitory event
– Some receptor proteins are also ion channels
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Slide 5
Focus Figure 11.3 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon terminal.
2 Voltage-gated Ca2+ channels
Mitochondrion
open and Ca2+ enters the axon
terminal.
Ca2+
Ca2+
Ca2+
Ca2+
3 Ca2+ entry causes
synaptic vesicles to
release neurotransmitter
by exocytosis.
4 Neurotransmitter
diffuses across the synaptic
cleft and binds to specific
receptors on the
postsynaptic membrane.
© 2016 Pearson Education, Inc.
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
Postsynaptic
neuron
Slide 6
Focus Figure 11.3 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon terminal.
2 Voltage-gated Ca2+ channels
Mitochondrion
open and Ca2+ enters the axon
terminal.
Ca2+
Ca2+
Ca2+
Ca2+
Synaptic
cleft
3 Ca2+ entry causes
Axon
terminal
synaptic vesicles to
release neurotransmitter
by exocytosis.
Synaptic
vesicles
4 Neurotransmitter
diffuses across the synaptic
cleft and binds to specific
receptors on the
postsynaptic membrane.
Postsynaptic
neuron
Ion movement
Graded potential
5 Binding of neurotransmitter opens ion channels,
resulting in graded potentials.
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Chemical Synapses (cont.)
6. Neurotransmitter effects are terminated
• As long as neurotransmitter is binding to receptor,
graded potentials will continue, so process needs to
be regulated
• Within a few milliseconds, neurotransmitter effect is
terminated in one of three ways
– Reuptake by astrocytes or axon terminal
– Degradation by enzymes
– Diffusion away from synaptic cleft
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Slide 7
Focus Figure 11.3 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon terminal.
2 Voltage-gated Ca2+ channels
Mitochondrion
open and Ca2+ enters the axon
terminal.
Ca2+
Ca2+
Ca2+
Ca2+
Synaptic
cleft
3 Ca2+ entry causes
Axon
terminal
synaptic vesicles to
release neurotransmitter
by exocytosis.
Synaptic
vesicles
4 Neurotransmitter
diffuses across the synaptic
cleft and binds to specific
receptors on the
postsynaptic membrane.
Postsynaptic
neuron
Ion movement
Enzymatic
degradation
Graded potential
Reuptake
Diffusion away
from synapse
5 Binding of neurotransmitter opens ion channels,
resulting in graded potentials.
6 Neurotransmitter effects are terminated
by reuptake through transport proteins,
enzymatic degradation, or diffusion away
from the synapse.
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Chemical Synapses (cont.)
• Synaptic delay
– Time needed for neurotransmitter to be released,
diffuse across synapse, and bind to receptors
• Can take anywhere from 0.3 to 5.0 ms
– Synaptic delay is rate-limiting step of neural
transmission
• Transmission of AP down axon can be very quick, but
synapse slows transmission to postsynaptic neuron
down significantly
• Not noticeable, because these are still very fast
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Electrical Synapses
• Less common than chemical synapses
• Neurons are electrically coupled
– Joined by gap junctions that connect cytoplasm
of adjacent neurons
– Communication is very rapid and may be
unidirectional or bidirectional
– Found in some brain regions responsible for eye
movements or hippocampus in areas involved in
emotions and memory
– Most abundant in embryonic nervous tissue
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11.8 Postsynaptic Potentials
• Neurotransmitter receptors cause graded
potentials that vary in strength based on:
– Amount of neurotransmitter released
– Time neurotransmitter stays in cleft
• Depending on effect of chemical synapse, there
are two types of postsynaptic potentials
– EPSP: excitatory postsynaptic potentials
– IPSP: inhibitory postsynaptic potentials
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Excitatory Synapses and EPSPs
• Neurotransmitter binding opens chemically gated
channels
– Allows simultaneous flow of Na+ and K+ in
opposite directions
• Na+ influx greater than K+ efflux, resulting in local
net graded potential depolarization called
excitatory postsynaptic potential (EPSP)
• EPSPs trigger AP if EPSP is of threshold strength
– Can spread to axon hillock and trigger opening of
voltage-gated channels, causing AP to be
generated
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Figure 11.17a Postsynaptic potentials can be excitatory or inhibitory.
Membrane potential (mV)
Excitatory postsynaptic potential (EPSP)
An EPSP is a local
depolarization of the
postsynaptic membrane.
+30
0
• EPSPs bring the neuron
closer to AP threshold.
Threshold
55
70
Stimulus
10
20
30
Time (ms)
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• Neurotransmitter binding
opens chemically gated ion
channels, allowing Na+ and
K+ to pass simultaneously.
Inhibitory Synapses and IPSPs
• Neurotransmitter binding to receptor opens
chemically gated channels that allow
entrance/exit of ions that cause
hyperpolarization
– Makes postsynaptic membrane more permeable
to K+ or Cl–
• If K+ channels open, it moves out of cell
• If Cl– channels open, it moves into cell
– Reduces postsynaptic neuron’s ability to produce
an action potential
• Moves neuron farther away from threshold (makes it
more negative)
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Figure 11.17b Postsynaptic potentials can be excitatory or inhibitory.
Membrane potential (mV)
Inhibitory postsynaptic potential (IPSP)
An IPSP is a local
hyperpolarization of the
postsynaptic membrane.
+30
0
• IPSPs drive the neuron
away from AP threshold.
Threshold
55
70
Stimulus
10
20
30
Time (ms)
© 2016 Pearson Education, Inc.
• Neurotransmitter binding
opens chemically gated ion
channels permeable to
either K+ or Cl.
Integration and Modification of Synaptic
Events
• Summation by the postsynaptic neuron
– A single EPSP cannot induce an AP, but EPSPs
can summate (add together) to influence
postsynaptic neuron
• IPSPs can also summate
– Most neurons receive both excitatory and
inhibitory inputs from thousands of other neurons
• Only if EPSPs predominate and bring to threshold will
an AP be generated
– Two types of summations: temporal and spatial
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Integration and Modification of Synaptic
Events (cont.)
– Temporal summation
• One or more presynaptic neurons transmit impulses in
rapid-fire order
– First impulse produces EPSP, and before it can
dissipate another EPSP is triggered, adding on top of
first impulse
– Spatial summation
• Postsynaptic neuron is stimulated by large number of
terminals simultaneously
– Many receptors are activated, each producing EPSPs,
which can then add together
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Figure 11.18a Neural integration of EPSPs and IPSPs.
Membrane potential (mV)
E1
0
Threshold of axon of
postsynaptic neuron
Resting potential
55
70
Excitatory synapse 1 (E1)
E1
E1
Time
No summation:
2 stimuli separated in
time cause EPSPs that
do not add together.
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Excitatory synapse 2 (E2)
Inhibitory synapse (I1)
Figure 11.18b Neural integration of EPSPs and IPSPs.
Membrane potential (mV)
E1
0
55
70
Excitatory synapse 1 (E1)
E1 E1
Time
Temporal summation:
2 excitatory stimuli close
in time cause EPSPs
that add together.
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Excitatory synapse 2 (E2)
Inhibitory synapse (I1)
Figure 11.18c Neural integration of EPSPs and IPSPs.
E1
Membrane potential (mV)
E2
0
55
70
Excitatory synapse 1 (E1)
E1 + E2
Time
Spatial summation:
2 simultaneous stimuli at
different locations cause
EPSPs that add together.
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Excitatory synapse 2 (E2)
Inhibitory synapse (I1)
Figure 11.18d Neural integration of EPSPs and IPSPs.
E1
Membrane potential (mV)
I1
0
55
Excitatory synapse 1 (E1)
70
I1
E1 + I1
Time
Spatial summation of
EPSPs and IPSPs:
Changes in membrane
potential can cancel each
other out.
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Excitatory synapse 2 (E2)
Inhibitory synapse (I1)
Integration and Modification of Synaptic
Events (cont.)
• Synaptic potentiation
– Repeated use of synapse increases ability of
presynaptic cell to excite postsynaptic neuron
• Ca2+ concentration increases in presynaptic terminal,
causing release of more neurotransmitter
• Leads to more EPSPs in postsynaptic neuron
– Potentiation can cause Ca2+ voltage gates to
open on postsynaptic neuron
• Ca2+ activates kinase enzymes, leading to more
effective response to subsequent stimuli
– Long-term potentiation: learning and memory
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Integration and Modification of Synaptic
Events (cont.)
• Presynaptic inhibition
– Release of excitatory neurotransmitter by one
neuron is inhibited by another neuron via an
axoaxonal synapse
– Less neurotransmitter is released, leading to
smaller EPSPs
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Table 11.2-1 Comparison of Graded Potentials and Action Potentials
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Table 11.2-2 Comparison of Graded Potentials and Action Potentials (continued)
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Table 11.2-3 Comparison of Graded Potentials and Action Potentials (continued)
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Table 11.2-4 Comparison of Graded Potentials and Action Potentials (continued)
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11.9 Neurotransmitters
• Language of nervous system
• 50 or more neurotransmitters have been
identified
• Most neurons make two or more
neurotransmitters
– Neurons can exert several influences
• Usually released at different stimulation
frequencies
• Classified by:
– Chemical structure
– Function
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Classification of Neurotransmitters by
Chemical Structure
• Acetylcholine (ACh)
– First identified and best understood
– Released at neuromuscular junctions
• Also used by many ANS neurons and some CNS
neurons
– Synthesized from acetic acid and choline by
enzyme choline acetyltransferase
– Degraded by enzyme acetylcholinesterase
(AChE)
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Classification of Neurotransmitters by
Chemical Structure (cont.)
• Biogenic amines
– Catecholamines
• Dopamine, norepinephrine (NE), and epinephrine:
made from the amino acid tyrosine
– Indolamines
• Serotonin: made from the amino acid tryptophan
• Histamine: made from the amino acid histidine
– All widely used in brain: play roles in emotional
behaviors and biological clock
– Used by some ANS motor neurons
• Especially NE
– Imbalances are associated with mental illness
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Classification of Neurotransmitters by
Chemical Structure (cont.)
• Amino acids
– Amino acids make up all proteins: therefore, it is
difficult to prove which are neurotransmitters
– Amino acids that are proven neurotransmitters
•
•
•
•
Glutamate
Aspartate
Glycine
GABA: gamma ()-aminobutyric acid
© 2016 Pearson Education, Inc.
Classification of Neurotransmitters by
Chemical Structure (cont.)
• Peptides (neuropeptides)
– Strings of amino acids that have diverse
functions
• Substance P
– Mediator of pain signals
• Endorphins
– Beta endorphin, dynorphin, and enkephalins: act as
natural opiates; reduce pain perception
• Gut-brain peptides
– Somatostatin and cholecystokinin play a role in
regulating digestion
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Classification of Neurotransmitters by
Chemical Structure (cont.)
• Purines
– Monomers of nucleic acids that have an effect in
both CNS and PNS
• ATP, the energy molecule, is now considered a
neurotransmitter
• Adenosine is a potent inhibitor in brain
– Caffeine blocks adenosine receptors
• Can induce Ca2+ influx in astrocytes
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Classification of Neurotransmitters by
Chemical Structure (cont.)
• Gases and lipids
– Gasotransmitters
• Nitric oxide (NO), carbon monoxide (CO), hydrogen
sulfide gases (H2S)
• Bind with G protein–coupled receptors in brain
• Lipid soluble and are synthesized on demand
• NO involved in learning and formation of new
memories, as well as brain damage in stroke patients,
and smooth muscle relaxation in intestine
• H2S acts directly on ion channels to alter function
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Classification of Neurotransmitters by
Chemical Structure (cont.)
– Endocannabinoids
• Act at same receptors as THC (active ingredient in
marijuana)
• Most common G protein–linked receptors in brain
• Lipid soluble
• Synthesized on demand
• Believed to be involved in learning and memory
• May be involved in neuronal development, controlling
appetite, and suppressing nausea
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Classification of Neurotransmitters by
Function
• Neurotransmitters exhibit a great diversity of
functions
• Functions can be grouped into two
classifications:
– Effects
– Actions
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Classification of Neurotransmitters by
Function (cont.)
• Effects: excitatory versus inhibitory
– Neurotransmitter effects can be excitatory
(depolarizing) and/or inhibitory (hyperpolarizing)
– Effect determined by receptor to which it binds
• GABA and glycine are usually inhibitory
• Glutamate is usually excitatory
• Acetylcholine and NE bind to at least two receptor
types with opposite effects
– ACh is excitatory at neuromuscular junctions in skeletal
muscle
– ACh is inhibitory in cardiac muscle
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Classification of Neurotransmitters by
Function (cont.)
• Actions: direct versus indirect
– Direct action: neurotransmitter binds directly to
and opens ion channels
• Promotes rapid responses by altering membrane
potential
• Examples: ACh and amino acids
– Indirect action: neurotransmitter acts through
intracellular second messengers, usually G
protein pathways
• Broader, longer-lasting effects similar to hormones
• Biogenic amines, neuropeptides, and dissolved gases
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Classification of Neurotransmitters by
Function (cont.)
• Actions: direct versus indirect (cont.)
– Neuromodulator: chemical messenger released
by neuron that does not directly cause EPSPs or
IPSPs but instead affects the strength of synaptic
transmission
• May influence synthesis, release, degradation, or
reuptake of neurotransmitter
• May alter sensitivity of the postsynaptic membrane to
neurotransmitter.
• May be released as a paracrine
– Effect is only local
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Neurotransmitter Receptors
• Channel-linked receptors
– Ligand-gated ion channels
– Action is immediate and brief
– Excitatory receptors are channels for small
cations
• Na+ influx contributes most to depolarization
– Inhibitory receptors allow Cl– influx that causes
hyperpolarization
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Figure 11.19 Channel-linked receptors cause rapid synaptic transmission.
Ion flow blocked
Ions flow
Ligand
Closed ion
channel
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Open ion
channel
Neurotransmitter Receptors (cont.)
• G protein–linked receptors
– Responses are indirect, complex, slow, and
often prolonged
– Involves transmembrane protein complexes
– Cause widespread metabolic changes
– Examples:
• Muscarinic ACh receptors
• Receptors that bind biogenic amines
• Receptors that bind neuropeptides
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Neurotransmitter Receptors (cont.)
• G protein–linked receptors (cont.)
– Mechanism:
• Neurotransmitter binds to G protein–linked receptor,
activating G protein
• Activated G protein controls production of second
messengers, such as cyclic AMP, cyclic GMP,
diacylglycerol, or Ca2+
• Second messengers can then:
– Open or close ion channels
– Activate kinase enzymes
– Phosphorylate channel proteins
– Activate genes and induce protein synthesis
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Slide 2
Figure 11.20 G protein–linked receptors cause the formation of intracellular second messengers.
G protein signaling
mechanisms are like a
molecular relay race.
Ligand (1st Receptor G protein
messenger)
Enzyme
2nd
messenger
1 Neurotransmitter
(1st messenger) binds
and activates receptor.
Receptor
Nucleus
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Slide 3
Figure 11.20 G protein–linked receptors cause the formation of intracellular second messengers.
G protein signaling
mechanisms are like a
molecular relay race.
Ligand (1st Receptor G protein
messenger)
Enzyme
2nd
messenger
1 Neurotransmitter
(1st messenger) binds
and activates receptor.
Receptor
G protein
GTP
GDP
GTP
2 Receptor
activates G
protein.
Nucleus
© 2016 Pearson Education, Inc.
Slide 4
Figure 11.20 G protein–linked receptors cause the formation of intracellular second messengers.
G protein signaling
mechanisms are like a
molecular relay race.
Ligand (1st Receptor G protein
messenger)
Enzyme
1 Neurotransmitter
(1st messenger) binds
and activates receptor.
2nd
messenger
Adenylate cyclase
Receptor
G protein
GTP
GDP
GTP
GTP
2 Receptor
activates G
protein.
© 2016 Pearson Education, Inc.
3 G protein
activates
adenylate
cyclase.
Nucleus
Slide 5
Figure 11.20 G protein–linked receptors cause the formation of intracellular second messengers.
G protein signaling
mechanisms are like a
molecular relay race.
Ligand (1st Receptor G protein
messenger)
Enzyme
1 Neurotransmitter
(1st messenger) binds
and activates receptor.
2nd
messenger
Adenylate cyclase
Receptor
G protein
ATP
GTP
GDP
GTP
cAMP
GTP
2 Receptor
activates G
protein.
© 2016 Pearson Education, Inc.
3 G protein
activates
adenylate
cyclase.
4 Adenylate
cyclase converts
ATP to cAMP
(2nd messenger).
Nucleus
Slide 6
Figure 11.20 G protein–linked receptors cause the formation of intracellular second messengers.
G protein signaling
mechanisms are like a
molecular relay race.
Ligand (1st Receptor G protein
messenger)
Enzyme
1 Neurotransmitter
(1st messenger) binds
and activates receptor.
2nd
messenger
Adenylate cyclase
Closed ion channel
Open ion channel
Receptor
G protein
ATP
GTP
GDP
GTP
5a cAMP changes membrane
permeability by opening or
cAMP closing ion channels.
GTP
2 Receptor
activates G
protein.
© 2016 Pearson Education, Inc.
3 G protein
activates
adenylate
cyclase.
4 Adenylate
cyclase converts
ATP to cAMP
(2nd messenger).
Nucleus
Slide 7
Figure 11.20 G protein–linked receptors cause the formation of intracellular second messengers.
G protein signaling
mechanisms are like a
molecular relay race.
Ligand (1st Receptor G protein
messenger)
Enzyme
1 Neurotransmitter
(1st messenger) binds
and activates receptor.
2nd
messenger
Adenylate cyclase
Closed ion channel
Open ion channel
Receptor
G protein
ATP
GTP
GDP
GTP
5a cAMP changes membrane
permeability by opening or
cAMP closing ion channels.
5b cAMP activates
GTP
2 Receptor
activates G
protein.
© 2016 Pearson Education, Inc.
3 G protein
activates
adenylate
cyclase.
4 Adenylate
cyclase converts
ATP to cAMP
(2nd messenger).
enzymes.
Active enzyme
Nucleus
Slide 8
Figure 11.20 G protein–linked receptors cause the formation of intracellular second messengers.
G protein signaling
mechanisms are like a
molecular relay race.
Ligand (1st Receptor G protein
messenger)
Enzyme
1 Neurotransmitter
(1st messenger) binds
and activates receptor.
2nd
messenger
Adenylate cyclase
Closed ion channel
Open ion channel
Receptor
G protein
ATP
GTP
GTP
5a cAMP changes membrane
permeability by opening or
cAMP closing ion channels.
5c cAMP activates
specific genes.
GDP
5b cAMP activates
GTP
2 Receptor
activates G
protein.
© 2016 Pearson Education, Inc.
3 G protein
activates
adenylate
cyclase.
4 Adenylate
cyclase converts
ATP to cAMP
(2nd messenger).
enzymes.
Active enzyme
Nucleus
11.10 Neural Integration
• Neural integration: neurons functioning
together in groups
• Groups contribute to broader neural functions
• There are billions of neurons in CNS
– Must have integration so that the individual parts
fuse to make a smoothly operating whole
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Organization of Neurons: Neuronal Pools
• Neuronal pool: functional groups of neurons
– Integrate incoming information received from
receptors or other neuronal pools
– Forward processed information to other
destinations
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Organization of Neurons: Neuronal Pools
(cont.)
• Simple neuronal pool
– Single presynaptic fiber branches and synapses
with several neurons in pool
– Discharge zone: neurons closer to incoming
fiber are more likely to generate impulse
– Facilitated zone: neurons on periphery of pool
are farther away from incoming fiber; usually not
excited to threshold unless stimulated by another
source
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Figure 11.21 Simple neuronal pool.
Presynaptic
(input) fiber
Facilitated
zone
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Discharge
zone
Facilitated
zone
Patterns of Neural Processing
• Serial processing
– Input travels along one pathway to a specific
destination
• One neuron stimulates next one, which stimulates
next one, etc.
– System works in all-or-none manner to produce
specific, anticipated response
– Best example of serial processing is a spinal
reflex
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Patterns of Neural Processing (cont.)
• Serial processing (cont.)
– Reflexes
• Rapid, automatic responses to stimuli
• Particular stimulus always causes same response
• Occur over pathways called reflex arcs that have five
components:
– Receptor
– Sensory neuron
– CNS integration center
– Motor neuron
– Effector
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Figure 11.22 A simple reflex arc.
Stimulus
1 Receptor
Interneuron
2 Sensory neuron
3 Integration center
4 Motor neuron
5 Effector
Spinal cord (CNS)
Response
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Patterns of Neural Processing (cont.)
• Parallel processing
– Input travels along several pathways
– Different parts of circuitry deal simultaneously
with the information
• One stimulus promotes numerous responses
– Important for higher-level mental functioning
– Example: A sensed smell may remind one of an
odor and any associated experiences
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Types of Circuits
• Circuits: patterns of synaptic connections in
neuronal pools
• Four types of circuits
– Diverging
– Converging
– Reverberating
– Parallel after-discharge
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Figure 11.23a Types of circuits in neuronal pools.
Input
Many outputs
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Diverging circuit
• One input, many outputs
• An amplifying circuit
• Example: A single neuron
in the brain can activate
100 or more motor neurons
in the spinal cord and
thousands of skeletal
muscle fibers
Figure 11.23b Types of circuits in neuronal pools.
Input 1
Input 2
Input 3
Output
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Converging circuit
• Many inputs, one output
• A concentrating circuit
• Example: Different sensory
stimuli can all elicit the
same memory
Figure 11.23c Types of circuits in neuronal pools.
Input
Output
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Reverberating circuit
• Signal travels through a
chain of neurons, each
feeding back to previous
neurons
• An oscillating circuit
• Controls rhythmic activity
• Example: Involved in
breathing, sleep-wake
cycle, and repetitive motor
activities such as walking
Figure 11.23d Types of circuits in neuronal pools.
Input
Output
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Parallel after-discharge
circuit
• Signal stimulates neurons
arranged in parallel arrays
that eventually converge
on a single output cell
• Impulses reach output cell
at different times, causing
a burst of impulses called
an after-discharge
• Example: May be involved
in exacting mental processes
such as mathematical
calculations
Developmental Aspects of Neurons
• Nervous system originates from neural tube
and neural crest formed from ectoderm
• The neural tube becomes CNS
– Neuroepithelial cells of neural tube proliferate
into number of cells needed for development
– Neuroblasts become amitotic and migrate
– Neuroblasts sprout axons to connect with targets
and become neurons
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Developmental Aspects of Neurons
• Growth cone: prickly structure at tip of axon
that allows it to interact with its environment via:
– Cell surface adhesion proteins (laminin, integrin,
and nerve cell adhesion molecules, or N-CAMs),
which provide anchor points
– Neurotropins that attract or repel the growth
cone
– Nerve growth factor (NGF), which keeps
neuroblast alive
– Filopodia are growth cone processes that follow
signals toward target
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Developmental Aspects of Neurons
• Once axon finds its target, it then must find right
place to form synapse
– Astrocytes provide physical support and the
cholesterol needed for construction of synapses
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Developmental Aspects of Neurons
• About two-thirds of neurons die before birth
– If axons do not form a synapse with their target,
they are triggered to undergo apoptosis
(programmed cell death)
– Many other cells also undergo apoptosis during
development
• Neurons are amitotic after birth; however, there
are a few special neuronal populations that
continue to divide
– Olfactory neurons and hippocampus
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Figure 11.24 A neuronal growth cone.
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