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7/31/2019 1.2. Neurophysiology-Conduction, Transmission, And the Integration of Neural Signals (Slide Presentation)
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Chapter 3: Neurophysiology: Conduction, Transmission,and the Integration of Neural Signals
> Communication Within a Neuron
> Communication Between Neurons
copyright D.P. Devine
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> Electricity:
negative pole = greater number of electrons, greater negative charge
positive pole = fewer electrons, less negative charge
current = flow of electrons from negative to positive pole (measured in amperes)
electrical potential = difference in electrical charge (measured in volts)
between negative and positive poles
Communication Within a Neuron
> Recording the Membrane
Potential of a Neuron:
Resting Potential = -70mV
(varies from one neuron to another)
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> Stimulating the Neuronal Membrane
with a Microelectrode:
Communication Within
a Neuron
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> Stimulate with
microelectrode> Record with secondmicroelectrode
> HyperpolarizationApply small negativecurrent to increasenegative membranepotential
0
-20
-40-60
-80
-100
Communication Within a Neuron
time (ms)
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> Depolarization
Apply depolarizingcurrent to decreasemembrane potentialtoward neutrality
0
-20
-40-60
-80
-100
> Stimulate with
microelectrode> Record with secondmicroelectrode
Communication Within a Neuron
time (ms)
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> Depolarization:
Apply a slightly largerdepolarizing current to reach-55mV threshold
> Action Potential:
A disproportionately
large response,constant regardless ofmagnitude of stimulation
above -55mV
20
0
-20-40
-80
-120
All - or - none
Communication Within a Neuron
time (ms)
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> Concentration Gradient:- Molecules are in constant motion.- In the absence of external forces or
barriers, molecules diffuse accordingto their concentration gradient.
Communication Within a Neuron
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> Voltage Gradient / Electrostatic Potential:- Electrolytes dissociate into ions in solution.- For example, NaCl dissociates into Na+
(a cation) and Cl-
(an anion)..- Like ions (i.e. those with the same charge)
will repel each other in solution.Na+
Na+
Na+
Na+
Cl-
Cl-
Cl-
Cl-
Communication Within a Neuron
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> Dispersion of charged particles with an impermeable and asemipermeable membrane:
Communication Within a Neuron
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> Positive ions (cations):
sodium (Na+), potassium (K+)
> Negative ions
(anions):
chloride (Cl-),
proteins
-
--
-+
+
+
++ +
+
Communication Within a Neuron
Ion Exchange
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+
+
> Channel proteins: Cylindrical proteins that permitcontrolled exchange of ions across the membrane.
++
+
+
+
+
+
-
-
-
-
-
-
-
+
+
+
+
+
+
++
Communication Within a Neuron
Ion Exchange
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> Resting potential: In the absence of disturbance themembrane maintains a slightly negative electrical
potential (i.e.balanceof ionic charges) insidethe neuron, with
respect to the outside. +
+
++
+
+
+
+
+
-
--
-
-
-
-
Communication Within a Neuron
Ion Exchange
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> Sodium (Na+): More than ten times more concentrated outsidethe cell (extracellular) than inside the cell (intracellular)
Na+
Na+Na+ Na+
Na+
Na+
Na+Na
+
Na+
Na+
Na+ Na+Na+ Na+ Na+
Na+Na+
Na+
Na+
Na+Na+ Na
+
Na+
Na+
Na+Na+
Na+Na+
Na+
Na+
Na+Na+
Na+
Na+
Na+
Na+
Communication Within a Neuron
Ion Exchange
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> Potassium (K+): More than twenty times more concentratedinside the cell (intracellular) than outside the cell (extracellular)
K+
K+
K+K+
K+K+
K+K+
K+
K+K+
K+
K+
K+
K+
K+
K+ K+K+
K+
K+
K+
Communication Within a Neuron
Ion Exchange
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Na+
> [Na+] > [K+]: There are many more sodium ions thanpotassium ions, providing a net positive extracellular potential.
K+
K+
K+K+
K+K+
K+K+
K+
K+K+
K+
K+
K+
K+
K+
K+ K+K+
K+
K+
Na+
Na+
Na+ Na+Na+ Na+
Na+
Na+
Na+Na+ Na
+
Na+
Na+
Na+Na+Na+
Na+
Na+
Na+Na+
Na+Na+
Na+
Na+
Na+Na+ Na+
Na+
Na+Na+
Na+Na
+Na+
Na+
K+
Communication Within a Neuron
Ion Exchange
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Cl-
Cl-
Cl-
Cl-Cl-
Cl-
> Chloride (Cl-): More concentrated in the extracellular spacethan the intracellular space
Cl-
Cl-Cl-
Cl-Cl-
Cl-
Communication Within a Neuron
Ion Exchange
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> Resting Potential: Difference between the net charge(considering all the positive and negative charges) inside
the cell, relative tothe net charge outsidethe cell (approx.
-70mV in the giantsquid axon).
Na+
Na+Na+
Na+Na+
Na+
Na+
Na+
Na+Na+ Na
+
Na+
Cl-
Cl-
Cl-
Cl-Cl-
Cl-
K+
K+
K+
K+
AAA
AAA
AA
AAA
AA
AAAAAAAA
AAAAA
AAAAAA
AA A
AAAA
Communication Within a Neuron
Ion Exchange
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> Selective Permeability: Some molecules can freely cross thecell membrane (e.g. O2, CO2, urea, water).
Most larger molecules (e.g. negatively charged proteins) andions (e.g. Na+) are prevented from freely crossing the
membrane.
CO2
CO2CO2
CO2
urea
ureaurea
urea
H2O H2O
H2O
H2O
O2
O2
O2
O2
O2
O2
H2O
H2OH2O
H2O
Na+ Na+
Na+
Na+
AAAAAA
AA A
AAAAAAAAAA
AA AAAAA
Communication Within a Neuron
Ion Exchange
intracellular extracellular
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Sodium-Potassium Pump:Na+ and K+ are actively transportedacross the membrane by specific Na+/K+ transport proteins
> Na+
:Na+
/K+
pump actively transports 3 Na+
out of the cell.Na+ concentration gradient would push Na+back in.Electrical gradient would push Na+back in.BUT the membrane is almost impermeable to Na+.
> K+:Na+/K+pump actively transports2 K+ into the cell.
K+ concentration gradient would push
K+back out.The membrane is semipermeable toK+, so K+ could leak back out.
BUT the electrical gradient keeps
K+ inside the cell.
Communication Within a NeuronIon Exchange
membrane
Na+-K+
transporter
extracellular
intracellular
3 Na+ out
2 K+ in
Na+ Na+
Na+
K+ K+
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+
-
At Resting Potential
Summary of Forces on Charged Particles
Communication Within a Neuron
membrane
extracellular
intracellular
+
-
+
-
+
-
+
-
+
--
+
-
+
proteins-
cannotleave cell
K+
force ofdiffusion
electrostaticpressure
K+low
conc Cl-
force ofdiffusion
electrostaticpressure
Cl-
Na+
force ofdiffusion
electrostaticpressure
Na+
highconc
highconc
lowconc
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Hyperpolarization and Depolarization
Communication Within a Neuron
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> Extremely high energy expenditure: Very energyexpensive, approximately 40% of neurons energy
resources
20
0
-20
-40
-80-120
> Extremely rapid, strong response: By maintaining a highconcentration gradient and electrostatic potential, the neuron
is prepared to exert a very rapid and powerful response whencalled upon - THE ACTION POTENTIAL!!
Communication Within a Neuron
Why a Resting Potential?
time (ms)
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> Axon Hillock:
Electrochemical input from
soma arrives at axon hillock.If above threshold, actionpotential is initiated.
20
0
-20
-40
-80-120
All - or - none
Axon hillock
Axon
Soma
Dendrites
The Action Potential and the Axon Hillock
Communication Within a Neuron
time (ms)
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The All-Or-None-Law
Communication Within a Neuron
For all stimuli that exceed threshold
The size and shape of the action potential are independent of theintensity of the stimulus that initiated it.
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> Voltage-Gated Ion Channels:
Respond by opening or closing
according to the value of themembrane potential
> At -70 to -55mV
Some Na+ channels openSmall Na+ influxSome K+ channels openSmall K+ effluxDriven by conc. gradient
& electrostatic pressure.
Communication Within a Neuron
The Action Potential
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> Voltage-Gated Ion Channels:
Respond by opening or closing
according to the value of themembrane potential
> At -55mV
Na+ channels openNa+ rushes inK+ channels openK+ exitsDriven by conc. gradient
& electrostatic pressure.
Communication Within a Neuron
The Action Potential
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> Voltage-Gated Ion Channels:
Respond by opening or closing
according to the value of themembrane potential
> Depolarization &
Reverse Polarization
Rapid change inmembranepotential from-70mV to +40mV
Communication Within a Neuron
The Action Potential
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> Voltage-Gated Ion Channels:
Respond by opening or closing
according to the value of themembrane potential
> Reverse polarization
Na+ channels becomerefractoryCannot open againuntil resting potentialis re-established
Communication Within a Neuron
The Action Potential
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Refractory Period
> Voltage-Gated Ion Channels:
Respond by opening or closing
according to the value of themembrane potential
> After-hyperpolarization
Neuron overshoots restingpotential.External K+diffuses, restoringresting potentialNa+/K+pump restores ion
balance
The Action Potential
Communication Within a Neuron
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The Action Potential
Communication Within a Neuron
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> Propagated signal retains intensity
As action potentialis transmitted down
axon, it is constantlyrenewed- depolarization ofarea around actionpotential createsnew action potential.
Propagation of The Action Potential
Communication Within a Neuron
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> Speed of conduction varies:
Thin unmyelinated -> less than1 m/s
Thick unmyelinated -> 10m/sThick myelinated -> 100 m/sElectricity -> 300,000,000 m/s
Propagation of The Action Potential
Communication Within a Neuron
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> Action Potential jumps from one node to the next:
AP cannot regenerateat myelin due to1- insulation2- Na+ channels
mostly at nodes
Positive charges repel
to next node
AP re-established
Saltatory conduction = fast propagation of AP
Nodes of Ranvier
Myelin
Axon
Saltatory Conduction
Communication Within a Neuron
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> Interneurons:Lack axon or short axon.Depolarize or hyperpolarize in
proportion to the intensity of thestimulus.Alterations in membrane potential
decay rapidly as they areconducted.
Graded Potentials
Communication Within a Neuron
X
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Communication Between Neurons
> Charles Scott Sherrington Discovery of the Synapse- (1906) demonstrated gaps between neurons, behaviorally- studied the leg flexion reflex in a dog
- measured conduction velocity in sensory & motor neurons- measured distance of input to spinal cord- measured distance of output to muscle- pinched foot, measured delay until flexion- found delay longer than expected- reasoned gaps between neurons- called gaps synapses (after Cajal)
A
C
B
D E
40 m/sec
~15 m/sec
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> Charles Scott Sherrington Discovery of the Synapse1) Reflexes are slower than conduction along an axon. Consequently,there must be some delay at synapses
2) Several weak stimuli presented at slightly different times or slightlydifferent locations produce a stronger reflex than a single stimulusdoes. Therefore, the synapses must be able to summate stimuli3) When one set of muscles is excited,another set is relaxed. Accordingly, theinput can simultaneously excite outputsat some synapses while inhibiting
outputs at other synapses
A
C
B
D E
40 m/sec
~15 m/sec
Communication Between Neurons
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Communication Between Neurons
> Otto Leowi Discovery of Chemical Neurotransmission- (1921) demonstrated neurons transmit using a chemical messenger- stimulated frog vagus nerve
- transferred bath fromstimulated heart tosecond heart
- both hearts decreased rateof beating
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> The Structure of Synapses- electron microscopy reveals synaptic structure
Synaptic vesiclesMitochondria
Neurotransmitters
GolgiComplex
Microtubules
Communication Between Neurons
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> The Structure of Synapses- electron microscopy reveals synaptic structure
Communication Between Neurons
Microtubules
transportSynaptic vesicles
storage/release
Cisternae (golgi)recycling
neurotransmitter
Mitochondria
energy
Synaptic cleft
site of release
Postsynaptic
Membrane &Receptors
site of action ofneurotransmitter
Synaptic cleft is approx. 200 .Neurons have an average of 1000 synapses each.
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Communication Between Neurons
> Most common types of synapses
Axodendritic
Axosomatic
Soma
AxonAxon
Dendrites
> Synapses are junctions between axon terminals and cellmembranes of other neurons
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Communication Between Neurons
> Excitatory and Inhibitory Messages- Specific synapses provide excitatory (depolarizing) input- Other synapses provide inhibitory (hyperpolarizing) input
- Type I synapses = located primarily on shafts or spines of dendrites,round vesicles, thick presynaptic density, wide synaptic cleft, largeactive zone, excitatory input
- Type II synapses = located primarilyon soma, flattened vesicles, thinpresynaptic density, narrow synapticcleft, small active zone, inhibitory input
Type I Type II
C i i
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Communication Between Neurons
> The Types of Receptors for Neurotransmitters- two main classes of receptors, ionotropic and metabotropic
Ionotropicreceptors:
Open a neurotransmitter-
dependent ion channelwhen a molecule ofneurotransmitter binds
This changes the localpostsynaptic membranepotential.
C i i B N
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Communication Between Neurons
> The Types of Receptors for Neurotransmitters
Na+ channels:
Most importantexcitatory input
(EPSP)
K+ channels:
Inhibitory input
(IPSP)
Different receptors are coupled to different ion channels
The type of ion channel determines whether input is excitatory or
inhibitory
C i i B N
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Ca++ channels:
Excitatory input
(EPSP)
Communication Between Neurons
> The Types of Receptors for Neurotransmitters
Different receptors are coupled to different ion channels
The type of ion channel determines whether input is excitatory or
inhibitory
Cl- channels:
Decrease thedepolarization ofexcited neurons
(neutralize EPSP)
C i ti B t N
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Communication Between Neurons
> The Types of Receptors for Neurotransmitters
Neurons exhibit a basal rate of firing of action
potentials:
basal or spontaneous firing rate
excitatory input
inhibitory input
C i ti B t N
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Communication Between Neurons
> The Types of Receptors for NeurotransmittersMetabotropic receptors: activate an associated protein (G protein)which triggers the opening of an ion channel.
This changes the local postsynaptic membrane potential or changes chemicalactivities within the cell.
C i ti B t N
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Communication Between Neurons
> The Types of Receptors for Neurotransmitters
C i ti B t N
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Excitatory Postsynaptic Potential (EPSP) andInhibitory Postsynaptic Potential (IPSP)
Communication Between Neurons
> EPSP:
Depolarizing input to the somaor a dendrite produces a localgraded EPSP
> IPSP:
Hyperpolarizing input to the
soma or a dendrite producesa local graded EPSP
C i ti B t N
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Summation of EPSPs and IPSPs
Communication Between Neurons
Summation of Excitatory Post Synaptic Potentials
MembranePotential(mV)
-90
-80
-70
-60
-50
-40
threshold
Summation of Inhibitory Post Synaptic Potentials
MembranePotential(mV)
-90
-80
-70
-60
-50
-40
threshold
EPSPEPSP
IPSPIPSP
C i ti B t N
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Summation of EPSPs and IPSPs
Communication Between Neurons
> EPSPs summate to
produce an ActionPotential
> IPSPs counteract the
effects of EPSPs to blockthe Action Potential
C i ti B t N
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Spatial Summation
Communication Between Neurons
Summation
Summation
Cancellation
excitatory
synapsesinhibitory
synapses
A
B C
D
A B
C D
A C
C i ti B t N
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TemporalSummation
Communication Between Neuronsinhibitory
synapseA B
A
B
excitatory
synapse
A
B
A A
B B
No Summation
No Summation
Summation
Summation
C i ti B t N
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Temporal and Spatial Summation
Communication Between Neurons
> EPSPs and IPSPs:
Excitatory and inhibitory inputsdiffuse along the interior surface ofthe cell membrane, summate (orcancel) and the net potential
registered at the axon hillock mayinitiate an action potential.
Comm nication Bet een Ne rons
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Communication Between Neurons
> Axoaxonic synapses A Special Case:Axoaxonic synapses do not contribute directly to neural integration.
Rather, they modulate the amount of neurotransmitter release fromthe terminal boutons of the postsynaptic neuron.
Ordinarily the number of quanta of
neurotransmitter release per action potentialis constant.
presynaptic inhibition: decrease in neurotransmitter releasepresynaptic facilitation: increase in neurotransmitter release
due to actions of axoaxonic synapses
Axoaxonic
Other Types of Synapses
Communication Between Neurons
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Communication Between Neurons
varicosities
electrical synapses
Other Types of SynapsesDendrodendritic synapses :Occur on some very small interneurons.May participate in regulatory functions
- e.g. organization of groups of neurons
small size, difficult to study, function unknown
Varicosities:
Not really synapses, beadlike swellings along
axon where neurotransmitter is releasedGap Junctions (Electrical Synapses) :
narrow gapion channels communicate directly between cells
common in invertebrates, less common invertebrates.
functions largely unknown in vertebrates- may participate in neuroadaptive processes
such as sensitization.
Communication Between Neurons
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Communication Between Neurons
> Nonsynaptic Chemical Communication:Neurons have membrane-bound receptorsall over their membranes. Neurons also
have cytosolic and nuclear receptors.
These non-synaptic receptors bind avariety of specific neurotransmitters,neuromodulators, and hormones.
Most non-synaptic membrane-bound
receptors are metabotropic. Some areionotropic. All known cytosolic andnuclear receptors are metabotropic.
Other Types of Synapses
Communication Between Neurons
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axon collateral oscillator circuit
Communication Between NeuronsTypes of Circuits
simple neural
chain
convergence and
divergence
Communication Between Neurons
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> Seven Stages in Neurotransmitter Function-
Communication Between Neurons
1. Neurotransmitters are synthesized.
2. Neurotransmitters are stored in vesicles.
3. Neurotransmitters that leak fromvesicles are destroyed by enzymes.
4. Action potentials cause vesicles to fusewith membrane and releaseneurotransmitters into the synapse.
5. Released neurotransmitters bind toautoreceptors and inhibit further synthesis
and release.6. Released neurotransmitters bind to
postsynaptic receptors.
7. Released neurotransmitters are removedby reuptake or enzymatic degradation.
Communication Between Neurons
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> Seven Stages in Neurotransmitter Function-
Communication Between Neurons
1. Neurotransmitters are synthesized.
Protein and peptide neurotransmitters are
synthesized from DNA template in thesoma. These proteins/peptides may bealtered after synthesis
Other neurotransmitters are synthesized bymodification of ingested substances. Thesemay be manufactured right in the axonterminal.
Energy for these actions is provided bychemical reactions in the mitochondria.
Communication Between Neurons
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> Seven Stages in Neurotransmitter Function-
Communication Between Neurons
2. Neurotransmitters are stored in vesicles.
Vesicular packaging occurs in the golgiapparatus in the cell body or in the axonterminal.
Some vesicles are further packaged intostorage granules that hold many vesicles.
Communication Between Neurons
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> Seven Stages in Neurotransmitter Function-
Communication Between Neurons
3. Neurotransmitters that leak fromvesicles are destroyed by enzymes.
Catabolizing enzymes (proteins) digest any
neurotransmitter molecules that leak out ofvesicles.
Communication Between Neurons
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> Seven Stages in Neurotransmitter Function-
Communication Between Neurons
4. Action potentials cause vesicles to fuse
with membrane and releaseneurotransmitters into the synapse.
Action potentials actually cause vesicles tomigrate toward the presynaptic membrane
and to fuse to the membrane.
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Action
Potential
docked synaptic vesiclepresynapticmembrane
proteins
calcium entryopens fusion
pore
fusionpore opens neurotransmitter release
omega figures
> Seven Stages in Neurotransmitter Function
Communication Between Neurons
Released neurotransmitters diffuse passively across the synapse.
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> Seven Stages in Neurotransmitter Function-
Communication Between Neurons
5. Released neurotransmitters bind toautoreceptors and inhibit further synthesis
and release.Autoreceptors are located on the
presynaptic neuron that releases theneurotransmitter. They activatemechanisms in the neuron that inhibitfurther synthesis and release.
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> Seven Stages in Neurotransmitter Function-
Communication Between Neurons
6. Released neurotransmitters bind topostsynaptic receptors.
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> Seven Stages in Neurotransmitter Function- The released neurotransmitter binds to a specific site on apostsynaptic receptor protein.
- Depending upon which type of receptorthe neurotransmitter binds to, it will either:1) cause excitation (depolarization) of the
postsynaptic neuron, or
2) cause inhibition (hyperpolarization) of thepostsynaptic neuron, or
3) produce changes in chemical activities insideof the postsynaptic neuron
- The effect from releasing one vesicle fullof neurotransmitter on the postsynaptic neuron is very small a
quantum effect. Many quanta are required to significantly alter theactivity of the postsynaptic neuron.
Communication Between Neurons
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> Seven Stages in Neurotransmitter Function-
Communication Between Neurons
7. Released neurotransmitters are removedby reuptake or enzymatic degradation.
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> Seven Stages in Neurotransmitter Function
Communication Between Neurons
> Reuptake
> Enzymatic
Degradation
> AChE
> MAO
> transporters
2 Mechanisms
of deactivation:
Reading Assignment
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Reading Assignment
Before next classChapter 4: The Chemical Basis of Behavior: Neurotransmitters
and Neuropharmacology
Rosenzweig, Breedlove, & Watson
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