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Synaptic Transmission

The process of information transfer at a synapse

Electrical Synapses

Synapse where an electrical current is transfered from one neuron to another which occurs at gap junctions and are usually bidirectional. Common in the mammalian brain and in every part of the mammalian CNS.

Chemical Synapses

Synapse where a chemical neurotransmitter transfered information from one neuron to another

Gap Junctions

Location where electrical synapses happen. The membrane of two cells are about 3 nm apart and has clusters of specialized proteins called connexins. Two connexins combine to form a gap junction channel.

Postsynaptic Potential (PSP)

An action potential in the second neuron caused by an action potential in the first neuron to send a small amount of ion current to flow through the gap junction channels.
Usually small so several PSPs are required to excite a neuron.

Synaptic Cleft

Region between the presynaptic and postsynaptic membrane thats about 20-50nm apart filled with a matrix of fibrous extracellular protein.

Synaptic Vesicles

Vesicles that store neurotransmitters on on the presynaptic side (usually an axon terminal)

Secretory Granules

Larger vesicles in the axon terminals that contain soluble protein.

Dense-core Vesicles

Another name for Secretory Granules

Membrane Differentiations

Dense regions of the presynaptic and postsynaptic membranes packed with proteins

Active Zones

The membrane differentiation of the presynaptic membrane that contains pyramid-like proteins where neurotransmitters are released

Postsynaptic Density

membrane differentiation of the postsynaptic membrane that contains a lot of NT receptors which convert intercellular chemical signals into an intracellular signal


The synapse in the CNS if the postsynaptic membrane is on a dendrite


The synapse in the CNS if the postsynaptic membrane is on a axon


The synapse in the CNS if the postsynaptic membrane is on a soma

Dendrodendritic Synapses

The synapse in the CNS if the presynaptic and postsynaptic membrane are on dendrites

Gray's Type I Synapses

Synapses where the membrane differentiations of the presynaptic membrane is thinner than of the postsynaptic membrane. Usually means synapse is excitatory.

Gray's Type II Synapses

Synapses where the membrane differentiations is of similar thickness. Usually means synapse is inhibitory.

Neuromuscular Junction

Synapse between the axons of a motor neuron of the spinal cord and skeletal muscle. It has many of the structural features of chemical synapses in the CNS.
One of the largest synapses in the body

Motor end-plate

Post-synaptic membrane of the muscle that contains a series of shallow folds so more receptors can be there.

What are the three categories of neurotransmitters?

amino acids, amines, and peptides

What are the major amino acid neurotransmitters? (3)

Gamma-amino-butyric Acid (GABA), Glutamate (Glu), Glycine (Gly)

What are the major amine neurotransmitters? (6)

Acetylcholine (Ach), Dopamin (DA), Epinephrine, Histamine, Norepinephrine (NE), Serotonin (5-HT)

What are the major peptide neurotransmitters? (9)

Cholecystokinin (CCK), Dynorphin, Enkephalins (Enk), N-acetlaspartylglutamate (NAAG), Neuropeptide Y, Somatostatin, Substance P, Thyrotropin-releasing hormone, Vasoactive intestinal polypeptide (VIP)

Which neurotransmitters are in fast synaptic transmissions at most CNS synapses?

Gamma-amino-butyric Acid (GABA), Glutamate (Glu), Glycine (Gly)

What neurotransmitter mediates fast synaptic transmission at all neuromuscular junctions?

Acetylcholine (ACh)

Which neurotransmitters are in slower forms of synaptic transmissions in the CNS and in the periphery?



Special proteins embedded in the vesicle membrane that transports NTs into the vesicles

How are peptides put in the secretory granules?

Budded off from the Golgi apparatus.

Voltage-Gated Calcium Channels

Permeable to calcium which allows calcium into the presynaptic membrane that causes NTs to be released from synaptic vesicles


The process by which vesicles release their content


The process where the vesicle membrane is recovered and refilled with NTs

What are the two types of NTs receptors?

Transmitter-gated ion channels and G-protein-coupled receptors

Transmitter-gated ion channels

Membrane-spanning proteins with four or five subunits that when activated, allows pore to be open to let specific ions in/out.

Excitatory Post-synaptic Potential (ESSP)

When the presynaptic NTs causes a depolarization in the postsynaptic membrane.
Synaptic activation of ACh-activated and glutamate-gated ion channels causes them.

Inhibitory Post-Synaptic Potential (IPSP)

When te presynaptic NTs causes hyperpolarization in the postsynaptic membrane.
Synaptic activation of Glycine-activated and GABA-gated ion channels causes them.

G-Protein Coupled Receptors

Receptors that can have a slower, longer-lasting and much more diverse postsynaptic actions.


small proteins that are free to move along the intracelular face of the postsynaptic membrane that can activate "effector" proteins to lead to postsynaptic actions

Second messengers

molecules that diffuse away and activate additional enzymes in the cytosol that can regulate ion channels and alter cellular metabolism.

Metabotropic Receptors

Another word for G-Protein Coupled Receptors because they can trigger widespread metabolic effects


Presynaptic receptors that are sensitive to the NT released by the presynaptic terminal.
Used to regular itself so it doesn't release too much NT

Why must NTs be recovered and degraded quickly after its been released and how?

Ot must be cleared so that the next action potential can cause another synaptic transmission.
There are NT transports on the presynaptic membrane that allows for re-uptake so enzyme can destroy them or be released back into vesicles. Glial cells also have these so that they can regular the cleft. There also could be enzymes in the cleft that destroys the NTs.
This prevents desensitization where the transmitter-gated channels won't respond to the NTs on the postsynaptic side.


Study of the effects of drugs on nervous system tissue


Drugs that inhibit the normal function of specific proteins

Receptor Antagonists

Inhibitors of NT receptors that bind to and block the receptors

Receptor Agonists

Drugs that mimic the normal actions of the naturally occurring NT

Synaptic Integration

Process where multiple postsynaptic potentials combine within one postsynaptic neuron.

Why at NTs quantally released?

A single vesicle has the same number of NT molecules (several thousands) so depending on how many vesicles are released, it will always be a multiple of that number.

Miniature PostSynaptic Potential

A response generated by one vesicle of NTs

Quantal Analysis

We can compare the amplitudes of miniature and evoked postsynaptic potentials to figure out how many vesicles is released during a synapse transmission.
Neuromuscular = about 200 vesicles
CNS = about 1 vesicle

EPSP Summation

The simplest form of synaptic integration in the CNS. There are two types: spatial and temporal

Spatial Summation

addition of EPSPs generated simultaneously at many different synapses on a dendrite

Temporal Summation

addition of EPSPs generated at the same synapses as they occur in rapid succession (1/15 m sec apart)

Length Constant (λ)

Distance from the synapse where the depolarization is 37% of that at the origin

Internal resistance

The resistance of current flowing longitudinally down the dendrite. It depends only on the diameter of the dendrite & electrical properties of the cytoplasm (constant)
Higher = Lower λ

Membrane resistance

The resistance of current flowing across the membrane. It depends on the number of ion channels number (changes)
Higher = Higher λ

Why do some dendrites have VG ion channels?

They can act as small postsynaptic potential amplifiers generated far out on dendrites

What factors effect an EPSP's output on a neuron?

Number of coactive excitatory synapses, the distance the synapse is form the spike-initiation zone, and the properties of the dendritic membrane.

Shunting Inhibition

When an inhibitory synapse acts as an electrical shunt and prevents current from flowing through the soma to the axon hillock which is usually inward movement of negatively charged Cl- ions.

Where are inhibitory synapses located usually?

On the soma and near the axon hillock where they have a powerful position to affect the neuron action


A synapse where it affects the effectiveness of EPSPs generated by other synapses with transmitter-gated channels

Norepinephrine (NE)

an amine NT that binds to ß receptors and triggers a cascade of biochemical events within the cell that leads to modulation

Adenylyl Cyclase

enzyme that catalyzes the reaction to convert ATP into cAMP

Cyclic Adenosine Monophosphate (cAMP)

a second messenger molecule that can stimulate other enzymes such as a protein kinase

Protein Kinases

enzyme that catalyzes phosphorylation to change another protein


process that transfers phosphate groups from ATP to a specific site on cell proteins to change its conformation and activity

What is an important effect of phosphorylation in neurons?

It can close a particular type of K channels and thus lowering the membrane's K+ conductance which increases the dendritic membrane resistance and increases λ

Otto Loewi

Discovered effect of acetylcholine, ACh (vagusstoff) through experiments with frog hearts...He used this discovery to prove chemical synapses


Introduced by Henry Dale. Described cells that produce and release ACh as this.


Dale termed neurons that use the amine neurotransmitter norepinephrine this...


Synapses that use glutamate


Synapses that use GABA


Synapses that use peptides

Criteria for a neurotransmitter

Must be synthesized and stored in the presynaptic neuron.
Must be released by presynaptic axon terminal upon stimulation
When experimentally applied, must produce a response in postsynaptic cell that minus the response produced by the release of neurotransmitter from presynaptic neuron

Important techniques to satisfy criteria for neurotransmitter

Immunuocytochemistry and in situ hybridization
Immunochemistry: method for viewing location of specific molecules in sections of brain tissue
In situ hybridization: Method for localizing specific mRNa transcripts for proteins. Together, these methods enable us to see whether a neuron contains and synthesizes a transmitter candidate.


Used to anatomically localize particular molecules to particular cells. Once neurotransmitter candidate has been chemically purified, it is injected into the bloodstream of an animal where it stimulates an immune response (to evoke response, molecule is chemically coupled to larger molecule). One aspect of immune response is generation of antibodies which tightly bind to specific sites of foreign molecule (transmitter candidate). Better antibodies bind tightly to transmitter of interest and very little or not at all to other chemicals. Antibodies are recovered from blood sample of immunized animal and chemically tagged with marker to see under microscope. Used to localize molecule for which antibody can be generated (synthesizing enzymes for transmitter candidates). Demonstration that transmitter candidate and its synthesizing enzyme are contained in the same neuron or axon terminal can help satisfy criterion.

In Situ Hybridization

Chemically label probe (complementary strand of mRNA), apply it to section of brain tissue, allow time for probes to stick to any complementary RNA strands, then wash away all extra probes that haven't stuck. Finally, search for neurons that contain label. Probes are usually labeled by making them radioactive.

Technique for viewing distribution of radioactivity


Stimulate release in in vitro brains

Slices bathed in solution containing hihg potassium concentration. Causes large membrane depolarization, stimulating transmitter release from axon terminals in the tissue.


Assesses postsynaptic actions of transmitter candidate. Inject charged candidate ions next to postsynaptic membrane and use microelectrode to record effects on membrane potential.

No two neurotransmitters

bind to the same receptor

One neurotransmitter

can bind to many different receptors.

Receptor subtype

Each of the different receptors a neurotransmitter binds to. ACh acts on two different cholinergic receptor subtypes: One present in skeletal muscle and the other in heart muscle. Both subtypes are also present in many other organs and within the CNS

Neuropharmacological Analysis

Distinguishing receptor types through the use of drugs

Nicotinic ACh receptors

Skeletal muscle. Also exists in brain.

Muscarinic ACh receptors

Heart muscle. Also exists in brain.

Selective Antagonists

Inhibit action of certain receptors to distinguish receptor subtypes

Subtypes of glutamate receptors

Meidate much of synaptic excitation in CNS. AMPA receptors, NMDA receptors, kainate receptors (named for different chemical agonists. Glutamate activates all three receptor subtypes, but AMPA acts only at AMPA receptor, NMDA at NMDA receptor, etc.


Broad class of drugs that are both medically important and commonly abused. Effects include pain relief, euphoria, depressed breathing, and constipation


Any chemical compound that binds to a specific site ona receptor

Ligand binding method

Technique of studying receptors using radioactively labeled ligands. Specific ligands were invaluable for isolating neurotransmitter receptors and determining their chemical structure. Important for mapping anatomical distribution of different neurotransmitters.

Molecular Analysis

Discovered that each subunit could be replaced/substituted by different polypeptides...leading to a very large diversity of receptors

Dale's Principle

The idea that a neuron has only one neurotransmitter. Many peptide containing neurons violate Dale's principle because these cells usually release more than one transmitter: an amino acid or amine AND a peptide. True for most cases, though.


When two or more transmitters are released from one nerve terminal, they are called...


Neurotransmitter at neuromuscular junction. Synthesized by all motor neurons int he spinal cord and brain stem. Other cholinergenic cells contribute to functions of specific circuits in PNS and CNS.

ACh synthesis

Requires choline acetyltransferase ChAT (manufactured in soma and transported to axon terminal) Only cholinergic neurons contain ChAT, so this enzyme is a good marker for cells that use ACh. Ex: Immunocytochem with ChAT specific antibodies can identify cholinergic neurons. ChAT synthesizes ACh in cytosol of axon terminal, and neurotransmitter is concentrated in synaptic vesicles by ACh transporter. ChAT transfers acetyl group from acetyl CoA to choline(from extracellular fluid taken up by transporter).

Rate Limiting Step in ACh synthesis

Transport of choline into neuron. (Availability of chlorine limits how much ACh can be made)

AChE (Acetylcholinesterase)

degradative enzyme. secreted into synaptic cleft and is associated with cholinergic axon terminal membranes. Also manufactured by some noncholinergic neurons (not useful as a market for cholinergic synapses)

Catecholaminergic Neurons

Amino acid(Tyrosine) is a precursor for three different amine neurotransmitters that contain a chemical structure called a catechol. Called catecholamines: Dopamine, norepinephrine, epinephrine. Found in regions of nervous system involved in the regulation of movement, mood, attention, and visceral attention. All catecholaminergic neurons contain tyrosine hydroxylase (TH [activity of this is rate limiting step for synthesis]), which catalyzes the firs step in catecholamine synthesis (conversion of tyrosine to dopa which is converted into dopamine via dopa decarboxylase).

Parkinson's disease

Dopaminergic neurons in brain slowly degenerate and eventually die.

Dopamine B-hydroxylase

Converts dopamine to norepinephrine. Located in synaptic vesicles

Phentolamine N Methyltransferase

Converts norepinephrone to epinephrine. In cytosol.

Monoamine oxidase (MAO)

enzyme found on outer membrane of mitochondria that functions in enzymatic destruction of catecholamines and serotonin once inside axon terminal


Amine neurotransmitter (also called 5 hydroxyltryptamine) and abbreviated 5-HT and derived from tryptophan amino acid.

Serotonergic Neurons

Few in number but regulate mood, emotional behavior, and sleep

Synthesis of serotonin

1) Tryptophan converted into intermediary (5 HTP) through tryptophan hydroxylase
2) 5 HTP converted to 5 HT by 5 HTP decarboxylase
* Synthesis is limited by availability of tryptophan in extracellular fluid bathing neurons. Dietary.

Glutamate, glycine, GABA

Serve as neurotransmitters at most CNS synapses


Unique to neurons that use it as neurotransmitter; synthesized only in neurons that use it since it's not one of the 20 amino acids used in protein syntehsis. Precursor: glutamate...Glutamic acid decarboxylase.

GABAergic neurons

Marked well by GAD. Distributed widely in brain. Major source of synaptic inhibition in ervous system.

GABA transaminase

Enzyme that metabolizes GABA.

ATP as a neurotransmitter

Concentrated in vesicles at many synapses (CNS and PNS). Released into cleft through spikes in Ca-dependent manner. Packaged in vesicles along with another transmitter. (Co transmitter with catecholamine) directly excites some neurons by gating cation channel...function similarly like glutamate) Binds to purinergic receptors


Can be released from postsynaptic neurons. Act on presynaptic terminals.Retrograde messengers. Serve as feedback system to regulate conventional forms of synaptic transmission. Vigorous firing of action potentials in postsynaptic neuron causes voltage gated CA channels to open, Ca enters cell and intracellular Ca rises. Elevated Ca stimulates synthesis of endo-cannabinoid from membrane lipids.

Retrograde signaling

Communication from post to presynpatic.

Unusual qualities about endocannabinoids

They are not packaged in vesicles like most other neurotransmitters; manufactured rapidly on demand
They are small and membrane permeable; can diffuse rapidly across membrane of cell origin to contact neighboring cells
Bind selectively to CB1 type of cannabinoid receptor located on presynaptic terminals

CB1 Receptors

G protein coupled receptors. Main effect is to reduce opening of presynaptic calcium channels. Slows down neurotransmitter release.

Cannabis Sativa

Botanical name for hemp, fibrous plant used through ages for making rope and cloth. Active gredient is THC. It binds to specific G-Protein cannabinoid receptors in brain (particularly motor control, cerebral cortex, pain pathways). Led to discovery of THC like neurotransmitters called endocannabinoids (anandamide and arachidonylglycerol. Potentially useful for relieving nausea, suppressing pain, relaxing muscles, treating seizures, decreasing intraocular pressure of glaucoma.

Nitrous oxide

Gaseous molecule. Interceullular communication. May be another example of retrograde messenger. Since it's small and membrane permeable, it can diffuse more freely than most other transmitter molecules. Also evanescent and breaks down very rapidly.

Dual function

Some neurotransmitters may exist elsewhere other than nervous system and serve another purpose. For example, amino acids and ATP serve other acids other than being neurotransmitters.

Transmitter Gated Channels

Most studied is nicotinic ACh receptor at neuromuscular junction in skeletal muscle. Each receptor subunit has different primary structure (some portions are similar). Pentameric complexes (mostly) minus glutamate which are tetramers. Different transmitter binding sites let one channel respond to Glu while another respond to GABA..

Similarities in structure of subunits for different transmitter gated ion channels

Amino acid gated channels

Mediate most of the fast synaptic transmission in the CNS.
1) Pharmacology of their binding sites describes which transmitters affect them and how drugs interact with them.
2) Kinetics of the transmitter binding process and channel gating determine the duration fo their effect
3) Selectivity of ion channels determines whether they produce excitation or inhibition and whether Ca enters the cell in significant amounts
4) conductance of open channels helps determine the magnitude of their effects

Glutamate-Gated channels

Three glubtamate receptor subytpes: AMPA, NMDA, kainate. AMPA and NMDA mediate bulk of fast excitatory synaptic transmission in the brain. Kainate receptors also exist but functions are not understood. *AMPA permeable to both sodium and potassium, most aren't permeable to calcium. Net effect at rest: admit Na into cell, causing rapid depolarization. AMPA receptors at CNS synapses mediate excitatory transmission in same way as nicotinic receptors at neuromuscular junctions.

AMPA Receptors

Permeable to both sodium and potassium, most aren't permeable to calcium. Net effect at rest: admit Na into cell, causing rapid depolarization. AMPA receptors at CNS synapses mediate excitatory transmission in same way as nicotinic receptors at neuromuscular junctions.

NMDA Receptors

Permeable to calcium, inward ionic current through NMDA gated channels is voltage dependent. When NMDA gated channels open, Ca and Na enter cell while K leaves. Magnitude of inward current depends on postsynaptic membrane potential. When glutamate binds to NMDA receptor, pore opens as usual but at normal negatie resting membrane potentials, channel becomes clogged by Mg ions (magnesium block prevents other ions from passing freely through NMDA channel). Mg pops out only when depolarized. Therefore, both glutamate and depolarization must coincide before the channel passes current.


Toxins that overstimulate glutamate release and cause neuron suicide.

GABA Gated channels

GABA mediates most of the synaptic inhibition in the CNS. Gates a chlorine channel. Similar structure to nicotinic ACh receptors despite different ion selectivity.

Glycine gated channels

Glycine mediates most of the rest of the inhibition of the CNS after GABA. Gates a chlorine channel. Similar structure to nicotinic ACh receptors despite different ion selectivity.

Benzodiazepines (diazepam, Valium)

Binds to specific site on outside face of GABA A channel. With GABA present, increase frequency of channel openings. Cause stronger inhibitory signals. Only receptors with gamma GABAA subnit (in addition to alpha and beta subunits) respond to these

Barbituates (phenobarbital, sedatives)

Binds to own distinct site on outside face of GABA A channel. With GABA present, increase duration of channel openings. Cause stronger inhibitory singals.


Affects NMDA, glycine, ACh, and serotonin receptors. Effects on GABA A channels depends on specific structure. *alpha, beta, and gamma sub units necessary for ethanol sensitive ethanol GABA A receptor. Enhances inhibition on some brain areas but not others.


Natural metabolites of steroid hormones that are synthesized from cholesterol primarily in gonads and adrenal glands but also glial cells of brain. Some enhance inhibitory function while others suppress it. Bind to own site on GABA A receptor.

Structure of G Protein coupled receptors

Single poly peptide containing seven membrane spanning alpha helices. Two extracellular loops form transmitter binding sites. Two intracellular loops bind to and activate G Proteins.


Guanosine triphosphate binding protein. Some types of g proteins can be activated by many receptors.
1) Each G protein has three subunits, alpha, beta, gamma. In resting state, GDP molecule is bound to G alpha subunit and the whole complex floats around on inner surface of membrane
2) If this GDP bound G protein bumps into proper type of receptor and that receptor has a transmitter molecule bound to it, G protein releases GDP and exchanges it for GTP from cytosol
3) The activated GTP bound G protein splits into two parts: G alpha subunit + GTP, and G beta-gamma complex. Both move on to influence effector proteins
4) G alpha subunit is itself an enzyme that eventually breaks down GTP into GDP. Terminates its own activity by converting bound GTP to GDP
5) G alpha and G beta-gamma subunits come back together, allowing cycle to begin again

Gs and Gi

Stimulatory and inhibitory G-proteins

The Shortcut Pathway

Fastest of G protein coupled systems (response 30-100 msec of neurotransmitter binding) Not as fast as transmitter gated, and very localized.
Example: Muscarinic receptors in heart. ACh receptors coupled via G proteins to potassium channels, explaining why ACh slows heart rate. Beta=gamma subunits migrate laterally along membrane until they bind to right type of potassium channel.
Another example: Neuronal GABA B receptors..coupled by shortcut pathway to potassium channels.
Fastest of G protein coupled systems.

Second Messenger Cascades

Happen when G proteins exert effects by activating certain enzymes, which trigger series of biochemical reactions. Second messengers are between first and last enzyme. The whole process that couples neurotransmitter, via multiple steps, to activation of a downstream is called this...

Protein Kinase A

Downstream enzyme activated by rise in cAMP in cytosol

Protein phosphatases

Act rapidly to remove phosphate groups from phosphorylated proteins

Channel phosphorylation

when ATP is bound and changes the conformation of protein channels to inhibit or excite

Signal amplification

Activation of one G protein coupled receptor can lead to activation of many ion channels

Signal cascade benefits

Allows signaling at a distance, signal amplification, provides more sites of regulation, and generation of long lasting chemical changes are possible


Ability of one transmitter to activate mor ethan one subtype of receptor and cause more than one type of postsynaptic response


Multiple transmitters, each activating their own receptor type, converge to affect the same effector systems. Can occur at level of G protein, second messenger cascade, or type of ion channel.

A. Carries particles away from equilibrium
B. Requires energy
c. occurs due to random motion of particles
d. is a type of active transport
e. only occurs in biological systems

c. Diffusion occurs due to random motion of particles

The Nernst equation
a. Calculates equilibrium potential across the membrane
b. Takes into account ionic concentration
c. Does not take into account the permeability of the membrane
d. Both a and b
e. All of the above

e. The nernst equation calculates equilibrium potential across the membrane, takes into account ionic concentration, and does not take into account the permeability of the membrane

An example of a neurotransmitter that is typically released at excititory synapses is:
a. Glutamate
b. Acetyl choline
c. dopamine
d. Both a and b.
e. all of the above

e. glutamate, acetyl choline, and dopamine are all examples of neurotransmitters that are typically released at excitatory synapses

Action potentials are propagated along an axon due to the
a. opening of voltage-gated Na+ channels
b. passive diffusion of electrical current
c. membrane depolarization
d. all of the above
e. none of the above

d. action potentials are propagated along an axon due to the opening of voltage-gated Na+ channels, passive diffusion of electrical current, and membrane depolarization

When a muscle cell is held at its reversal potential of 0 mV, the release of Ach no longer produces a current because:
a. nicotinic AchRs won't open unless the membrane depolarizes past 0 mV
b. The rate of Ach degradation is increased
c. The equilibrium potential for Na+ (Ena) and for K+ (Ek) is 0 mV in muscle cells
d. an influx of Na+ is balanced by an equal efflux of K+
e. none of the above - d. When a muscle cell is held at its reversal potential of 0 mV, the release of ACh no longer produces current because an influx of Na+ is balanced by an equal efflux of K+


Ionotropic receptors:
a. are ion channels.
b. produce slower, longer lasting postsynaptic responses compared to metabotropic receptors.
c. cause ion channels to open or close by activating intermediate proteins.
d. All of the above
e. None of the above

a. Ionotropic receptors are ion channels.

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