1 synaptic transmission

7/31/2019 1 Synaptic Transmission

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Synaptic

Transmission

Expiratory neuron(top trace) andinspiratory neuron

(bottom trace) werelabeled with dyeduring intracellularrecording from theventrolateralmedulla. Clearly,activity in each oneof these cells affectsactivity in the other

one.


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Outline

A. Electrical synapses

B. Overview of chemical synapses

C. Synaptic transmission via acetylcholine

D. Diversity of chemical synapses

E. Norepinephrine/serotonin and depression


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Synapses

Cellular junctions where signals are

transmitted from neurons to target cells

These are communicating junctions

Target cells: Other neurons, muscle cell,

gland cells

Two types of communicating junctions or

synapses: Electrical synapses via gap

junctions, chemical synapses involving

neurotransmitters


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Part A: Electrical synapses


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Electrical synapse and gap

junctions Recall that this involves channels comprised

of connexons that link cells


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Gap junctions

A patch where cells are separated by a

narrow gap of 2-4 nm

Connexons, Connexins

Each connexon is comprised of six identical

subunits (connexins)

Permeability of junction mediated by

conformation of the connexons


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Impulse transmission across

synapses Some terminology:

Presynaptic cellNeuron carrying action potential

Postsynaptic cell Target cell receiving signal

Transmission of signal can result in a depolarization ofthe postsynaptic cell - an excitatory postsynaptic

potential (EPSP),Or hyperpolarization, or simply stabilization, of the

membrane potential of the postsynaptic cellaninhibitory postsynaptic potential (IPSP)


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Structure of an electrical synapse


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Impulse transmission across

electrical synapses is almost

instantaneous Ions move directly from presynaptic cell to

postsynaptic cell via gap junctions

Transmission occurs in a few microseconds

Over a hundred times faster than in chemical

synapses


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Transmission of an action potential

across an electrical synapse


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Under what circumstances are

electrical synapses important? Invertebrate escape responses

Also escape responses in vertebrates such as

goldfish

Large number of electrical synapses in

fishes living at low temperature

Can also be used to electrically couple

groups of cells so they are synchronized


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Summary

Transmission of signals across electrical

synapses is rapid This involves movement of ions via gap

junctions

Used when rapid conduction of signals isessential or to synchronize cells


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Part B: Overview of chemical

synapse


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Structure of a chemical synapse


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Chemical synapses

Overall:

Action potential of presynaptic cell causes release

of neurotransmitter into the synaptic cleft Binding of neurotransmitter to postsynaptic cell

results in a depolarization at excitatory synapses

(an excitatory postsynaptic potential EPSP) or

stabilization or hyperpolarization at inhibitory

synapses (an IPSP).


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chemical synapse transmission-


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Step 1


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Step 2

N Ca++

channels


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Release

ofsynaptic

vesicles

S f th l i ( ) d ki (b) f i


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Some of the players in (a) docking (b) fusion

preparation and (c) Ca++-sensitive exocytosis


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Freeze-Fracture view of vesicle release

Docking proteins and N-type Ca++ channels are visible in the picture

at left. In the picture at right we are looking into the mouths of

several open vesicles.


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Vesicle Membrane Conservation: a kiss-and-run process - the motor

protein dynamin pinches and the coating protein clathrin forms a cage

around the membrane


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Toxins and synaptic vesicle fusion

Synaptobrevin and SNAP-25 are targets of the

clostridial neurotoxins: tetanus toxin acts in the

Central Nervous System (CNS) and botulinumtoxin acts at neuromuscular synapsesparalysis is

caused by blockage of transmitter release.

Neurexin is targeted by a-latrotoxin, the black

widow spider toxin, which induces massivetransmitter release independent of Ca++ levels.


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S 4


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Step 4


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Transmission of an action potential across chemical synapse

Most of the synaptic delay (1-2 msec) is due to the

time it takes to organize the presynaptic processes


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Part C: Transmission via

acetylcholine

A fairly well-understood example


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I. Storage of acetylcholine (ACh)

in synaptic vesicles 40 nm diameter membrane bounded

vesicles

Contain 1000 to 10,000 molecules ofacetylcholine

A single axon terminus may contain a

million or more vesicles contacting thetarget cell at several hundred points

Anatomy: Skeletal Muscle Synapse


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Anatomy: Skeletal Muscle Synapse


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Synaptic

vesicles at

a nerve-muscle

synapse

What neuromuscular synapse anatomy


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What neuromuscular synapse anatomy

reveals:

The area of contact at the neuromuscular synapse is

very extensive.

Glia cover the area of the synapse.

Highly specialized regions exist in both cells:

1. The neurons have the large accumulations ofsynaptic vesicles and associated release system

2. The muscle cell has an accumulation of receptors

and other response elements that will allow thesignal to spread over the membrane and within the

cell.

Acetylcholine


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Acetylcholine

(ACh) and the

neuromuscular

synapse:

In 1921 Otto Loewi

showed that ACh was

released at synapses(and also into the

saline) by the vagus

nerve: andtransfer of

the solution slowed

the heartbeat of a

second frog heart.

A t l h li


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AcetylcholineACh is a transmitter that is in a class by itself:

It is synthesized in terminals from acetyl CoA and cholineby choline acetyltransferase.

It is packaged in vesicles in the axon terminals.

It can bind to two distinct receptor types: nicotinic andmuscarinic. Nicotinic receptors are seen in the skeletalmuscle synapse and at synapses within the CNS.Muscarinic receptors for ACh are also seen in the CNS andat parasympathetic synapses on target tissues.

After release, ACh is degraded by the enzymeacetylcholinesterase into acetate and choline.

The choline is taken back into the terminal by Na+-drivenfacilitated uptake.

Recycling is always good!


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Recycling is always good!


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Synthesis of acetylcholine

Takes place in cytosol of axon terminals

Accumulation of acetylcholine in


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Accumulation of acetylcholine in

synaptic vesicles

Involves active transport

Vacuolar-type H+ATPase

A l ti f t l h li


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Accumulation of acetylcholine

V type ATPase in vesicle membrane is used

to reduce vesicle pH Low vesicle pH powers a

proton/neurotransmitter (NT) antiporter

1 synaptic transmission

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