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8/6/2019 GEK1532 Nerve Pulses
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GEK1532
Nerve impulses
Thorsten Wohland
Dep. Of Chemistry
S8-03-06
Tel.: 6516 1248E-mail: [email protected]
http://www.yorku.ca/eye/toc-sub.htmhttp://www.sirinet.net/~jgjohnso/neuronphysiology.html
http://psych.hanover.edu/Krantz/neural/diffuse1.html
http://www.mrothery.co.uk/vision/EyeNotes.htm
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Revision: G-protein coupled
receptors (GPCRs)
All GPCRs have 7 transmembrane
spanning -helices and are sometimes
called 7TM receptors.
These receptors are involved in many
functions, e.g. vision, olfaction, taste,
response to hormones.
More than 50% of drugs on the market
target these receptors.
The ligands that activate the GPCRs are
therefore depending on the particular
GPCR: chromophores, molecules that
convey smells and tastes or hormones.
The G-proteins are activated by G-protein coupled receptors. The receptors in
turn are regulated by ligands.
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Revision: Transducin
http://www.med.ufl.edu/biochem/rcohen/transduc.html
opsin
11-cis-retinal
Transducin(G-protein)
rhodopsin
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Revision: The activation cycle for
rhodopsin
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Revision: The vision cycle at
different light intensities
Kurt Nassau, Fig 14.4 Kurt Nassau, Fig 14.5 Kurt Nassau, Fig 14.6
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Revision: Rhodopsin in the eye
http://www.uchc.edu/dsp/rodcone.html
http://webvision.med.utah.edu/photo1.html#phagocytosis
http://education.vetmed.vt.edu/
Curriculum/VM8054/EYE/RETINA.HTM
http://webvision.med.utah.edu/photo1.htmlhttp://webvision.med.utah.edu/photo1.html8/6/2019 GEK1532 Nerve Pulses
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Revision: Rods and Cones
Rod
Cones
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Cone opsin differences
Red-Pigment Blue-Pigment
Differences to Green-Pigment are indicated in dark shading.
From Scientific American, Special on Color (German Version)
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Neurons
http://www.drugabuse.gov/
MOM/TG/momtg-introbg.html
Soma: the cell body; its task is the
production of neurotransmitters andthe summation of the signal. As
well some input.
Dendrites: The dendrites are the
points of input of the neuron.
Axon: The axon is responsible forthe output of the neuron.
Synapse: Connection between two
neurons from an axon (presynaptic)
to a dendrite or cell body
(postsybaptic).
Network of neurons: Neurons can
have many inputs on the dendrites
or cell body from other neurons.
And through the axon they can
communicate to many other
neurons.
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The neuron
Input from other neuronsover dendrites.
Summation of all
signals and decision
whether the neuron
should fire or not.
Output: action potentials onthe axon trigger the
synapses and hand on the
signal to other neurons.
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Neural activity: Excitation and
Inhibition
time
Nerve
Impulse
Normal
activity,
random
firing
time
Nerve
Imp
ulse
Inhibition
time
Nerv
e
Impu
lse
Excitation
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Summation of signals
http://zeus.rutgers.edu/~ikovacs/SandP/c_fig2.jpg
The upper synapse is excitatory,
the lower synapse inhibitory.Their signal strength is indicated
by the black arrows.
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The Synapse of light sensitive cellsInput: Light
Output: Nerve signal
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Axon: Nerve impulses, the action
potential
The action potential is a depolarization
of the membrane traveling along the
axon.
Remember: The inside of the
cell is more negative than theoutside, resulting in a resting
potential of -70 mV.
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The Axon: Voltage gated cation
channelsResting potential of the membrane is -70 mV. Na+ ions are more abundant
outside the cell, and K+ ions are more abundant inside the cell. A difference inconcentartion between the two creates the resting potential
1. At a synapse channels open and
depolarize the membrane due to a
neurotransmitter.
2. Because of the depolarization,voltage-gated cation channels open and
let Na+ ions into the cell, further
depolarizing the cell.
3. This opens more voltage-gated cation
channels and the impulse can travelalong the membrane.
4. After a short opening the voltage-
gated cation channels close
automatically and stay inactive for a few
milliseconds.
http://www.accessexcellence.org/AB/GG/action_Potent.html
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Synapse (chemical)
http://science-education.nih.gov/nihHTML/ose/snapshots/multimedia/ritn/spinal/axon.html
Presynaptic cell
Postsynaptic cell
Neurotransmitter: A chemical that
transmits a signal from one cell to
another. The signal can be
inhibitory or excitatory depending
on the synapse.
The synapse is controlled by depolarizing or hyperpolarizing the membrane
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The overall picture of an synaptic
event
http://web.mit.edu/rujira/www/4.206/neuron/synapse.html
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1. Action potential
http://web.mit.edu/rujira/www/4.206/neuron/synapse.html
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2. Influx of Ca2+
http://web.mit.edu/rujira/www/4.206/neuron/synapse.html
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3. Fusion of vesicles containing
neurotransmitters
http://web.mit.edu/rujira/www/4.206/neuron/synapse.html
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Fusion of neurotransmitter containing vesicles
Intracellular side, presynaptic cell
Extracellular side, synaptic cleft
- - - - - - -
+ + + + + +
Resting potential
-70 mV
-
-
-
--
-
-
-
-
neurotransmitters
--
Fusion and
neurotransmitter
release
- -
+ +
depolarization
up to +50 mV
Ca2+ influx, helps fusion
Ca+
+
-
-
-
--
-
-
-
-
Ca2+
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4. Release of neurotransmitters
http://web.mit.edu/rujira/www/4.206/neuron/synapse.html
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5. Action potential in postsynaptic
cell (depolarization)
http://web.mit.edu/rujira/www/4.206/neuron/synapse.html
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Remember, Lecture 13:
Receptors: ligand-gated channels
Neurotransmitter frompresynaptic cell
Postsynaptic membrane
Resting potential -70 mV
+ + + + + + + +
- - - - - - - -
+ + + +
- - - -
Ions can flow
Cation channels (Ca2+, Na+, K+ etc.)
Depolarization
(up to +50 mV)
+ + + + + + + +
- - - - - - - -
Depolarization is then
registered by neuron and
depending on all its inputs
on dendrites the cell will
decide whether to fire or not.
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6. Closing of channels and
recycling of neurotransmitters
http://web.mit.edu/rujira/www/4.206/neuron/synapse.html
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Excitation and Inhibition
time
Nerve
Impulse
Normal
activity,
random
firing
time
Ne
rve
Impulse
Inhibition
time
Nerv
e
Impu
lse
Excitation
Up to now we talked only about excitation. How can we have inhibition?
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Inhibition can happen in two ways
1. Presynaptic: By the control of the membrane potential: de- or
hyperpolarization
Depolarization: fusion of neurotransmitter containing vesicles,
neurotransmitter release
Hyperpolarization: no fusion of these vesicles, no neurotransmitter
release
F i f i i i i l
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Fusion of neurotransmitter containing vesiclesIntracellular side, presynaptic cell
Extracellular side, synaptic cleft
- - - - - - -
+ + + + + +
Resting potential
-70 mV
-
-
-
--
-
-
-
-
neurotransmitters
+ + + + + + + + + + + + +
- - - - - - - - - - - - - - - - - -
Hyperpolarization
-70 to -90 mV
No Ca2+ influx
-
-
-
--
-
-
-
-
No fusion
--
Fusion and neurotransmitter
release
- -
+ +
DepolarizationCa2+ influx, helps fusion
Ca+
+
-
--
--
--
-
-
Ca2+
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Remember the rhodposin
activation?
Control of Cation channel. Since rhodopsin activation leads to the synthesis of
GMP from cGMP, the cGMP concentration decreases.
When the cGMP concentration decreases cation channels close. Themembrane will be hyperpolarized.
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Hyperpolarization after light
activation of rhodopsinIntracellular side
Extracellular side
+ + + + + + + +
- - - - - - - -
Flow of Na+
cGMPcGMP
No light light
++++++++++++++++++
No flow of Na+
anymore results inhyperpolarization of membrane
- - - - - - - - - - - - - - - - -
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Inhibition can happen in two ways
2. Postsynaptic: By the type of neurotransmitter and receptor at a
synapse
Neurotransmitter that bind to excitatory or inhibitory receptors
Cation channels: depolarizing, excitatory
Anion channels: polarizing inhibitory
R b L t 13
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Remember, Lecture 13:
Receptors: ligand-gated channels
Neurotransmitter frompresynaptic cell
Postsynaptic membrane
Resting potential -70 mV
+ + + + + + + +
- - - - - - - -
+ + + +
- - - -
Ions can flow
Cation channels (Ca2+, Na+, K+ etc.)
Depolarization
(up to +50 mV)
++++++++++++++++++
- - - - - - - - - - - - - - - - -
Anion channels (e.g. Cl-)
Hyperpolarization
(up to -90 mV)
+ + + + + + + +
- - - - - - - -
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The overall pictureA. Depolarization or hyperpolarization determines whether neurotransmitters are
released into a synaptic cleft from the presynaptic neuron.
B. Neurotransmitters can activate cation or anion channels on the postsynaptic
neuron and have thus an excitatory (depolarizing) or inhibitory (hyperpolarizing)
effect, respectively.
1. Input on dendrites: excitation (depolarization by
neurotransmitter-gated cation channels or inhibition(hyperpolarization by neurotransmitter-gated anion
channels)
2. The soma/cell body
collect the signal and
decides to apply an
action potential on the
axon or not.
3. Output: The axons are
connected to other neurons
whose dendrites it will then
excite or not depending onits own input
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Signal from light sensitive cells
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Summary
Nerve cells (dendrite, soma, axon) Depolarization and hyperpolarization decide
over neurotransmitter release from presynapticneurons
Neurotransmitters can activate excitatory orinhibitory receptors on postsynaptic neurons
Depending on excitatory and inhibitory signalsapplied to a neuron the neuron will fire or not
Light sensitive cells inhibit the release of(inhibitory) neurotransmitters after activation andthus create an action potential on bipolar cells.