gek1532 nerve pulses

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.html


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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



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2. Influx of Ca2+



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3. Fusion of vesicles containing

neurotransmitters



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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



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5. Action potential in postsynaptic

cell (depolarization)



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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



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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.

gek1532 nerve pulses

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