ion channels and resting potential

8/20/2019 ion channels and resting potential

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Neural Communication: 1Communication within and

between cells


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Transmission o inormation

Inormation must be transmitted

within each neuron

and between neurons


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The Membrane

The membrane surrounds the neuron!

It is com"osed o lipid and protein!


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The #esting Potential

There is an electrical charge across the membrane! This is the membrane potential! The resting potential $when the cell is not iring% is a

&'mV dierence between the inside and the outside!

inside

outside

Resting potential of neuron = -70mV

+

-

+

-

+

-

+

-

+

-


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(rtist)s rendition o a t*"ical cell membrane


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Ions and the #esting Potential

Ions are electrically-charged molecules e!g! sodium $Na+%, "otassium $+%,chloride $Cl-%!

The resting "otential e.ists because ions are concentrated on dierent sides othe membrane!

Na+ and Cl- outside the cell! K+ and organic anions inside the cell!

inside

outsideNa+Cl-Na+

K+

Cl-

K+Organic anions (-)

Na+Na+

Organic anions (-)

Organic anions (-)


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Ions and the #esting Potential

Ions are electrically-charged molecules e!g! sodium $Na+%, "otassium $+%,chloride $Cl-%!

The resting "otential e.ists because ions are concentrated on dierent sides othe membrane!

Na+ and Cl- outside the cell! K+ and organic anions inside the cell!

inside

outsideNa+Cl-Na+

K+

Cl-

K+Organic anions (-)

Na+Na+

Organic anions (-)

Organic anions (-)


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Maintaining the #esting

Potential Na+ ions are actively transported $this uses

energ*% to maintain the resting "otential!

The sodium-potassium pump $a membrane"rotein% e.changes three Na+ ions or two + ions!

inside

outside

Na+

Na+

K+ K+

Na+


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Neuronal iring: the action

"otential The action potential is a ra"id

depolarization o the membrane!

It starts at the axon hillock and "asses

/uic0l* along the axon!

The membrane is /uic0l* repolarized to

allow subse/uent iring!


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Beore e"olari2ation


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(ction "otentials: #a"id

de"olari2ation 3hen "artial de"olari2ation reaches the activation

threshold, voltage-gated sodium ion channels o"en! 4odium ions rush in! The membrane "otential changes rom -&'mV to +5'mV!

Na+

Na+

Na+

-

+

+

-


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e"olari2ation


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(ction "otentials: #e"olari2ation

4odium ion channels close and become refractory! e"olari2ation triggers o"ening o voltage-gated potassium ion channels. K+ ions rush out o the cell, re"olari2ing and then h*"er"olari2ing the

membrane!

K+ K+

K+Na+

Na+

Na+

+

-


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#e"olari2ation


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The (ction Potential

The action "otential is all-or-none!!

It is alwa*s the same si2e!

6ither it is not triggered at all - e!g! too

little de"olari2ation, or the membrane is

7reractor*89

Or it is triggered com"letel*!


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Course of the Action Potential• The action potential begins with a partial depolarization (e.g. from firing of another

neuron ) [A].

• When the excitation threshold is reached there is a sudden large depolarization [B].

• This is followed rapidly by repolarization [C] and a brief hyperpolarization [D].

• There is a refractory period immediately after the action potential where nodepolarization can occur [E]

Membranepotential(mV)

[A]

[B] [C]

[D] excitation threshold

Time (msec)

-70

+40

0

0 1 2 3

6;


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(ction Potential


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Conduction o the action

"otential! "assive conduction will ensure that ad=acent

membrane de"olari2es, so the action "otential7travels8 down the a.on!

But transmission b* continuous action "otentialsis relativel* slo# and energy-consuming $Na+@+ "um"%!

( aster, more eicient mechanism has evolved:saltatory conduction. $yelination "rovides saltator* conduction!


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M*elination

Most mammalian a.ons are myelinated!

The m*elin sheath is "rovided b* oligodendrocytes and

%ch#ann cells!

M*elin is insulating, "reventing "assage o ions over the

membrane!


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4altator* Conduction

M*elinated regions o a.on are electrically insulated! 6lectrical charge moves along the axon rather than across the

membrane! (ction "otentials occur onl* at unmyelinated regions: nodes of

&anvier !

Node of RanvierMyelin sheath


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4*na"tic transmission Inormation is transmitted rom the presynaptic

neuron to the postsynaptic cell. Chemical neurotransmitters cross the synapse,

rom the terminal to the dendrite or soma.

The s*na"se is ver* narrow, so transmission isfast.


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terminal

dendritic spine

synaptic cleftpresynaptic membrane

postsynaptic membrane

extracellular fluid

4tructure o the s*na"se (n action "otential causes neurotransmitter

release rom the presynaptic membrane. Neurotransmitters diffuse across the synaptic

cleft!

The* bind to receptors within the postsynapticmembrane, altering the membrane "otential!


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Neurotransmitter release

CaA+ causes vesicle membrane to fuse with"res*na"tic membrane!

Vesicle contents em"t* into clet: exocytosis! Neurotransmitter diuses across s*na"tic

clet!

Ca2+


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'onotropic receptors (ligand gated)

4*na"tic activit* at ionotro"ic rece"torsis fast and brief $milliseconds%!

*cetylcholine $(ch% wor0s in this wa*

at nicotinic rece"tors! Neurotransmitter binding changes the

rece"tor)s sha"e to o"en an ion

channel directl*!ACh ACh


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Ionotro"ic #ece"tors


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Metabotro"ic #ece"tors $-Protein%


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6.citator* "osts*na"tic

"otentials $6P4Ps% O"ening o ion channels which leads to

depolarization ma0es an action "otential more likely ,hence 7e.citator* P4Ps8: "%"s. Inside o "ost-s*na"tic cell becomes less negative!

Na+ channels (NB remember the action potential) Ca+ ! $(lso activates structural intracellular changes -

learning!%

inside

outsideNa+

Ca2+

+

-


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Inhibitor* "osts*na"tic

"otentials $IP4Ps% O"ening o ion channels which leads to

hyperpolarization ma0es an action "otential lesslikely , hence 7inhibitor* P4Ps8: '"%"s. Inside o "ost-s*na"tic cell becomes more negative! K+ (NB remember termination of the action potential)

Cl- $i alread* de"olari2ed%

K+

Cl- +

-inside

outside


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Posts*na"tic Ion motion


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#e/uirements at the s*na"se

Dor the s*na"se to wor0 "ro"erl*, si. basic events need to ha""en: Production o the Neurotransmitters

4*na"tic vesicles $4V% 4torage o Neurotransmitters

4V #elease o Neurotransmitters Binding o Neurotransmitters


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Integration o inormation

P4Ps are small! (n individual 6P4P will not "roduceenough de"olari2ation to trigger an action "otential! IP4Ps will counteract the eect o 6P4Ps at the

same neuron! %ummation means the eect o man* coincident

IP4Ps and 6P4Ps at one neuron! I there is suicient de"olari2ation at the axon

hillock, an action "otential will be triggered!

axon hillock


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Three Nobel Pri2e 3inners on

4*na"tic Transmission*rvid Carlsson discovered do"amine is a neurotransmitter!

Carlsson also ound lac0 o do"amine in the brain o

Par0inson "atients!

"aul reengard studied in detail how neurotransmitters

carr* out their wor0 in the neurons! o"amine activated a

certain "rotein $(#PP-EA%, which could change the unction

o man* other "roteins!

ric Kandel "roved that learning and memor* "rocessesinvolve a change o orm and unction o the s*na"se,

increasing its eicienc*! This research was on a certain

0ind o snail, the 4ea 4lug $("l*sia%! 3ith its relativel* low

number o A',''' neurons, this snail is suitable or

neuron research!


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Neuronal iring: the action

"otential The action potential is a ra"id

depolarization o the membrane!

It starts at the axon hillock and "asses/uic0l* along the axon!

The membrane is /uic0l* repolarized to

allow subse/uent iring!


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Overview

Course introduction

Neural Processing: Basic Issues

Neural Communication: Basics

Vision, Motor Control: Models


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Motor Control: Basics


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Hierarchical Organization of

Motor System

• Primary Motor Cortex and Premotor Areas


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Primary motor cortex (M1)

Foot

Hip

Trunk

Arm

Hand

Face

Tongue

Larynx


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Motor Control Basics

• Reflex Circuits

– Usually Brain-stem, spinal cord based

– Interneurons control reflex behavior

– Central Pattern Generators

• Cortical Control


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postsynaptic

neuron

science-education.nih.gov

Dle.or


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Dle.or-

Crossed

6.tensor

#ele.

$4heridan

1F''%

Painul 4timulus

#ele.

Circuits

3ith

Inter-neurons


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ion channels and resting potential

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