7membrane potential

8/3/2019 7Membrane Potential

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Membrane Potential -1

Membrane Potential


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Two Types of Ion Channels

Leakage (nongated) channels always open Gated channels open and close

voltage-gatedligand-gatedmechanically-gated


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Membrane Permeability

1. Ligand-gated ion channelsNa+

Open Na+channel

Closed Na+channel

Acetylcholine boundto receptor sites

Na+ diffuse through

the open channel

Receptor sitefor acetylcholine

Acetylcholine

ligand molecule thatbinds to a receptor

ion channels open inresponse to ligand

binding


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Gated Ion Channels


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MEMBRANE POTENTIALS

Like charges repel and unlike attract anion = negative, cation = positive separated : move back together creates electrical force separation of charge = potential difference The resting potential is what you find in a

cell at


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CytoplasmCell

- - - - - - - - - - - - -

+ + + + + + + + + + + +

Voltmeter of Science

Cytoplasm

Plasma membrane

Extracellular fluid

-40 to -70 mV

Plasma

Membrane


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Electrical Signals in Neurons Neurons are electrically excitable due to the

resting potential

2 types of electric signals action potentials local or graded potentials

In living cells, a flow of ions occurs throughion channels in the cell membrane


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Electrical Signals

Electrical properties due toionic concentrationspermeability

Concentration highlightsCations:

Potassium (K+) > inside Sodium (Na+) > outside

Anions: proteins > inside


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Why are there concentration

differences? Na-K pump

active transport 2 K+ in and 3 Na+ outproteins are made in the cell permeability : gates & channels

number of open channels size of ions number of gated channels


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Sodium-Potassium Exchange

PumpConcentration gradient maintained by Na+/K+ pumpExtracellular fluid

Cytoplasm

Na+ K+

P

ATP binding siteATP

ADP

K+

K+

Na+

Na+

P


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Resting Membrane Potential

- ions along inside of cell membrane & + ions alongoutside

difference at rest is -70 mV cell is polarized

Resting potential exists because concentration of ions different inside & outside

extracellular lots of Na+ and Cl cytosol full of K+ & amino acids

membrane permeability differs for Na+ and K+ 50-100 greater permeability for K+ inward flow of Na+ cant keep up with outward flow of K+ Na+/K+ pump removes Na+ as fast as it leaks in


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Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

K+

K+

K+

K+

K+

K+

ATP

ADP

PiNa+

Na+

K+

K+

Na+

Na+

Na+

K+

K+K

+

K+

Na+/K+ ATPase

(Sodium Pump)

1. ATP binds

3. Pump activated

and 3Na+ exit and 2K+ enter cell

2. ATP converted to

ADP and Pi

Leakage of Na+ and

K+ needs to be

addressed.

4. ADP and Pi

dissociate and pump

stops


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Local or Graded Potentials

Small deviations from resting potential of -70mV hyperpolarization (i.e. -70 mV to -90 mV) Depolarization (i.e. -70 mV to -40 mV)


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How do Graded Potentials

Arise? Source of stimulimechanical stimulation (pressure)chemical stimulation (neurotransmitter)

What really happens?ions flow through ion channels & change

membrane potential locally

amount of change varies Flow of current (ions) is local change only


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Action Potential

change then restore themembrane potential

Ion channels open,depolarization thenrepolarization

All-or-none principal Travels over surface of

cell without dying out:Not local!!

+200

90

(m

V)

Depolarization Repolarization

Local

potential Afterpotential

Threshold

Time (ms)


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Depolarizing Phase of Action

Potential graded potential reaches

threshold

Voltage-gated Na+ channelsopen & Na+ rushes into cell in resting membrane, inactivation gate of sodium channel

is open & activation gate is closed (Na+ can not get in)

At threshold, both open & Na+ enters inactivation gate closes


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Repolarizing Phase of Action

Potential

When -55mV reached, voltage-gatedK+ channels open

K+ channel opening is slow When K+ channels open, the Na+ channels close K+ outflow returns us back to -70mV If enough K+ leaves the cell, it will reach -90mV

K+ channels close : back to resting potential of -70mV


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Refractory Period of Action

Potential neuron can not generate

another action potential

Absolute

even very strong stimulus willnot begin another AP

inactivated Na+ channels must return to the restingstate

Relative a suprathreshold stimulus will start AP K+ channels open, but Na+ channels closed


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Propagation of Action Potential

An action potential spreads over thesurface of axon

as Na+ flows into the cell duringdepolarization, the voltage of adjacent areas

is effected and their voltage-gated Na+

channels open

self-propagating along the membrane

The traveling action potential = nerveimpulse


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Action Potential

Frequency

Threshold

Supra-maximalstimulus

Maximalstimulus

Sub-maximalstimulus

Sub-maximalstimulus

Thres-holdstimulus

Sub-thresholdstimulus

Same frequency ofaction potentials

Increasing frequencyof action potentials

Actionpotential

Localpotential

Increasing stimulus strength

Time (ms)

90

0

+20

(mV

)

7membrane potential

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