Transcript Ion channels

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Neurophysiology
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Opposite electrical charges attract each other
In case negative and positive charges are
separated from each other, their coming
together liberates energy
Thus, separated opposing electrical charges
carry a potential energy
Neurophysiology
• Voltage (V)
measure of differences in electrical potential
energy generated by separated charges
• Current (I)
the flow of electrical charge between two points
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• Resistance (R)
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hindrance to charge flow
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Ohm’s law
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Current: ions
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Voltage: potential across the membrane
Resistance: membrane permeability
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Resistance: membrane permeability
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How can ions move across the membrane?
Ion channels
Ion channels
1) Leak channels
2) Chemically (ligand)
– gated channels
- Can be ion-specific or
not (e.g. the
Acetylcholine receptor at
the neural-muscular
junctions is permeable to
all cations)
3) Voltage – gated channels
- Ion selective
- Gates can open (and close) at different speeds
4) Mechanically – gated channels
- Found in sensory receptors
The driving force:
the electrochemical gradient
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The driving force:
the electrochemical gradient
K+
Na+
K+
Na+
Cations are the key players here , as anions
are actually negatively charged proteins
that cannot move through channels
Potassium wants to
go out, but also
wants to go in
Potassium will
diffuse via leak
channels until
balanced (higher
concentrations
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Potassium wants to go out
Sodium wants to go in
K+
Na+
K+
Na+
Na+/K+ pump
- The neuronal membrane is much less permeable
to Na+ than to K+ . The result- Na+ stays out
- How do we keep this gradient?
The sodium/potassium pump acts to reserve
an electrical gradient
- Requires ATP
- Throwing 2 K+ in,
while throwing 3
Na+ out
The resting membrane potential is
Negative
K+
Na+
K+
Na+
The Membrane is Polarized
Depolarization
Making the cell less polarized
Hyperpolarization
Making the cell more polarized
This is the resting membrane potential
How can we change it?
Stimulus
How can we depolarize a cell?
Example
A chemical stimulus
Cell body
Axon
Dendrites
Sodium channels opening
leads to depolarization
-70 mV
- Generation of a graded potential
measure of differences in electrical potential
energy generated by separated charges
The graded potential is increased with a
stronger stimulus
Think about a membrane with 50 channels
Stimulating them with 4 ligand molecules or 40 will
make a difference
A graded potential can spread locally
- Cations will move towards
a negative charge
- The site next to the original
depolarization event will
also depolarize, creating
another graded potential
Membrane potential
- Graded potentials
measure of differences in electrical potential
energy generated by separated charges
- Graded potentials spread locally but die out
Membrane potential
Who said you have to depolarize?
A stimulus can lead to hyperpolarization
How would that occur?
• Graded potentials
- Proportional to the stimulus size
- Act locally, starting from the stimulus site
- Attenuate with distance
- Spread in both directions
- Take place in many types of cells
• Action potentials do/are NOT
- Proportional to the stimulus size
- Act locally
- Attenuate with distance
- Spread in both directions
- Take place in many types of cells
Action potential can be generated and
propagated ONLY in:
- Neurons (only at the axon)
- Muscles
Why only there?
Function follows form
Voltage - gated channels are found mainly on
the axon and the axon hillock
Axon hillock
(trigger zone)
Axon
Dendrites
Cell body
Voltage - gated channels are found mainly on
the axon and the axon hillock
Axon hillock
(trigger zone)
Axon
Dendrites
Cell body