Neurophysiology

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

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• Voltage (V)measure of differences in electrical potential energy

generated by separated charges• Current (I)the flow of electrical charge between two points• Resistance (R) hindrance to charge flow

Neurophysiology

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Ohm’s law

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-Current: ions

Resistance: membrane permeability

Voltage: potential across the membrane

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Resistance: membrane permeability

How can ions move across the membrane?

Ion channels

2) Chemically (ligand) – gated channels

1) Leak channels

- Can be ion-specific or not (e.g. the Acetylcholine receptor at the neural-muscular junctions is permeable to all cations)

Ion channels

3) Voltage – gated channels

4) Mechanically – gated channels

- Ion selective- Gates can open (and close) at different speeds

- Found in sensory receptors

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The driving force: the electrochemical gradient

Na+

K+

K+

Na+

The driving force: the electrochemical gradient

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 INSIDE)

Na+

K+

K+

Na+

Potassium wants to go outSodium wants to go in

- The neuronal membrane is much less permeable to Na+ than to K+ . The result- Na+ stays out- How do we keep this gradient?

Na+/K+ pump

The sodium/potassium pump acts to reserve an electrical gradient

- Requires ATP

- Throwing 2 K+ in, while throwing 3 Na+ out

Na+

K+

K+

Na+

The resting membrane potential is Negative

The Membrane is Polarized

DepolarizationMaking the cell less polarized

HyperpolarizationMaking the cell more polarized

This is the resting membrane potential

How can we change it?

Stimulus

ExampleA chemical stimulus

How can we depolarize a cell?

AxonCell body

Dendrites

Sodium channels opening leads to depolarization

-70 mV

- Generation of a graded potentialmeasure of differences in electrical potential energy

generated by separated charges

Think about a membrane with 50 channelsStimulating them with 4 ligand molecules or 40 will make a difference

The graded potential is increased with a stronger stimulus

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

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- Graded potentialsmeasure of differences in electrical potential energy

generated by separated charges

- Graded potentials spread locally but die out

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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

Axon

Cell bodyDendrites

Axon hillock (trigger zone)

Voltage - gated channels are found mainly on the axon and the axon hillock

Axon

Cell bodyDendrites

Axon hillock (trigger zone)

Voltage - gated channels are found mainly on the axon and the axon hillock