Neurophysiology Opposite electrical charges attract each other - + inside outside 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

Neurophysiology 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 - + inside outside

Ohm’s law

Voltage: potential across the membrane - + inside outside Current: ions Voltage: potential across the membrane Resistance: membrane permeability

How can ions move across the membrane? - + inside outside Resistance: membrane permeability How can ions move across the membrane?

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 electrochemical gradient The driving force: the electrochemical gradient outside + + + + + + + + + + - - - - - - - - - inside

In a resting state, Potassium is the key player The driving force: the electrochemical gradient K+ Na+ K+ Na+ In a resting state, Potassium is the key player

Potassium wants to go out (chemical force), but also wants to go in (electric force) Potassium will diffuse via leak channels until equilibrium is reached (higher concentrations INSIDE)

Potassium wants to go out Sodium wants to go in K+ Na+ K+ Na+ - The neuronal membrane is much less permeable to Na+ than to K+ . The result: Na+ stays out - How do we maintain 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+

This is the resting membrane potential But we can change it

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 (aka local) A short-range change in a membrane potential upon a stimulus

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

Graded potentials spread locally but die out Membrane potential - A Graded/local potentialA short-range change in a membrane potential upon a stimulus Graded potentials spread locally but die out

How would that occur? Who said you have to depolarize? A stimulus can lead to hyperpolarization Membrane potential 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