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 inside outside

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 inside outside

Ohm’s law

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

inside outside 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

inside outside The driving force: the electrochemical gradient

Na + K+K+ K+K+ 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+ K+K+ Potassium wants to go out Sodium 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+ K+K+ The resting membrane potential is Negative

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

Example A chemical stimulus How can we depolarize a cell?

Axon Cell body 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

Think about a membrane with 50 channels Stimulating 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

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

Axon Cell body Dendrites Axon hillock (trigger zone) Voltage - gated channels are found mainly on the axon and the axon hillock

Axon Cell body Dendrites Axon hillock (trigger zone) Voltage - gated channels are found mainly on the axon and the axon hillock