Day 2

RMP (-70 mV)

A real state — where the resting neuron's voltage actually sits, moment to moment

Threshold (-55 mV)

A real tipping point — the specific voltage that, once reached, triggers the action potential

E(Na+) (+40 mV)

A theoretical number — the voltage where sodium's chemical and electrical forces would balance if only Na+ could move; the AP peak gets pulled toward this value

E(K+) (-90 mV)

A theoretical number — the voltage where potassium's chemical and electrical forces would balance if only K+ could move; the AP dips toward this value during repolarization/hyperpolarization

Why is RMP close to E(K+)?

Because the resting membrane is mostly permeable to K+ (leaky K+ channels), so resting voltage is pulled much closer to K+'s equilibrium than Na+'s

Equilibrium Potential (Eq)

The voltage at which the chemical force balances the electrical force for a specific ion; every ion has its own Eq

Vg (voltage gate)

A channel that opens/closes in response to voltage changes; general rule is they open quickly and close slowly

Voltage-gated Na+ channels

Open quickly at threshold; close quickly at the peak (+40 mV), creating the sharp peak of the AP

Voltage-gated K+ channels

Open quickly, but close slowly

Action Potential (AP)

A massive, momentary reversal of the resting membrane potential

At the axon hillock — the connection zone between the soma and the axon

Where does the AP occur?

AP: all-or-none

What matters is whether depolarization crosses threshold; there is no partial action potential

AP amplitude

Stays the same every time the AP fires (unlike PSPs, which vary in size)

Polarization

The state where the inside of the neuron is more negative than the outside; this is the resting condition of the membrane (-70 mV)

Depolarization

The inside of the neuron becomes less negative (more positive), typically because Na+ enters; moves the neuron closer to threshold/firing

Hyperpolarization

The inside of the neuron becomes more negative than resting, typically because K+ exits; moves the neuron further from threshold, less likely to fire

At rest RMP

sitting at -70 mV, polarized

AP Phase 1

Depolarization : Threshold (-55 mV) reached, voltage-gated Na+ channels open quickly, Na+ rushes in, inside becomes more positive, rising slope toward +40 mV

AP Phase 2

Peak / Na+ channels close : At +40 mV, voltage-gated Na+ channels close quickly, creating the sharp peak

AP Phase 3

Repolarization : Voltage-gated K+ channels open, K+ rushes out, inside becomes negative again, falling slope back toward -70 mV

AP Phase 4

Hyperpolarization (overshoot) : K+ channels close slowly, so K+ keeps leaving too long, dipping below -70 mV toward -90 mV before returning to rest

AP Phase 5

Sodium in, then potassium out

Simple summary of AP ion movement

Absolute refractory period

The moment when sodium channels are closed (inactivated); no action potentials are possible no matter how strong the stimulus

Relative refractory period

Threshold has increased (harder to reach); an action potential is possible but less likely, needs a stronger-than-normal stimulus

Saltatory propagation

How the action potential moves down the axon to the terminal; vertical/jumping movement of ions between myelinated gaps (nodes of Ranvier), sped up by myelin

Postsynaptic Potential (PSP)

A graded, decremental electrical change in the postsynaptic neuron produced when neurotransmitter binds to receptors

PSP: Decremental

PSPs start big when produced, then decay over time and distance as they travel

PSP: Graded

The strength of the stimulation determines the amplitude — strong stimulation makes a large amplitude, weak stimulation makes a small amplitude

AP amplitude is always the same (all-or-none); PSP amplitude is graded and varies with stimulus strength

How do AP and PSP amplitude differ?

EPSP (Excitatory Postsynaptic Potential)

Caused by Na+ moving into the channel; results in depolarization; makes the inside more positive and increases the likelihood of crossing threshold

IPSP (Inhibitory Postsynaptic Potential)

Caused by K+ moving out of the channel; results in hyperpolarization; makes the inside more negative and decreases the likelihood of crossing threshold

Summation

The neuron receives many PSPs and adds them together, generally around the soma/axon hillock (same idea as convergence)

Spatial summation

PSPs from different locations on the neuron add together

Temporal summation

PSPs fire in rapid succession (same location, close in time) and add together