PSL 310 Unit 1: Intro to Membrane Potential and Graded Potentials

Equilibrium potential

Membrane potential at which a specific ion is at equilibrium across the plasma membrane (between ISF and ICF)

Chemical gradient

Gradient of concentration

Electrical gradient

Gradient of charge

Resting membrane potential

Membrane potential at which a neuron is at rest

3 things responsible for neuron RMP

K+ leak channels, Na+/K+ pump, anionic molecules

K+ leak channels

Once cell is depolarized, K+ leaks out, helps to repolarize the neuron to make the outside more positive and inside more negative

Na+/K+ pump

Makes cell more negative on inside than outside, contributes not a lot in neurons

Anionic molecules

Cytosolic proteins and RNA are generally negative, contribute to inside of cell being more negative

How important are each of the things responsible for neuron RMP?

1. K+ leak channels

2. Na+/K+ pump

3. Anionic molecules

Graded potential (GP)

Local change in membrane potential, magnitude of change is variable and directly proportional to stimulus

ΔVm

Change in membrane potential

Where do GPs happen?

Local, only in very small region of plasma membrane, typically around channels

What channels could open for a depolarization?

Opening Na+ or Ca+ channel, both more outside than inside

What channels could open for a hyperpolarization?

Opening Cl- or K+ channel, Cl- comes in (more on outside) and K+ leaves (more on inside)

Repolarization

Return of TMP to RMP after depolarization

Transmembrane potential

Electrical difference of plasma membrane

How are graded potentials generated?

Opening of ligand or mechanically-gated ion channels results in flow of i(ion) across membrane

Spike initiation zone

Axon hillock and initial segment of axon, GPs summate at hillock

Transmembrane ionic current

i(ion), movement of ions in the cell

i(ion)

Transmembrane ionic current, ions moving across the membrane

How does the magnitude of GPs change?

If there is more Na+ in, more trigger, higher magnitude

How does the magnitude of GPs change in relation to i(ion)?

Magnitude of GPs dependent on amount of i(ion)

How do ligand channels open for GPs?

2 ACh bind to ligand channel, receptor has conformational change, opens and allows Na+ in due to gradient, on membrane

How do mechanical channels open for GPs?

Channels are pried open with pressure, Na+ in due to gradient, on dendrite

Where are ligand channels for GPs located?

On membrane of cell body

Where are mechanical channels for GPs located?

Dendrites

Axial current

ia, longitudinal current

What initiates axial current (ia)?

GP creating a voltage gradient along the membrane

How does ia move and what does it do?

Forwards in ICF and backwards in ISF, completes local circuits along membrane

What does movement of ia in ICF and ISF cause?

Axial resistance (Ra)

Ra

Axial resistance

What does ia cause?

Change in membrane potential in adjacent membrane, each change causes more ia, allowing depolarization to spread longitudinally

Electronic conduction

Passive sppread of change of membrane potential, drives axial current, flip-flop of charges

How do axial current and membrane potential relate?

Change in membrane potential → change in axial current → separation of charge → change in membrane potential

Ohm’s Law

V = IR, or voltage = current x resistance, therefore axial current = change in voltage/axial resistance, ia = ΔV/Ra

Transmembrane capacitive current

ic, as membrane has Cm some spreading of ia becomes ic used to charge membrane

Capacitance

Cm, ability to store charge

What does ic result in?

ΔVm develops gradually at each membrane segment as ic is used to charge membrane, membrane potential changes

What is the speed of electrotonic spread determined by?

Cm and Ra

What is the distance of electrotonic spread determined by?

Rm and Ra

How does Ra affect the distance of electrotonic spread?

ia spreads and is limited by Ra, ΔVm decreases with distance as ia gets weaker and weaker

How does Rm affect the distance of electrotonic spread?

ia leaves as i(ion) as ions leak through K+ leak channels, less remains to depolarize the rest of the membrane, ΔVm decreases with distance

How can ia lost as i(ion) be prevented?

By increasing resistance of the axon using myelin

What are the two forms in which ia is lost?

as i(ion) and ic