Lecture 10: Membrane Potential - Graded Potentials and Action Potentials

Neuronal activity depends on

changes in membrane potential

Membrane potential

voltage across a membrane

Resting membrane potential

membrane potential of an undisturbed cell

Graded potential

localized change in resting membrane potential which decreases with distance away from the stimulus

Action potential

electrical event that starts at one location on the excitable membrane and spreads along the surface of an axon toward the axon terminals

Synaptic activity

produces graded potentials in the plasma membrane of the postsynaptic cell

Information processing

the integration of stimuli at the level of the individual cell

Differences in electrochemical gradients determine

the resting membrane potential

Leak channels

channels that are always open and allow ions to move along their gradient

Sodium-Potassium Exchange Pump

ejects 3 Na+ ions for every 2 K+ recovered from the extracellular fluid (ratio of 3:2); maintains stable resting potential

Chemical gradient

concentration gradient for an ion across the plasma membrane

Electrical gradient

difference in electrical charges between the inside and outside of the cell

Electrochemical gradient

The diffusion gradient of an ion, representing a type of potential energy that accounts for both the concentration difference of the ion across a membrane and its tendency to move relative to the membrane potential

Equilibrium potential

the membrane potential at which chemical and electrical forces are balanced for a single ion

Potassium ion gradients

K+ dominated inside of cell; forces K+ outside of cell

Sodium ion gradients

Na+ dominates outside the cell; at normal resting potential, both the chemical and electrical gradients cause Na+ to move into the cell

Types of gated ion channels change

membrane permeability

Chemically gated ion channels aka

ligand gated ion channels

Chemically gated ion channels

open in response to binding of the appropriate neurotransmitter

Voltage gated ion channels

open in response to changes in membrane potential

Mechanically gated ion channels

respond to mechanical vibration or pressure

Distribution of gated ion channels on a neuron

Chemically gated channels: neuron cell body and dendrites

Voltage gated ion channels: axon of neuron

Depends on myelination or demyelination

Graded potential are

localized changes in the membrane potential

Graded potentials aka

local potentials

Graded potentials

changes in the membrane potential that cannot spread far from the site of stimulation

Depolarization

inrush of positive charges reduces the membrane potential

Local current

movement of positive charges parallel to the inner and outer surfaces of the membrane

Repolarization

Return of the cell to resting state, caused by reentry of potassium into the cell while sodium exits the cell

Hyperpolarization

increasingly negative membrane potential as additional potassium leaves the cytosol; shift in membrane potential past the resting levels

Graded potential characteristics

1. the membrane potential is most affected at the site of stimulation, and the effect decreases with distance

2. the effect spreads passively through local currents

3. the graded change in membrane potential may involve either depolarization or hyperpolarization. the nature of the change is determined by the properties of the gated channels involved. for example, in a resting membrane, the opening of sodium ion channels will cause depolarization, whereas the opening of potassium ion channels will cause hyperpolarization. thus, the change in membrane potential reflects whether positive charges enter or leave the cell

4. the stronger the stimulus, the greater the change in the membrane potential, and the larger the area affected

Action potentials are

all or none events for communication that begin with membrane potential reversal

Action potentials affect

the entire excitable membrane

Action potential steps

1. depolarization to threshold

2. Activation of sodium ion channels and rapid depolarization

3. Inactivation of sodium ion channels and activation of potassium ion channels

4. potassium ion channels close

5. return to resting membrane potential

Continuous propagation

action potentials along an unmyelinated axon (axolemma); affects one segment of axon at a time

Continuous propagation steps

1. as an action potential develops in the initial segment, the transmembrane potential depolarizes to +30 mV

2. a local current depolarizes the adjacent portion of the membrane to threshold

3. an action potential develops at this location, and the initial segment enters the refractory period

4. a local current depolarizes the adjacent portion of the membrane to threshold, and the cycle is repeated

Saltatory propagation

the movement of an action potential along a myelinated axon, "jumping" from node to node

Saltatory propagation steps

1. action potential develops at the initial segment

2. local current produces a graded depolarization that brings the axolemma at node 1 to threshold

3. action potential develops at node 1; initial segment begins repolarization (and is now refractory)

4. local current produces a graded depolarization that brings the axolemma at node 2 to threshold

Cotransporters are what type of transport?

secondary active transport

Pumps are what type of transport?

primary active transport

Ion channels are what type of transport?

passive transport

Ions move according to

concentration gradient, electrochemical gradient

[All/some] cells have transmembrane potential

All

Excitable cells can

alter transmembrane potential to send and/or receive signals

Excitable cells

neurons and myocytes

transmembrane potential signals occur by proteins that allow ions to [rapidly/slowly] cross the plasma membrane

rapidly

Types of transport proteins involved in membrane potentials

ion channel

Transmembrane potentials are described in terms of

changes from RMP

Depolarization aka

excitation

Gating factors

- ligand

- voltage

- mechanical

RMP arises from

unequal permeability of ions due to different leak channel abundance

Potassium ion channels are [more/less] permeable to ions

more

There are [more/less] potassium leak channels than sodium leak channels

more

At an ions equilibrium potential

- inward and outward movements of the ion are equal

- there is no net force on the ion

Equilibrium potential for K+

-90mV

Equilibrium potential for Na+

+66m

At rest [K+/Na+/Cl-] is the most permeable ion

K+

Differences in ion concentration are maintained by

active transport

Na+ and K+ gradients are maintained by

primary active transport; Na-K-ATPase pump

Cl- gradient is maintained by

secondary active transport; co-transport with K+

Post synaptic potential is found at

dendrite

Receptor/generator potential is found at

dendrite

Action potential is found at

axon

Graded potentials can have different

sizes, shapes, amplitudes, grades, duration, polarity (direction)

Size of graded potential depends on

how many ion channels are open

Transmembrane potential is determined by

permeability; number of channels open

Graded potentials produce transmembrane potential changes from RMP for

as long as gated channels are open

Membrane potentials spread away from induction site, get [stronger/weaker] the further they spread

weaker

Electrotonic current

the passive movement of ions caused by similar electrical charges that oppose each other

Action potentials are transmitted [short/long] distances

long

Key features of an action potential

- all or none (threshold)

- propagation

- stereotyped dimensions

- all or none (refractory)

Action potentials are generated by

opening and closing of voltage gated sodium and potassium channels

VGNC gates

activation and inactivation

VGNC activation gate

opens rapidly in response to depolarization

VGNC inactivation gate

starts open and closes

VGKC channels

open slowly after a suprathreshold depolarization and slowly close after membrane repolarizes

At RMP, VGNC and VGKC are

both closed

At depolarization, VGNC and VGKC are

VGNC open; VGKC closed

At repolarization, VGNC and VGKC are

VGNC inactivating; VGKC open

At hyperpolarization, VGNC and VGKC are

VGNC inactivated; VGKC open

Co-transporters

Active transporters that use the energy from ionic gradients to carry multiple ions across the membrane in the same direction.

Pumps

Molecules are "pumped" by a transport protein to get across the membrane, requiring energy

Ion channels

channel proteins that transport ions

Diffusive force

Drives ions down the concentration gradient

Ions move across membranes according to

electrochemical gradient

Graded potential vs Action potential

Graded potential: small deviation from the resting membrane potential that makes the membrane either more polarized or less polarized; occurs when a stimulus causes mechanically-gated or ligand-gated channels to open or close in an excitable cell's plasma membrane

Action potential (aka impulse): a sequence of rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state; occurs in the axon of a neuron when depolarization reaches a certain level termed the threshold (-55mV)

Membrane potentials are generated by

the K+ that leaks from the inside of the cell to the outside via leak K+ channels and generates a negative charge in the inside of the membrane vs the outside

RMP

resting membrane potential; -70mV

Leak Channels

ungated; always open

RMP is created by

leak channels

Gated Channels

open and close depending on gating factors

ICF has more [Na+/K+]

K+

ECF has more [Na+/K+]

Na+

Ion concentration differences are maintained by

active transport

Potentials in an excitable membrane are driven by the opening and closing of

gated ion channels

Size of graded potential is affected by

number of ion channels that open

Graded potentials last for

tens of milliseconds

Graded potentials can have different _____ and _______

polarity (direction); amplitude

Membrane potential changes spread away from site where they are first induced but get _______ the further they spread

weaker

AP Key Features

all or none, propagation, stereotyped dimensions, refractory period

Action potentials are generatd in an axon by

opening and closing of Na+ and K+ channels