Membrane potentials

characteristics of neurons resting membrane potential

neurons can rapidly change resting membrane potential → highly excitable

the two sides of the membrane have opposite charges

outside = positive, inside = negative

the charge difference across the plasma membrane results in

potential (voltage difference)

greater charge difference = higher voltage

current

flow of electrical charge (ions) between two point

can be used to do work: higher voltage = higher current

resting membrane potential

differences in charges across a membrane at rest

graded potential

localized change in resting potential that decreases with distance from stimulus

voltage variable

action potential

electrical impulse propagated along the membrane surface

voltage constant

large proteins serve as

selective membrane ion channels

K+ ion channel allows only K+ to pass through

two main types of ion channels

leakage channels

gated channels

leakage (nongated) channels

always open

gated channels

part of the protein changes shape to open/close the channel

ligand-gated, mechanically gated, or voltage gated

ligand-gated channels

gated channels that open in response to binding of ligand (chemical) stimulus

mechanically-gated channels

gated channels that open in response to mechanical stimulus

such as touch, pressure, vibration, or tissue stretching

voltage-gated channels

gated channels that open in response to voltage stimulus (change in membrane potential)

when gated channels are open, ions diffuse

quickly

ions diffuse along chemical concentration gradients

from higher concentration to lower concentration

ions diffuse along electrical gradients

toward opposite electrical charge

electrochemical gradient

electrical and chemical gradients combines

ion flow creates

an electrical current, and voltage changes across membrane

resting membrane potential of a resting neuron is approximately

-70 mV = POLARIZED

the cytoplasmic side of membrane is negatively charged relative to the outside

potential is generated by

1. ion gradients

2. inability of most anions to leave cell

3. electrogenic nature of (Na+/K+ ATPase)

1: ion gradients

unequal distributions of ions

IDF (High K+) and ECF (High Na+)

2: inability of most anions to leave cell

usually attached to non-diffusible molecules (ATP, protein)

3: electrogenic nature of (Na+/K+ ATPase)

membrane is 25 times more permeable to K+ than Na+ (more leakage channels)

More K+ diffuses out than Na+ diffuses in

as a result, the inside of the cell is more negative

establishes resting membrane potential

Na+/K+ ATPase stabilizes resting membrane potential

maintains concentration gradients for Na+ and K+

3 Na+ are pumped out of the cell while 2 K+ are pumped back in

Membrane potential changes when

concentrations of ions across membrane change

membrane permeability to ions changes

Open/close ion channels

• bind neurotransmitter (ligand-gated)

• Distort membrane (mechanically-gated_

• Change membrane potential (voltage-gated)

  • have “activation” or “inactivation” gates - can be open, closed, or “inactivated”

Changing resting membrane - changes produce two types of signals

graded potentials and action potentials

graded potentials

incoming signals operating over short distances (voltage variable)

action potentials

long distance-signals of axons (voltage cosntant)

changes in membrane potential are used as

signals to receive, integrate, and send information

terms describing membrane potential changes relative to resting membrane potential (RMP)

depolarization

hyperpolarization

repolarization

depolarization

decrease in membrane potential (moves toward zero and above)

inside of membrane becomes less negative than RMP

probability of producing impulse increases

hyperpolarization

increase in membrane potential (away from zero)

inside of membrane becomes more negative than RMP

probability of producing impulse decreases

repolarization

returns/resets the membrane potential back to RMP

Graded potentials

short-lived localized changes (small deviations) in membrane potential

where do graded potentials occur

in dendrites and cell body

graded potentials are triggered by

stimulus that opens a gated ion channel

graded potentials result in

depolarization or hyperpolarization

For graded potentials - the change in local potential depends on

stimulus strength

as stimulus increases, more channels open

more channels = larger change in local potential

stronger stimulus = more voltage changes and the farther current flows

graded potentials can be added together to become

larger in amplitude

Action potentials

main way neurons send signals

long-distance communication

action potentials occur

only in muscle cells and axons of neurons

during an action potential, a neuron’s membrane potential rapidly flips from

-70 mV to +30 mV - about a 100 mV swing - before returning to rest

do action potentials decay over distance

no, they are voltage constant

action potentials in neurons are called

nerve impulses

action potentials involve the opening of

specific voltage-gated channels

true or false: all depolarization events produce APs

FALSE - not all depolarization events produce APs

threshold

membrane potential at which AP is generated (i.e. axon “fires)

for a graded potential to become an AP, the membrane but be depolarized by

10-15 mV (from 70 mV to -55mV)

what does it mean that action potentials are “all-or-none”?

An action potential either happens completely, or does not happen at all

Stimulus strength and size of action potential

size of action potential does NOT depend on the strength of the stimulus

an action potential will be generated if the threshold is met