Membrane potential

nerve cell membrane is a

double layer lipid membrane

a nerve cell membrane is mostly

impermeable to ions

when is a nerve cell membrane permeable to ions

when ion channels or transporter proteins are present

ion channel

protein that creates a hole in the membrane when open

can be selective for specific ions or classes (Na channel)

allows ions to flow passively diffuse according to concentration and electrical charge

ion transporters

actively transport ions across the membrane against the concentration gradient

create ion concentration gradients

use energy (ATP)

resting membrane potential

the voltage (electrical charge) difference between the inside of the neuron and the extracellular fluid, when the neuron is at rest (no stimulus).

neurons have a negative resting potential

inside of the cell is more negatively charged than the outside of the cell

creation of resting potential

1. extracellular fluid is arbitrarily considered to be at 0 volts

2. if intracellular ions = extracellular ions (equal to ion charges) resting potential would be 0

3. most cells have different concentrations inside vs outside, voltage difference across the membrane

4. most cells have more negative ions inside than positive ions → negative resting potential

membrane potential changes when

ions move across the membrane

depolarization

1. if + ions move in (or - ions move out) = more positive inside than it was at rest

2. as its potential moves closer to 0 =depolarized

hyperpolarization

1. if - ions move in (or + ions move out) = more negative inside than it was at rest

2. as its potential moves farther from 0 = hyperpolarized

ions move across the membrane through

ion channels

ion channels

1. openings in the cell membrane formed by proteins

2. change configuration so they are open, closed, or inactivated

3. channels are specific for type of ions (ex: Na channels, Cl channels)

what makes an ion channel open?

channel gating

chemically gated channels are called

ligand- gated

ligand gated channels

1. opened by binding of neurotransmitter

2. can be directly or indirectly gated

directly gated

neurotransmitter opens channel

indirectly gated

neurotransmitter activated a second messenger that opens that channel

ion channels that are opened by voltage change across the membrane are called

voltage gated ion channels

voltage gated channels

changes configuration

voltage gated Na channels at the nodes of Ranvier and axon hillock

produce Action potentials

voltage gated Ca channels - in the axon terminal for neurotransmitter release

mechanically gated channels trigger is

touch, pressure, stretch, or vibration

mechanically- gated channels are used as

sensory receptors

which channels are activated by electrical stimulation

voltage gated channels

Is it a normal mode of action for the neuron to use voltage gated channels (ex: when E stim is used)?

No, neuron usually uses ligand gated channels (chemically gated)

leakage channels

the neuron does NOT have a totally impermeable membrane, a little “leaky”

movement of ions

potassium > sodium

what direction does sodium and potassium generally leak through leakage channels

sodium (Na+) : leaks in

Potassium (K+) : leak out

what charge is inside of the cell due to leaking

negatively

Na/K pump attempts to return

leaking ions back across the cell membrane

what is the Na/K pump operated by

“carrier proteins” in the membrane

At rest, which way does Na+ and K+ leak

Na+: leaks in

K+: leaks out

The pump protein moves Na+ ions

back out

The pump protein carries K+

back in

How much total energy of a neuron does it take to run the Na+/K+ pump

10-20%

what causes a concentration gradient

ions not distributed equally on both sides of the membrane

Sodium and chloride (NaCl) ions are concentrated outside or inside neurons

outside

Potassium (K+) and protein anions (A-) are outside or inside neurons

inside

ions want to diffuse from areas of blank concentration to blank concentration

High

Low

due to the concentration gradient, does K+ want to move inside or out of the neuron

inside

due to the concentration gradient, does Na+ want to move inside or out of the neuron

inside

due to the concentration gradient, does Cl- want to move inside or out of the neuron

outside

Ions are NOT always free to follow their concentration gradient due to

voltage gradient

voltage gradient

ions have an electrical charge (positive or negative)

attracted or repelled by other charged particles

large protein ions are trapped inside the neuron (too big)

negative charge proteins attract positively charged small ions & repels negatively charged ions

this process creates electrochemical force or a voltage gradient

due to the voltage gradient, does Cl- want to move inside or out of the neuron

outside

due to the voltage gradient, does Na+ want to move inside or out of the neuron

inside

due to the voltage gradient, does K+ want to move inside or out of the neuron

inside

definition of ionic equilibrium potential

equilibrium potential is the neuron membrane potential at which the concentration gradient and the electrochemical gradient will be equally balanced, and no net ion flow occurs

some ions, both the concentration and the electrochemical gradients will act in the same direction (moving the ion into or out of the cell), while for other ions, the two forces will act in opposite directions

K+ ionic equilibrium potential

K+ concentration is out and voltage is in

-90mV

Chloride ionic equilibrium potential

concentration is in and voltage is out

-90mV

Sodium ionic equilibrium potential

concentration in and voltage in

+55mV

what equation can be used to calculate the equilibrium potential for any ion

Nernst Equation

driving force definition

ion driving force is equal to the difference between the equilibrium potential for the individual ion (ex: Na+) vs. the present neuron potential

larger the voltage gradient means what for the driving force?

a greater tendency for ions to move across the membrane

at rest K+ and Cl- have a blank driving force

small

this means they leak a little

at rest, Na+ has a blank driving force

large

so it leaks a lot

If the neuron membrane was permeable to only one of these ions the neuron resting potential would

be the same as the ion’s membrane potential

The membrane is permeable to

all 3 ions

resting potential of a neuron= average equilibrium potential of ALL ions to which the neuron is permeable

normal resting potential is

-65mV

lies between the equilibrium potentials for K+ and Cl- (-90mV) and the equilibrium potential for Na+ (+55mV)

the normal resting potential is closer to what ions’ equilibrium potential

K+/Cl-

what equation can calculate the ionic basis of resting potential

goldman equation