Neurophysiology Lecture 3

Membranes, Diffusion, Transport, Membrane Potentials, APs

Cell membranes have ____-________ barriers, with _________ within them and ___ outside of the barrier

semi-permeable; cytoplasm; ECF

Cell membranes envelop organelles; what does this allow organelles to do?

Organelles can become discrete compartments that perform specialized biochemical functions

Cell or “plasma” membranes have ________ head groups and _______ interiors; allowing _____, _________, and relatively ____-_______ molecules to diffuse through the lipid bilayer, like ______.

hydrophilic; hydrophobic

small; uncharged; lipid-soluble; steroids

More than half of the mass of a cell membrane is comprised of ______ which must be crossed through by _______, or _____ transport mechanisms

Proteins

Passive; active

Simple Diffusion

What type of transport is it?

What ions can diffuse through easily? Through what?

Two things simple diffusion does NOT require?

• Passive transport

• Small charged ions diffuse through ion channel proteins

• No energy required

  • Cannot move up a gradient

• Does not require binding of the particle to a carrier

Ion channels

Definition

Are _____-_____ pores

Do they allow diffusion across the membrane up or down the electrochemical gradient?

Usually allow only ___ type of ion

Some are _____ and let ions slip through, some are opened and closed by _____

• Pore-forming membrane proteins that allow the passage of ion

• water-filled

• Down

• One

• leaky; gates

  • K+ channels are especially leaky

Voltage-Gated Channels

Controlled by changes in _________ across the membrane

Example 

What is the name of the gate that opens the channel? Closes it?

• Voltage

• Voltage-gated Na+ channels

• Activation gate; inactivation gate

  • Inactivation gate has a recovery period after closing 

Chemically-gated channels

What type of molecule needs to bind to the receptor to open this gate?

Example

• A ligand, like a neurotransmitter

• Chemical/ligand-gated Ca2+ channels

  • Acetycholine receptor 

Mechanically-Gated Channels

Open in response to _______ force, or a ________ of membrane

These channels are also called _____-activated channels

mechanical; distortion

Stretch-activated

• This is how touch or pressure is turned into an electrical signal that neurons can transmit

Active Transport

Movement of substances _____ their cxn gradients, which usually requires ___

Typically mediated by an ______, like (examples)

against; ATP

enzyme

• Na+/K+ ATPase, Na+/H+ ATPase

• Sodi-OUT Potassi-IN

This transport is high active in cells that need to move ions

• like neurons, kidney, muscle, intestine

Secondary Active Transport

Examples

Rely on _______ _____ transport to produce __________ gradients

Used stored energy to move other molecules _______ their cxn gradients

_______ consumption of ___

Usually done by symporter or antiporter _______

• Co-transporters and counter-transporters

• Primary active; concentration

• Against

• Indirect; ATP

  • Rely on leftover energy consumption from primary active transport mechanisms

• proteins

Leaky K+ channels help to establish and maintain a _______ in ________ across the membrane, leading to the idea that _______ determines ____ but only with the ion that _____

difference in concentration

concentration determines charge

• the membrane remains permeable to K to maintain charges

ion that moves 

• K+ moves, Cl- does not, so K+ is the ion that determines the charge across the membrane

The flux of any permeable ion across the membrane is determined by the _______ _____ and the electric potential, creating a ________ _________

concentration gradient; electrochemical gradient/balance

The Nernst Equation

This equation allows you to calculate:

1) The charge in a membrane if you know the ______ of permeable ____

2) The _______ of ions if you know the _____

1) concentration; ion

2) Concentration; charge

ONLY APPLIES WHEN ION INVOLVED CAN MOVE ACROSS THE MEMBRANE

The charge inside membrane at rest relative to outside is known as the ______ ________ _______ and is __mV

Resting Membrane Potential (RMP)

-91mV

Movement of K+ and the Membrane Potential

If charge of cell interior becomes less negative, what happens?

If it becomes more negative what happens?

K+ _____ to _______ the RMP in neurons when the charge changes

Less negative

• Electrochemical gradient will drive K+ out and the charge will decrease (become more negative)

More negative

• Electrochemical gradient will pull K+ in, charge will rise (become less negative)

Moves to restore

• At rest the membrane is only permeable to K+ (leaky) so the RMP is near Ek

• Same principle applies to other ions, like Cl-, but in the SKELETAL MUSCLE

Na+ Across the Cell Membrane

The RMP is influenced ONLY by ions that ___ _____ _____ the membrane

At rest, the membrane is NOT ________ to Na+ so it has no effect (only K+ is)

Under specific circumstances, the membrane becomes far more permeable to Na+ and Na+ rushes in. What are these circumstances?

can move across

permeable

Excitation, or depolarization

• they result in an ACTION POTENTIAL

• Na+ sets the charge on the membrane from this

• Transient permeability to sodium - quick and in abundance 

The change in the RMP is called a _____ potential

When these become large enough, they cause _____-_____ Na+ channels to open

What is the threshold value at which this happens?

graded potential

voltage-gated Na+ channels open

The threshold value at which this happens is -55mV

Does the charge from an action potential span the whole length of one neuron?

NO!

APs are VERY LOCAL in a small area of the membrane, and causes a chain reaction that moves down the whole neuron like a wave 

All-or-None Principle

To trigger an action, you need a local potential stimulus great enough to open voltage-gated Na channels (-60 to -55mV)

• Below threshold, nothing happens

• Above threshold, AP occurs

After the threshold is met, voltage-gated Na+ channels open and the membrane becomes much more _______ to Na+, causing the membrane potential to move toward the _______ potential for Na+ which is __ to __ mV

This ______ the membrane briefly in a wave

more permeable to Na+

membrane potential to move toward the equilibrium potential for Na+

+65mV to +75mV

this depolarizes the membrane

Excitation: Opening and Closing of Gates in V-G Channels

1) At rest, the ______ gate is closed

2) At threshold value, the _____ gate opens, and ___ flows into the cell, causing further _______

3) The _____ gate closes and _____ the ___ after ~1ms, causing membrane ___________

What is it called when after this more K+ leaves the cell as additional K+ channels open?

The RMP gets re-established based on which ions’ concentration?

1) The activation gate is closed

• Inactivation gate is open

2) activation gate opens, and Na+ flows into the cell causing further depolarization

3) The activation gate closes and blocks the pore; causing membrane repolarization 

When more K+ leaves out of the leaky ion channel after repolarization is hyperpolarization

The RMP gets reestablished based on K+ concentration

• Concentration continues long term to bring membrane potential back to resting long-term through Na/K pump

  • refractory period; AP cannot happen until out of this period

    • This ensures the AP does not travel backwards through the neuron from where it came from

Saltatory Conduction

Definition

Do APs occur at myelinated internodes or unmyelinated nodes?

Action potential does NOT move linearly across axons; jumps from node to node between them

APs occur in the unmyelinated nodes

• Faster - ion channels in between do NOT have to move and open; impulse jumps in between