BIO-249 Lecture Exam 7: Membrane Potentials & The Nervous System

What is the voltage & membrane potential?

• Voltage:

  • measure of potential energy generated by separated electrical charges

  • measured in mV

• Membrane Potential:

  • Relative DIFFERENCE in charge between the inside & outside of the cell

  • we can use this potential energy that’s held in the separation of charge across the cell membrane

  • charge of the inside - charge of the outside

• What are the four most common ions that can contribute to a cell’s membrane potential?

• For each of these ions, what is their charge?

1. Na⁺ : positive 1

2. K⁺: positive 1

3. Cl^-: negative 1

4. Ca²⁺: positive 2

What is the electrochemical gradient for a given ion?

• “chemical”: ions moving in and out of the cell due to their concentration gradient

  • INFLUENCES electrical gradient

  • focusing on the ion concentration

  • STRONGER than the electrical gradient

• “electrical”: the charge gradient

  • focusing on all the ions and proteins (negative charge) inside and outside the cell

  • electrical gradient dictates HOW MANY ions should come back inside the cell

Describe how ions would move along the electrochemical gradient.

• Channel Proteins

  1. Leaky Channels (facilitated diffusion)

     1. Na+ Channel

     2. K+ Channel

  2. Na+/K+ Exchanger (active transportation)

     • Uses ATP

     • [Na⁺]_outside HIGH and [Na⁺]_inside LOW

     • [K⁺]_outside LOW and [K⁺]_inside HIGH

ION MOVEMENT

1. Chemical Gradient

   • Na⁺ ions move DOWN their concentration gradient (move outside the cell)

2. Electrical gradient

   • There’s a difference in charges now

   • Outside: MORE POSITIVE

   • Inside: MORE NEGATIVE

   • Opposite charges attract → Na+ cations will move BACK into the cell

3. Summary

   • Chemical gradient pushes ions OUT, electrical gradient moves ions back INTO the cell

What two factors determine the electrochemical gradient?

1. Concentration Gradient of Ions

2. Permeability (ability for ions to move across the cell membrane)

• In a human cell, where is potassium ion (K) concentration the highest?

• Where is the chloride ion (Cl) concentration the highest?

• Where is Sodium (Na) concentration the highest?

• K⁺

  • Highest: Inside the cell

  • Lowest: Outside the cell

• Cl^-

  • Highest: Outside the cell

  • Lowest: Inside the cell

• Na⁺

  • Highest: Outside

  • Lowest: Inside

Are the intracellular fluid (ICF) and extracellular fluid (ECF) themselves electrically neutral?

YES

• Intracellular Fluid

  • Negative charge: proteins

  • Positive charge: K⁺ ions

• Extracellular Fluid

  • Negative Charge: Cl^-

  • Positive Charge: Na⁺

• In a sentence or two, describe what the resting membrane potential of a cell is.

• What is the resting membrane potential of neurons (give units)?

• Resting Membrane Potential

  • Membrane potential BEFORE it changes (NOT conducting electricity, NOT excited)

  • -70mV

  • Has the ability to trigger a response, but the neuron must be excited first

What two factors generate the resting membrane potential?

1. Concentration/Chemical Gradient

   • ions moving down their concentration gradients starts off the electrochemical gradient

2. Permeability (ion channels)

   • How WELL can ions pass through the cell membranes

   • Na⁺ Leaky Channel

   • K⁺ Leaky Channel

   • Na⁺/K⁺ Exchanger

How does the cell maintain the chemical gradient of Na⁺ and K⁺?

Na⁺/K⁺ Exchanger:

• Using ENERGY to:

• REMOVE Na⁺ from the cell

• ADD K⁺ to the cell

Scenarios that change the overall resting membrane potential (discussion slides)

add pic

Changing the Ion Concentration

• ADD Na⁺to the ECF:

  • Overall increase the extracellular [Na⁺] → FASTER diffusion → increase the resting membrane potential

• ADD Na ⁺ leakage channels

  • Increase the permeability of Na⁺ → increase the opportunity for Na⁺ to diffuse into the cell → increase the resting membrane potential

• What does it mean for a membrane to depolarize or to hyperpolarize?

• Describe events that can hyperpolarize or depolarize a neuron?

• NOTE: How can you change ion concentration or permeability to hyperpolarize or depolarize a neuron?

• add pic

• Depolarization: membrane potential is CLOSER to 0 (more positive)

  1. Increase Na⁺ Permeabilty: OPEN Na⁺ chemically-gated channels

  2. Decrease K⁺ Permeability: REMOVE K⁺ leaky channels

• Hyperpolarization: membrane potential is FURTHER AWAY from 0 (more negative)

  1. OPEN K⁺ chemically-gated channels

Describe how a chemically gated ion channel works.

1. Stimulus: ligand (neurotransmitter- a chemical signal)

   • ligand/neurotransmitter BINDS to the active site of the chemically gated ion channel

2. Chemically gated ion channel OPENS

3. Ions can flow into the cell

What can be a stimulus to a graded potential?

Neurotransmitters (chemical signals)

• Briefly describe what electrical current is.

• Is current faster or slower than chemical diffusion?

• Electrical Current: Flow of electrical charge from one point to another point

• FASTER than chemical diffusion

How can a graded potential hyperpolarize or depolarize a cell?

• Depolarization (closer to 0, more positive)

  • OPEN Na⁺ chemically gated channel to let Na⁺ in

  • Positive charge spreads out throughout the cell (attracted to negatively charged proteins)

• Hyperpolarization (moves AWAY from 0, more negative)

  • Open Cl^- chemically gated channels

    • Negative charge spread throughout the cell → membrane potential becomes more negative

  • Opening K⁺ Channels

    • K+ ions move OUT of the cell

  • Close Na⁺ chemically gated channels

How can a graded potential vary the amount of depolarization or hyperpolarization that occurs in the neuron?

verify w/ nair

Graded Potential Strength varies by:

1. Time

   • ↑ time: ↓ strength

2. Distance

   • ↑ distance: ↓ strength

3. Ion Permeability

   • ↑ Na⁺ permeability (↑ # of channels): ↑ strength

4. Concentration

   • ↑ [Na⁺] = ↑ strength

Where in the neuron does a graded potential occur?

Dendrites & axon

• What is the “strength” of a graded potential?

• Can the “strength” of a graded potential vary?

• What can cause that variation?

• Strength: depolarization ability

  • Trying to reach the -50 mV or above threshold

• Strength Variation

  1. Time

     • ↑ time: ↓ strength

  2. Distance

     • ↑ distance: ↓ strength

• For a given portion of a neuron’s cell membrane, describe how the membrane potential varies over time given a hyperpolarizing or depolarizing graded potential.

• Describe the movement of ions across the membrane patch for either situation.

Depolarization

1. Stimulus (neurotransmitter) triggers Na⁺ chemically-gated channel to open

2. Rush of Na⁺ ions INTO the cell depolarizes the cell

Hyperpolarization

1. K⁺ chemically gated channel opens after Na⁺ chemically-gated channels

2. K⁺ ions LEAVE the cell

3. Na⁺ions move AWAY from the chemically-gated channels

4. Na⁺ ions leave via leaky Na⁺ channels

• In a small paragraph, describe how the membrane potential varies over time AND distance as a graded potential moves across the cell body.

• Describe the movement of ions within the cell body as the graded potential moves away from the stimulus.

Membrane Potential

• Peaks QUICKLY but decreases over time

• Decays as time and distance increase

• Travels as a “wave” through the cell body

Movement of ions

• RUSH of Na⁺ ions depolarize the cell QUICKLY, but as Na⁺ & K⁺ leave the cell, it hyperpolarizes

Describe how a voltage-gated ion channel works.

1. Requires at least -55 mV to open

2. Depolarizes the membrane potential

3. Increases the permeability of ions (lets them into/out the cell)

Where do action potentials occur in a neuron?

Axon

4 Phases of the action potential

1. Resting: -70 mV

2. Depolarization: more than -50 mV

3. Repolarization: -70mV

4. Hyperpolarization: -90 mV, then -70mV

Action Potential Resting Phase

• Occurs BEFORE the action potential

• MINIMAL permeability

  • Na⁺ and K⁺ voltage-gated channels are CLOSED

  • leaky ion channels allow very little ion diffusion

Action Potential Depolarization Phase

1. Membrane Potential increases to -55mV

2. Na⁺ voltage gate opens & allows Na⁺ INTO the cell

   • permeability INCREASES the membrane potential

   • Na⁺ positive charge brings the membrane potential closer to 0

3. K⁺ voltage-gated channels open up after Na⁺

4. Membrane potential plateaus/flattens

   • K⁺ voltage-gated channels are still open & K⁺ ions are escaping the cell

Action Potential Repolarization Phase

Membrane Potential starts to decrease

1. Na⁺ voltage-gated channels CLOSE UP

   • DECREASE Na⁺ permeability QUICKLY

2. K⁺ voltage gate channels are still open

   • always delayed

   • K⁺ ions are still escaping & decreasing the membrane potential

Action Potential Hyperpolarization Phase

Membrane Potential is LESS than -70 mV (usually -90 mV)

1. Na⁺ voltage-gated channels are closed

2. SOME K⁺ voltage-gated channels are open

   • K⁺ ions are LEAVING the cell

3. K⁺ channels CLOSE

4. Na⁺/K⁺ ATPase Exchanger brings the membrane potential BACK to -70 mV

What causes the pattern of permeability in an action potential?

verify w/ nair

• Ion voltage-gated channels opening & closing

• Concentration gradient??

At the beginning of the axon, how does the first action potential get triggered?

Suprathreshold (at least -55 mV) graded potentials at the axon hillock trigger the first action potential

• What does it mean for a graded potential to be a subthreshold stimulus?

• What is the voltage needed to start an action potential at the axon hillock typically?

• Subthreshold stimulus: less than -55 mV

• at LEAST-55mV to trigger an action potential

What does it mean for an action potential to be an “all-or-none” phenomenon?

Action potentials either happen OR NOT

• have the EXACT amount of depolarization & repolarization

• All action potentials (in a series) look the same

In what direction along the axon do action potentials propagate?

• Propagate across the axon (left → right)

• from the axon hillock/trigger zone TO the axon terminal

• What is a neurotransmitter, and where is it released from?

• What triggers the release of neurotransmitters?

• Neurotransmitter: chemical released by axon terminals of neurons

• Action potentials trigger a release of neurotransmitters at the axon terminal

  1. Action potential depolarizes the axon terminal

  2. Ca²⁺ ENTRY (into the presynaptic neuron) tiggers exocytosis of neurotransmitters

  3. Neurotransmitter diffuses across the synaptic cleft & binds with receptors on the postsynaptic cell

How does a neuron code for stimulus strength in action potentials? Note: Action potentials are “all-or-none.” Note: There is no such thing as “small or large” action potentials.

Stimulus Strength: frequency of action potentials

• Weak stimulus: LOW frequency of action potentials

• Strong stimulus: HIGH frequency of action potentials

How does a presynaptic neuron express signal strength to a postsynaptic neuron? Note: Consider what is happening at the synapse between the two neurons.

Signal Strength: amount of neurotransmitters released

• Weak signal: LOW release of neurotransmitters

• Strong signal: HIGH release of neurotransmitters

Describe how a neurotransmitter, once released from the axon terminal, can trigger (or prevent) a graded potential in the postsynaptic neuron.

• Trigger a graded potential: Neurotransmitter DEPOLARIZES the postsynaptic neuron

• Inhibited/prevent a graded potential: neurotransmitter HYPERPOLARIZES the membrane potential of the postsynaptic neuron

  • shifted the membrane potential so it won’t reach the -50 mV threshold

  • Bind/open K⁺ channels

What does it mean for a neurotransmitter to be excitatory or inhibitory?

• Excitatory: Depolarizes the neuron

• Inhibit: Hyperpolarize the neuron

How can the effect of Acetylcholine (ACh) and Norepinephrine (NE) vary between excitatory and inhibitory in the body?

• Acetylcholine (ACh): binds to K⁺ ion → opens them → decreases the membrane potential → hyperpolarizes the cell → inhibits the neuron

• Norepinephrine (NE): binds to Na⁺ ions → opens them → INCREASES membrane potential → depolarizes the cell → excites the neuron

• Excitatory & inhibitory effect vary with the types of receptors

  • Adrenergic receptors (alpha & beta)