CVM 711 Lecture 3: Equilibrium, Resting Membrane, and Action Potentials

What is the potential difference across the membrane when the cell is not generating an action potential?

Resting membrane potential (RMP)

What do resting membrane potentials primarily result from?

Diffusion potentials

How do K+ leak channels generate RMP?

Potassium diffuses out of the cell carrying a positive charge so it leaves behind a negative charge in the ICF

What maintains ion concentration gradients that allow diffusion potentials to develop?

Na+/K+ ATPase

What ion is the largest contributor to RMP?

K+

What ion is the second largest (minor) contributor to RMP?

Cl-

What ions contribute very little at rest?

Na+ and Ca2+

Was is the typical value of resting membrane potential?

-70 to -90 mV

What is resting membrane potential always close to?

Potassium equilibrium potential

Is the membrane more permeable to K+ or Na+?

K+

What does K+ leave behind when diffusing out of the cell?

Impermeant intracellular anions

What is K+ (potassium) equilibrium potential?

-85 to 90 mV

What is Na+ equilibrium potential?

+65 mV

What is Ca2+ equilibrium potential?

+120 mV

What is the point at which movement of K+ into cell because of negative electrical potential is balanced by diffusion of K+ out of cell due to concentration gradient?

K+ equilibrium potential

What is the difference between the measured membrane potential and the ion’s calculated equilibrium potential?

Driving force

When will an ion enter a cell if it is a cation (positive) and leave the cell if it is an anion (negative)?

Driving force is negative

What predicts both the direction and strength of ion movement?

Driving force

What transmits information in the nervous system and all muscle?

Action potentials

What is rapid depolarization followed by repolarization?

Action potentials

Membrane potential becomes less negative than usual at what point?

Depolarization

What is the most positive (least negative) point of membrane potential?

Peak action potential

Membrane potential becomes more negative up until resting potential at what point?

Repolarization

Membrane potential becomes more negative than usual at what point?

Hyperpolarization

What ion is involved in the inward current (flow of positive charge into cell)?

Na+

What ion is involved in the outward current (flow of positive charge out of cell)?

K+

-65/-75 mV

Resting membrane potential

-55 mV

Threshold of excitation

-65/-70 mV to 0 mV

Depolarization

0 mV to +30/+40 mV

Overshoot

+30/+40 mV to -65/-70 mV

Repolarization

-65/-70 mV to -90 mV (from RMP to K+ equilibrium potential)

Hyperpolarization/Undershoot

What is the membrane potential at which voltage-gated Na+ channels open rapidly, initiating an action potential? (-55 mV)

Threshold potential

What is the portion of AP where membrane potential is positive?

Overshoot

What is the portion of AP where membrane potential is more negative than RMP (less than -65 mV / -70 mV)?

Undershoot

What is the period during which another AP can’t be generated?

Refractory period

RMP is -70 mV, K+ conductance is high, Na+ conductance low

First event of AP

Membrane is depolarized to threshold (around -55 mV / -60 mV). Voltage-gated Na+ channels rapidly open. Na+ flows in.

Second event of AP: Rapid depolarization (upstroke)

Inactivation gates on Na+ channels close (terminate upstroke) and K+ channels open (allow outward K+ current to repolarize to make mV negative again)

Third event of AP: Repolarization

K+ conductance is higher than at rest

Fourth event of AP: Undershoot (hyperpolarization)

When are excitable cells unable to produce another normal AP?

Refractory period

When can no second AP occur because Na+ channels are inactivated?

Absolute refractory period

Which refractory period overlaps with most of the action potential (up until the peak of K+ conductance, before mV is back to resting membrane potential?

Absolute refractory period

Which refractory period lasts from the end of the absolute refractory period until through most of hyperpolarization?

Relative refractory period

When would a stronger-than-normal stimulus be required to trigger an AP because the membrane is hyperpolarized (more negative mV so needs a stronger simulus to reach threshold)

Relative refractory period

What does the sodium channel look like at rest (RMP)?

Activation gate closed

What does the sodium channel look like when activated by a signal?

Activation gate opens

What does the sodium channel look like when inactivated?

Inactivation gate closes

What is required to reset the sodium channel?

Time and returning to RMP

_______ and _____ remain constant for the AP of a given cell type

Amplitude and shape

Depolarization occurs at adjacent areas of membrane

Propagation

Stimulus either reaches threshold and generates a full AP or it does not generate an AP at all

All-or-none response

Where do APs start?

Close to the cell body of a nueron

Where do APs go after starting?

Spread down the axon via local currents

Initial area of axon depolarized to threshold, AP fires, cell interior positive

First step of propagation of APs

Positive charges inside cell flow towards negative charges in adjacent areas of cell - those areas then depolarized to threshold

Second step of propagation of APs

Initial area has repolarized

Third (last) step of propagation of APs

True or False: As local current spreads, some positive current lost across membrane due to current leakage since the membrane isn’t a perfect insulator

True

What two properties does conduction velocity depend on?

Time constant and length constant

What determines how quickly a membrane depolarizes? (How fast the membrane responds)

Time constant

Which time constant means the membrane reaches threshold faster and results in faster action potential and conduction?

Smaller time constant

What is the time constant influenced by?

Membrane resistance and membrane capacitance

Which membrane resistance leads to less current leakage but membrane voltage changes slowly. Causes a larger time constant.

High membrane resistance

Which membrane capacitance requires more charge before voltage changes. Causes a larger time constant.

High membrane capacitance

What determines how far local current spreads before it dies out?

Length constant

Which length constant causes adjacent membrane to reach threshold more easily? Conduction velocity increases.

Larger length constant

What influences length constant?

Membrane resistance and internal resistance

Which membrane resistance causes the current to stay inside the axon so it spreads farther? Length constant increases.

Membrane resistance

Which internal resistance causes the current to flow easily inside the axon so the current spreads farther? Length constant increases.

Low internal resistance

Which time constant maeks the membrane change slowly so conduction velocity decreases?

Large time constant

Which length constant makes conduction velocity increase?

Large length constant

How does the body increase conduction velocity?

Myelination

Why can’t the body just increase axon diameter to increase conduction velocity?

Requires very large nerves

Does myelination increase or decrease membrane resistance?

Increase

Does myelination increase or decrease membrane capacitance?

Decrease

Does myelination make currents spread farther or less far?

Farther

Does myelination make membrane depolarize faster or slower?

Faster

What is the insulation that wraps around nerves?

Myelin

How does myelin force currents to travel inside the axon?

Prevents leakage (increases membrane resistance)

How does myelin decrease myelin capacitance?

Increases thickness of membrane

What is membrane capacitance?

The ability to store electrical charge

What type of conduction is described when AP jumps for node to node?

Saltatory conduction

What are the nodes between myelin sheath cells called?

Nodes of Ranvier

What increases conduction velocity because less internal resistance causes the current to travel farther?

Greater Axon diameter

What increases conduction velocity because more membrane resistance causes less current to leak out?

Greater myelination

What increases conduction velocity because less membrane capacitance causes the membrane to depolarize faster?

Greater myelination

What can the body use to increase conduction velocity?

Myelinaion and larger axon diameter