action potential I

Question 1

passive electrical properties of axons render them

Answer 1

poor conductors of electrical signals over distances greater than a few millimeters.

Question 2

Axons are poor conductors of electrical signals because they are

Answer 2

essentially very dilute salt solutions.

Question 3

ECF is essentially the same solution, however; the

Answer 3

cross-sectional area is very small for a nerve axon compared to the ECF.

Question 4

So for a given length of axon, the intracellular resistance is

Answer 4

much greater than ECF resistance which has more area.

Question 5

ion channels act like

Answer 5

resistors

Question 6

Ions continue to leak out after ____

The result of this is that ______

Answer 6

positive charges have ceased flowing into the cell.

less current is delivered down the axon than was originally inserted.

Question 7

Booster stations act by applying an

Answer 7

energy boost to a decaying electrical signal when the incoming signal is detected.

Action potentials work in essentially the same way.

Question 8

The detector that senses the decaying message is the ______ and the energy source that propagates the message is the _______

Answer 8

voltage-gated sodium channel

Na “battery”

Question 9

the Na battery is the

Answer 9

the Na gradient that can inject extra positive charge into the cell.

Question 10

The larger the membrane resistance becomes.

Answer 10

the farther the voltage can spread down the axon, because increased membrane resistance means there are fewer ion channels for charges to leak out of

Question 11

Membrane capacitance is the ability.

Answer 11

of charge to accumulate on two sides of a membrane

Question 12

These charges ______ create an electrical force that that pulls oppositely charged ions toward it.

Answer 12

membrane capacitance and membrane resistence

Question 13

Reducing membrane capacitance increases the

Answer 13

passive spread of voltage.

Question 14

When current is injected into the cell, it must follow either

Answer 14

through the ion channels (as resistors) or into the membrane capacitor.

Question 15

Increased internal resistance will slow the

Answer 15

spread of voltage, as charge is lost to the surrounding fluid.

Question 16

Resistance increases as cross-sectional area

Answer 16

decreases.

Question 17

cells communicate infor via

Answer 17

action potentials

Question 18

how is AP generated?

Answer 18

Vm and current are related to one another, more current results in more Vm, change in potential. If you have enough current, then you end up with an action potential characteric shape

Question 19

Phases of AP

Answer 19

1. resting potential
2. depolarization
3. repolarization
4. after hyperpolarization

Question 20

What is the basis for membrane potential?

Answer 20

1) Electrochemical gradient for each ion: Eion, the ionic equilibrium potential
[Na+]o&raquo_space; [Na+]i [K+]o &laquo_space;[K+]I Nernst Equation: ENa ≈ +60 mV
Ek ≈ -90 mV
2) Ionic membrane permeability: How easy is it for an ion to cross the cell membrane?

Question 21

Electrochemical gradients

Answer 21

• K+ “wants” to exit the cell
– If the membrane were permeable to K+ only:
Vm = Ek ≈ -90mV
• Na+ “wants” to enter the cell
– If the membrane were permeable to Na+ only: Vm = ENa ≈ +60mV

Question 22

ionic permeability

Answer 22

Permeability is determined by ion channels in the membrane- are they open or closed?

Question 23

resting membrane potential

Answer 23

Vm is determined by Ek, ENa, and the relative membrane permeability to K+ and Na+
(PR = PNa / PK)

• Since the membrane is much more permeable to K+ than Na+, Vm is much closer to Ek: Vm ≈ -70mV

Question 24

resting membrane potential is

Answer 24

-70mV

Question 25

What if na+ channels open?

Answer 25

1. Relative permeability to NA increase 2. Na+ enters the cell (inward current) 3. Vm moves close to Ena, ~60mV

Question 26

in reality, small initial depolarization causes

Answer 26

specialized Na+ channels to open

Question 27

depolarization means

Answer 27

the membrane potential becomes less negative

Question 28

some _____ open in response to depolarization

Answer 28

voltage gated Na+ channels

Question 29

Does Vm exceed a critical threshold: Is inward flow of Na+ > outward flow of K+?

Answer 29

– If no: Vm returns to resting potential | – If yes: More Na+ channels open, causing further depolarization

Question 30

positive feedback loop leads to

Answer 30

all or none AP more Na+ channels open--> more depot --> more Na+ channels open

Question 31

What causes the depolarization in the first place?

Answer 31

initial depolarization: synaptic transmission

Question 32

Why doesn't the cell stay depolarized?

Answer 32

1. depol is transient, 2. voltage gated Na+ channels inactivate and Na+ stops entering the cell 3. Also K+ channels open and K+ exits the cell, so Vm is pulled back to Ek

Question 33

depol is transient

Answer 33

1. voltage gated Na+ channels inactivate - -Na+ stops entering the cell 2. Voltage gated K+ channels open - -Channel opening is delayed from when the threshold is reached - -K+ exits the cell 3. Vm is pulled bck to Ek

Question 34

Return to resting potential

Answer 34

1. voltage gated channels close - -Overshoot/ hyperpol/ afterhyperpol 2. Relative permeability to Na+ and K+ returns to resting baseline

Question 35

What is the role of the Na+/K+ pump?

Answer 35

it is responsible for concentration gradients of Na and K inside and outside the cel and the potential difference between the two

Question 36

refractory period is

Answer 36

1. short period (3ms) rolling return to baseline which initiation of anther AP is difficult or impossible 2. some Na+ channels are inactivated and some K+ channels are still open

Question 37

absolute refractory period

Answer 37

AP initiation is impossible

Question 38

Relative refractory period

Answer 38

AP initiation is less likely

Question 39

function consequence of Refractory period

Answer 39

limits the max possible firing rate of the cell

Question 40

Effect of hyperkalemia on AP

Answer 40

Ek increases -> Membrane is depolarized -> More action potentials are generated(?)

Question 41

More action potentials are generated in hypekalemia?

Answer 41

No: it is more difficult to generate an action potential - because Na+ channels are inactivated

Question 42

summary of AP stages

Answer 42

1) Small depolarization to threshold 2) Large depolarization: Voltage-gated Na+ channels open 3) Repolarization: Na+ channels inactivate; K+ channels open 4) K+ channels close; concentrations return to baseline levels

Question 43

Shape of AP is determined by

Answer 43

ion concentrations and conductances

Question 44

Action potentials are affected by:

Answer 44

– Ionic concentrations – Membrane potential – Relative membrane permeability – Specialized voltage-sensitive channels

Question 45

Action potential generation is the output of

Answer 45

a computation that can be communicated with downstream cells

Question 46

Describe the positions of the activation and inactivation gates in sodium channels during an action potential.

Answer 46

1) At rest: activation gate (m) is shut and inactivation gate (h) is open 2) As the membrane depolarizes: activation gate (m) opens as inactivation gate (h) closes •activation gate swings faster than the inactivation gate 3) At peak of action potential: activation gate (m) and inactivation gate (h) are open 4) As membrane repolarizes: activation gate (m) is closing and inactivation gate (h) is closed 5) Refractory period: activation gate (m) and inactivation gate (h) are closed

Question 47

intracellular concentrations of sodium and potassium do not change much after

Answer 47

a single action potential.

Question 48

During a single action potential,

Answer 48

thousands of sodium ions enter the axon through the sodium channels and thousands of potassium ions exit through potassium channels during repolarization.

Question 49

In most cells, the influx and efflux of potassium and sodium, respectively,

Answer 49

do not significantly impact the overall concentrations of the ions.

Question 50

_____ before the Na+/K+ pump is required to restore proper ion balances

Answer 50

many AP's must be generated

Question 51

The Na+/K+ pump does not play a direct role in the _______. Instead, the Na+/K+ pump functions to _________

Answer 51

generation of an action potential. restore ion balances after many action potentials have been fired

Question 52

The refractory period is the

Answer 52

time after an action potential is fired during which an axon cannot fire another action potential (absolute refractory period) or requires a more extreme stimulus to fire another action potential (relative refractory period).

Question 53

The number of action potentials that can be fired before Na+/K+ pump activity is required depends on _____

Answer 53

an axon’s surface area relative to its volume.

Question 54

An axon with a relatively small surface area compared to volume can fire

Answer 54

more action potentials before pump activity is needed than an axon with a large surface area to volume ratio.

Question 55

A smaller surface area to volume ratio means

Answer 55

less influx or efflux per unit volume, resulting in a smaller percent change in ion concentrations.

Question 56

The refractory period exists as a result of two mechanisms:

Answer 56

1. after an axon depolarizes, both he activation gate (m) and inactivation gate (h) of the sodium channel are closed. The inactivation gate requires time to reopen before another activation potential can fire 2. After an axon depolarizes, the K+ channels are still open. K+ is still leaving the axon, which makes it more difficult to depolarize

Question 57

Accomodation refers to a

Answer 57

nerve cell’s loss of excitability as a result of applying a stimulus gradually, rather than all at once.

Question 58

During normal depolarization (in the absence of accommodation), the inactivation gate of the sodium channel is ______ This allows sodium to ______.

Answer 58

closing as the activation gate is opening. rush into the axon and generate the action potential

Question 59

With accommodation, the stimulus is applied slowly, _______. As a result, sodium cannot enter the cell and an

Answer 59

causing the inactivation gate to close before the activation gate opens action potential is not generated.

Question 60

Threshold is the

Answer 60

membrane potential at which sodium and potassium influx and efflux, respectively, are equal. threshold is the membrane potential at which sodium and potassium currents are equal and opposite.

Question 61

Threshold is achieved by a

Answer 61

slight initial depolarization of the axon.

Question 62

The threshold must be reached in order for an

Answer 62

action potential to be fired.

Question 63

Once threshold is reached, the potential can either be

Answer 63

tipped in the direction of sodium or potassium equilibrium potentials.

Question 64

Because sodium channels are voltage gated, a momentary, slight influx of sodium will cause

Answer 64

the potential to tip in favor of the sodium equilibrium potential, causing more channels to open. Sodium will then rush into the cell and an action potential is generated.

Question 65

An action potential is generated due to ______ This is an explosive, ______ mechanism because sodium channels are voltage gated.

Answer 65

depolarization of an axon caused by sodium influx. positive-feedback

Question 66

Once an axon is depolarized to threshold,

Answer 66

1. voltage gated sodium channels begin to open 2. sodium moves into the cell with further depolarizes the cell 3. Further depolarization causes more voltage gated sodium channels to open

Question 67

This positive feedback (more channels opening in response to increased influx of sodium) repeats, resulting in

Answer 67

a rapid depolarization and the firing of an action potential