Action Potential SDL

Question 1

What is the job of the neuronal AP?

Answer 1

to initiate release of neurotransmitters at the synapse

Question 2

What has to be reached in order for an AP to initiate and propagate along the axon?

Answer 2

threshold

Question 3

What are the 5 phases of an AP?

Answer 3

rising phase, overshoot, peak, falling phase, and after hyperpolarization

Question 4

Which phase of the AP lasts the longest?

Answer 4

the after hyper polarization phase

Question 5

What is the resting membrane potential?

Answer 5

-70 mV

Question 6

What causes the rising phase of the AP?

Answer 6

influx of Na+ ions

Question 7

What is the threshold (in mV)?

Answer 7

about 15 mV above the resting MP (-55 mV)

Question 8

What happens at threshold?

Answer 8

some voltage-gated Na+ selective channels open (activate), increasing the permeability of the membrane to Na+

Question 9

What does influx of Na+ do to the membrane?

Answer 9

depolarizes the membrane, causing more Na+ channels to open (positive feedback condition)

Question 10

The fast voltage-gated Na+ channels possess what two gates?

Answer 10

an activation/deactivation gate and an inactivation/deinactivation gate

Question 11

An increase in K+ permeability begins when?

Answer 11

during the rising phase of the AP (it increases more slowly than that for Na+ and continues after Na+ channel inactivation)

Question 12

The efflux of K+ (caused by the positive MP) results in what phase and consequence?

Answer 12

the falling phase resulting in repolarization, it overshoots the resting potential causing the after hyperpolarization

Question 13

What is the equilibrium potential of K+?

Answer 13

-90 mV

Question 14

What is the equilibrium potential for Na+?

Answer 14

+60 mV

Question 15

What toxin blocks voltage-gated Na+ channels?

Answer 15

Tetrodotoxin (found in puffer fish)

Question 16

What ions block voltage-gated K+ channels?

Answer 16

Tetraethylammonium (TEA)

Question 17

Why are axons and dendrites considered poor conductors?

Answer 17

their high internal resistance (ions moving in a water environment) rapidly dissipates the energy of an ionic current

Question 18

What happens to potentials, depolarizing or hyper polarizing, that do not reach threshold?

Answer 18

they are passively conducted along the axon and die out within a few millimeters

Question 19

Longer distance transmission of APs requires a system which _____ the AP along the axon.

Answer 19

regenerates

Question 20

At threshold, voltage-gated Na+ channels open and Na+ rushes in causing ____ _____?

Answer 20

eddy currents

Question 21

How is the AP propagated?

Answer 21

entering Na+ ions are attracted to nearby negative regions, this depolarizes the nearby membrane to threshold generating an AP, the Na+ ions entering from this AP evoke more eddy currents which depolarize to threshold the adjacent membrane

Question 22

In what kind of fibers is conduction slow because the AP is generated sequentially on every piece of the membrane?

Answer 22

unmyelinated fibers

Question 23

Why is propagation of APs in myelinated fibers faster?

Answer 23

because the eddy currents skip the internal segment and depolarize only the membrane at the node where the voltage-gated Na+ channels are congregated

Question 24

What is saltatory conduction?

Answer 24

(saltare = to leap), the AP skips from node to node

Question 25

Where are the voltage-gated K+ channels in myelinated nerve fibers?

Answer 25

they are sequestered under the myelin, so they normally have no part in the AP

Question 26

In the case of myelinated nerve fibers, repolarization of the AP is due to what?

Answer 26

K+ leakage channels that are not actively opened (thus no after-hyperpolarization)

Question 27

What is the benefit of not having after-hyperpolarization?

Answer 27

an axon can conduct AP's at a faster rate

Question 28

What is the danger in demyelinating diseases, such as MS?

Answer 28

As the AP in the myelinated part of the nerve fiber enters the unmyelinated region, its velocity greatly decreases or the AP ceases because it doesn't have enough energy to continue

Question 29

What is an absolute refractory period?

Answer 29

during the spike part of the AP, the membrane cannot initiate another AP

Question 30

What is the relative refractory period?

Answer 30

during the early part of the after-depolarization, only some of the inactivated Na+ channels are reset and all K+ channels are still open, so another AP can be initiated if the stimulus is increased in strength

Question 31

The after hyperpolarization phase corresponds to what?

Answer 31

the subnormal period

Question 32

What is happening during the subnormal period?

Answer 32

K+ channels are still open and the membrane potential is near the equilibrium potential for K+, but another AP can be elicited by a stimulus if it is large enough to shift the MP to threshold (voltage-gated Na+ channels are reset by this time)

Question 33

The normal conduction direction for the AP, away from the soma

Answer 33

orthodromic conduction

Question 34

conduction of the AP toward the soma

Answer 34

antidromic conduction

Question 35

What is the benefit of refractory periods?

Answer 35

they insure the forward progress of the AP

Question 36

Where is the AP normally initiated?

Answer 36

the axon hillock

Question 37

What site has the largest concentration of voltage-gated Na+ channels?

Answer 37

the axon hillock

Question 38

What will happen if a nerve fiber is electrically stimulated somewhere along its length?

Answer 38

APs will conduct in both directions but they cannot reverse directions

Question 39

What are electrotonic potentials?

Answer 39

changes in the membrane potential which occur in a small region of the cytoplasmic membrane

Question 40

the amplitude of the response is proportional to what?

Answer 40

the amplitude of the stimulus

Question 41

All responses, whether de- or hyperpolarizing, that do not move the membrane potential to threshold are called what? they do not propagate and dissipate quickly.

Answer 41

electrotonic potentials

Question 42

More positive charges on one side of the cytoplasmic membrane attract more ____ charges on the other side, hence it behaves like an electrical capacitor

Answer 42

negative

Question 43

The change in membrane potential can be either positive (depolarizing) or negative (hyperpolarizing) without what?

Answer 43

without actual flow of ions across the membrane

Question 44

Actual current across the membrane only occurs when...?

Answer 44

when the permeability (due to opening of channels) of the membrane changes

Question 45

If the cathode (negative pole) of an electrical source, such as a battery, is placed near a nerve fiber and a brief stimulus is applied, the nerve fiber adjacent to the cathode will be _____.

Answer 45

depolarized (+) (this is due to the attraction of positive ions toward the membrane internally depolarizing it)

Question 46

Electrical stimulation of nerve fibers in a peripheral nerve produces a _____ AP, the summation of many APs in individual axons.

Answer 46

compound AP

Question 47

Since larger diameter fibers have faster conduction velocities, when do their peaks arrive at the recording electrode?

Answer 47

they arrive first (alpha type), followed by peaks of slower conducting groups of axons

Question 48

The largest diameter fibers have the ____ threshold for electrical stimulation, and the smallest fibers have the ____ electrical threshold.

Answer 48

lowest; highest

Question 49

An unhealthy nerve fiber has slowed conduction velocity and is said to be affected by a form of _____.

Answer 49

neuropathy

Question 50

If the MP remains above threshold, the neuron is considered to be in ______ _____. this occurs due to hypoxia or anoxia.

Answer 50

depolarization block

Question 51

Does the soma of a neuron or a nerve fiber tolerate anoxia for longer periods of time?

Answer 51

nerve fiber

Question 52

If the MP is maintained below the resting potential level, the neuron is considered to be in ______ _____.

Answer 52

hyperpolarization block

Question 53

What do local anesthetics block?

Answer 53

voltage-gated Na+ channels

Question 54

What do electrical currents do when you cut a nerve fiber?

Answer 54

electrical currents flow across a gap in the membrane