Exam 2: Ch5 Book Since Exam 1

Question 1

every cell that is in a nonexcited or “resting” state has…

Answer 1

a potential difference (Vrest) across the membrane

usually lies between -20 and -100 mC depending on the kind of cell and ionic envionment

Question 2

2 factors govern resting potential

Answer 2

presence of open selective ion channels

unequal extracellular and intracellular distribution of ions maintained by active transport (allows membrane potential)

Question 3

ions influence the ________ across a membrane roughly in proportion to the _______ of the membrane to each ion species

Answer 3

potential, permeability

Question 4

if a membrane is only permeable to one ion, will the distribution of that ion dictate the membrane potential?

Answer 4

yes, use Nernst equation

Question 5

Vrest in frog skeletal muscle

Answer 5

-90 to -100

Question 6

resting potential depends on which ions

Answer 6

Mostly K and some Na

Question 7

resting potentials of muscle, nerve, and other cells are more sensitive to changes in [ ]… than the concentrations of other _____

Answer 7

[K] out, cations

consistent finding with high permeability of plasma membranes to K, compared to other cations

Question 8

why do large changes in [Na] out have ______ effect on Vrest

Answer 8

little

b/c resting membrane is relatively impermeable to Na

Question 9

how to calculate force acting on an ion

Answer 9

emf(x) = Vm - E(x)

depends on how different membrane potential is from equilibrium potential

Question 10

based on [ ] gradient, there is a steady influx of which ion into the cell if not maintained by active transport?

Answer 10

Na, large electric driving force (extra: 120mM; intra: 10mM)

would accumulate inside cells if not removed, depolarizing the cell

Question 11

if Na were allowed to leak in at rest, what would happen to K

Answer 11

it would leak out b/c internal negatively less able to hold K in

K high intracell, low extracell

Question 12

which pump contributes to Vrest and how

Answer 12

Na/K pump (3 Na out / 2K in)

indirectly maintains high internal [K]

directly (very small effect) moves net pos charge out of cell

Question 13

if you inhibit Na/K pump…

Answer 13

concentration gradients diminished

Na goes in, K eventually goes out and no Vrest b/c equilibrium will be reached

Question 14

what is an action potential

Answer 14

a large, brief change in Vm that is propagated along an axon without decrement

all or none

Question 15

3 key elements to producing an AP

Answer 15

active transport of ions to maintain [ ] gradients

[ ] gradient generates an electrochemical gradient that provides a reservoir of potential energy

electrochemical gradient drives ions across membrane when selective channels open (changes Vm)

Question 16

what two voltage-gated ion channels are responsible for all the features of the AP

Answer 16

voltage gated Na and K

Question 17

APs are generated by the membranes of ….

Answer 17

neurons

muscle cells

receptor cells, secretory cells, and some single-celled animals

Question 18

local response

Answer 18

when a depolarization stimulus brings the membrane almost to threshold, there is an abortive, non-propagated excitation

beginning of an AP that died out before it reached threshold

Question 19

threshold current

Answer 19

intensity of depolarizing stimulus that is just sufficient to bring membrane to threshold and elicit an AP

Question 20

most neurons have threshold currents between…

Answer 20

-30 and -50 mV

Question 21

what does regenerative mean

Answer 21

self-perpetuating AP

Question 22

overshoot

Answer 22

brief period when interior of membrane is positive

Question 23

after-hyperpolarization

Answer 23

transient period when Vm is more negative than Vrest

Question 24

refractory

Answer 24

for a short time after an AP, it is difficult or impossible to trigger another AP

absolute refractory period

relative refractory period

Question 25

absolute refractory period

Answer 25

if 2nd stimulus is delivered during an AP or immediately after, no AP is triggered

Question 26

relative refractory period

Answer 26

slightly after the absolute refractory period, if stim is delivered an AP may be triggered but stim must be stronger than usual

amplitude of AP may be smaller (breaks all or none rule)

Question 27

threshold potential decreases during the _____ refractory period until the membrane returns to its normal resting state

Answer 27

relative

Question 28

what happens when a neuron is stimulated with a series of subthreshold depolarization

Answer 28

excitability progressively decreases

the stronger the series of subthreshold currents, the stronger they actually need to be to reach threshold

Question 29

accommodation

Answer 29

the phenomena that the slower the rate of increase in the intensity of the stimulating current, the greater the increase in threshold potential

caused by time-dependent changes in the way membrane channels respond to depol

Question 30

can some neurons accommodate rapidly?

Answer 30

yes

some generate 1-2 APs (phasic response)

some fire repeatedly but with decreasing frequency (tonic response)

Question 31

adaptaion

Answer 31

the decrease in frequency of APs seen in a neuron that responds tonically during sustained stimulus

visualized as increasing distance between APs

Question 32

what causes the rapid depolarization of an AP

Answer 32

inward Na current caused by a sharp increase in Na conductance (rising phase)

opening of voltage-gated Na channels

Question 33

when voltage-gated Na channels close and permeability of Na drops (peak of AP), what happens to K

Answer 33

memb. much more permeable to K b/c voltage-gated K channels are still open

falling phase

Question 34

why do animals have giant axons

Answer 34

rapid conduction of AP

escape response

Question 35

what did Hodgkin and Huxley discover about resistance and capacitance

Answer 35

membrane resistance during an AP decreases, but capacitance remains constant

therefore, change in conductance is entirely responsible for change in ionic current

Question 36

what did Hodgkin and Huxley discover about voltage during AP

Answer 36

goes past 0 toward ENa of nernst equation (58 mV in squid giant axon)

when Na removed (artificial seawater), magnitude of AP decreased as more Na removed

Question 37

why use a squid giant axon (1mm in diameter)

Answer 37

thin electrode wires could be threaded longitudinally

Question 38

equation for current carried by ionic species

Answer 38

I(x) = g(x)*(Vm-E(x))

I(x) = current carried by ionic species x

g(x) = membrane conductance for X (proportional to # of open channels selective for x)

V(m) = membrane potential

E(x) = equilibrium potential of x

Question 39

what does equation for current carried by ionic species demonstrate

Answer 39

there is only an ionic current across the memb. if there is a force driving the movement of ion x, and a conductance for x

if g(x) or emf acting on X is -0, there is no I(x)

Question 40

what does it mean if g(x) is 0

Answer 40

no channels are open

Question 41

changes in ionic ________ play a major role in controlling the electric currents carried across biological membranes

Answer 41

conductance

Question 42

4 pieces of evidence that Na is the major ionic species responsible for AP

Answer 42

[Na] out exceeds [Na] by ~10, ENa ~55-60 mV so emf acting on Na is large and drives Na into cell

entry of pos charged Na produced pos shift in Vm

overshoot of AP approaches ENa

magnitude of overshoot changes as a function of [Na] out (artificial seawater)

Question 43

voltage clamping

Answer 43

method used on giant squid axon allows you to change the Vm abruptly to any value, and hold it there

can therefore measure ionic current flowing across memb.

Ohms law to calculate conductance

Question 44

when Hodgkin and Huxley abruptly depolarized an axonal memb. from its resting level they were able to measure…

Answer 44

initial transient inward current of Na to drive rising phase followed by a sustained outward current of K for falling phase

Question 45

voltage clamping method

Answer 45

electronic feedback system holds voltage difference across plasma membrane at a constant

electrode inserted into neuron and control amplifier compares potential across membrane

use solutions of different ion [ ] or agents that block particular ion channels

Question 46

tetrodotoxin (TTX) pufferfish fugu

Answer 46

blocks voltage-gated Na channels (know from radiolabel)

Question 47

are there many voltage gated sodium channels in cell memb

Answer 47

no, makes up small surface area

however, 10^7 Na ions or more per second can pass through

Question 48

unitary current

Answer 48

current through a single channel

Question 49

which technique elucidated how voltage gated Na channels operate

Answer 49

patch-clamping

Question 50

gating current

Answer 50

I(g)

small measurable current associated in time with opening and closing of Na channels

Question 51

mechanism of voltage gated Na channels

Answer 51

gating takes place with movement of charges

activation and inactivation are coupled processes

Question 52

selectivity of channels

Answer 52

determine relative permeability to each ion

channel filters partly by size, net charge, and ease of dehydration

selectivity filter: part of channel that determines selectivity

Question 53

positive feedback loop during AP (Hodgkin’s cycle)

Answer 53

Na channels respond to depol but opening, allowing Na to enter cell, which depol it more

this extra depol causes more channels to open, allowing even more Na to enter causing an explosive regenerative event

once AP started it needs no more stimulus to continue

Question 54

limits of Hodgkin’s cycle

Answer 54

as membrane potential approaches ENa, driving force on Na is reduce so less is driven into the cell

open Na channels become inactivated after a short time even if depol is maintained

Question 55

how are Na channels opened

Answer 55

activation gate closed and inactivating particle located away from channel

when voltage clamped to depol, activation gate opens and inward current is maxed when most Na channels are open

as depol continues, inactivating particle blocks channels

after membrane repolarization, inactivation particle unblocks channels and activation gate closes (memb. must be inside-negative)

Question 56

what is patch-clamping

Answer 56

variation of voltage clamping that allows currents through a single membrane channel to be recorded

Question 57

do voltage gated K channels respond to changes more quickly or slowly than Na channels

Answer 57

slowly

Question 58

when does membrane conductance of K change in an AP

Answer 58

only near its peak (very slow rise before then)

remains elevated during falling phase

Question 59

during after-hyperpolarization, membrane potential moves even closer to ___

Answer 59

E(K) equilibrium potential of K

Question 60

are voltage gated K channels needed to generate APs?

Answer 60

no

Question 61

benefit of rapid membrane repolarization produced by current through K channels

Answer 61

shortens AP, but allows neurons to generate them with higher frequency

Question 62

how do glial cells help maintain Vrest

Answer 62

remove K from extracellular fluid surrounding axons

slowly release it allowing it to be reclaimed by neurons

Question 63

lidocaine blocks

Answer 63

Na channels

dentists use for pain… no signals conducting

Question 64

K channel structure

Answer 64

each gene codes for a subunit (4) monomeric alpha arranged around a central pore

4 beta subunits associated with each channel modulate function

Question 65

Na channel structure

Answer 65

1 large alpha protein with 4 same transmembrane domains

central pore

Question 66

Ca channels

Answer 66

help depol w/ inward current a little

open more slowly and conduct less current

Question 67

Ca can act as a

Answer 67

intracellular messenger