SAKMANN AND NEHER INVENT PATCH-CLAMPING

Question 1

A method to measure the behavior of a single ion channel

Answer 1

SAKMANN AND NEHER

PATCH-CLAMPING

Question 2

Erwin Neher and Bert Sakmann invented a new technique called patch
clamping, which allowed them to study the

Answer 2

behavior of a single ion channel

Question 3

1991 Nobel Prize in Medicine or Physiology

Answer 3

Neher and

Sakmann

Question 4

The ion channel is

inserted in the

Answer 4

membrane that is

in the lumen of the electrode,

Question 5

However, whether
open for a longer or shorter period, the conductance of the Na channel (the amount of current passed
during any instant in time) is always the same. T/F

Answer 5

True

Question 6

individual channel opens and closes

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very rapidly

Question 7

channels have ___ conductances

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unitary

Question 8

If the same depolarizing pulse was presented a
second time, or a third or fourth time, the same inward current pattern was evoked. This pattern
of current flowing through the entire axon is called the

Answer 8

macroscopic current

Question 9

patterns of inward current evoked in a single Na channel, called the

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microscopic current,

Question 10

The opening and closing of an individual channel was variable because the behavior of any
channel is

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probabilistic.

Question 11

What the membrane potential does is to change the ___that a

channel will open.

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probability

Question 12

In the case of the Na channel, depolarization increases the probability that the

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activation gate will open.

Question 13

hyperpolarization

increases the probability of

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closing the Na activation gate

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depolarization increases the probability that the inactivation gate will

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close

Question 15

hyperpolarization

increases the probability that the inactivation gate will

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open

Question 16

How then do we reconcile the stereotyped current response of the squid giant axon to a
depolarization of -10 mV, the shape of the macroscopic current, with the highly variable
response of an individual Na channel, the microscopic current, to exactly the same
depolarization?

Answer 16

squid giant
axon during voltage clamp shows the summated (or averaged) currents through many ion
channels in the entire membrane of the axon rather than the current through a single channel.

Question 17

was simple to calculate

the probability of a Na channel opening as a function of

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membrane potential

Question 18

function of membrane potential. That function is
shown in Fig. 4E, and has a \_\_\_shape

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sigmoidal

Question 19

inward currents are

shown as

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downward deflections

Question 20

Current through a single

channel is called the

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microscopic

current

Question 21

The sum of many such
microscopic currents shows that most
channels open in the initial\_\_\_after
which the probability of channel
openings diminishes because of channel
inactivation.

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1-2 ms,

Question 22

The macroscopic
current resulting from the current
flowing through all of the Na channels
while

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voltage clamping the entire axon

Question 23

is very similar

to the summed microscopic current in C.

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macroscopic current

Question 24

The probability of Na channel

opening depends on the

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membrane

potential,

Question 25

Notice that the probability is virtually zero (few or no channels open) at

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-60 to -80 mV

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probability of channel opening is about 80% even when the membrane is depolarized to

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+40 mV

Question 27

Depolarizing steps cause outward currents which are seen as

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(upward | deflections

Question 28

Notice that the K channel opens with different latencies to the depolarization and that the channels stay open for the length of the

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depolarization,

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the probability-membrane potential function is similar to both the K conductance-membrane potential function and to the Na conductance-membrane potential function T/F

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T

Question 30

Strong hyperpolarization virtually ensures that 52 activation gates are closed, since the probability of a Na or K channel being open at ____ is just about zero (

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-70-80 mV

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the open probability increases | sharply with depolarization at these values

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-40 to 0 mV

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max probability at

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+30 mV.

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What is somewhat surprising however is that the maximum open probability is not 100% but rather only

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70-80%

Question 34

hyperpolarization | guarantees gate closure whereas depolarization only makes gate opening more likely. T/F

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T

Question 35

he probability that the channel can pass Na+ ions decreases after the depolarization

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been present for a period of time due to the closure of the inactivation gate

Question 36

The delayed closure can be appreciated from the records in Fig. 2 and Fig. 4B, since there were no channel openings after the first

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first 8.0 msec or so of the depolarizing pulse.

Question 37

It has been known that if a nerve axon is hyperpolarized for a period of time, an action potential is often set off when the hyperpolarization is released and the cell quickly brought back to rest; this is called

Answer 37

anode break excitation or rebound excitation.

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explanation for rebound excitation is that

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hyperpolarization of the membrane increases | the number of Na channels available for opening in response to depolarization

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why the states of the activation and | inactivation gates are “probabilistic”.

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This “flickering” between states | even when the membrane potential is not changing,

Question 40

The larger number of Na channels open when the membrane potential was brought back to rest allowed a correspondingly

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larger influx of Na+ ions

Question 41

two features of axons that determine velocity

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axon diameter; and 2) myelination

Question 42

larger the axon diameter, the ___ the conduction velocity

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faster

Question 43

The longer the advance of the current, the ____ the action potential propagates

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faster

Question 44

How far the local current spreads down the axon depends upon two features that act as resistances to current flow. They are;

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membrane resistance, and 2) the internal resistance of the axon

Question 45

The membrane resistance is determined simply by the

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number of open ion channels in the membrane. (K+ channels that set the resting potential, that is, the K+ channels that are NOT gated by voltage but are always open.)

Question 46

refers to how difficult it is for current to flow down the axon.

Answer 46

internal resistance

Question 47

The internal resistance refers to how difficult it is for current to flow down the axon. Obviously, the larger the diameter, the ___resistance there is to flow.

Answer 47

less

Question 48

larger axons have a ____ internal resistance, and hence it is easier for current to flow down the axon and depolarize long segments of the axon

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lower

Question 49

the larger the axonal diameter, the ___the membrane resistance and the more current that leaks out along the axon

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smaller

Question 50

membrane resistance acts to oppose the | effects of

Answer 50

internal resistance

Question 51

Thus decreasing membrane resistance ___the time it takes for the axon to reach threshold

Answer 51

lengthens slow conduction velocity

Question 52

The question is, which of the two features dominates, internal resistance or membrane resistance?

Answer 52

internal resistance

Question 53

Membrane resistance is | determined by the

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axonal | circumference

Question 54

internal | resistance is determined by

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area of | the axon.

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changes as the square of the radius and thus changes faster with diameter

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Internal resistance

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Circumference of circle

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2 x pi x r

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area of a circle

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pi x r^2

Question 58

are those that are voltage insensitive, i.e., are not influenced by voltage

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Passive | properties

Question 59

Passive | properties are

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conduction velocity, membrane resistance, internal | resistance and capacitance

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active properties that are voltage sensitive, | such as

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voltage gated Na+ and K+ channels

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electrical component that simply is an insulator that separates and thereby stores charges

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capacitor

Question 62

The quantitative feature of a capacitor is called its

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capacity.

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simply the amount of charge | required to change the potential across the capacitor by 1 millivolt.

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capacity.

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Capacity =

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Capacity = | Q/ V, where Q is charge (coulombs) and V is voltage.

Question 65

So the passive properties are determined by three principal features:

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1) membrane resistance; 2) | membrane capacitance and 3) internal resistance.

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each | patch is connected to the next patch of membrane through the

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internal resistance (ri).

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other words, the charge across capacitor in patch 1 experiences the

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``` greatest change (the largest depolarization). ```

Question 68

Each successive patch of membrane | receives less current than the previous patch because; 1

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1) much of the current has already been used to depolarize the set of capacitors closest to the current source; and 2) some of the current has leaked out in the initial patches through open channels that form the membrane resistance.

Question 69

If the depolarization of the membrane potential at patch 1 is sufficiently large, the signal is prevented from dying out by

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active processes that are voltage dependent

Question 70

nature has done in vertebrates is to add

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myelin sheaths to smaller axons

Question 71

Since all membranes | are made of lipids, the fatty wrapping, called myelin, forms an

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insulation, just

Question 72

The insulation prevents current from leaking out along the length of the insulated portion, i.e., it massively increases

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membrane resistance

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The entire length of the axon is not insulated without | interruption. Rather, there are bare spots located periodically along the axon called

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nodes of Ranvier.

Question 74

All voltage gated Na+ and K+ channels are densely packed at the

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nodes of Ranvier.

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What | happens, in essence, is that the action potential in a myelinated axon

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skips from node to node

Question 76

This type of conduction used by myelinated axons is called

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saltatory conduction

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saltatory conduction Latin saltare,

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hop or leap

Question 78

The largest myelinated axons in our nervous system have diameters of only

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20 micrometers (recall that the diameter of the squid giant axon is 800-1000 micrometers

Question 79

an unmyelinated fiber would have to be___ in diameter to conduct as fast as the most rapid myelinated fibers.

Answer 79

4 mm

Question 80

``` Multiple sclerosis (abbreviated MS) is an inflammatory disease in which the fatty myelin sheaths around the axons of the brain and spinal cord are ```

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damaged or degenerate

Question 81

The name multiple | sclerosis refers to scars (scleroses—better known as plaques or lesions) particularly in the

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white matter | of the brain and spinal cord, which is mainly composed of myelin

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In MS, the body's own immune | system attacks

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oligodendrocytes,

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there are non-voltage gated K+ channels but no voltage-gated | Na+ or K+ channels under the

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myelin sheaths (

Question 84

conduction | velocity in vertebrates is determined by both

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myelination and axon diameter.

Question 85

Our unmyelinated axons have conduction velocities of around

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1 meter/sec,

Question 86

myelinated nerve fibers conduct from

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10-120 meters/sec

Question 87

The recording electrodes are on the surface of the nerve and record the summated action potentials of all the fibers as they travel along the nerve. This recording is

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compound action potential

Question 88

compound action potential has three major components

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first component evoked reflects the arrival of the fastest (largest) myelinated axons and is called the A component. smaller myelinated axons and is called the B component. slowest axons are unmyelinated

Question 89

touch, joint position, | muscle stretch and sharp pain

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A component

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slow pain and temperature | receptors,

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C component. | slowest axons are unmyelinated

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The fastest fibers are called

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Aa fibers

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next fastest are

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Ab fibers

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slowest A fibers are | the

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Ag fibers

Question 94

The nerves of the | peripheral nervous system are

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mixed nerves;

Question 95

action potentials in the | axons of motor neurons move

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orthodromically) toward their muscles in the thumb

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action potentials in the axons of the sensory fibers move | backward

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antidromically

Question 97

the conduction velocity of both sensory and motor myelinated fibers is slowed down, or even abolished, due to demyelination, whereas the conduction velocity of unmyelinated fibers is normal

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MS

Question 98

selective degeneration of motor neurons; sensory neurons are not affected. The selective pathology and death of motorneurons is detectable by observing decreased, slowed, or jittery motor components (at the base of the thumb) with normal sensory components (at the fingertips since no motoneurons are present in the fingertips).

Answer 98

ALS also called Lou Gehrig’s | disease)