CHAPTER 5

Question 1

Within a millisecond or so, the
potential difference between the inside and outside, called
the

Answer 1

Diffusion potential

Question 2

becomes great enough to block further net potassium diffusion to the exterior, despite the high
potassium ion concentration gradient.

Answer 2

Diffusion potential,

Question 3

becomes great enough to block further net potassium diffusion to the exterior, despite the high potassium ion concentration gradient. In the normal mammalian nerve fiber,
-the potential difference required is about 94 millivolts, with negativity inside the fiber membrane.

Question 4

Diffusion of the positively charged sodium
ions to the inside creates a membrane potential of opposite polarity to that in, with negativity outside
and positivity inside.

Question 5

The diffusion potential level across a membrane that exactly opposes the net diffusion of a particular ion through the membrane is called the

Answer 5

Nernst potential

Question 6

The magnitude of this Nernst potential is determined by
the –
of the concentrations of that specific ion on the two sides of the membrane.

Answer 6

ratio

Question 6

The greater this ratio, the
greater the tendency for the ion to diffuse in one direction, and therefore the greater the Nernst potential required
to prevent additional net diffusion.

Question 6

Can be used to calculate
the Nernst potential for any univalent ion at normal body temperature of 98.6°F (37°C):

Answer 6

Nernst equation

Question 7

When a membrane is permeable to several different ions, the diffusion potential that develops depends on three factors:

Answer 7

(1) the polarity of the electrical charge of each ion,
(2) the permeability of the membrane (P) to each ion, and
(3) the concentrations (C) of the respective ions on the inside (i) and
outside (o) of the membrane.

Question 8

Formula, gives the calculated membrane potential on the inside of the membrane when two univalent positive
ions, sodium (Na+) and potassium (K+), and one univalent negative ion, chloride (Cl−), are involved.

Answer 8

Goldman equation, or the Goldman-HodgkinKatz equation,

Question 9

Is placed in the extracellular fluid,
and the potential difference between the inside and outside of the fiber is measured using an appropriate voltmeter

Answer 9

“indifferent electrode,”

Question 10

is a highly sophisticated electronic apparatus that is capable of measuring small voltages despite extremely high resistance to electrical flow through the tip of the micropipette, which has a lumen diameter usually less than 1 micrometer and a resistance more than a million ohms.

Answer 10

voltmeter

Question 11

Then, as the recording electrode passes through the voltage change area at the
cell membrane (called the

Answer 11

Electrical dipole layer

Question 12

Note that this is an electrogenic pump because more positive charges are pumped to the outside than to the inside(three Na+ ions to the outside for each two K+ ions to the inside), leaving a net deficit of positive ions on the inside; this causes a negative potential inside the cell membrane.

Question 12

in the nerve membrane through which potassium can leak even in a resting cell.

Answer 12

“tandem pore domain,”
potassium channel, or
potassium (K+) “leak” channel,

Question 13

Nerve signals are transmitted by

Answer 13

Action potentials

Question 14

Which are rapid changes in the membrane potential that spread rapidly along the nerve fiber membrane.

Answer 14

action potentials

Question 15

This is the resting membrane potential before the action potential begins. The membrane is said to be “polarized” during this stage because of the −90
millivolts negative membrane potential that is present.

Answer 15

Resting Stage

Question 16

At this time, the membrane suddenly becomes permeable to sodium ions, allowing tremendous numbers of positively charged sodium ions to diffuse to the interior of the axon.

Answer 16

Depolarization Stage

Question 17

immediately neutralized
by the inflowing positively charged sodium ions, with the potential rising rapidly in the positive direction

Answer 17

depolarization

Question 18

after the membrane becomes highly permeable to sodium ions, the sodium channels begin to close and the
potassium channels open more than normal

Answer 18

Repolarization Stage.

Question 19

Then, rapid diffusion of potassium ions to the exterior re-establishes the normal negative resting membrane potential

Answer 19

repolarization

Question 20

The necessary actor in causing both depolarization and
repolarization of the nerve membrane during the action
potential is the

Answer 20

voltage-gated sodium channel

Question 21

also plays an important role in
increasing the rapidity of repolarization of the membrane

Answer 21

voltage-gated potassium channel

Question 22

This channel
has two gates—one near the outside of the channel called

Answer 22

activation gate

Question 23

during this state, sodium ions can pour inward through
the channel, increasing the sodium permeability of the
membrane as much as 500- to 5000-fold

Answer 23

activated state

Question 23

and another near the inside called the

Answer 23

inactivation gate

Question 24

which is used to measure flow of ions through the
different channels.

Answer 24

voltage clamp

Question 24

-For instance, the sodium channels can be blocked by a toxin called
-by applying it to the outside
of the cell membrane where the sodium activation gates
are located.

Answer 24

tetrodotoxin

Question 25

Therefore, a sudden increase
in the membrane potential in a large nerve fiber from −90 millivolts up to about −65 millivolts usually causes the explosive development of an action potential. This level of −65 millivolts is said to be the

Answer 25

threshold for stimulation

Question 25

In fact, the calcium ion
concentration needs to fall only 50 percent below normal before spontaneous discharge occurs in some peripheral
nerves, often causing muscle

Answer 25

muscle “tetany

Question 25

tetraethylammonium ion

Answer 25

Blocks the potassium channels when it is applied to the interior of the nerve fiber.

Question 26

This transmission of the depolarization process along a nerve or muscle fiber is

Answer 26

nerve or muscle impulse

Question 27

Once an action potential has been elicited at any point on the membrane of a normal fiber, the depolarization process travels over the entire membrane if conditions are right, or it does not travel at all if conditions are not right. This is called the

Answer 27

all-or-nothing principle

Question 28

when the spread of depolarization stops?

Answer 28

action potential reaches a
point on the membrane at which it does not generate sufficient voltage to stimulate the next area of the membrane.

Question 28

Therefore, for continued propagation of an impulse to occur, the ratio of action potential to threshold for excitation must at all times be greater than 1. This “greater than 1” requirement is called the

Answer 28

“safety factor” for propagation

Question 28

This type of action potential occurs in

Answer 28

heart muscle fibers

Question 29

The cause of the plateau is a combination of several factors. First, in heart muscle, two types of channels enter into the depolarization process:

Answer 29

(1) the usual
voltage-activated sodium channels, called fast channels, and
(2) voltage-activated calcium-sodium channels, which are slow to open and therefore are called
slow channels.

Question 30

These rhythmical discharges cause:

Answer 30

These rhythmical
discharges cause

Question 31

This is not enough negative voltage to keep the sodium and calcium channels totally closed. Therefore, the following sequence occurs:

Answer 31

(1) some sodium and
calcium ions flow inward;
(2) this increases the membrane
voltage in the positive direction, which further increases
membrane permeability;
(3) still more ions flow inward
(4) the permeability increases more, and so on, until an action potential is generated.

Question 32

This continues for nearly a second after the preceding action potential is over, thus drawing the membrane potential nearer to the potassium Nernst potential. This is a state called

Answer 32

Hyperpolarization

Question 33

But the increased potassium conductance (and the state of hyperpolarization) gradually disappears, as shown after
each action potential is completed in the figure, thereby allowing the membrane potential again to increase up to the

Answer 33

threshold

Question 34

The large fibers are

Answer 34

myelinated

Question 35

and the small ones are

Answer 35

unmyelinated

Question 36

A typical myelinated fiber. The central
core of the fiber is the

Answer 36

axon

Question 36

The axon is filled in its center with a
-which is a viscid intracellular fluid.

Answer 36

axoplasm

Question 36

Surrounding the axon is a

Answer 36

myelin sheath

Question 36

About once every 1 to 3 millimeters along the length of the myelin sheath is a

Answer 36

node of Ranvier

Question 37

The myelin sheath is deposited around the axon by

Answer 37

Schwann cells

Question 38

Then the Schwann cell rotates around the axon many times, laying down multiple layers of Schwann cell membrane containing the lipid subtance

Answer 38

sphingomyelin

Question 39

Yet the action potentials
are conducted from node to node, as
this is called

Answer 39

saltatory conduction

Question 40

Thus, the nerve impulse jumps along the fiber, which is the origin of the term

Answer 40

“saltatory.”

Question 40

mechanical pressure to excite

Answer 40

nerve endings in the skin

Question 41

chemical neurotransmitters to

Answer 41

transmit signals from one neuron to the next in the brain

Question 41

Electrical current to transmit signals between

Answer 41

successive muscle cells in the heart and intestine.

Question 41

These local potential changes are called

Answer 41

Acute local potentials

Question 42

and when they fail to elicit an action potential, they are called

Answer 42

acute subthreshold potentials

Question 42

Now the local potential has barely reached the level required to elicit an action potential, called the

Answer 42

threshold level

Question 43

The period during which a second action potential cannot be elicited, even with a strong stimulus, is called the

Answer 43

Absolute refractory period

Question 44

In contrast to the factors that increase nerve excitability, still others, called

Answer 44

membrane-stabilizing factors, can
decrease excitability

Question 44

decreases membrane permeability to sodium ions and simultaneously reduces excitability

Answer 44

high extracellular fluid calcium ion concentration

Question 45

Among the most important stabilizers are the many substances used clinically as local anesthetics, including p

Answer 45

procaine and tetracaine

Question 46

The cathode ray tube itself is composed
basically of an

Answer 46

electron gun and a fluorescent screen

Question 46

When excitability has been reduced so low that the ratio of action potential
strength to excitability threshold (called the

Answer 46

“safety factor”