Chapter 8

Question 1

Information flow through the nervous system follows the basic pattern
of a reflex

Answer 1

timulus S sensor S input signal S integrating center S

output signal S target S response

Question 2

CNS, which

is t

Answer 2

the integrating center for neural reflexes. CNS neurons integrate
information that arrives from the sensory division of the PNS and
determine whether a response is needed.

Question 3

multipolar

Answer 3

ified by the number of processes
that originate from the cell body. The model neuron that is commonly used to teach how a neuron functions is multipolar, with many
dendrites and branched axons (Fig. 8.2e). Multipolar neurons in the
CNS look different from multipolar efferent neurons

Question 4

Two factors influence the membrane potential:

Answer 4

The uneven distribution of ions across the cell membrane. Normally, sodium (Na+), chloride (Cl-), and calcium (Ca2+) are
more concentrated in the extracellular fluid than in the cytosol. Potassium (K+) is more concentrated in the cytosol than
in the extracellular fluid.
2. Differing membrane permeability to those ions. The resting cell membrane
is much more permeable to K+ than to Na+ or (Ca2+) This makes
K+ the major ion contributing to the resting membrane potential

Question 5

However, an average value for the resting membrane potential of neurons
is -70 mV (inside the cell relative to outside), more positive than
predicted by the potassium equilibrium potential. This means that

Answer 5

other ions must be contributing to the membrane potential. Neurons
at rest are slightly permeable to Na+, and the leak of positive Na+ into
the cell makes the resting membrane potential slightly more positive
than it would be if
the cell were permeable only to K+.

Question 6

The Goldman-Hodgkin-Katz (GHK)

equation calculates

Answer 6

s the membrane potential that results from the
contribution of all ions that can cross the membrane. The GHK
equation includes membrane permeability values because the permeability of an ion influences its contribution to the membrane
potential.

Question 7

The resting membrane potential of living cells is determined
primarily by

Answer 7

the K+ concentration gradient and the cell’s resting

permeability to K+, Na+, and Cl-.

Question 8

A change in either the K+ concentration gradient or ion permeabilities changes

Answer 8

the membrane
potential. If you know numerical values for ion concentrations and
permeabilities, you can use the GHK equation to calculate the new
membrane potential.

Question 9

For example, at rest, the cell membrane of
a neuron is only slightly permeable to Na+. If the membrane
suddenly increases its Na+ permeability,

Answer 9

, Na+ enters the cell, moving down its electrochemical gradient

Question 10

The addition of

positive Na+ to the intracellular fluid does what

Answer 10

depolarizes the cell membrane

and creates an electrical signal. (good graph on pg 272)

Question 11

To appreciate how a tiny change can have a large effect, think
of getting one grain of beach sand into your eye.

Answer 11

There are so
many grains of sand on the beach that the loss of one grain is
not significant, just as the movement of one K+ across the cell
membrane does not significantly alter the concentration of K+.
However, the electrical signal created by moving a few K+ across
the membrane has a significant effect on the cell’s membrane
potential, just as getting that one grain of sand in your eye creates
significant discomfort.

Question 12

The ease with which ions flow through a channel is called the
channel’s

Answer 12

conductance (G)

Question 13

Channel conductance varies with

Answer 13

s with the gating state of the channel and with the channel protein isoform. Some ion channels, such as the K+ leak channels
that are the major determinant of resting membrane potential,
spend most of their time in an open state. Other channels have
gates that open or close in response to particular stimuli.

Question 14

Most

gated channels fall into one of three categories [

Answer 14

Mechanically gated ion channels are found in sensory
neurons and open in response to physical forces such as pressure or stretch.
2. Chemically gated ion channels in most neurons respond
to a variety of ligands, such as extracellular neurotransmitters
and neuromodulators or intracellular signal molecules.
3. Voltage-gated ion channels respond to changes in the
cell’s membrane potential. Voltage-gated Na+ and K+ channels play an important role in the initiation and conduction
of electrical signals along the axon.

Question 15

An inactivated channel returns

to its normal closed state shortly after

Answer 15

the membrane repolarizes.
The specific mechanisms underlying channel inactivation vary
with different channel types.

Question 16

Many channels that open in response to depolarization close

only

Answer 16

when the cell repolarizes. The gating portion of the channel
protein has an electrical charge that moves the gate between open
and closed positions as membrane potential changes. This is like
a spring-loaded door: It opens when you push on it, then closes
when you release it.

Question 17

The speed with which a gated channel opens and closes also
differs among different types of channels. Channel opening to
allow ion flow is called

Answer 17

d channel activation. For example, voltagegated Na+ channels and voltage-gated K+ channels of axons are
both activated by cell depolarization

Question 18

The Na+ channels open

very rapidly, but… and what is the result of this

Answer 18

the K+ channels are slower to open. The result
is an initial flow of Na+ across the membrane, followed later by
K+ flow

Question 19

Ohm’s law

Answer 19

Current flow, whether across a membrane or inside a cell,
obeys a rule known as Ohm’s law [p. A-00]. Ohm’s law says that
current flow (I) is directly proportional to the electrical potential difference (in volts, V) between two points and inversely proportional
to the resistance (R) of the system to current flow: I = V * 1/R
or I = V/R. In other words, as resistance R increases, current
flow I decreases. (You will encounter a variant of Ohm’s law when
you study fluid flow in the cardiovascular and respiratory systems.)

Question 20

resistance

Answer 20

you study fluid flow in the cardiovascular and respiratory systems.)
Resistance in biological flow is the same as resistance in
everyday life: It is a force that opposes flow. Electricity is a form of
energy and, like other forms of energy, it dissipates as it encounters resistance. As an analogy, think of rolling a ball along the
floor. A ball rolled across a smooth wood floor encounters less
resistance than a ball rolled across a carpeted floor. If you throw
both balls with the same amount of energy, the ball that encounters less resistance retains energy longer and travels farther along
the floor

Question 21

In biological electricity, resistance to current flow comes from
two main sources:

Answer 21

the resistance of the cell membrane (Rm) and
the internal resistance of the cytoplasm (Ri
). (The extracellular fluid also creates resistance Ro, but it is so small compared to
Rm and Ri
that it is usually ignored.)

Question 22

The phospholipid bilayer

of the cell membrane is normally an excellent

Answer 22

insulator, and a
membrane with no open ion channels has very high resistance
and low conductance.

Question 23

Voltage changes across the membrane can be classified into

two basic types of electrical signals:

Answer 23

graded potentials and action

potentials

Question 24

graded potential

Answer 24

are variable-strength
signals that travel over short distances and lose strength as they
travel through the cell. They are used for short-distance communication. If a depolarizing graded potential is strong enough
when it reaches an integrating region within a neuron, the
graded potential initiates an action potential

Question 25

action potential

Answer 25

are very brief, large depolarizations that travel for long
distances through a neuron without losing strength. Their function is rapid signaling over long distances, such as from your toe
to your brain

Question 26

Graded potentials in neurons are… and why are these changes in membrane potential called “graded”

Answer 26

e depolarizations or hyperpolarizations that occur in the dendrites and cell body or, less frequently,
near the axon terminals. These changes in membrane potential are
called “graded” because their size, or amplitude {amplitudo, large}, is
directly proportional to the strength of the triggering event. A large
stimulus causes a strong graded potential, and a small stimulus
results in a weak graded potential.

Question 27

s local current flow.

Answer 27

The wave of depolarization that moves through the cell is
known as local current flow. By convention, current in biological systems is the net movement of positive electrical charge

Question 28

Why do graded potentials lose strength as they move through

the cytoplasm? Two factors play a role

Answer 28

Current leak. The membrane of the neuron cell body has open
leak channels that allow positive charge to leak out into the
extracellular fluid. Some positive ions leak out of the cell
across the membrane as the depolarization wave moves
through the cytoplasm, decreasing the strength of the signal
moving down the cell.
2. Cytoplasmic resistance. The cytoplasm provides resistance to
the flow of electricity, just as water creates resistance that
diminishes the waves from the stone. The combination
of current leak and cytoplasmic resistance means that the
strength of the signal inside the cell decreases over distance

Question 29

excitability

Answer 29

The ability of a neuron to respond to a stimulus and fire an action potential is called
the cell’s excitability

Question 30

The high-speed movement of an action potential along the axon is called

Answer 30

conduction of the action potential.

Question 31

! A neuron without a functional Na+@K+ pump could

Answer 31

d fire a thousand or more action potentials

before a significant change in the ion gradients occurred.

Question 32

These voltage-gated Na+ channels have two gates to regulate ion
movement rather than a single gate. The two gates, known as

Answer 32

activation and inactivation gates, flip-flop back and forth to
open and close the Na+ channel.

Question 33

The double gating of Na+ channels plays a major role in the phenomenon known as the refractory period

Answer 33

The “stubbornness” of the neuron refers to the fact that once an action potential
has begun, a second action potential cannot be triggered for about
1–2 msec, no matter how large the stimulus. This delay, called the absolute refractory period, represents the time required for
the Na+ channel gates to reset to their resting positions (FIG. 8.12).
Because of the absolute refractory period, a second action potential cannot occur before the first has finished. Consequently, action
potentials moving from trigger zone to axon terminal cannot overlap and cannot
travel backward

Question 34

A relative refractory period

Answer 34

follows the absolute refractory period. During the relative refractory period, some but not all
Na+ channel gates have reset to their original positions. In addition,
during the relative refractory period, K+ channels are still open

Question 35

saltatory

conduction,

Answer 35

Sodium ions entering
at a node reinforce the depolarization and restore the amplitude of
the action potential as it passes from node to node. The apparent
jump of the action potential from node to node is called saltatory
conduction, from the Latin word saltare, meaning “to leap.”
(pg 284)

Question 36

In summary, action

potentials travel through different

Answer 36

axons at different rates, depending on the two parameters of axon diameter and myelination

Question 37

In demyelinating diseases, the loss of myelin from vertebrate neurons can have devastating effects on neural signaling.

Answer 37

In the central
and peripheral nervous systems, the loss of myelin slows the conduction of action potentials. In addition, when current leaks out of
now-uninsulated regions of membrane between the channel-rich
nodes of Ranvier, the depolarization that reaches a node may no
longer be above threshold, and conduction may fail (Fig. 8.16b).

Question 38

If Na+ channels are not functional,

Answer 38

Na+ cannot enter the axon.

Question 39

When myelin is lost in demyelinating diseases,

Answer 39

the membrane
capacitance increases and voltage changes across the membrane take longer. This contributes to slower action potential
conduction or even failure of action potentials to reach the
axon terminal in diseases such as multiple sclerosis.

Question 40

hyperkalemia

Answer 40

An increase in blood K+ concentration—
hyperkalemia {hyper-, above + kalium, potassium + -emia, in the
blood}—shifts the resting membrane potential of a neuron closer
to threshold and causes the cells to fire action potentials in response
to smaller graded potentials (Fig. 8.17c).

Question 41

hypokalemia

Answer 41

If blood K+ concentration falls too low—a condition known
as hypokalemia—the resting membrane potential of the cells
hyperpolarizes, moving farther from threshold. In this case, a stimulus strong enough to trigger an action potential when the resting
potential is the normal -70 mV does not reach the threshold value
(Fig. 8.17d). This condition shows up as muscle weakness because
the neurons that control skeletal muscles are not firing normally

Question 42

Information flow through the nervous system using electrical and
chemical signals is one of the most active areas of neuroscience
research today because so many devastating diseases affect this
process. The specificity of neural communication depends on several factors:

Answer 42

the signal molecules secreted by neurons, the target
cell receptors for these chemicals, and the anatomical connections
between neurons and their targets, which occur in regions known
as synapses.

Question 43

In a

neural reflex, information moves from

Answer 43

presynaptic cell to postsynaptic cell. The postsynaptic cells may be neurons or non-neuronal
cells. In most neuron-to-neuron synapses, the presynaptic axon
terminals are next to either the dendrites or the cell body of the
postsynaptic neuron.

Question 44

Electrical synapses

Answer 44

pass an electrical signal, or current, directly from the cytoplasm of one cell to another through the pores of gap junction proteins.

Question 45

When we examine the axon terminal of a presynaptic cell with an
electron microscope, we find many

Answer 45

small synaptic vesicles filled
with neurotransmitter that is released on demand (FIG. 8.18). Some
vesicles are “docked” at active zones along the membrane closest to
the synaptic cleft, waiting for a signal to release their contents. Other
vesicles act as a reserve pool, clustering close to the docking sites. Axon
terminals also contain mitochondria to produce ATP for metabolism
and transport. In this section, we discuss general patterns of neurotransmitter synthesis, storage, release, and termination of action

Question 46

Electrical signaling patterns in the CNS are more variable.

Brain neurons show

Answer 46

different electrical personalities by firing
action potentials in a variety of patterns, sometimes spontaneously, without an external stimulus to bring them to threshold. For
example, some neurons are tonically active [p. 182], firing regular
trains of action potentials (beating pacemakers). Other neurons
exhibit bursting, bursts of action potentials rhythmically alternating
with intervals of quiet (rhythmic pacemakers)

Question 47

spatial summation

Answer 47

The combination of several nearly simultaneous graded
potentials is called spatial summation. The word spatial
{spatium, space} refers to the fact that the graded potentials originate at different locations (spaces) on the neuron.

Question 48

postsynaptic inhibition

Answer 48

Spatial summation is not always excitatory. If summation prevents an action potential in the postsynaptic cell, the summation is
called postsynaptic inhibition. This occurs when presynaptic
neurons release inhibitory neurotransmitter. For example, Figure
8.24e shows three presynaptic neurons, two excitatory and one
inhibitory, converging on a postsynaptic cell. The neurons fire, creating one IPSP and two EPSPs that sum as they reach the trigger
zone. The IPSP counteracts the two EPSPs, creating an integrated
signal that is below threshold. As a result, no action potential is
generated at the trigger zone.

Question 49

temporal summation

Answer 49

Summation of graded potentials does not
always require input from more than one presynaptic neuron. Two
subthreshold graded potentials from the same presynaptic neuron
can be summed if they arrive at the trigger zone close enough
together in time. Summation that occurs from graded potentials
overlapping in time is called temporal summation {tempus,
time}. Let’s see how this can happen.

Question 50

In many situations, graded potentials in a neuron incorporate
both temporal and spatial summation. The summation of graded
potentials demonstrates a key property of neurons: postsynaptic
integration. W

Answer 50

When multiple signals reach a neuron, postsynaptic
integration creates a signal based on the relative strengths and
durations of the signals. If the integrated signal is above threshold,
the neuron fires an action potential. If the integrated signal is
below threshold, the neuron does not fire

Question 51

Information transfer and communication depend on

Answer 51

electrical signals that
pass along neurons, on molecular interactions between signal molecules and
their receptors, and on signal transduction in the target cells.

Question 52

This chapter introduces the nervous system, one of the major control
systems responsible for maintaining homeostasis. The divisions of the
nervous system correlate with the steps in a reflex pathway. Sensory
receptors monitor

Answer 52

r regulated variables and send input signals to the
central nervous system through sensory (afferent) neurons. Output signals, both electrical and chemical, travel through the efferent divisions
(somatic motor and autonomic) to their targets throughout the body