Chapter 1

Question 1

Camillo Golgi

Answer 1

Then the Italian investigator Camillo Golgi found a way to stain nerve cells with silver salts. This method, which completely stained some cells without affecting others at all, enabled researchers to examine the structure of a single cell.

Question 2

Santiago ramón y cajal

Answer 2

Cajal used Golgi’s methods but applied them to infant brains, in which the cells are smaller and therefore easier to examine on a single slide. Cajal’s research demonstrated that nerve cells remain separate instead of merging into one another.

Question 3

membrane

Answer 3

The surface of a cell is its membrane (or plasma membrane), a structure that separates the inside of the cell from the outside environment. It is composed of two layers of fat molecules that are free to flow around one another.
Most chemicals cannot cross the membrane, but specific protein channels in the membrane permit a controlled flow of water, oxygen, sodium, potassium, calcium, chloride, and other important chemicals.

Question 4

nucleus

Answer 4

Except for mammalian red blood cells, all animal cells

have a nucleus, the structure that contains the chromosomes.

Question 5

mitochondrion

Answer 5

A mitochondrion (pl.: mitochondria) is the struc-
ture that performs metabolic activities, providing the energy
that the cell requires for all other activities. Mitochondria
require fuel and oxygen to function.

Question 6

Ribosomes & endoplasmic reticulum

Answer 6

Ribosomes are the sites
at which the cell synthesizes new protein molecules. Proteins provide building materials for the cell and facilitate various chemical reactions. Some ribosomes float freely within the cell. Others are attached to the endoplasmic reticulum, a network of thin tubes that transport newly synthesized proteins to other locations.

Question 7

larger neurons have these components

Answer 7

dendrites, a soma (cell body), an axon, and presynaptic terminals.

Question 8

The tiniest neurons lack

Answer 8

axons, and some lack well-defined dendrites

Question 9

motor neuron

Answer 9

A motor neuron has its soma in the spinal cord. It receives excitation from other neurons through its dendrites and conducts impulses along its axon to a muscle. (p. 31)

Question 10

sensory neuron

Answer 10

A sensory neuron is specialized at one end to
be highly sensitive to a particular type of stimulation, such as light, sound, or touch. Tiny branches lead directly from the receptors into the axon, and the cell’s soma is located on a little stalk off the main trunk.(p. 31)

Question 11

dendrites & dendritic spines

Answer 11

Dendrites are branching fibers that get narrower near
their ends. (The term dendrite comes from a Greek root word meaning “tree.” A dendrite branches like a tree.) The dendrite’s surface is lined with specialized synaptic receptors, at which the dendrite receives information from other neurons. (Chapter 3 concerns synapses.) The greater the surface area of a dendrite, the more information it can receive. Some dendrites branch widely and therefore have a large surface area. Many also contain dendritic spines, the short outgrowths that increase the surface area available for synapses

Question 12

soma

Answer 12

The cell body, or soma (Greek for “body”; pl.: somata),
contains the nucleus, ribosomes, and mitochondria. Most of the metabolic work of the neuron occurs here. Cell bodies of neurons range in diameter from 0.005 mm to 0.1 mm in mammals and up to a full millimeter in certain invertebrates. Like the dendrites, the cell body is covered with synapses on its surface in many neurons.

Question 13

axon, myelin sheath, nodes of Ranvier & presynaptic terminal

Answer 13

The axon is a thin fiber of constant diameter, in most 
cases longer than the dendrites. (The term axon comes from a Greek word meaning “axis.”) The axon is the neuron’s information sender, conveying an impulse toward other neurons or an organ or muscle. Many vertebrate axons are covered  with  an  insulating  material  called  a  myelin	 sheath with interruptions known as nodes	of	Ranvier. Invertebrate axons do not have myelin sheaths. An axon  has  many  branches,  each  of  which  swells  at  its  tip, 
forming a presynaptic	terminal, also known as an end bulb 
or  bouton  (French  for “button”).

Question 14

afferent, efferent & intrinsic axons

Answer 14

An afferent axon brings information into
a structure; an efferent axon carries information away from
a structure. Every sensory neuron is an afferent to the rest of the nervous system, and every motor neuron is an effer- ent from the nervous system. Within the nervous system, a given neuron is an efferent from one structure and an afferent to another. (You can remember that efferent starts with e as in exit; afferent starts with a as in admit.) For example, an axon might be efferent from the thalamus and afferent to the cerebral cortex (Figure 2.8, p. 32). If a cell’s dendrites and axon are entirely contained within a single structure, the cell is an interneuron or intrinsic neuron of that structure. For example, an intrinsic neuron of the thalamus has its axon and all its dendrites within the thalamus.

Question 15

What are the widely branching structures of a neuron
called? And what is the long thin structure that carries
information to another cell called?

Answer 15

The widely branching structures of a neuron are
called dendrites, and the long thin structure that carries
information to another cell is called an axon.

Question 16

Which animal species would have the longest axons?

Answer 16

The longest axons occur in the largest animals. For example, giraffes and elephants have axons that extend from the
spinal cord to the feet, nearly two meters away.

Question 17

Variation among neurons, widely branching & short branching dendrites

Answer 17

The function of a neuron relates to its shape
Neurons with wider branching connect with more targets. For example, the widely branching dendrites of the Purkinje cell of the cerebellum (Figure 2.9a, p. 32) enable it to receive a huge number of inputs—up to 200,000 in some cases. By contrast, certain cells in the retina (Figure 2.9d, p. 32) have only short branches on their dendrites and therefore pool input from only a few sources.

Question 18

glia

Answer 18

Glia (or neuroglia), the other major components of the ner- vous system, do not transmit information over long distances as neurons do, although they perform many other functions. The term glia, derived from a Greek word meaning “glue,” reflects early investigators’ idea that glia were like glue that held the neurons together.
The brain has several types of glia with different functions (e.g. astrocytes, microglia, oligodendrocytes, Schwann cells & radial glia)

Question 19

astrocytes

Answer 19

The star-shaped astrocytes wrap around the
presynaptic terminals of a group of functionally related axons, as shown in Figure 2.11 (p. 34). By taking up ions released by axons and then releasing them back to axons, an astrocyte helps synchronize the activity of the axons, enabling them to send messages in waves. Astrocytes also remove waste material created when neurons die and control the amount of blood flow to each brain area. An additional function is that during periods of heightened activity in some brain areas, astrocytes dilate the blood vessels to bring more nutrients into that area. Uncertainty surrounds another possible function: Neurons communicate by releasing certain transmitters, such as glutamate. After a neuron releases much glutamate, nearby glia cells absorb some of the excess. We know that the glia
convert most of this glutamate into a related chemical, glutamine, and then pass it back to the neurons, which convert it back to glutamate, which they get ready for further release. (It’s a recycling system.) The uncertain question is whether glia cells also release glutamate and other chemicals themselves. If so, they could be part of the brain’s signaling system.

Question 20

microglia

Answer 20

very small cells, also remove waste material

as well as viruses, fungi, and other microorganisms. In effect, they function like part of the immune system

Question 21

oligodendrocytes & Schwann cells

Answer 21

Oligodendrocytes in the brain and spinal cord and Schwann cells in the periphery of the body are specialized types of glia that build the myelin sheaths that surround and insulate certain vertebrate axons.

Question 22

radial glia

Answer 22

guide the migration of neurons and their axons and dendrites during embryonic development. When embryological development finishes, most radial glia differentiate into neurons, and a smaller number differentiate into astrocytes and oligodendrocytes

Question 23

presynaptic terminal/end bulb/bouton

Answer 23

An axon has many branches, each of which swells at its tip, forming a presynaptic terminal, also known as an end bulb or bouton (French for “button”). This is the point from which the axon releases chemicals that cross through the junction between one neuron and the next.

Question 24

Identify the four major structures that compose a neuron.

Answer 24

Dendrites, soma (cell body), axon, and presynaptic

terminal.

Question 25

Which kind of glia cell wraps around the synaptic terminals of axons?

Answer 25

Astrocytes.

Question 26

a virus that enters

your nervous system…?

Answer 26

probably remains with you for life. For
example, the virus responsible for chicken pox and shingles enters spinal cord cells. No matter how effectively the im-mune system attacks that virus outside the nervous system, virus particles remain in the spinal cord, from which they can emerge decades later. The same is true for the virus that causes genital herpes.

Question 27

blood-brain barrier

Answer 27

The mechanism that excludes most chemicals from the vertebrate brain. The blood –brain barrier (Figure 2.12, p. 35) depends on the endothelial cells that form the walls of the capillaries. Outside the brain, such cells are separated by small gaps, but in the brain, they are joined so tightly that virtually nothing passes between them.The brain has several mechanisms to get useful chemicals across the blood-brain barrier.

Question 28

active transport

Answer 28

a protein-mediated process that expends energy to
pump chemicals from the blood into the brain. Chemicals that are actively transported into the brain include glucose (the brain’s main fuel), amino acids (the building blocks of proteins), purines, choline, a few vitamins, iron, and certain hormones

Question 29

Identify one major advantage and one disadvantage of having a blood–brain barrier.

Answer 29

The blood–brain barrier keeps out viruses (an advan-

tage) and also most nutrients (a disadvantage).

Question 30

Which chemicals cross the blood–brain barrier passively?

Answer 30

Small, uncharged molecules such as oxygen, carbon
dioxide, and water cross the blood–brain barrier pas-
sively. So do chemicals that dissolve in the fats of the
membrane.

Question 31

mechanisms to get useful chemicals across the blood-brain barrier.

Answer 31

First, small uncharged molecules, including oxygen and carbon dioxide, cross freely. Water crosses through special protein channels in the wall of the endothelial cells. Second, molecules that dissolve in the fats of the membrane also cross passively. Examples include vitamins A and D and all the drugs that affect the brain—from antidepressants and other psychiatric drugs to illegal drugs such as heroin. For a few other chemicals, the brain uses active transport, a protein-mediated process that expends energy to pump chemicals from the blood into the brain. Chemicals that are actively transported into the brain include glucose
(the brain’s main fuel), amino acids (the building blocks of proteins), purines, choline, a few vitamins, iron, and certain

Question 32

Which chemicals cross the blood–brain barrier by active transport?

Answer 32

Glucose, amino acids, purines, choline,

certain vitamins, iron, and a few hormones.

Question 33

why do neurons depend so heavily on glucose & oxygen?

Answer 33

vertebrate neurons depend almost entirely on glucose. Because the metabolic pathway that uses glucose requires oxygen, neurons need a steady supply of oxygen. Although neurons have the enzymes necessary to metabolize other fu-
els, glucose is practically the only nutrient that crosses the blood–brain barrier after infancy, except for ketones (a kind of fat), and ketones are seldom available in large amounts

Question 34

How much percent of oxygen consumed by the body does the brain use?

Answer 34

20%

Question 35

why is glucose shortage

rarely a problem?

Answer 35

The liver makes glucose from many kinds of
carbohydrates and amino acids, as well as from glycerol, a breakdown product from fats. The only likely problem is an inability to use glucose.

Question 36

What does the body need to use glucose?

Answer 36

To use glucose, the body needs vitamin
B 1 , thiamine. Prolonged thiamine deficiency, common in chronic alcoholism, leads to death of neurons and a condition called Korsakoff’s syndrome, marked by severe memory impairments (Chapter 13).

Question 37

How does the visual system compensate for the fact that some parts of the retina are slightly closer to your brain than other parts are?

Answer 37

Without some sort of compensation, simultaneous flashes arriving at two spots on your retina would reach your brain at different times, and you might perceive movement inaccurately. What prevents this illusion is the fact that axons from more distant parts of your retina transmit impulses slightly faster than those closer to the brain!

Question 38

All parts of a neuron are covered by a membrane about 8 nanometers (nm) thick composed of…?

Answer 38

two  layers  (an  inner  layer  and  an  outer  layer)  of 
phospholipid molecules (containing chains of fatty acids and a phosphate group). Embedded among the phospholipids are cylindrical protein molecules through which various chemicals can pass (see Figure 2.3, p. 30). The structure of the membrane provides it with a combination of flexibility and firmness and controls the flow of chemicals between the inside and outside of the cell.

Question 39

electrical gradient/polarization

Answer 39

an electrical gradient, also known as polarization—a difference in electrical charge between the inside and outside of the cell.

Question 40

resting potential

Answer 40

The difference in voltage in a resting neuron is called the resting potential. In the absence of any outside disturbance, the membrane maintains an electrical gradient. The neuron inside the membrane has a
slightly negative electrical potential with respect to the outside, mainly because of negatively charged proteins inside the cell.

Question 41

how to measure resting potential?

Answer 41

Researchers measure the resting potential by inserting a very thin microelectrode into the cell body, as Figure 2.13 (p. 38) shows. The diameter of the electrode must be as small as possible so that it enters the cell without causing damage. The most common electrode is a fine glass tube filled with a concentrated salt solution and tapering to a tip diameter of 0.0005
mm or less. A reference electrode outside the cell completes the circuit. Connecting the electrodes to a voltmeter, we find that the neuron’s interior has a negative potential relative to its exterior. A typical level is –70 millivolts (mV), but it varies from one neuron to another.

Question 42

selectively permeable membranes

Answer 42

Some chemicals pass through it more freely than others do. Oxygen, carbon dioxide, urea, and water cross freely through channels that are always open. Most large or electrically charged ions and molecules do not cross the membrane at all. A few biologically important ions, such as sodium, potassium, calcium, and chloride, cross through membrane channels (or gates) that are sometimes open and sometimes closed.

Question 43

When membrane is at rest sodium channel & potassium channel..?

Answer 43

When the membrane is at rest, the sodium chan-
nels are closed, preventing almost all sodium flow, as shown on the right side of Figure 2.14 (p. 38). Certain kinds of stimulation can open the sodium channels, as in the center of that figure. When the membrane is at rest, potassium channels are nearly but not entirely closed, so potassium flows slowly. Stimulation opens them more widely also, as it does for sodium channels.

Question 44

The sodium–potassium pump and its results

Answer 44

The sodium–potassium pump, a protein complex, repeatedly transports three sodium ions out of the cell while drawing two potassium ions into it. The sodium –potassium pump is an active transport that requires energy. As a result of the sodium–potassium pump, sodium ions are more than 10 times more concentrated outside the membrane than inside. The sodium –potassium pump is effective only because of
the selective permeability of the membrane, which prevents the sodium ions that were pumped out of the neuron from leaking right back in again. When sodium ions are pumped out, they stay out. However, some of the potassium ions pumped into the neuron slowly leak out, carrying a positive charge with them. That leakage increases the electrical gradient across the membrane, as shown in Figure 2.15 (p. 39).

Question 45

When the neuron is at rest, two forces act on sodium

Answer 45

both tending to push it into the cell. First, consider the electrical gradient. Sodium is positively charged and the inside of the cell is negatively charged. Opposite electrical charges attract, so the electrical gradient tends to pull sodium into the cell. Second, consider the concentration gradient, the difference in distribution of ions across the membrane. Sodium is more concentrated outside than inside, so just by the laws of probability, sodium is more likely to enter the cell than to leave it. (By analogy, imagine two rooms connected by a door. There are 100 cats in room A and only 10 in room B. Cats are more likely to move from A to B than from B to A. The same principle applies to the movement of ions across a mem-
brane.)

Question 46

When the membrane is at rest, sodium..

Answer 46

The sodium channels are closed when the membrane is at rest, and almost no sodium flows except for the sodium pushed out of the cell by the sodium–potassium pump.

Question 47

When the neuron is at rest, two forces act on potassium

Answer 47

Potassium is subject to competing forces. Potassium is positively charged and the inside of the cell is negatively charged, so the electrical gradient tends to pull potassium in. However, potassium is more concentrated inside the cell than
outside, so the concentration gradient tends to drive it out. (Back to our cat analogy: Imagine some female cats tethered inside a room. Male cats can enter the room or leave through a narrow door. They are attracted to the female cats, but when the males get too crowded, some of them leave.)

Question 48

What if the potassium channels were wide open at rest?

Answer 48

If the potassium channels were wide open, potassium would have a small net flow out of the cell. That is, the electrical gradient and concentration gradient for potassium are almost in balance, but not quite. The sodium –potassium pump pulls more potassium into the cell as fast as it flows out of the cell, so the two gradients cannot get completely in balance.

Question 49

In a resting membrane, potassium gates

Answer 49

retard the flow of potassium outside the membrane

Question 50

The body invests much energy to operate the sodium –potassium pump, which maintains the resting potential. Why is it worth so much energy?

Answer 50

The resting potential prepares the neuron to respond rapidly.

Question 51

When the membrane is at rest, are the sodium ions more
concentrated inside the cell or outside? Where are the
potassium ions more concentrated?

Answer 51

Sodium ions are more concentrated outside the

cell; potassium is more concentrated inside.

Question 52

When the membrane is at rest, what tends to drive the
potassium ions out of the cell? What tends to draw them
into the cell?

Answer 52

When the membrane is at rest, the concentration gradient tends to drive potassium ions out of the cell; the electrical gradient draws them into the cell. The sodium–potassium pump also draws them into the cell.

Question 53

Who is responsible for the membranes polarization?

Answer 53

Negatively charged proteins inside the cell are responsible for the membrane’s polarization. Chloride ions, being negatively charged, are mainly outside the cell. When the membrane is at rest, the concentration gradient and electrical gradient balance, so opening chloride channels produces little effect. However, chloride does have a net flow when the membrane’s polarization changes.

Question 54

What happens if we disturb the resting potential with a microelectrode? (hyperpolarization, depolarization, subthreshold, threshold of excitation)

Answer 54

If we now use an electrode to apply a negative charge, we can further increase the negative charge inside the neuron. The change is called hyperpolarization, which means increased polarization. When the stimulation ends, the charge returns to its original resting level.
Applying a current to depolarize the neuron—that
is, reduce its polarization toward zero. If we apply a small depolarizing current the potential rises about the factor we applied.
Any subthreshold stimulation produces a small response proportional to the amount of current.
Stimulation beyond the threshold of excitation produces a massive depolarization of the membrane. When the potential reaches the threshold, the membrane opens its sodium channels and permits sodium ions to flow into the cell. The potential shoots up far beyond the strength of the stimulus.

Question 55

What is the difference between a hyperpolarization and a depolarization?

Answer 55

A hyperpolarization is an exaggeration of the
usual negative charge within a cell (to a more negative
level than usual). A depolarization is a decrease in
the amount of negative charge within the cell.

Question 56

What is the relationship between the threshold and an

action potential?

Answer 56

A depolarization that passes the threshold produces an

action potential. One that falls short of the threshold does not produce an action potential.

Question 57

voltage-gated channels

Answer 57

cylindrical proteins on a neuron’s membrane whose permeability depends on the voltage difference across the membrane (e.g. potassium channels, sodium channels)

Question 58

3 principles to understand action potential

Answer 58

1. At the start, sodium ions are mostly outside the neuron and potassium ions are mostly inside.
2. When the membrane is depolarized, sodium and potassium channels in the membrane open.
3. At the peak of the action potential, the sodium channels close.

Question 59

What happens (with the channels) when the threshold is passed?

Answer 59

When the depolarization reaches the threshold of the membrane, the sodium channels open wide enough for sodium to flow freely. Driven by both the concentration gradient and the electrical gradient, the sodium ions
enter the cell rapidly, until the electrical potential across the membrane passes beyond zero to a reversed polarity as shown in the following diagram (p.41)
At the peak of the action potential, the sodium gates
snap shut and resist reopening for the next millisecond.
depolarizing the membrane also opens potassium channels.
At first, opening those channels made little difference. However, after so many sodium ions have crossed the membrane, the inside of the cell has a slight positive charge instead of its usual negative charge. At this point both the concentration gradient and the electrical gradient drive potassium ions out of the cell. As they flow out of the axon, they carry with them a positive charge. Because the potassium channels remain open after the sodium channels close, enough potassium ions leave to drive the membrane beyond its usual resting level to a temporary hyperpolarization.
Figure 2.16 (p. 41) summarizes the key movements of ions during an action potential.
At the end of this process, the membrane has returned to its resting potential, but the inside of the neuron has slightly more sodium ions and slightly fewer potassium ions than before. Eventually, the sodium–potassium pump restores the original
distribution of ions, but that process takes time.

Question 60

Local anesthetic drugs

Answer 60

such as Novocain and Xylocaine, attach
to the sodium channels of the membrane, preventing sodium ions from entering, and thereby stopping action potentials

Question 61

An action potential in many cases “back-propagates” into the cell body and dendrites

Answer 61

The cell body and dendrites do
not conduct action potentials in the same way that axons do, but they passively register the electrical event happening in the nearby axon. This back-propagation is important in some neurons, as we shall see in Chapter 13: When an action potential back-propagates into a dendrite, the dendrite becomes more susceptible to the structural changes responsible for learning.

Question 62

all-or-none law

Answer 62

is that the amplitude and velocity of an action potential are independent of the intensity of the stimulus that initiated it, provided that the stimulus reaches the threshold. By analogy, imagine flushing a toilet: You have to make a press of at least a certain strength (the threshold), but pressing harder does not make the toilet flush faster or more vigorously.

Question 63

During the rise of the action potential, do sodium ions

move into the cell or out of it? Why?

Answer 63

During the action potential, sodium ions move
into the cell. The voltage-dependent sodium gates
have opened, so sodium can move freely. Sodium is
attracted to the inside of the cell by both an electri-
cal and a concentration gradient.

Question 64

As the membrane reaches the peak of the action poten-
tial, what brings the membrane down to the original rest-
ing potential?

Answer 64

After the peak of the action potential, potassium ions exit the cell, driving the membrane back to the resting potential.
Important note: The sodium–potassium pump is NOT
responsible for returning the membrane to its resting
potential. The sodium–potassium pump is too slow for
this purpose.

Question 65

refractory period

Answer 65

Immediately after an action potential, the cell is in a
refractory period during which it resists the production of further action potentials. In the first part of this period, the absolute refractory period, the membrane cannot produce an action potential, regardless of the stimulation. During the second part, the relative refractory period, a stronger than
usual stimulus is necessary to initiate an action potential. The refractory period has two mechanisms: The sodium channels are closed, and potassium is flowing out of the cell at a faster than usual rate.

Question 66

State the all-or-none law.

Answer 66

According to the all-or-none law, the size and
shape of the action potential are independent of the
intensity of the stimulus that initiated it. That is, every
depolarization beyond the threshold of excitation pro-
duces an action potential of about the same amplitude
and velocity for a given axon.

Question 67

Does the all-or-none law apply to dendrites? Why or why not?

Answer 67

The all-or-none law does not apply to dendrites because they do not have action potentials.

Question 68

Suppose researchers find that axon A can produce up to
1,000 action potentials per second (at least briefly, with
maximum stimulation), but axon B can never produce
more than 100 per second (regardless of the strength of
the stimulus). What could we conclude about the refrac-
tory periods of the two axons?

Answer 68

Axon A must have a shorter
absolute refractory period, about 1 ms, whereas B has
a longer absolute refractory period, about 10 ms.

Question 69

axon hillock

Answer 69

In a motor neuron, an action potential begins on the axon hillock, a swelling where the axon exits the soma (Figure 2.5, p. 31). During the action potential, sodium ions enter a point on the axon.

Question 70

propagation of the action potential

Answer 70

describes
the transmission of an action potential down an axon.
During the action potential, sodium ions enter a
point on the axon. Temporarily, that spot is positively charged in comparison with neighboring areas along the axon. The positive ions flow within the axon to neighboring regions. The positive charges slightly depolarize the next area of the membrane, causing it to reach its threshold and open its voltage-gated sodium channels. Therefore, the membrane regenerates the action potential at that point. In this manner, the action potential travels along the axon, as
in Figure 2.17 (p. 44).

Question 71

what prevents an action potential near the
center of an axon from reinvading the areas that it has just
passed?

Answer 71

the areas just passed are still in

their refractory period.

Question 72

myelin shealth/ myelinated axons

Answer 72

An insulating material composed of fats and proteins. Increase the speed of vertebrate axons.
The myelin sheath is interrupted periodically by short sections of axon called nodes of Ranvier, each one about 1 micrometer wide, as shown in Figure 2.18 (p. 45). In most cases, the action potential starts at the axon hillock, but in some cases it starts at the
first node of Ranvier.

Question 73

saltatory conduction

Answer 73

from the Latin word saltare, meaning “to jump.” (The same root shows up in the word somersault.) Suppose an action potential starts at the axon hillock and
propagates along the axon until it reaches the first myelin segment. The action potential cannot regenerate along the membrane between nodes because sodium channels are virtually absent between nodes. After an action potential occurs at a node, sodium ions enter the axon and diffuse within the axon, pushing a chain of positive ions along the axon to the next node, where they regenerate the action potential (Figure 2.19, p. 45). This flow of ions is considerably faster than the regeneration of an action potential at each point along the axon.

Question 74

When you have multiple sclerosis, what happens with your myelin sheaths

Answer 74

In multiple sclerosis, the immune system attacks myelin
sheaths. An axon that never had a myelin sheath conducts impulses slowly but steadily. An axon that has lost its myelin is not the same. After myelin forms along an axon, the axon loses its sodium channels under the myelin
If the axon later loses its myelin, it still lacks sodium
channels in the areas previously covered with myelin, and most action potentials die out between one node and the next. People with multiple sclerosis suffer a variety of impairments, ranging from visual impairments to poor muscle coordination.

Question 75

In a myelinated axon, how would the action potential
be affected if the nodes were much closer together?
How might it be affected if the nodes were much farther
apart?

Answer 75

If the nodes were closer, the action potential
would travel more slowly. If they were much farther
apart, the action potential would be faster if it could
successfully jump from one node to the next. When
the distance becomes too great, the current cannot
diffuse from one node to the next and still remain
above threshold, so the action potentials would stop.

Question 76

local neurons

Answer 76

Axons produce action potentials. However, many small neurons have no axon. Neurons without an axon exchange information with only their closest neighbors. We therefore call them local neurons. Because they do not have an axon, they do not follow the all-or-none law. (graded potential)

Question 77

graded potential

Answer 77

When a local neuron receives information from other neurons, it has a graded potential, a membrane potential that varies in magnitude in proportion to
the intensity of the stimulus. The change in membrane potential is conducted to adjacent areas of the cell, in all directions, gradually decaying as it travels. Those various areas of the cell contact other neurons, which they excite or inhibit through synapses.

Question 78

orthodromic direction & antidromic direction

Answer 78

An action potential moving in the usual
direction, away from the axon hillock, is said to be
traveling in the orthodromic direction. An action potential traveling toward the axon hillock is
traveling in the antidromic direction.