Chapter 42: Cells of the Nervous System (Part 2, Week 10)

Question 1

[Start 42.3 Generation and Transmission of Electrical Signals Along Neurons]

What is the change in the membrane potential that occurs when a cell membrane becomes less polarized, that is, less negative inside the cell relative to the surrounding fluid?

Answer 1

Depolarization

When a neuron is stimulated, one or more types of membrane ion channels open.

Often, these are Na+ channels; when they open, Na+ ions diffuse into the cell, bringing with them their
positive charge.

This makes the new membrane potential somewhat less negative than the resting membrane potential. Consequently, the membrane is said to be depolarized.

Question 2

What do you call the change in the membrane potential that occurs when the cell membrane becomes more polarized?

Answer 2

Hyperpolarization

For example, opening more K+ channels would result in an increase in the rate of diffusion of K+ out of a cell. This would make the charge along the inside of the cell membrane more negative than it normally
is while at the resting membrane potential.

Question 3

What are considered excitable cells since they can generate electrical signals by changing their membrane potentials?

All cells have membrane potential but only some can generate electrical signals by changes their membrane potentials.

Answer 3

Neurons and muscle cells

This is accomplished by gated ion channels, so called because they open and close in a manner analogous to a gate in a fence.

Question 4

What are ion channels that open nd close in response to changes in voltage across a cell membrane?

Answer 4

Voltage-gated ion channels

Imagine: this gate has a majority of charge on one side, and a different one on the other. When these charges flip sides (probably due to diffusion), the gate opens.

Question 5

What is a type of cell surface receptor that binds a ligand and functions as an ion channel? These can open or close a channel?

Answer 5

Ligand-gated ion channels

A ligand is defined as any molecule or atom that irreversibly binds to a receiving protein molecule, otherwise known as a receptor.

These ligands can be neurotransmitters.

Question 6

The opening and closing of ligand-gated and voltage-gated ion channels are responsible for WHAT two types of changes in a neuron’s membrane potential?

Answer 6

Graded potentials and action potentials

Question 7

What is the depolarization or hyperpolarization of a neuron’s plasma membrane that varies with the strength of a stimulus?

Answer 7

Graded potential

A large change in membrane potential occurs when a strong stimulus opens many channels, whereas a
weak stimulus causes a small change because only a small number of channels open.

Question 8

Fun Fact: Graded Potentials

Graded potentials occur locally on a particular area of the plasma membrane, such as on dendrites or the cell body, where an electrical or chemical stimulus opens ion channels.

From this area, a graded potential spreads a small distance across a region of the plasma membrane. In a short time, the membrane potential returns to the
resting potential because ion pumps restore the ion concentration gradients, and the ion channels close again.

Graded potentials occur on all neurons and are particularly important for the function of sensory neurons, which must distinguish between strong
and weak environmental stimuli. As discussed next, graded potentials can act as triggers for the long distance type of electrical signal, the action
potential.

Answer 8

N/A

Question 9

What is an electrical signal along a cell’s plasma membrane; occurs in animal neuron axons and muscle cells and in some plant cells?

Answer 9

Action potential

Action potentials are the electrical events that carry a signal along an axon. In contrast to a graded potential, an action potential is always a large depolarization.

Question 10

What is so different about action potential versus graded potential?

Answer 10

Once an action potential has been triggered, it occurs in an all-or-none manner. In other words, it
cannot be graded.

Unlike a graded potential, an action potential is actively propagated along the axon, regenerating itself as it travels. Action potentials travel rapidly down the axon from the axon hillock to the axon terminals,
where they initiate a response at a junction with another cell.

Question 11

What is the membrane potential, typically around -55 to -50 mV, which is sufficient to trigger an action potential in an electrically excitable cell such as a neuron?

Answer 11

The threshold potential

Question 12

What is a string of amino acids that juts out from a channel protein into the cytosol and blocks the movement of ions through the channel?

Answer 12

Inactivation gate

Question 13

What is the period during an action potential when the inactivation gate of the voltage-gated sodium channel is closed; during this time, it is impossible to generate another action potential?

Answer 13

Absolute refractory period

During this time, that portion of membrane is
unresponsive to another stimulus. A change in voltage cannot open the Na+ channels while they are inactivated. As the membrane repolarizes, the
inactivation gate is eventually released and the voltage-gated Na+ channels revert to the closed state.

Question 14

What is the period near the end of an action potential when voltage-gated potassium channels are still open; during this time a new action potential can be generated if a stimulus is sufficiently strong to raise the membrane potential to threshold?

Answer 14

Relative refractory period

Therefore, the refractory periods place limits on the frequency with which a neuron can generate and transmit action potentials. As we will
see, the refractory periods also ensure that the action potential does not “retrace its steps” by moving backward toward the cell body.

Question 15

Describe the process as a neuron recieves a signal.

This is from the membrane, to the axon hillock, through the axon to the terminals.

Answer 15

1. Neuron receives stimuli from other cells, this causes a graded potential in the cell body of the neuron that reaches the axon hillock.
2. If the change in membrane potential is sufficient to reach threshold, an action potential will be triggered.
3. An action potential first occurs with the abrupt opening of several voltage-gated Na+ channels just beyond the axon hillock, where voltage-gated
   Na+ channels are abundant.
4. This action potential, in turn, triggers the
   opening of nearby Na+ channels farther along the axon, which allows even more Na+ to flow into the neuron and depolarize a region closer to the axon
   terminals, leading to another action potential.
5. In this way, the sequential opening of Na+ channels along the axon membrane conducts a wave of
   depolarization from the axon hillock to the axon terminals.

Question 16

When it comes to direction of propagation of an action potential down the axon of a neuron, why does it never reverse?

And when does it become restored?

Answer 16

The absolute refractory period momentarily permanently closes the Na+ voltage-gated channel preventing it from being opened until the K+ channels are open to restore the resting potential.

The wave of repolarzation by the K+ channels also travels from the axon hillock to the axon terminals.

Question 17

What is the speed of propagation for action potentials and what is the speed determined by?

Answer 17

As fast as 100 m/sec or as slow as a centimeter or two per second.

The speed is determined by two factors: the axon diameter and the presence or absence of myelin.

Question 18

How does the axon diameter affect the speed of the action potential down the axon?

Answer 18

It influences the rate at which incoming ions can spread along the inner surface of the plasma membrane.

The flow of ions meets less resistance in a wide axon than it does in a thin axon, just as water moves
more easily through a wide hose than a narrow one.

Like squids and lobsters, their motor neurons are very large in diameter which allows for them to move quicker.

Question 19

T/F Unmyelinated axons conduct action potentials faster than myelinated axons.

Answer 19

False.

Myelination also influences the speed at which action potentials travel along an axon. Myelinated axons conduct action potentials at a faster rate than do
unmyelinated axons.

Question 20

What does the insulating layer of myelin reduce?

Answer 20

It reduces charge leakage across the membrane of the axon.

Question 21

What are the unmyelinated areas of an axon, the nodes of Ranvier, characterized as AND, as a result, are the only areas that can generate an action potential which aids in its continous travel without burning out?

Answer 21

This gaps, the nodes of Ranvier, are characterized by a large number of voltage-gated Na+ channels that are able to generate an action potential, a very large depolarization!

Question 22

What is the conduction of an action potential along an axon in which the action potential is regenerated at each node of Ranvier instead of along the entire length of the axon?

Answer 22

Saltatory conduction

Question 23

What is a major advantage of saltatory

conduction?

Answer 23

Saltatory conduction allows an action potential to move faster down an axon. This effect is especially important for long axons, such as those that carry signals from the spinal cord to distant muscles.

Question 24

[Start 42.4 Electrical and Chemical Communication at Synapses]

What is a junction where an axon terminal meets another neuron, a muscle cell, or a gland cell and through which an electrical or chemical signal passes?

Answer 24

A synapse

At a synapse, an electrical or chemical signal passes from anaxon terminal to the next cell.

Question 25

What is the extracellular space between a neuron and a recieving cell that is sometimes included with a synapse?

Answer 25

The synaptic cleft.

Question 26

What is the name of the neuron that sends an electrical or chemical signal to another cell?

Answer 26

Presynaptic cell

A given cell may be presynaptic to onecell, and postsynaptic to another.

Question 27

What is the name of the cell that receives the electrical or chemical signal sent from a neuron?

Answer 27

Postsynaptic cell

A given cell may be presynaptic to onecell, and postsynaptic to another.

Question 28

What are the two different types of synapses?

Answer 28

Electrical and chemical

Question 29

What is a synapse that directly passes electric current from the presynaptic to the postsynaptic cell via gap junctions?

Answer 29

Electrical synapse

An electrical synapse does not have asynaptic
cleft, but a small intercellular gap is found between the presynaptic and postsynaptic membranes.

Question 30

What is a synapse in which a chemical called a neurotransmitter is released from an axon terminal and acts as a signal from the presynaptic to the postsynaptic cell?

Answer 30

Chemical synapse

Chemical synapses appear to be more common than electrical synapses, particularly in vertebrates. Some neurons release only one type of neurotransmitter, and some neurons can release two or more different ones.

Chemical synapses are slower than electrical synapses. However, unlike electrical synapses, a major advantage of these synapses is that they allow for complex modulation of the responses of postsynaptic cells.

Question 31

Explain the Process of a Chemical Synapse and their Neurotransmitters.

Answer 31

Background Info!

Axon terminal of a presynaptic cell contains contains vesicles—small, membrane-enclosed packets, each containing thousands of molecules of neurotransmitter.

The membranes of axon terminals contain voltage-gated Ca2+ channels.

1. Action potential arrives at an axon terminal, the voltage change from the AP opens the Ca2+ channels.
2. Ca2+ ions diffuse down the electrochemical gradient into the cell (remember: its negative inside) where the calcium binds to a protein associated with the vesicle membrane.
3. This triggers exocytosis (vacoule contents fuse with presynaptic membrane and are released outside the cell) and they are released into the synaptic cleft.
4. Neurotransmitter molecules diffuse across the 10- to 20-nm-wide synaptic cleft and bind to ligand-gated ion channels or other receptor proteins in the postsynaptic cell membrane.

Question 32

What does the binding of neurotransmitter molecules accomplish within a chemical synapse?

Answer 32

The binding of neurotransmitter molecules opens or closes ligand-gated ion channels, thereby changing the membrane potential of the postsynaptic cell.

In some cases, neurotransmitters directly bind to ion channels and cause the channels to open or close. In other cases, neurotransmitters bind to receptor proteins on the postsynaptic cell membrane, which leads to the accumulation of second messengers in the cytosol of the cell.

These messengers, in turn, open or close ion channels by any of several mechanisms.

Question 33

What is the response to an excitatory neurotransmitter that depolarizes the postsynaptic membrane?

The depolarization brings the membrane potential closer to the threshold potential that would trigger an action potential and the reason for its name.

What are the two ways that this response can be caused by?

Answer 33

Excitatory postsynaptic potential (EPSP)

An EPSP is agraded potential that can be caused by the opening of Na+ channels in a neuronal membrane or by the closing of K+ channels. In both cases, positive charges accumulate inside the cell.

Question 34

What is the response to an inhibitory neurotransmitter that hyperpolarizes the postsynaptic membrane; this hyperpolarization reduces the likelihood of an action potential?

Answer 34

Inhibitory postsynaptic potential (IPSP)

An IPSP is a graded potential that can be caused by, for example, the opening of Cl– channels.

The equilibrium potential for Cl–
is typically close to or more negative than the resting membrane potential. Therefore, when Cl– channels are opened, Cl– moves into cells, making them more negatively charged and bringing them closer to the equilibrium potential for Cl–.

Hyperpolarization is the most common way in which neurons are inhibited.

Question 35

What happens to the neurotransmitter molecules after stimulation of the postsynaptic cell?

Is it efficient? What does it prevent?

Answer 35

They are broken down by enzymes or transported back to their origin cell via an event called reuptake.

Reuptake is an efficient mechanism for recapturing and reusing excess neurotransmitters that were released into the synaptic cleft.

It also prevents these neurotransmitters from diffusing away from the synapse and possibly interacting with other distant cells.

Question 36

What occurs when two or more postsynaptic potentials are generated at one time along different regions of the dendrites and their depolarizations and hyperpolarizations sum together to initiate an action potential?

Answer 36

Spatial summation

Question 37

What occurs when two or more postsynaptic potentials arrive at the same location in a dendrite in quick succession and their depolarizations and hyperpolarizations sum together to initiate an action potential?

Answer 37

Temporal summation

Question 38

Even though the number of synapses that stimulate the postsynaptic membrane, what is also important?

Answer 38

The location of the synapses. Synapses that occur far from the axon hillock are less effective than synapses on the cell body nearer the axon hillock.

Question 39

How many different neurotransmitters have neuroscientists identified in animals?

Answer 39

More than 100.

Question 40

How are neurotransmitters categorized?

Answer 40

Generally, neurotransmitters are categorized by size or structure.

The changing balance between excitatory and inhibitory neurotransmitters controls the state of nervous system circuits at any one time.

All nervous systems operate in this way, with combined excitatory and inhibitory actions of neurotransmitters.

Question 41

What neurotransmitter is one of the most widespread neurotransmitters in animals and is released at the synapses of neuromuscular juntions, which is the contact point between an axon terminal of a motor neuron and a skeletal or cardiac muscle fiber?

When is it excitatory? When is it inhibitory?

Answer 41

Acetylcholine (ACh)

It is also released at synapses within the brain and elsewhere. Acetylcholine acts as an excitatory neurotransmitter in the brain and on skeletal muscle cells andc ertain gland cells, but it is inhibitory when released from neurons that control cardiac muscle contraction.

Question 42

What are compounds containing amine groups that are formed from amino acids such as the catecholamines–dopamine, norepinephrine, and epinephrine–and serotonin and histamine and have physiological effects such as control of heart and lung function while some, such as catecholamines and serotonin, are psychoactive meaning they affect mood, attention, behavior, and learning?

Answer 42

Biogenic Amines

The catecholamines are formed from the amino acid tyrosine, serotonin is formed from tryptophan, and histamine from histidine.

In humans, for example, abnormally high or low levels of catecholamines and serotonin have been associated with a variety of mental illnesses, including schizophrenia, attention-deficit disorder, and depression.

Histamine is well known as a component of allergic reactions in people, but this is not related to its neurotransmitter functions.

In the brain, neurons that produce histamine are important in modulating sleep. They are most active during waking and are nearly inactive during sleep. This explains why certain antihistamines (drugs that block the ability of histamine to bind to its receptor, thereby inhibiting its action) used to treat colds and allergies also induce drowsiness.

Question 43

Amino Acid Neurotransmitter Facts

The amino acids glutamate, aspartate, glycine, and γ-aminobutyric acid (GABA) function as neurotransmitters.

Glutamate is the most widespread excitatory neurotransmitter found in animal nervous systems, whereas GABA is the most common inhibitory neurotransmitter.

GABA hyperpolarizes the postsynaptic membrane by opening Cl– channels, allowing negatively charged Cl– to diffuse into the cell

In this way, GABA brings neurons further away from the threshold potential required to generate an action potential, and thus it acts as the major “brake” on the CNS.

Answer 43

N/A

Question 44

Which neurotransmitter can be excitatory or inhibitory and are often called neuromodulators, because they can alter or modulate the response of the postsynaptic neuron to other neurotransmitters?

Answer 44

Neuropeptides

For example, a neuropeptide may stimulate synthesis of receptors for another neurotransmitter, which makes a cell more responsive to that neurotransmitter.

One group of neuropeptides is called the opiate peptides because opium-like drugs, such as morphine, bind to their receptors.

Opiate peptides include the endorphins, a group of peptides that decrease pain and cause natural feelings of euphoria (extreme happiness and sense of invulnerability).

Question 45

What neurotransmitter is unlike other neurotransmitters, these molecules are not sequestered into vesicles and are produced locally as required and are short-acting and influence other cells by diff usion from a presynaptic cell?

Answer 45

Gaseous Neurotransmitters such as nitric oxide (NO) and carbon monoxide (CO)

In humans, NO is responsible for relaxing the smooth muscle surrounding blood vessels, including those in the penis.

When a male becomes sexually aroused, NO levels increase in this tissue, dilating the vessels and increasing blood flow into the penis, producing an erection. Several drugs used to treat male sexual dysfunction enhance erections by increasing or mimicking the action of NO on smooth muscle.

Question 46

What are the two major types of postsynaptic membrane receptors?

Answer 46

Ionotropic and metabotropic

Many neurotransmitters, such as acetylcholine, act on both.

Question 47

What consists of a ligand-gated ion channel that opens in response to binding of a neurotransmitter?

Answer 47

Ionotropic receptor

Question 48

What is a g-protein-coupled receptor that initiates a signaling pathway in response to a neurotransmitter?

Answer 48

Metabotropic receptor (GPCRs)

G-protein-coupled receptors

A common type of response is the phosphorylation of plasma membrane ion channels for sodium, potassium, or calcium ions.

One example of a function of metabotropic receptors is the activation of sensory cells that respond to visual and other stimuli.

Question 49

What is the most common mood disorder?

Answer 49

A neurological disorder characterized by feelings of despair and sadness, resulting from an imbalance in neurotransmitter levels in the brain. aka Major Depressive Disorder or frankly, depression.

This illness results in prolonged periods of sadness, despair, and lack of interest in daily activities without alternating episodes of euphoria. Depression aff ects 5–12% of men and 10–25% of women at some time during their lives.

This condition is thought to result from decreased activity of synapses that release biogenic amines, such as serotonin, which changes neuronal activity within specific areas in the brain involved in processing emotion.

Question 50

What are drugs used to treat major depressive disorder that act by increasing the concentrations of serotonin in the brain?

Answer 50

Selective serotonin reuptake inhibitors (SSRIs)

Prozac, Zoloft, and Paxil, which reduce the reuptake of serotonin into the presynaptic terminal after it is released.

This allows serotonin to accumulate in the synaptic cleft, counteracting the deficit that causes the alteration in mood.

Question 51

How do illicit drugs disrupt normal neurotransmission?

Answer 51

At the presynaptic terminal, such drugs can decrease neurotransmitter release by reducing Ca2+
entry into the cell or by preventing the exocytosis of vesicles containing stored neurotransmitters.

In the synaptic cleft, drugs can slow the rate at which the neurotransmitter is broken down into an inactive form or taken back up into the presynaptic neuron, thereby prolonging the action of the neurotransmitter in the synaptic cleft.

Some substances act on the postsynaptic membrane by either preventing the neurotransmitter from binding to its receptor or by acting as a substitute for the neurotransmitter by stimulating the receptor.

In effect, these drugs produce changes or imbalances in neurotransmission similar to those observed in some neurological disorders.

These substances can induce euphoria, increase activity, alter mood, and produce hallucinations. They can also have potentially life-threatening effects and may be highly addictive.

Some drugs, such as cocaine, block the removal of dopamine and norepinephrine from the synaptic cleft by preventing their reuptake into the presynaptic terminal.

Morphine and marijuana mimic the actions of biological substances already in the brain, binding to receptors on the postsynaptic membrane.

With these drugs, the resulting effects are much stronger than are the effects of natural neurotransmitters. It is no surprise that many of these drugs are mind-altering. They do, after all, change the ways in which neurons communicate with each other.

Question 52

T/F Some human diseases are caused by the inability of certain axons to properly conduct an action potential. This occurs most commonly because an axon fails to become myelinated or because a myelinated axon becomes demyelinated.

Answer 52

True

Question 53

Facts!

In congenital hypothyroidism, axons fail to become wrapped with myelin during fetal development, which leads to slow conduction speeds and abnormal connections between brain neurons.

This results in profound mental defects that cannot be reversed unless treatment begins immediately after birth.

Congenital hypothyroidism is caused by a deficiency of thyroid hormone in the fetus. Among its many actions, thyroid hormone stimulates the formation of myelin during fetal development. However, thyroid hormone cannot be synthesized without the element iodine, which is part of its structure.

The iodine in the fetus comes from the mother’s diet. If a mother’s dietary intake of iodine is too low, the fetus will not have enough iodine to make its own thyroid hormone, and therefore will not be able to make normal amounts of myelin. Congenital hypothyroidism is rare in the U.S. and many other countries since the advent of iodized table salt. However, it is a serious public health concern in many parts of the world.

Answer 53

Unlike congenital hypothyroidism, multiple sclerosis (MS)
is a myelin-related disease that usually begins between the ages of 20 and 50 in individuals with apparently healthy nervous systems.

With MS, a person’s own immune system, for reasons unknown, attacks and destroys myelin as if it were a foreign substance.

Eventually, these repeated attacks leave multiple scarred (sclerotic) areas of tissue in the nervous system and impair the function of myelinated neurons that control movement, speech, memory, and emotion.

Multiple sclerosis is a serious and unpredictable disease, characterized by flare-ups followed by periods of remission in which symptoms are reduced or absent.