Module 4

Question 1

The membrane potential is established by what two forces?

Answer 1

1) Electrostatic pressure (i.e., opposite charges attract; identical charges repel) and;

2) Pressure for ions to move down their concentration gradients. (Concentration gradients induce diffusion (passive movement) of ions from areas of high concentration to areas of relatively low concentration.)

Question 2

The resting membrane potential is ______

a) negative

b) positive

Answer 2

a) negative

It’s negative (typically between -65 to -70 mV) because there is a higher concentration of cations (positively charged ions) in the extracellular fluid compared to the cytoplasm).

Question 3

What is depolarization?

Answer 3

When there is a predominant influx of positive ions, the neuron becomes less negative, which we call depolarization (in fact, the neuron may even become positive, as during the generation of an action potential).

Question 4

What is hyperpolarization?

Answer 4

When there is a net addition of negative charges to the inside of a neuron, it becomes more negative, an event termed hyperpolarization.

Question 5

For neurons, the membrane potential changes over time, and these changes are caused by ______moving in and out of the neuron.

Answer 5

ions

(As ion channels in the cell membrane open and ions move in and out of the neuron according to their electrochemical gradients, the membrane potential will become more or less negative, depending on the net movement of charge. This is the process of depolarization and hyperpolarization.)

Question 6

Because the phospholipid bilayer that makes up the cell membrane is an effective block against the movement of charged molecules, specialized membrane proteins are required to facilitate ion movement.

The membrane proteins responsible for facilitating ion movement are __________ and _____________.

Answer 6

ion channels, transporters.

Question 7

What are ion channels?

What are transporters?

Answer 7

An ion channel is a protein that facilitates the passive movement of ions from regions of high concentration to low concentration (diffusion) through a pore.

Transporters are membrane proteins that facilitate active (energy-dependent) movement of ions and molecules in the absence of a pore.

Question 8

What are the 3 main types of transporters and what do they do?

Answer 8

1) Pumps: move ions in or out of the neuron, against their concentration gradient, using energy derived from the breakdown of adenosine triphosphate (ATP, the main source of energy for all cells, obtained by the metabolism of glucose).

2) Co-transporters: move ions in or out of the neuron, against their concentration gradients, using energy derived from the diffusion of other ions in the same direction.

3) Exchangers: move ions in or out of the neuron, against their concentration gradients, using energy derived from the diffusion of other ions in the opposite direction.

Question 9

The plasma membrane contains proteins that allow both passive and active ion movement. The ion channels facilitate the ______ movement of ions down concentration gradients.

In contrast, exchangers, co-transporters, and pumps facilitate the movement of ions ________ their concentration gradients in an energy-dependent manner.

Answer 9

passive

against

Question 10

The pore of an ion channel is opened and closed via a gaiting mechanism, limiting ion movement and membrane permeability for certain ion species. The gaiting mechanism of most ion channels is a change in protein conformation (shape) that opens and closes the pore.

Most ion channels can be categorized into what 2 categories?

Answer 10

1) Voltage-gated channels - open and close in response to changes in membrane potential (depolarization or hyperpolarization)

2) Ligand-gated channels - open in response to the binding of an extracellular chemical, such as a neurotransmitter or neuromodulator to a receptor associated with the channel.

Question 11

The membrane potential is maintained by concentration gradients and electrostatic pressure. The ion species that contribute most strongly to the membrane potential are ____, _____ and ______.

Answer 11

Na+, K+ and Cl-

Question 12

Ions move in and out of neurons via ________ and __________.

Answer 12

ion channels and transporters.

Question 13

Ion channels may open and close in response to changes in ___________ or the binding of a _______.

Answer 13

membrane potential

ligand

Question 14

When Cl- channels open in a neuron at rest, which of the following will occur?

a) Cl- will flow out of the cell and the cell will be hyperpolarized

b) Cl- will flow out of the cell and the cell will be depolarized

c) Cl- will flow into the cell and the cell will be hyperpolarized

d) Cl- will flow into the cell and the cell will be depolarized

Answer 14

c) Cl- will flow into the cell and the cell will be hyperpolarized

The reason for the correct answer above is:

Cl- follows its concentration gradient.

Question 15

If a neuron at rest becomes highly permeable to sodium (i.e., Na+ channels open), the membrane potential will become:

a) polarized

b) depolarized

c) hyperpolarized

d) repolarized

Answer 15

b) depolarized

Question 16

Researchers often record the activity of neurons in fresh, living brain slices. Preparing brain slices without killing all of the neurons in the slice is extremely difficult. One strategy for maintaining neuron health when preparing a slice is to prevent them from firing action potentials. How might this be accomplished?

a) bathing the slices in a solution lacking K+

b) bathing the slices in a solution lacking Cl-

c) bathing the slices in a solution that contains high levels of Na+

d) bathing the slices in a solution lacking Na+

Answer 16

d) bathing the slices in a solution lacking Na+

Question 17

While recording from a neuron in a slice of brain tissue, you identify a novel membrane protein that facilitates the movement of Na+ using energy derived from glucose. What type of membrane protein have you likely identified?

a) Na+ ion channel

b) co-transporter

c) exchanger

d) pump

Answer 17

d) pump

Question 18

True or False?

Even small changes in voltage-gated ion channel function can have dramatic effects on neuron activity and health.

Answer 18

True.

Given the sensitivity of a neuron’s membrane potential to current flow through open ion channels, even small changes in voltage-gated ion channel function can have dramatic effects on neuron activity and health.

Question 19

How do a number of toxins derived from plants and animals (or synthesized in a lab) disrupt nervous system function?

Answer 19

By altering the activity of voltage-gated ion-channels, transporters, and receptors.

These chemicals disrupt the ability of neurons to maintain ion gradients and generate action potentials. Toxins can alter ion channel activity by blocking or opening them, or by altering the amount of time that the channels remain open.

Question 20

What are two kinds of naturally-occurring toxicants?

Answer 20

1) Tetrodotoxin - originally isolated from pufferfish found along the coast of Japan. It’s highly lethal.
Tetrodotoxin blocks voltage-gated Na+ channels, preventing neuron membranes from depolarizing and reaching the threshold for generating action potentials. Tetrodotoxin can be lethal because it prevents neurons from exciting muscles, resulting in paralysis of muscles throughout the body, including those of the respiratory tract.

2) Palytoxin - produced by marine species including certain corals. It’s highly lethal. Rather than preventing the opening of Na+ channels, palytoxin disrupts neuron activity by converting the Na+ /K+ pump into a non-selective cation channel (that is, it allows for the non-selective movement of positive
ions).

Question 21

Which of the following drugs would reduce the ability of a neuron to generate an action potential?

a) tetrodotoxin, which prevents the opening of Na+ channels

b) DDT, an insecticide that opens Na+ channels

c) batrachotoxin, which results in persistent activation of Na+ channels

d) lorazepam, a drug that opens ligand-gated Cl- channels

Answer 21

a) tetrodotoxin, which prevents the opening of Na+ channels

Question 22

Here is a list of four (hypothetical) drugs and their respective LD50 values. Which of these drugs is the safest for you to consume?

a) drug A: LD50 = 3 kg/kg body weight

b) drug B: LD50 = 4 g/kg body weight

c) drug C: LD50 = 5 mg/kg body weight

d) drug D: LD50 = 6 µg/kg body weight

Answer 22

a) drug A: LD50 = 3 kg/kg body weight

Question 23

You are a biopsychologist investigating a newly discovered brain area (area X). Your experiment involves (a) exciting and (b) inhibiting neurons in area X and observing effects on the behavior in mice. To cause neuronal excitation and inhibition, you use drugs with the following effects on ion channels:

a). excitation: open K+ channels; inhibition: open Na+ channels

b). excitation: open Na+ channels; inhibition: open K+ channels

c). excitation: open Na+ channels; inhibition: open Cl- channels

d). excitation: open Cl- channels; inhibition: open K+ channels

e.) both (b) and (c) are correct

Answer 23

e.) both (b) and (c) are correct

Question 24

Which of the following drugs are used clinically to alleviate chronic (neuropathic) pain by modifying the activity of Na+ channels?

a). selective serotonin reuptake inhibitors

b). tricyclic antidepressants

c). local anesthetics

d). tetrodotoxin

e). both (b) and (c) are correct

Answer 24

e). both (b) and (c) are correct

Question 25

Hodgkin and Huxley made several important discoveries about the nature and induction of action potentials. List 3 of those discoveries.

Answer 25

A neuron’s membrane potential rapidly reverses during an action potential; the membrane potential is transiently positive (approximately +40 mV). Influx of Na+ through open voltage-gated channels depolarizes the membrane, thus triggering the action potential. During the action potential, voltage-gated K+ channels open and K+ flows out of the neuron, eventually returning the membrane potential to the resting membrane potential.

Question 26

Regarding the video in module 4.3...... What are three different changes in ion channel conductance induced by the drug that would have an effect to depolarize the neuron and increase firing rate?

Answer 26

1) INCREASED FLOW OF NA+ INTO THE NEURON 2) DECREASED FLOW OF CL- INTO THE NEURON 3) DECREASED FLOW OF K+ OUT OF THE NEURON

Question 27

What are field potentials?

Answer 27

Potentials generated by the summed activity of a relatively large "field of neurons", as opposed to potentials in a single neuron, as discussed so far. These field potentials are sufficiently large that they can be recorded by electrodes attached to the scalp. This is the basis for electroencephalography (EEG).

Question 28

EEG signals, often referred to as “brain waves”, have high _______ resolution, meaning that changes in neuronal activity are very quickly reflected by changes in EEG signals.

Answer 28

temporal

Question 29

EEG signals can be categorized on the basis of their _____________.

Answer 29

frequencies (i.e., how fast the signal oscillates). EEG signals are often associated with specific behavioral states (e.g., low-frequency “slow waves” are readily observed during deep sleep, whereas higher frequency signals occur more frequently during active waking).

Question 30

Given that action potentials are all-or-none, what measure might a researcher use to determine whether a drug is excitatory or inhibitory while recording from a single neuron? a) The amplitude of action potentials b) The frequency of action potentials c) The shape of action potentials d) None of the above measures are informative in this case

Answer 30

b) The frequency of action potentials

Question 31

How might a researcher induce the generation of action potentials when recording from a neuron? a). Injecting positive (depolarizing) current into the neuron b). Injecting negative (hyperpolarizing) current into the neuron c). Applying an excitatory drug to the neuron d). Both A and C

Answer 31

d). Both A and C

Question 32

The EEG can be recorded with varying numbers of electrodes attached to the skull, ranging from as few as two electrodes to hundreds. The more electrodes are used, the more time-consuming the procedure will be. Assume that you are a very busy neurologist who sees 20-30 patient a day. Today, one of your patients is suspected of having a tumor, which interferes with their auditory processing. What would you suggest to do for the EEG assessment? a) Use a smaller number of electrodes, since it will save valuable time b) Use many electrodes, spaced out over the entire brain (i.e., all four lobes of the cortex) c) Use an intermediate number of electrodes, all spaced out over the occipital cortex d) Use an intermediate number of electrodes, all spaced out over the temporal cortex

Answer 32

d) Use an intermediate number of electrodes, all spaced out over the temporal cortex

Question 33

Which of the following is NOT an example of a neuroprosthetic? a) Deep brain stimulation b) A retinal implant that bypasses retinal neurons to transmit visual signals to the brain c) A device that electrically stimulates the spinal cord to restoring movement d) An EEG cap that records brain activity to detect seizures

Answer 33

d) An EEG cap that records brain activity to detect seizures

Question 34

Which of the following is NOT involved in deep-brain stimulation? a) Stereotactic surgery to implant leads and electrodes into the brain b) Surgical implantation of a pulse generator to control brain stimulation c) Programming of electrical pulse parameters to optimize brain stimulation d) Anticonvulsant medications to enhance the effects of brain stimulation

Answer 34

d) Anticonvulsant medications to enhance the effects of brain stimulation

Question 35

A neurologist and neurosurgeon implant deep-brain stimulation electrodes into a patient diagnosed with short-term memory deficits to enhance brain signals in a relevant brain area. Which brain area should they target? a) hippocampus b) neocortex c) prefrontal cortex d) basal ganglia

Answer 35

c) prefrontal cortex

Question 36

Based on your understanding of neuroanatomy (Module 3), where in Gertrude’s brain was the Neuralink most likely implanted? a) the motor cortex b) the somatosensory cortex c) the auditory cortex d) the olfactory cortex

Answer 36

b) the somatosensory cortex

Question 37

How does one record a neuron’s membrane potential?

Answer 37

Position the tip of one electrode inside the neuron and the tip of another electrode outside the neuron in the extra- cellular fluid. Although the size of the extracellular electrode is not critical, the tip of the intracellular electrode must be fine enough to pierce the neural membrane without damaging it. The intracellular electrodes are called microelectrodes.

Question 38

When the tip of the intracellular electrode is inserted into a neuron that is at rest (not receiving signals from other cells), a steady potential of about __________ millivolts (mV) is recorded.

Answer 38

−70 This indicates that the potential inside the resting neuron is about 70 mV less than that outside the neuron. This steady membrane potential of about −70 mV is called the neuron’s resting potential. In its resting state, with the −70 mV charge built up across its membrane, a neuron is said to be polarized (it has a membrane potential that is not zero).

Question 39

What is membrane potential?

Answer 39

The difference in electrical charge between the inside and the outside of a cell.

Question 40

Like all salts in solution, the salts in neural tissue separate into positively and negatively charged particles called ______.

Answer 40

ions.

Question 41

In resting neurons, there are more ______ ions outside the cell than inside and more ________ ions inside than outside.

Answer 41

Na+ , K+

Question 42

These unequal distributions of Na and K ions are maintained even though there are specialized pores in the neural membrane, called ___________.

Answer 42

ion channels. Each type of ion channel is specialized for the passage of particular ions (e.g., Na+ or K+). For example, some ion channels are specialized for the passage of Na+ ions, K+ ions, or other ions.

Question 43

There is substantial pressure on Na+ ions to enter the resting neurons. This pressure is of what two types?

Answer 43

1) Electrostatic pressure - from the resting membrane potential: Because opposite charges attract, the positively charged Na+ ions are attracted to the −70 mV charge inside resting neurons. 2) Pressure from random motion for Na+ ions to move down their concentration gradient. ( particles in random motion tend to become evenly distributed because they are more likely to move down their concentration gradients than up them; that is, they are more likely to move from areas of high concentration to areas of low concentration than vice versa.)

Question 44

So, why then do Na+ ions under electrostatic pressure and pressure from random movement not come rushing into neurons, thus reducing the resting membrane potential?

Answer 44

The sodium ion channels in resting neurons are closed, thus greatly reducing the flow of Na+ ions into the neuron. (In contrast, the potassium ion channels are open in resting neurons, but only a few K+ ions exit because the electrostatic pressure that results from the negative resting membrane potential largely holds them inside.)

Question 45

The sodium ion channels in resting neurons are closed, thus greatly reducing the flow of Na+ ions into the neuron. HOWEVER, some Na+ ions do manage to enter resting neurons despite the closed sodium channels and some K+ ions do exit. Why then does the resting membrane potential stay fixed?

Answer 45

Because of sodium-potassium pumps. At the same rate that Na+ ions leaked into resting neurons, other Na+ ions were actively transported out; and at the same rate that K+ ions leaked out of resting neurons, other K+ ions were actively transported in. Such ion transport is performed by mechanisms in the cell membrane that continually exchange three Na+ ions inside the neuron for two K+ ions outside. These transporters are commonly referred to as sodium–potassium pumps.

Question 46

What are transporters?

Answer 46

Mechanisms in the membrane of a cell that actively transport ions or molecules across the membrane.

Question 47

Disturbances of the resting membrane potential are termed _______________.

Answer 47

postsynaptic potentials (PSPs).

Question 48

When neurotransmitter molecules bind to postsynaptic receptors, they typically have one of two effects, depending on the neurotransmitter, receptor, and postsynaptic neuron in question. What are the 2 effects?

Answer 48

1) They may depolarize the receptive membrane (decrease the resting membrane potential, from −70 to −67 mV, for example), or 2) They may hyperpolarize it (increase the resting membrane potential, from −70 to −72 mV, for example).

Question 49

Postsynaptic depolarizations are called _________________ .

Answer 49

Excitatory postsynaptic potentials (EPSPs) -they increase the likelihood that the neuron will fire.

Question 50

Postsynaptic hyperpolarizations are called __________________.

Answer 50

Inhibitory postsynaptic potentials (IPSPs) -they decrease the likelihood that the neuron will fire.

Question 51

All PSPs, both EPSPs and IPSPs, are graded potentials. What does this mean?

Answer 51

This means that the amplitudes of PSPs are proportional to the intensity of the signals that elicit them: Weak signals elicit small PSPs, and strong signals elicit large ones.

Question 52

EPSPs and IPSPs travel passively from their sites of generation at synapses, usually on the dendrites or cell body, in much the same way that electrical signals travel through a cable. Accordingly, the transmission of PSPs has what two important characteristics?

Answer 52

1) It is rapid - so rapid that it can be assumed to be instantaneous for most purposes. It is important not to confuse the duration of PSPs with their rate of transmission; although the duration of PSPs varies considerably, all PSPs, whether brief or enduring, are transmitted almost instantaneously. 2) It is is decremental - They decrease in amplitude as they travel through the neuron, just as a ripple on a pond gradually disappears as it travels outward. Most PSPs do not travel more than a couple of millimeters from their site of generation before they fade out completely.

Question 53

Why do the PSPs created at a single synapse typically have little effect on the firing of the postsynaptic neuron?

Answer 53

The receptive areas of most neurons are covered with thousands of synapses, and whether a neuron fires is determined by the net effect of their activity. More specifically, whether a neuron fires depends on the balance between the excitatory and inhibitory signals reaching its axon.

Question 54

True or False? Action potentials are generated at the axon hillock (the conical structure at the junction between the cell body and the axon).

Answer 54

False. They are actually generated in the adjacent section of the axon, called the axon initial segment.

Question 55

Ff the sum of the depolarizations and hyperpolarizations reaching the axon initial segment at any time is sufficient to depolarize the membrane to a level referred to as its _______________ —usually about −65 mV—an action potential is generated.

Answer 55

threshold of excitation

Question 56

How long does the action potential last?

Answer 56

It's a massive but momentary—lasting for 1 millisecond—reversal of the membrane potential from about −70 to about +50 mV.

Question 57

True or False? Unlike PSPs, APs are not graded responses.

Answer 57

True. Their magnitude is not related in any way to the intensity of the stimuli that elicit them. To the contrary, they are all-or-none responses; that is, they either occur to their full extent or do not occur at all.

Question 58

In effect, each neuron adds together all the graded excitatory and inhibitory PSPs reaching its axon initial segment and decides to fire or not to fire on the basis of their sum. The summation of PSPs occurs in two ways?

Answer 58

over space and over time. (spatial summation, temporal summation)

Question 59

Why can successive stimulations of a neuron add together over time?

Answer 59

The PSPs they produce often outlast them. Thus, if a particular synapse is activated and then activated again before the original PSP has completely dissipated, the effect of the second stimulation will be superimposed on the lingering PSP produced by the first. Accordingly, it is possible for a brief subthreshold excitatory stimulus to fire a neuron if it is administered twice in rapid succession. In the same way, an inhibitory synapse activated twice in rapid succession can produce a greater IPSP than that produced by a single stimulation.

Question 60

True or False? Stimulating a neuron more intensely increases the speed and/or amplitude of the resulting action potential.

Answer 60

False. The firing of a neuron is like the firing of a gun. Both reactions are triggered by graded responses. As a trigger is squeezed, it gradually moves back until it causes the gun to fire; as a neuron is stimulated, it becomes less polarized until the threshold of excitation is reached and firing occurs. Furthermore, the firing of a gun and neural firing are both all-or-none events. Just as squeezing a trigger harder does not make the bullet travel faster or farther - so stimulating a neuron more intensely DOES NOT increase the speed/amplitude of the action potential.

Question 61

_______ is a common chemical used to alleviate symptoms in people living with Parkinson’s disease.

Answer 61

L-dopa

Question 62

The difference in electrical charge between the inside and outside of a nerve cell is called _______ and is recorded using microelectrodes.

Answer 62

membrane potential,

Question 63

The resting potential inside the neuron is approximately _______ mV less than that outside the cell. This is called polarization.

Answer 63

70

Question 64

Sodium and potassium ions are both _______ charged.

Answer 64

positively

Question 65

In a resting neuron, there are more _______ ions outside the cell and more _______ ions inside the cell.

Answer 65

Na+, K+

Question 66

K+ ions are largely held inside the cell because of the membrane’s _______ resting potential.

Answer 66

negative

Question 67

_______ pumps ensure that at resting potential, three Na+ ions move inside the cell and two K+ ions move outside the cell.

Answer 67

Sodium–potassium

Question 68

_______ are released into the synaptic cleft, and they attach to receptor molecules on the postsynaptic membrane of the next cell.

Answer 68

Neurotransmitters

Question 69

The neurotransmitters may _______ the postsynaptic receptive membrane, which implies that the resting membrane potential will increase.

Answer 69

hyperpolarize

Question 70

_______ postsynaptic potentials increase the likelihood that a neuron will fire.

Answer 70

Excitatory

Question 71

Postsynaptic potentials _______ in amplitude as they travel through the neuron.

Answer 71

decrease

Question 72

A momentary increase of membrane potential to about +50 mV is called _______.

Answer 72

action potential

Question 73

Each neuron sums the number of excitatory and inhibitory postsynaptic potentials to create a single signal, a process called _______.

Answer 73

integration

Question 74

When postsynaptic potentials that are produced in rapid succession at the same synapse are added, we have _______ summation.

Answer 74

temporal

Question 75

When postsynaptic potentials that are produced simultaneously in different parts of the receptive membrane are added, we have _______ summation.

Answer 75

spatial

Question 76

The firing of neurons and the firing of a gun are both _______ responses.

Answer 76

all-or-none

Question 77

How are action potentials (APs) produced? How are they conducted along the axon? (Hint: The answer to both questions is the same)

Answer 77

Through the action of voltage-gated ion channels— ion channels that open or close in response to changes in membrane potential.

Question 78

Explain the ionic basis of an action potential.

Answer 78

When the membrane potential of the axon initial segment is depolarized to the threshold of excitation by a sufficiently large EPSP, the voltage-gated sodium channels in the axon membrane open wide, and Na+ ions rush in, suddenly reversing the membrane potential; that is, driving the membrane potential from about −70 to about +50 mV. The rapid change in the membrane potential associated with the influx of Na+ ions then triggers the opening of voltage-gated potassium channels. K+ ions near the membrane are driven out of the cell through these channels (first by their relatively high internal concentration and then, when the AP is near its peak, by the positive internal charge.) After about 1 millisecond, the sodium channels close. This closure marks the end of the rising phase of the AP and the beginning of the repolarization phase, which is the result of the continued efflux of K+ ions. Once repolarization has been achieved, the potassium channels gradually close, which marks the beginning of the hyperpolarization phase. Because they close gradually, too many K+ ions flow out of the neuron, and it is left hyperpolarized for a brief period of time.

Question 79

True or False? A single AP has little effect on the relative concentrations of various ions inside and outside the neuron.

Answer 79

True. The number of ions that flow through the membrane during an AP is extremely small in relation to the total number inside and around the neuron. The AP involves only those ions right next to the membrane. Therefore, a single AP has little effect on the relative concentrations of various ions inside and outside the neuron, and the resting ion concentrations next to the membrane are rapidly reestablished by the random movement of ions. The sodium–potassium pumps play only a minor role in the reestablishment of the resting potential.

Question 80

There is a brief period of about 1 to 2 milliseconds after the initiation of an AP during which it is impossible to elicit a second AP. This period is called the _______________ period.

Answer 80

absolute refractory

Question 81

The absolute refractory period is followed by the __________ refractory period—the period during which it is possible to fire the neuron again but only by applying higher-than-normal levels of stimulation.

Answer 81

relative The end of the relative refractory period is the point at which the amount of stimulation necessary to fire the neuron returns to baseline.

Question 82

Refractory periods are responsible for what two important characteristics of neural activity?

Answer 82

1) They are responsible for the fact that APs normally travel along axons in only one direction. Because the portions of an axon over which an AP has just traveled are left momentarily refractory, an AP cannot reverse direction. 2) They are responsible for the fact that the rate of neural firing is related to the intensity of the stimulation.

Question 83

If a neuron is subjected to continual high-intensity stimulation, it fires and then fires again as soon as its absolute refractory period is over—a maximum of about 1,000 times per second. However, if the level of continuous stimulation is of an intensity just sufficient to fire the neuron when it is at rest, what happens?

Answer 83

The neuron does not fire again until both the absolute and the relative refractory periods have run their course. Intermediate intensities of continuous stimulation produce intermediate rates of neural firing.

Question 84

The conduction of APs along an axon differs from the conduction of PSPs in what two important ways?

Answer 84

1) The conduction of APs along an axon is typically nondecremental; APs do not grow weaker as they travel along the axonal membrane. 2) APs are conducted more slowly than PSPs. (These two differences are the result of the important role played by voltage-gated sodium channels in AP conduction. Once an AP has been generated, it travels along the axon as a graded potential; that is, it travels rapidly and decrementally. However, when that graded potential reaches the next voltage-gated sodium channel along the axon, and if it is sufficiently large (i.e., it exceeds the threshold of excitation), then those channels open and Na+ ions rush into the axon and generate another full-blown AP. In essence, the AP is continually regenerated at each sodium channel along the length of the axon, again and again until a full-blown AP is triggered as the axon terminal buttons.)

Question 85

The nondecremental nature of AP conduction is readily apparent from this analogy; the last trap on the shelf strikes with no less intensity than did the first. This analogy also illustrates another important point: The row of traps can transmit in either direction, just like an axon. If electrical stimulation of sufficient intensity is applied to a midpoint of an axon, two APs will be generated:

Answer 85

1) One AP will travel along the axon back to the cell body—this is called antidromic conduction; 2) The second AP will travel along the axon towards the terminal buttons—this is called orthodromic conduction.

Question 86

The axons of many neurons are insulated from the extracellular fluid by segments of fatty tissue called _________.

Answer 86

myelin.

Question 87

In myelinated axons, ions can pass through the axonal membrane only at the _________.

Answer 87

nodes of Ranvier (the gaps between adjacent myelin segments)

Question 88

In myelinated axons, axonal voltage-gated sodium channels are concentrated at the nodes of Ranvier. So how then are APs transmitted in myelinated axons?

Answer 88

If we consider the mouse trap metaphor again, the answer is quite simple: It is just as if the mouse traps were placed further apart along the wobbly shelf. That is, because the sodium channels are concentrated at some distance from one another (at the nodes of Ranvier), the electrical signal generated at the sodium channels at one node of Ranvier travels instantly and decrementally (i.e., it is a graded potential) to the sodium channels at the next node, and so on down the length of the myelinated axon.

Question 89

What does myelination do?

Answer 89

Myelination increases the speed of axonal conduc- tion. Because conduction along the myelinated segments of the axon is instantaneous (i.e., it is a graded potential) the signal “jumps” along the axon from one node of Ranvier to the next. There is, of course, a slight delay at each node while the AP is regenerated, but conduction is still much faster in myelinated axons than in unmyelinated axons.

Question 90

The transmission of APs in myelinated axons is called _________________.

Answer 90

saltatory conduction (saltare means “to skip or jump”).

Question 91

At what speed are APs conducted along an axon? The answer to this question depends on what two properties of the axon?

Answer 91

1) Conduction is faster in large-diameter axons and 2) It is faster in those that are myelinated.

Question 92

Flip to see an explanation of CONDUCTION IN NEURONS WITHOUT AXONS.

Answer 92

APs are the means by which axons conduct all-or-none signals nondecrementally over relatively long distances. Remember that most neurons in mammalian brains either do not have axons or have very short ones, and many of these neurons do not normally display APs. Conduction in these interneurons is typically only through graded potentials.

Question 93

What are the 5 aspects of synaptic transmission?

Answer 93

(1) the structure of synapses; (2) the synthesis, packaging, and transport of neurotransmitter molecules; (3) the release of neurotransmitter molecules; (4) the activation of receptors by neurotransmitter molecules; and (5) the reuptake, enzymatic degradation, and recycling of neurotransmitter molecules.

Question 94

Most synapses in the brain form a tripartite synapse. What is a triparte synapse?

Answer 94

A synapse that involves two neurons and an astroglial cell. All three cells communicate with one another through synaptic transmission.