BSc 1407 MB Chapter 48: Neurons, Synapses, and Signaling

Resting neurons are most permeable to which of the following ions?

K+. Resting neurons are most permeable to K+ ions.

Scientific Skills Exercise: Interpreting Data Values Expressed in Scientific Notation Part C Compare the concentrations for methadone (2 ×× 10-8 M) and phenobarbital (10-4 M). Which concentration is higher and by how much?

Phenobarbital's concentration is 5,000 times higher.

Solve It: What Is Causing Episodes of Muscle Weakness in a Patient? Part G - What is your final diagnosis? Based on your analysis, what is your final diagnosis of the patient?

Patient has hyperthyroidism caused by a pituitary tumor. He has thyrotoxic periodic paralysis (TPP).

A stimulus has opened the voltage-gated sodium channels in an area of a neuron's plasma membrane. As a result, _____ rushes into the neuron and diffuses to adjacent areas; this in turn results in the _____ in the adjacent areas.

sodium ... opening of voltage-gated sodium channels This describes part of the process by which an action potential travels along an axon.

How Neurons Work (3 of 3): Conduction of an Action Potential (BioFlix tutorial) Part D - Conduction speed of an action potential There are two properties that affect the conduction speed of an action potential along an axon: the axon's diameter and whether or not the axon is myelinated. Rank the axons from slowest to fastest conduction speed. If two axons have the same conduction speed, place one on top of the other.

(slowest to fastest) non-myelinated invertebrate axon, 20 um diameter non-myelinated invertebrate axon, 30 um diameter non-myelinated invertebrate axon, 40 um diameter myelinated invertebrate axon, 30 um diameter For non-myelinated axons, the larger the diameter of the axon, the faster the conduction speed of an action potential. However, even the smallest myelinated axons are much faster than the largest non-myelinated axons (the squid axon, for example).

Identify the correct statement(s) about the resting membrane potential of a cell.

- Concentration gradients of potassium (K+) and sodium (Na+) across the plasma membrane represent potential energy. - Potassium (K+) and sodium (Na+) gradients are maintained by active transport in a resting mammalian neuron.

A neuron has a resting potential of about _____ millivolts.

-70 This is the resting potential, the charge difference found across the plasma membrane of a "resting" neuron.

Scientific Skills Exercise: Interpreting Data Values Expressed in Scientific Notation Part A - Understanding scientific notation The data from this experiment are expressed using scientific notation: a numerical factor times a power of 10. Remember that a negative power of 10 means a number less than 1. For example, the concentration 10-1 M (molar) can also be written as 0.1 M. What is the lowest concentration of morphine that blocked naloxone binding, in standard notation?

0.000000006 M 10-9 is 0.000000001, so 6 × 10-9 M is 0.000000006 M.

Solve It: What Is Causing Episodes of Muscle Weakness in a Patient? Part D - Understanding the effects of hypokalemia on muscle physiology

1. Group 1: Out of the cell 2. Group 2: Hyperpolarized 3. Group 3: Negative 4. Group 4: Harder 5. Group 5: Less 6. Group 4: Harder Group 2: Depolarized Ions flow from areas of high concentration to areas of low concentration (down a gradient). The bigger the difference between two areas, the stronger the driving force, and the more ions that move toward the lower concentration area. In patients with hypokalemia, there is less K+ outside the cell than normal. As a result, more K+ will flow out of the cell through K+ channels. The effect of more K+ leaving the cell is that the inside of the cell becomes more negative. As a result, the cell becomes hyperpolarized (outside becomes more positive, inside becomes more negative) and the resting membrane potential in these patients is more negative than normal. This means that it will be harder to trigger an action potential and more difficult to trigger muscle contraction. This explains why your patient is experiencing muscle weakness and paralysis.

Solve It: What Is Causing Episodes of Muscle Weakness in a Patient? Part E - What is causing your patient's hypokalemia? Use the data above to calculate your patient's TTKG and answer the following questions.

1. TTKG = 1.87 2. no 3. yes Your patient's TTKG is 1.87, which is low. This indicates that the kidneys are not secreting large amounts of K+, which is the normal response to hypokalemia. The patient does not have a kidney dysfunction and instead appears to have an ion channel disorder caused by a genetic mutation. Intermittent paralysis due to hypokalemia caused by an ion channel disorder is known as hypokalemic periodic paralysis (HPP).

Solve It: What Is Causing Episodes of Muscle Weakness in a Patient? Part A - Identifying possible diagnoses using MedlinePlus

1. _______ is an autoimmune disorder. Antibodies block acetylcholine receptors, preventing the opening of ion channels to depolarize the muscle cell. Patients first exhibit eyelid and eye muscle weakness or paralysis. Myasthenia gravis 2. ______ is a condition in which the level of potassium (K+) in the blood rises above normal. Hyperkalemia 3. ________ is a condition in which the level of potassium (K+) in the blood drops below normal. Hypokalemia 4.________ is caused by a bacterial infection. Bacterial toxin prevents the fusion of vesicles to the membrane, stopping neurotransmitter release. Botulism

How Neurons Work (3 of 3): Conduction of an Action Potential (BioFlix tutorial) Part B - Conduction of an action potential along an axon This diagram shows the changes in charge distribution that occur across an axon membrane as an action potential propagates from left to right. The region where the action potential is occurring is shown in orange. Seven locations along the axon are marked by the letters a through g. Match the letter of each location along the axon with the correct description of what is occurring at that position.

1. c 2. f 3. a 4. d 5. g 6. b 7. e As an action potential moves along an axon, one location reaches the rising phase of the action potential, while a nearby location reaches the peak, while another location reaches the falling phase, and so on. You can use the familiar graph of an action potential to pinpoint the stage of the action potential occurring at various locations on the axon as the action potential moves along. For example, at location (f), the action potential has just started—the membrane has reached threshold and the voltage-gated Na+ channels open. At location (d), the action potential is at its peak—the voltage-gated Na+ channels inactivate and the voltage-gated K+ channels open.

True or false? Action potentials travel in only one direction down an axon because potassium channels in the neuron are refractory and cannot be activated for a short time after they open and close.

False Action potentials travel in only one direction down an axon because sodium channels in the neuron are refractory.

Which term describes the difference in electrical charge across a membrane?

Membrane potential. Membrane potential is the difference in electrical charge across a membrane.

Which term describes an electrical signal generated by neurons?

Action potential. An action potential is a rapid electrical signal generated by neurons.

How Neurons Work (3 of 3): Conduction of an Action Potential (BioFlix tutorial) Part A - Depolarization of neighboring regions of the axon An action potential moves along an axon due to the sequential opening of voltage-gated Na+ channels. The diagram below shows voltage-gated Na+ channels separated by a short distance in the plasma membrane of an axon. Initially (left panel), only channel (a) is open. Within a very short time (right panel), channel (b) also opens. Which statement correctly describes what causes the second voltage-gated Na+ channel to open?

After the first channel opens, the movement of many types of ions (both inside and outside the cell) alters the distribution of charges near the second channel, causing it to open. When Na+ ions enter the cell through the first channel, the charge distribution across the membrane changes. Inside the cell, the increase in Na+ ions near the first channel makes that region more positive; as a result, negative ions are attracted to the region, while positive ions are repelled. Conversely, outside of the cell, the loss of Na+ ions makes the region near the first channel more negative; as a result, positive ions are attracted to that region, while negative ions are repelled. Together, all of these ion movements alter the charge (and thus the membrane potential) at the neighboring channel, allowing it to reach threshold.

Where in the neuron do action potentials begin?

Axon hillock. The axon hillock is the region where voltage-gated channels begin in a neuron, near the cell body.

Why is an action potential an all-or-none response to stimuli?

Because voltage-gated ion channels open when membrane potential passes a particular level

Solve It: What Is Causing Episodes of Muscle Weakness in a Patient? Part C - Confirming the diagnosis

ECG indicates hypokalemia. The patient's muscle weakness and paralysis are likely caused by a very low level of potassium in the blood, which is interfering with cell depolarization and normal muscle contraction.

True or false? The potential energy of a membrane potential comes solely from the difference in electrical charge across the membrane.

False The potential energy of a membrane potential comes both from the difference in electrical charge and from the concentration gradient of ions across a membrane.

Which of the following terms describes how a neuronal membrane's potential is altered in the presence of inhibitory signals?

Hyperpolarization. Inhibitory signals hyperpolarize the membrane and make the membrane potential even more negative than normal.

What causes the falling phase of the action potential? Select the best answer.

Inactivation of voltage-gated sodium channels and the opening of voltage-gated potassium channels

The plasma membrane of a neuron has voltage-gated sodium and potassium channels. What is the effect of membrane depolarization on these channels?

Membrane depolarization first opens sodium channels and then opens potassium channels.

Which structure is not part of a neuron?

Myelin sheath. The myelin sheath is the layer of Schwann cells wrapped around a neuron.

Scientific Skills Exercise: Interpreting Data Values Expressed in Scientific Notation Part E Would phenobarbital, atropine, or serotonin have blocked naloxone binding at a concentration of 10-5 M?

None of these drugs would have blocked naloxone binding at 10-5 M.

How Synapses Work (1 of 2): Chemical Synapses (BioFlix tutorial) Part A - Ion channels and chemical synapses Chemical synapses transmit information from the sending (presynaptic) cell to the receiving (postsynaptic) cell in the form of neurotransmitters. The release of neurotransmitter into the synaptic cleft and the resulting changes in the membrane potential of the postsynaptic cell (postsynaptic potentials) all depend on the presence of several different types of gated ion channels and the distribution of these channels in the pre- and postsynaptic cells. The image here illustrates a chemical synapse. Drag the labels onto the table to indicate which type(s) of gated ion channels are found in each membrane associated with a chemical synapse. (The letters in the table refer to the lettered structures in the image above.) Labels can be used once, more than once, or not at all.

PRESYNAPTIC CELL a. synaptic terminal ~ voltage-gated Na+ ~ voltage-gated K+ b. presynaptic membrane ~ voltage-gated Na+ ~ voltage-gated K+ ~ voltage-gated Ca2+ POSTSYNAPTIC CELL c. postsynaptic membrane ~ ligand-gated d. plasma membrane of cell body ~ none e. axon hillock ~ voltage-gated Na+ ~ voltage-gated K+ Voltage-gated Na+ and K+ channels are found only in membranes that propagate action potentials. Membranes found in a chemical synapse that propagate action potentials are the membrane of the synaptic terminal, including the presynaptic membrane, and the membrane of the axon hillock (and axon) of the postsynaptic neuron. Voltage-gated Ca2+ channels, which help regulate the release of neurotransmitter from the presynaptic neuron, are found only in the presynaptic membrane. Ligand-gated ion channels, which open in response to neurotransmitter in the synaptic cleft, are found only in the postsynaptic membrane. Finally, there are no gated ion channels in the plasma membrane of the cell body of the postsynaptic cell. For this reason, postsynaptic potentials are not propagated along the membrane of the postsynaptic cell in the same way that an action potential is propagated along a membrane containing voltage-gated Na+ and K+ channels.

Which channel is mainly responsible for the resting potential of a neuron?

Potassium leak channel. K+ ions flow along their concentration gradient to maintain the resting potential of a neuron.

What type of cell makes up the myelin sheath of a motor neuron?

Schwann cells Myelin sheaths are formed when Schwann cells wrap around the axons of motor neurons.

How is an action potential propagated down an axon after voltage-gated sodium channels open in a region of the neuron's membrane?

Sodium ions enter the neuron and diffuse to adjacent areas, resulting in the opening of voltage-gated sodium channels farther down the axon. The entry of sodium ions into the neuron and their diffusion to adjacent areas of the membrane causes those portions of the membrane to become depolarized and results in the opening of voltage-gated sodium channels farther down the axon, which release potassium ions to the outside, returning the charge to its previous state.

Solve It: What Is Causing Episodes of Muscle Weakness in a Patient? Part F - Is the thyroid involved in your patient's hypokalemic periodic paralysis? For each possible diagnosis, consider whether each test result supports or rejects that diagnosis. For help determining the tumor diagnoses, see Hint 1.

T3 & T4 Normal, not hyperthyroid: Rejects Graves Disease: Supports Thyroid Tumor: Supports Pituitary Tumor: Supports TSH Normal, not hyperthyroid: Rejects Graves Disease: Rejects Thyroid Tumor: Rejects Pituitary Tumor: Supports TSH receptor-activating autoantibody Normal, not hyperthyroid: Supports Graves Disease: Rejects Thyroid Tumor: Supports Pituitary Tumor: Supports The patient's high levels of T3, T4, and TSH reject a diagnosis of normal thyroid function. The patient's high levels of TSH reject a diagnosis of Graves disease or a thyroid tumor because, in both of those conditions, TSH levels should drop in response to elevated T3 and T4 levels. The patient's negative test for TSH receptor-activating autoantibodies rejects a diagnosis of Graves disease, which is an autoimmune disorder.

At rest, which of these plays a role in establishing the charge differential across a neuron's plasma membrane?

The sodium-potassium pump moving sodium ions out of the neuron and potassium ions into the neuron The sodium-potassium pump moves more sodium ions out of the cell than potassium ions into the cell; this net loss of positive ions establishes a charge differential across the plasma membrane.

Which event triggers the creation of an action potential?

The membrane depolarizes above a certain threshold potential. Influx of Na+ ions into the neuron can lead to membrane depolarization above the threshold potential; this event triggers the creation of an action potential.

What behavior is observed if the voltage across a neuronal membrane is set to -20 mV?

The sodium channel opens, and Na+ ions flow in. Sodium ions flow into the cell when the membrane potential is between -20 mV and 30 mV.

Which channel maintains the concentration gradients of ions across a neuronal membrane?

The sodium-potassium pump moving Na+ ions out and K+ ions in. This channel maintains the ion concentration gradients across a neuronal membrane.

Scientific Skills Exercise: Interpreting Data Values Expressed in Scientific Notation Part G When the researchers repeated the experiment using tissue from mammalian intestinal muscles rather than brains, they found no naloxone binding. What does this result suggest about opiate receptors in mammalian intestinal muscle tissue?

There are no opiate receptors in mammalian intestinal muscle tissue.

Which of the following statements about action potentials in a given neuron is false?

They are propagated down the length of the dendrite. They are propagated down the length of the axon NOT the dendrite

Scientific Skills Exercise: Interpreting Data Values Expressed in Scientific Notation Part F - Making inferences Morphine, methadone, and levorphanol blocked naloxone binding in this experiment. What do these results indicate about the brain receptors for naloxone?

They are specific for opiate drugs.

How Synapses Work (1 of 2): Chemical Synapses (BioFlix tutorial) Part B - Transmission of information across the synaptic cleft All chemical synapses exhibit the same general sequence of events during the transmission of information across the synaptic cleft. This sequence is always initiated by an action potential that travels down the presynaptic cell (the sending neuron) to its synaptic terminal(s). Drag the labels onto the flowchart to indicate the sequence of events that occurs in the presynaptic cell (orange background) and the postsynaptic cell (blue background) after an action potential reaches a chemical synapse.

a) Ca2+ channels in presynaptic membrane open briefly b) Ca2+ ions enter presynaptic cell c) neurotransmitter-containing vesicles fuse with presynaptic membrane d) neurotransmitter released into presynaptic cleft e) neurotransmitter binds to ligand-gated ion channels in postsynaptic membrane; channels open f) neurotransmitter degraded or removed from cleft; ligan-gated ion channels close The sequence of events at a chemical synapse is centered around the release of neurotransmitter into the synaptic cleft, in response to an action potential in the presynaptic cell. Recall that it is the neurotransmitter that "transmits" information from the presynaptic cell to the postsynaptic cell. The following steps occur as a result of an action potential reaching a synaptic terminal of the presynaptic cell. The diagram below indicates where each step occurs in a chemical synapse. Presynaptic cell1. Voltage-gated Ca2+ channels in the presynaptic membrane open briefly, allowing Ca2+ ions to enter the cell.2. This higher cytosolic Ca2+ concentration in the synaptic terminal causes some synaptic vesicles to fuse with the presynaptic membrane.3. By fusing with the presynaptic membrane, the synaptic vesicles release neurotransmitter into the synaptic cleft.Postsynaptic cell4. The increased concentration of neurotransmitter in the synaptic cleft causes it to bind to ligand-gated ion channels in the postsynaptic membrane. As a result, the channels open. Ions may then diffuse through the channels, causing a change in the membrane potential of the postsynaptic cell.5. Quickly, the neurotransmitter concentration in the synaptic cleft falls (due to degradation or removal), causing the release of neurotransmitter from the ligand-gated ion channels. As a result, the channels close.

How Synapses Work (2 of 2): Postsynaptic Potentials (BioFlix tutorial) Part B - Summation of postsynaptic potentials at the axon hillock The figure below shows a single postsynaptic neuron that has synapses with four other neurons via their synaptic terminals. Two of the presynaptic neurons produce EPSPs on the postsynaptic cell (green, E1 and E2), and the other two produce IPSPs (red, I1 and I2). The graph below shows the membrane potential measured at the axon hillock of the postsynaptic neuron. Each target indicates a change in the membrane potential at the axon hillock caused by a postsynaptic potential at one of the four synapses. Summation occurs when two or more postsynaptic potentials overlap in time; that is, one postsynaptic potential begins before the membrane has returned to resting potential after a previous postsynaptic potential. When such overlap occurs, the membrane potential measured at the axon hillock is the sum of the two (or more) overlapping postsynaptic potentials. Drag the labels onto the graph to indicate which presynaptic neuron produced each change in the axon hillock's membrane potential. Assume that the same number of ligand-gated ion channels opens at each synapse. (To review the effect of distance between the synapse and the axon hillock, see Hint 2.) Labels can be used once, more than once, or not at all.

a) I2 b) E2 c) E1 d) I1 e) E2 f) E1 By analyzing each change in the membrane potential at the axon hillock of the postsynaptic neuron, you can tell which presynaptic neuron produced the change. For example, when the membrane potential at the axon hillock becomes more negative (hyperpolarizes), you know that an inhibitory postsynaptic potential (IPSP) was produced at the synapse. Conversely, when the membrane potential at the axon hillock becomes less negative (depolarizes), you know that an excitatory postsynaptic potential (EPSP) was produced at the synapse.Because postsynaptic potentials decrease in magnitude with distance from the synapse, a smaller change in the axon hillock's membrane potential indicates that the presynaptic neuron that produced that potential is farther away. Conversely, a presynaptic neuron nearer the axon hillock will produce a larger change in the axon hillock's membrane potential.Finally, an action potential is generated in the postsynaptic cell only when the membrane potential at the axon hillock reaches threshold. In this example, both presynaptic neurons that produce EPSPs must produce potentials nearly simultaneously in order to bring the membrane potential at the axon hillock to threshold. This effect is called summation.

How Neurons Work (2 of 3): The Action Potential (BioFlix tutorial) Part A - Initiating an action potential Under most circumstances, once an axon's membrane potential reaches threshold (about -55 mV in mammals), an action potential is automatically triggered. The graph below shows the changes in membrane potential that occur in an axon membrane that is initially at resting potential. In response to a stimulus, the membrane slowly depolarizes until the membrane potential reaches a particular value, called threshold. At threshold, a rapid depolarization of the membrane occurs and an action potential is initiated. Drag the labels onto the flowchart to show the sequence of events that occurs once the membrane potential reaches threshold. You may use a label once or not at all.

a. many voltage-gated Na+ -channels open b. Na+ ions rush into the cell c. membrane potential rises (depolarizes) rapidly Once the membrane potential reaches threshold, the voltage-gated Na+ channels open and the Na+ ions move into the cell (moving down their electrochemical gradient). This influx of positive ions makes the inside of the cell less negative compared to the outside, and the membrane potential rises (depolarizes) rapidly.

How Neurons Work (2 of 3): The Action Potential (BioFlix tutorial) Part B - Voltage-gated channels and the action potential The fixed pattern of changes in membrane potential during an action potential is coordinated by the sequential opening and closing of voltage-gated ion channels. Can you identify the status (open/closed) of the voltage-gated Na+ and K+ channels during each phase of an action potential? Drag the appropriate labels onto the graph to indicate the status (open or closed) of the voltage-gated Na+ and K+ channels during each phase of an action potential. Labels may be used once, more than once, or not at all.

a. resting potential Na+ channels closed K+ channels closed b. rising phase Na+ channels open K+ channels closed c. falling phase Na+ channels closed K+ channels open d. undershoot Na+ channels closed K+ channels open e. resting potential Na+ channels closed K+ channels closed During the rising phase, the membrane potential becomes less negative because voltage-gated Na+ channels are open and Na+ ions enter the cell. During the falling and undershoot phases, the membrane potential becomes more negative because voltage-gated K+ channels are open (while voltage-gated Na+ channels are closed) and K+ ions leave the cell. At resting potential, both types of voltage-gated channels are closed and no ions move through the voltage-gated channels.

Which of these causes the release of neurotransmitter molecules?

an action potential reaching the end of the axon When an action potential reaches the end of an axon, vesicles fuse with the plasma membrane and release neurotransmitter into the synaptic cleft.

An action potential moves along a(n) _____.

axon An axon is the only portion of a neuron capable of generating an action potential.

A nerve impulse moves away from a neuron's cell body along _____.

axon Axons conduct a nerve impulse away from the cell body.

How Neurons Work (3 of 3): Conduction of an Action Potential (BioFlix tutorial) Part C - Direction of action potential conduction In the diagram, (a), (b), and (c) represent three points along a vertebrate axon where electrodes were implanted to detect action potentials. Under normal conditions, when this neuron produces an action potential, the action potential passes through point (a) first, followed by point (b), and then point (c). Suppose, however, that an action potential is artificially triggered at the point indicated by the red arrow. In what sequence would the action potential pass through points (a), (b), and (c)? Enter the sequence in which the action potential would pass through the points. Enter the letters in the correct order separated by commas. For example if the order is point (c), then (b), then (a), enter c, b, a. If the action potential would not pass though a point, do not include that point in your answer.

b,a,c Under the artificial conditions, the axon membrane is initially at resting potential everywhere except at the location indicated by the red arrow (where the membrane is artificially brought to threshold). Voltage-gated Na+ channels at that location open and Na+ ions rush into the cell. The depolarization caused by this influx of Na+ ions is "felt" both to the right and left of the open channels. Voltage-gated Na+ channels on both sides open, resulting in an action potential that moves in both directions. It reaches nearby locations (on both sides) first, then locations farther away.

A neuron's nucleus is located in its _____.

cell body The cell body is the region of a neuron where the nucleus is found.

If the membrane potential of a neuron decreases, the membrane potential _____.

becomes less negative.

A nerve impulse moves toward a neuron's cell body along _____.

dendrites Dendrites conduct an impulse from a synapse toward the cell body.

How Synapses Work (2 of 2): Postsynaptic Potentials (BioFlix tutorial) Part A - Types of postsynaptic potentials The binding of neurotransmitter to ligand-gated ion channels in the postsynaptic membrane causes these channels to open. As soon as the neurotransmitter is removed from the synaptic cleft, the ligand-gated ion channels close. In the brief time these channels are open, ions are able to diffuse across the postsynaptic membrane down their electrochemical gradient. The result is a postsynaptic potential, a brief change in the membrane potential of the dendrites and cell body of the postsynaptic cell. There are two types of postsynaptic potentials: excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). An EPSP is excitatory because it makes it more likely that the axon of the postsynaptic cell will trigger an action potential. Conversely, an IPSP is inhibitory because it makes it less likely that the axon of the postsynaptic cell will trigger an action potential. Sort the phrases into the appropriate bins depending on which type of postsynaptic potential they describe. If a phrase describes both types of potentials, drag it to the "both" bin.

excitatory postsynaptic potential (EPSP): -brings the postsynaptic membrane potential closer to threshold -depolarizes the postsynaptic membrane -results from the movement of Na+ ions into postsynaptic cell inhibitory postsynaptic potential (IPSP): -moves the postsynaptic membrane potential farther away from threshold -hyperpolarizes the postsynaptic membrane -results from the movement of K+ ions out of the postsynaptic cell both: -it is a graded potential Excitatory postsynaptic potentials (EPSPs) are excitatory because they make the postsynaptic neuron more likely to generate an action potential by depolarizing the membrane (making the membrane potential less negative) and bringing the membrane potential closer to threshold. This is often accomplished by opening ligand-gated Na+ channels in the postsynaptic membrane, which allows Na+ ions to enter the cell. In contrast, inhibitory postsynaptic potentials (IPSPs) make it more difficult for the postsynaptic neuron to produce an action potential by hyperpolarizing the membrane (making the membrane potential more negative) and moving the membrane potential farther from threshold. This is often accomplished by opening ligand-gated K+ channels in the postsynaptic membrane, which allows K+ ions to leave the cell. Regardless of whether they are excitatory or inhibitory, all postsynaptic potentials are graded, meaning that their magnitudes are variable. (Action potentials, on the other hand, are all-or-none events.) And because a postsynaptic potential is not propagated like an action potential, its magnitude decreases with distance from the synapse along the cell body.

How Neurons Work (2 of 3): The Action Potential (BioFlix tutorial) Part C - What determines how soon one action potential can follow another? The strength of a stimulus (for example, whether you feel a slight pain versus an intense pain) determines the number of action potentials sent along an axon. As the graphs show, a strong stimulus produces more action potentials spaced more closely together than a weak stimulus. The time between when a first action potential ends and a second action potential can be triggered is determined by the axon's refractory period. A second action potential cannot be triggered until the end of the refractory period. Which of the following characteristics determines when the refractory period ends?

how long it takes for the voltage-gated Na+ channels to reactivate at the end of an action potential During the refractory period, an action potential cannot be triggered even if the membrane potential reaches threshold because the voltage-gated Na+ channels are inactive. The Na+ channels must reactivate before Na+ ions can move into the cell again, and the rising phase of the second action potential can begin. In this way, the refractory period determines how closely one action potential can follow another.

The transmission first triggers the _____.

opening of voltage-gated sodium channels and the diffusion of sodium ions into the neuron This is the first of the events listed here. As a result of the inward flux of sodium ions, that region of the neuron depolarizes.

Solve It: What Is Causing Episodes of Muscle Weakness in a Patient? Part B - Narrowing down the diagnosis Based on this information, which condition is most likely?

hypokalemia The patient's bloodwork indicates that the potassium level in the blood is lower than normal, suggesting that the patient is hypokalemic, not hyperkalemic.

Scientific Skills Exercise: Interpreting Data Values Expressed in Scientific Notation Part D - Interpreting the data Which drugs blocked naloxone binding in this experiment?

morphine, methadone, and levorphanol only

Axons insulated by a(n) _____ are able to conduct impulses faster than those not so insulated.

myelin sheath Myelin sheaths, formed when Schwann cells wrap around an axon, allow such neurons to conduct impulses more rapidly than unmyelinated axons.

Scientific Skills Exercise: Interpreting Data Values Expressed in Scientific Notation Part B What result did the researchers obtain for atropine, in standard notation?

no effect at 0.0001 M 10-4 is 0.0001.

An impulse relayed along a myelinated axon "jumps" from _____ to _____.

node of Ranvier ... node of Ranvier In myelinated neurons the impulse jumps from node of Ranvier to node of Ranvier.

The space between an axon of one neuron and the dendrite of another neuron is called a(n) _____.

synaptic cleft "Synaptic cleft" is the name given to the space between two neurons that meet at a synapse.

What part of a neuron relays signals from one neuron to another neuron or to an effector?

synaptic terminal Synaptic terminals contain neurotransmitter molecules that relay the nerve impulse across a synapse

Neurons store neurotransmitter molecules in vesicles located within _____.

synaptic terminals Vesicles within synaptic terminals contain neurotransmitter that may be released into the synaptic cleft.