A-5 Mastering

The velocity of the action potential is fastest in which of the following axons?

a small myelinated axon

During depolarization, which gradient(s) move(s) Na+ into the cell?

both the electrical and chemical gradients

What causes calcium channels in the synaptic knob to open?

depolarization of the presynaptic membrane due to an arriving action potential

when the voltage-gated Na+ channels open, Na+ rushes into the cell causing __________________.

depolarization.

An action potential is self-regenerating because __________.

depolarizing currents established by the influx of Na+‎ flow down the axon and trigger an action potential at the next segment

Binding of the neurotransmitter to its receptor causes the membrane to __________.

either depolarize or hyperpolarize

The electrical and chemical forces for a particular ion combine to become a single force, the ____________________________ which causes the movement of that ion across the cell membrane.

electrochemical gradient

As a result of the Na+-K+ pump Na+ is pumped out of the cell against its _________________gradient and K+ is pumped into the cell against its _____________ gradient.

electrochemical; concentration

Because both ions have a single charge, the transmembrane potential affects them ___________ Therefore, the magnitude of the K+ electrical gradient cannot be ________ than the magnitude of the Na+ electrical gradient.

equally, less (or more)

Neurotransmitters exit the presynaptic cell via __________.

exocytosis

What type of membrane transport causes the depolarization phase of the action potential in neurons?

facilitated diffusion

In contrast to the internodes of a myelinated axon, the nodes __________.

have lower membrane resistance to ion movement

myelinated 3

high priority info fast saltatory

Compared to the electrical gradient for sodium at rest, the electrical gradient for potassium at rest is __________.

in the same direction and of the same magnitude.

Where is acetylcholinesterase (AChE) primarily located?

in the synaptic cleft

Myelin ____________ the membrane resistance of the axon section it surrounds, allowing local currents to travel between nodes, even though they are 1-2 mm apart. Removing myelin decreases the membrane resistance of internode regions. This shortens the distance that local currents travel because more charge now exits at the internode regions before it reaches the next node.

increases

Action potential propagation begins (is first generated at) what region of a neuron?

initial segment

In what part of the neuron does the action potential typically initiate?

initial segment of the axon

In a typical neuron, what is the equilibrium potential for sodium?

+66mV

Around what transmembrane potential does threshold commonly occur?

-60mv

What is the value for the resting membrane potential for most neurons?

-70 mV

In a typical neuron, what is the equilibrium potential for potassium?

-90mV

K+ comes to equilibrium when the membrane potential for the cell is

-90mV

Approximately how fast do action potentials propagate in unmyelinated axons in humans?

1 meter per second

What is the magnitude (amplitude) of an action potential?

100 mV

What is the typical duration of a nerve action potential?

2 ms

Na+-K+ ATPase also known as the Na+-K+ pump, or simply the pump, this transporter moves _______ Na+ ____ of the cell and ______K+ ______ the cell for every ATP it hydrolyzes. This pumping action prevents the Na+ and K+ gradients from running down as these ions passively move through leak channels.

3, out, 2, into

Which of the following best describes how ACh causes depolarization of the postsynaptic membrane?

ACh opens ACh receptors.

What is the primary role of acetylcholinesterase (AChE) at a cholinergic synapse?

AChE degrades acetylcholine in the synaptic cleft.

Which of the following best describes the order of events in synaptic activity?

An action potential arrives and depolarizes the synaptic knob. Extracellular calcium enters the synaptic knob, triggering exocytosis of ACh. ACh binds to receptors and depolarizes postsynaptic membrane. ACh is removed by AChE.

How is an action potential propagated along an axon?

An influx of sodium ions from the current action potential depolarizes the adjacent area. (the influx of sodium ions depolarizes adjacent areas, causing the membrane to reach threshold and cause an action potential. Thus, the action potential is regenerated at each new area.)

Which of the following best describes the role of calcium in synaptic activity?

Calcium enters the presynaptic cell and causes the release of ACh.

Ions move through leak channels because of ____________ and _______________ gradients.

Chemical and electrical

What changes occur to voltage-gated Na+ and K+ channels at the peak of depolarization?

Inactivation gates of voltage-gated Na+‎ channels close, while activation gates of voltage-gated K+‎ channels open.

The concentrations of ___ and organic anions are high inside the cell.

K

What ion causes repolarization of the neuron during an action potential?

K+

More ___ moves out of the cell than ____ moves into the cell, helping to establish a negative resting membrane potential.

K+; Na+

The membranes of neurons at rest are very permeable to _____ but only slightly permeable to _____.

K+; Na+

In which type of axon will velocity of action potential conduction be the fastest?

Myelinated axons with the largest diameter (The large diameter facilitates the flow of depolarizing current through the cytoplasm. The myelin sheath insulates the axons and prevents current from leaking across the plasma membrane.)

What ion is responsible for the depolarization of the neuron during an action potential?

Na+

The ______________________ is essential for maintaining the resting membrane potential in neurons.

Na+ K+ pump

The concentrations of which two ions are highest outside the cell.

Na+ and Cl-

Ions are unequally distributed across the plasma membrane of all cells. This ion distribution creates an electrical potential difference across the membrane. What is the name given to this potential difference?

Resting membrane potential (RMP)

The Na+-K+ pump actively transports both sodium and potassium ions across the membrane to compensate for their constant leakage. In which direction is each ion pumped?

Na+ is pumped out of the cell and K+ is pumped into the cell.

What prevents the Na+ and K+ gradients from dissipating?

Na+-K+ ATPase

What characterizes repolarization, the second phase of the action potential?

Once the membrane depolarizes to a peak value of +30 mV, it repolarizes to its negative resting value of -70 mV.

How do action potential propagation speeds in myelinated and unmyelinated axons compare?

Propagation is faster in myelinated axons.

The sodium-potassium exchange pump transports potassium and sodium ions in which direction(s)?

Sodium ions are transported out of the cell. Potassium ions are transported into the cell.

Why does the action potential only move away from the cell body?

The areas that have had the action potential are refractory to a new action potential. (sodium channels are inactivated in the area that just had the action potential.)

Why does regeneration of the action potential occur in one direction, rather than in two directions?

The inactivation gates of voltage-gated Na+‎ channels close in the node, or segment, that has just fired an action potential. At the peak of the depolarization phase of the action potential, the inactivation gates close. Thus, the voltage-gated Na+‎ channels become absolutely refractory to another depolarizing stimulus.

On average, the resting membrane potential is -70 mV. What does the sign and magnitude of this value tell you?

The inside surface of the plasma membrane is much more negatively charged than the outside surface.

At rest, why is the transmembrane potential of a neuron (-70 mV) closer to the potassium equilibrium potential (-90 mV) than it is to the sodium equilibrium potential (+66 mV)?

The membrane is much more permeable to potassium ions than to sodium ions.

What characterizes depolarization, the first phase of the action potential?

The membrane potential changes from a negative value to a positive value.

What event triggers the generation of an action potential?

The membrane potential must depolarize from the resting voltage of -70 mV to a threshold value of -55 mV.

What is the function of the myelin sheath?

The myelin sheath increases the speed of action potential conduction from the initial segment to the axon terminals.

Curare is a drug that prevents ACh from binding to ACh receptors. How would you expect curare to affect events at a cholinergic synapse?

The postsynaptic cell would not depolarize.

The resting membrane potential depends on two factors that influence the magnitude and direction of Na+ and K+ diffusion across the plasma membrane. Identify these two factors.

The presence of concentration gradients and leak channels

In an unmyelinated axon, why doesn't the action potential suddenly "double back" and start propagating in the opposite direction?

The previous axonal segment is refractory.

The plasma membrane is much more permeable to K+ than to Na+. Why?

There are many more K+ leak channels than Na+ leak channels in the plasma membrane. (More leak channels translates into more leakiness. Thus the outward flux of K+ is greater than the inward flux of Na+.)

What opens first in response to a threshold stimulus?

Voltage-gated Na+ channels

What is the first change to occur in response to a threshold stimulus?

Voltage-gated Na+ channels change shape, and their activation gates open.

During an action potential, after the membrane potential reaches +30 mV, which event(s) primarily affect(s) the membrane potential?

Voltage-gated sodium channels begin to inactivate (close) and voltage-gated potassium channels begin to open.

Multiple sclerosis (MS) is a disease that stops action potential propagation by destroying the myelin around (normally) myelinated axons. Which of the following best describes how MS stops action potential propagation?

Without myelin, the internode membrane resistance decreases, preventing local currents from reaching adjacent nodes.

The electrochemical gradient for potassium ions when the transmembrane potential is at the resting potential (-70 mV) is caused by what?

a chemical gradient going out of the cell and an electrical gradient going into the cell

The acetylcholine receptor is an example of what type of channel?

a chemically gated channel

The mechanism by which the neurotransmitter is returned to a presynaptic neuron's axon terminal is specific for each neurotransmitter. Which of the following neurotransmitters is broken down by an enzyme before being returned?

acetylcholine

Where are action potentials regenerated as they propagate along an unmyelinated axon?

at every segment of the axon

Where are action potentials regenerated as they propagate along a myelinated axon?

at the nodes

Where in the neuron is an action potential initially generated?

axon hillock

Leak channels allow the movement of potassium and sodium ions by what type of membrane transport?

channel-mediated diffusion

The electrochemical gradient for sodium ions in a neuron when the transmembrane potential is at the resting potential is caused by what?

chemical and electrical gradients both going into the cell

Binding of a neurotransmitter to its receptors opens __________ channels on the __________ membrane.

chemically gated; postsynaptic

What type of conduction takes place in unmyelinated axons?

continuous conduction

The internode segments of myelinated axons allow local currents to travel quickly between nodes where the action potential is regenerated. This leaping of action potentials from node to node is several times faster than the continuous propagation found in unmyelinated axons. Myelinated axons also tend to have ________ diameters, which enhances propagation speed.

larger

Sodium and potassium ions can diffuse across the plasma membranes of all cells because of the presence of what type of channel?

leak-channels

unmyelinated 3

low priority info slow Continuous

In non excitable cells, the ________________ depends only on ___.

membrane potential; K+

A positive ion is driven into the cell because the inside of the cell is _____________ compared to the outside of the cell, and Na+ is driven into the cell because the concentration of Na+ is ___________ outside the cell.

negative; greater

The _______________of a cell for ions depends on the number and type of ion channels in the cell membrane.

permeability

Potassium and sodium are both ___________ charged. Thus, at the resting potential of -70 mV, both ions are attracted to the ___________ charged interior of the neuron. Therefore, the electrical gradient for both ions must be in the same direction (_________).

positively, negatively, inward

In a synapse, neurotransmitters are stored in vesicles located in the __________.

presynaptic neuron

The ___________ _________________ _______________ in neurons depends on the distribution of Na as well as K across the cell membrane.

resting membrane potential

the myelination acts as insulation and the action potential is generated only at the nodes of Ranvier. Propagation along myelinated axons is known as ____________ conduction.

saltatory

The node-to-node "jumping" regeneration of an action potential along a myelinated axon is called __________.

saltatory propagation

Hyperpolarization results from __________.

slow closing of voltage-gated K+ channels

The influx of ___________ ions causes the rapid depolarization during the action potential. The influx of _________ ions through open channels is favored by two factors. (1) The sodium concentration inside the neuron is only about 10% of the sodium concentration outside the neuron. (2) Most of the time, the interior of the cell is electrically negative, which is attractive for the positively charged sodium ions.

sodium

When ACh receptors open, what ion causes depolarization of the postsynaptic membrane?

sodium

The movement of what ion is responsible for the local currents that depolarize other regions of the axon to threshold?

sodium (Na+)

The higher concentration of ___________ outside the cell than inside the cell, creates an inward chemical gradient. In addition, the electrical gradient for sodium is also inward because, at the resting potential, the inside of the cell is relatively more _______________ than the outside, which is attractive to the positively charged sodium ions.

sodium, negative

The repolarization phase of an action potential results from __________.

the opening of voltage-gated K+ channels

In a neuron, sodium and potassium concentrations are maintained by the sodium-potassium exchange pump such that __________.

the sodium concentration is higher outside the cell than inside the cell and the potassium concentration is higher inside the cell than outside the cell.

What is the electrochemical gradient of an ion?

the sum of the electrical and chemical gradients for that ion (these two gradients determine the net movement of a particular ion across the plasma membrane)

During an action potential of a neuron, what directly causes the different channels to open and close?

the transmembrane potential (voltage)

The initial segment has the lowest ____________ and, therefore, is the place where most action potentials are initiated.

threshold

In ____________________ axons, the action potential is regenerated continuously along every segment of the axon (continuous propagation). In humans, only small diameter axons (for example, type C fibers) are __________________. These neurons carry low-priority information, such as smell (olfaction) and temperature sensations. (1 answer)

unmyelinated

An action potential releases neurotransmitter from a neuron by opening which of the following channels?

voltage-gated Ca2+ channels

The depolarization phase of an action potential results from the opening of which channels?

voltage-gated Na+ channels

What is primarily responsible for the brief hyperpolarization near the end of the action potential?

voltage-gated potassium channels taking some time to close in response to the negative membrane potential