Chapter 7 Questions: Page 170 - 176

True or false: There are two types of Na+ channels?

False. All Na+ channels are gated channels.

Na+/K+ Pumps are always working and never stop pumping 3 Na+ and 2 K+?

True.

True or false:

True.

True or false: The ion channels for Na+ and K+ are specific?

True.

True or false: There are two types of K+ channels?

True.

What is the equilibrium potential for Na+ and why does it not reach it?

+66 mV is the equilibrium potential for Na+. It does not reach this level because during an action potential when the membrane reaches+30 voltage it triggers the voltage regulated Na+ channels to close.

What is the threshold level for a neuron?

-55 mV

During an action potential ______ open and ____ diffuses into the cell. After a brief period, ______ open and ___ diffuses out of the cell. An inward diffusion of ____ causes further depolarization which in turn causes further opening of ___ gates in a positive feedback fashion. The opening of ___ gates and outward diffusion of ____ makes the inside of the cell more negative.

1. Na+ / 2. Na+ / 3. K+ / 4. K+ / 5. Na+ / 6. Na+ / 7. K+ / 8. K+

For ion channels, the channels are closed at ___________________ but opens in response to a _______ level of depolarization.

1. Resting Membrane Potential 2. Threshold

Define a resting membrane potential

A RMP is a potential voltage difference across the membrane in which the inside of the cell is negatively charged in comparison to the outside of the cell.

What happens when you increase a membranes permeability for a specific ion?

A change in permeability for a specific ion will result in the diffusion of that ion down its electrochemical gradient.

Hyperpolarization

A change in the cells membrane potential that makes it more negative.

Depolarization

A change in the cells membrane potential that makes it more positive

Explain the Na+/K+ Pump

A protein pump which maintains a potential difference due to it pumping out 3 sodium ions for every 2 potassium ions it lets into the cell.

At what point are gated channels close?

During the resting membrane potential of -70 mV.

Depolarization of a dendrite or cell body is: Excitatory or inhibitory?

Excitatory

Where is K+ more highly concentrated for neurons?

K+ is more highly concentrated within the cell instead of the extracellular fluid.

What units are used to measure the electrical potential in a neuron?

Milli-volts or mV.

What stimulus causes gated K+ Channels to open?

The depolarization stimulus

How many of the Na+ ions rush in to the cell during an action potential?

The number of Na+ ions that actually rush in is relatively small compared to the total, so the extracellular Na+ concentration is not measurably changed. However, the increased Na+ within that tiny region of axon membrane greatly affects the membrane potential.

What is the process called when K+ is diffused out of the cell to restore the RMP?

The process is called repolarization.

What is -55 mV called?

Threshold, this is when the nerve fires

What happens when a neuron hits its threshold level?

When a neuron hits -55 mV it opens up its Na+ channels. As the Na+ rushes in a massive and rapid action potential is generated.

Electrochemical gradient

a difference in ion concentration between two points so that ions tend to move passively along it.

What does rmp stand for?

resting membrane potential

What is the average rmp that a neuron maintains?

-70 mV

The explosive increase in Na+ permeability results in a rapid reversal of the membrane potential in that region from _____ to _____.

-70 mV to +30 mV

What is the rmp of a muscle cell?

-85 mV

What is K+'s equilibrium point?

-90 mV

Once a cell hits its threshold now, for an instant, the plasma membrane is freely permeable to 1. _________. Because the inside of the cell is _________ charged relative to the outside, and the concentration of Na+ is _____ inside of the cell, the electrogchemical gradient for Na+ causes Na+ to rush into the cell. This causes the membrane potential to move rapidly towards the _______ __________ ____________.

1. Na+ 2. Negatively 3. Lower 4. Sodium Equilibrium potential

Depolarization to a ______ level causes the ___ channels to open.

1. Threshold 2. Na+

Hypopolarization

A flow of positive charges into the cell. Also called depolarization.

What is a relative refractory period? How long does it last? What marks the beginning and end of this period?

A relative refractory period is a period of time during which it is theoretically possible for another action potential to take place, but incredibly unlikely due to a continued outward diffusion of K+. This is because while Na+ gates are now closed instead of inactive (therefore able to be opened), K+ is currently being diffused out of the cell and an undershoot is present. If a second action potential were to take place it would need to be a very strong stimulus in order for Na+ to be able to overpower the outward rush of K+ and diffuse into the cell. The relative refractory period lasts for about under 4 milliseconds.The beginning of this period is marked by the membrane just reaching the RMP. The membrane potential then dips down into an undershoot. The end of this period is marked by the membrane potential once again reaching its resting potential.

Repolarization

A return to the resting membrane potential

How many Na+/K+ Pumps does a typical neuron possess? How many ions per second can they collectively transport?

A typical neuron possesses 1 million Na+/K+ Pumps which can collectively move 200 million ions per second. Because of this, any small changes in the neuron can be quickly corrected.

How is a voltage gated ion channel inactivated?

A voltage gated ion channel is inactivated or closed by a "ball and chain" portion of the polypeptide chain. The "ball" portion will actually block the channel.

There are two types of voltage-regulated gates. How do these gates different in their closing mechanisms when they become inactivated?

A voltage-regulated gate will either have a polypeptide chain and a "ball" on the end of its chain, or will possess the ability to rearrange its molecular structure. Therefore, either a "ball" will obstruct the channel, or a rearrangement will render the channel impassable.

What causes all action potential to have about the same duration?

Action potentials have a consist duration because Na+ and K+ gates are not closed, they are inactivated. Inactivation of a gate is an automatic process that happens when a cell has reached a certain level of membrane potential. (measured in millivolts) A membrane potential is determined by the levels of Na+ and K+ and because the concentration gradients of Na+ and K+ stay relatively consistent and the rate at which they diffuse into or out of the cell stays consist, action potentials have a very predictable duration.

What is an absolute refractory period? How long does this last? What marks the beginning and end of this period?

An absolute refractory period is a period of time during which it is impossible for a neuron to undergo another action potential due to inactivated Na+ channels. This period lasts one millisecond and it is during this period that the membrane potential overshoots up to +30 mV. The beginning of this period occurs the moment the membrane potential hits threshold and lasts until the membrane potential returns to its RMP.

Explain the process of an action potential.

An axon maintains a charge of -70 mV which is called its resting membrane potential. When an axon has been depolarized to its threshold level the Na+ gates open and the membrane becomes permeable to Na+. Na+ rushes into the cell and as the sodium rushes in the cell becomes depolarized even more resulting in even more Na+ channels being opened. This is called a positive feedback loop. The rush of Na+ causes an explosive increase in membrane potential from -70 mV to +30 mV. Once the inside reaches +30 mV Na+ channels close. As Na+ channels are becoming inactive, K+ channels open as a result of a time delayed effect of the depolarization. As the K+ ions rush out of the cell the membrane potential moves towards the K+ equilibrium point. Diffusion of the positive K+ makes the inside of the cell more negative which acts to restore the original resting membrane potential of -70 mV. This process is called repolarization. This entire even is called an action potential.

Why are Na+ and K+ channels considered voltage regulated?

Because a depolarization stimulus is required to stimulate them.

Why is a flow of positive charges into a cell called depolarization or hypopolarization?

Because the potential difference between the two recording electrodes is reduced.

How do cells restore RMP.

Diffusion of K+ out of the cell makes the inside of the cell less positive and more negative. This restores the original resting membrane potential.

For ion channels, the channels are closed at the resting membrane potential but opens in response to a threshold level of _________.

Depolarization

Na+ gates open and Na+ Diffuses into the cell. After a brief period, K+ gates open and K+ diffuses out of the cell. An inward diffusion of Na+ causes further ________ which in turn causes further ________ of Na+ gates in a ________ feedback fashion. The opening of K+ gates and outward diffusion of K+ makes the inside of the cell more negative and thus has a ______ feedback effect.

Depolarization, opening, positive, negative.

What are the three terms that explain all the different changes a cell's membrane potential can experience?

Depolarization, repolarization, and hyperpolarization

What is the name of the process that causes Na+ to enter the cell through its gated channels?

Diffusion

Why was cocaine disused as a local anesthetic? What replaced it?

Due to its toxicity and potential for abuse. The first synthetic analog of cocaine used for local anesthetics was procaine in 1905. Other local anesthetics of this type include lidocaine and tetracaine.

What is after-hyperpolarization?

During repolarization, the membrane potential slightly overshoots the resting membrane potential and produces an after-hyperpolarization.

True or false: Axons are excellent conducts when compared to metal wires.

False. Axons are very poor conductors of electricity. If you were to depolarize a localized area to -60 mV and then measure the membrane potential 3mm away from that the recorded area would remain at -70 mV.

True or False: Na+ channels remain shut?

False. Na+ channels occasionally flicker open and then quickly close which will sometimes allow Na+ to leak into the resting cell.

True or false: all cells can alter their membrane potential in response to stimulation?

False. Only neurons and muscle cells possess this unique ability.

True or false: The length of time that the Na+ and K+ channels stay open is dependent of the strength of the depolarization status.

False. The length of time that the Na+ and K+ channels stay open is independent of the strength of the depolarization status.

True or false: The neuron at the resting membrane potential is much more permeable to Na+ than to K+.

False. The membrane is much more permeable to K+ rather than to Na+ because some K+ ion channels are always open. These channels are called leakage channels.

Ions such as Na+ and K+ and others pass through ions channels in the plasma membrane that are said to be: _____ ______

Gated channels

What allows the axon membrane to be permeable to Na+ and K+?

Gated channels

What are gated channels?

Gated channels are a that possesses the ability to open or close its ion channels in response to a certain stimuli.

What are the two types of K+ Channels

Gated channels, which remain closed until a certain stimuli activates them, and leakage channels which remain open.

What causes an inactivated gate to become a resting or closed gate?

Hyperpolarization. The time at which hyperpolarization happens is consistent and therefore it could also be said that inactivated gates become resting gates after a set period of time.

Hyperpolarization of a dendrite or cell body is: Excitatory or inhibitory?

Inhibitory

How can you measure changes in the potential difference across a membrane?

It can be measured by observing the voltage that develops between two microelectrodes, one of which is placed inside the cell and the other is placed outside the plasma membrane at the region being recorded.

How does inactivating a gate differ from simply closing it?

It differs in that an inactivated gate is a process happens automatically and lasts until the membrane is repolarized.

What does it mean if a neuron is refractory to further stimulus?

It is incapable of responding.

How long does it take during an action potential for a cell to go from -70 mV to +30 mV and back down to - 70 mV?

It only takes about 3 milliseconds

How long can a membrane be in an absolute refractory period?

Just one millisecond.

When Na+ gates open the membrane becomes permeable to ______.

Na+

Where is Na+ more highly concentrated for neurons?

Na+ is more highly concentrated in the extracellular fluid rather than inside the cell itself.

What are spiked potentials?

Spiked potentials are what action potentials are sometimes called.

What is 0 mV called?

Neutral

At what point are all Na+ channels closed?

Once it reaches +30 mV all sodium channels have closed.

During an action potential, how long after Na+ channels open do they then close?

Only a fraction of a second.

Depolarization to a threshold level causes Na+ channels to ____.

Open.

What is +30 mV called?

Overshoot

What is -70 mV called?

Resting membrane potential

What determines the strength of a stimulus pertaining to action potentials. Include the terms AM and FM in your answer.

The cause or code of stimulus strength in the nervous system is not amplitude modulated (AM). Instead a stimulus strength is determined by the frequency of identical action potentials. Therefore, the cause or code of stimulus strength is frequency modulated or FM.

What causes after-hyperpolarization?

The continued movement of K+ past the resting membrane potential and towards its equilibrium point.

Ion Currents

The flow of ions that occur only across limited patches of membrane where specific ion channels are located.

What causes depolarization, repolarization, and hyperpolarization?

They are caused by changes in the net flow of ions through ion channels in the membrane.

How do local anesthetics work?

They block the conduction of action potentials in axons. They do this by reversibly binding to specific sites within the voltage-gated Na+ channels, reducing the ability of membrane depolarization to produce action potentials.

Why is it that a membrane can only fire one action potential at a time?

This is because during an action potential a membrane will open up its gated channels in order to reach the overshoot of +30 mV. When gated channels are stimulated to open they will open. Gates do not have varying levels of open, it is open or it is closed. Once the Na+ has rushed in these channels then close not due to a stimulus from the cell, but due to an automatic process called inactivation. This inactivation is determined by the membrane potential level of a cell which is measured in millivolts. Therefore, even if an axon receives a second stimulus during its current stimulus, the rate at which Na+ ions will diffuse is consist, therefore the duration until a cell reaches overshoot is consist, and therefore the time until a gated channel "closes" (inactivated) is also consistent. Even if the membrane receives a second stimulus during the time gates are open it will not speed up the rate ions diffuse and if a second stimulus is received after a gate has closed it cannot re-open it due to inactivation.

True or false: A neuron poisoned with cyanide so that it cannot produce ATP can still produce an action potential.

True. Action potentials are caused by diffusion of Na+ and K+ and not by any active (requiring ATP) process. However, after a neuron losses its ability to produce ATP it is not long before it loses its resting membrane potential of -70 mV. This is because the pump that maintains the neuron's RMP requires ATP in order to function. Without a proper resting membrane potential a neuron cannot fire.

True or false: When enough Na+ channels are in the closed rather than inactivated state, it is theoretically possible to again stimulate the axon with a sufficiently strong stimulus. Please explain answer.

True. If a gate is closed and not inactivated it is possible to once again open it. However this is only theoretically as you would need a very strong stimulus in order to allow the Na+ to overcome the rush of K+ ions out of the cell.

True or false: The axons do not rely on their cable properties in order to transmit signals.

True. If axons relied on their cable properties they would only be able to conduct signals no more than 1 millimeter in length.

True or false: The amplitude of an action potential is an all or nothing event.

True. If the depolarization is below the threshold value, the gated channels are closed.

True or false: Only a relatively small amount of Na+ and K+ more into and out of the axon during an action potential.

True. The movement is sufficient to cause changes in the membrane potential during an action potential but does not significantly affect the concentrations of these ions.

True or false: The resting membrane potential is a little less negative than the equilibrium potential for K+.

True. This is because of the slight inward movement of Na+.

How long can a membrane be in a relative refractory period?

Under 4 milliseconds

Explain how and in what way a positive feedback loop is present during an action potential.

When an axon membrane is depolarized during an action potential Na+ gates open and Na+ is allowed into the membrane. When this happens the membrane becomes depolarized to an even further extent. Because Na+ channels are voltage regulated this causes even more channels to open and allows even more Na+ into the cell which likewise will open more channels. This is a positive feedback loop.

Explain the process of recruitment in stimulus intensity

When an entire collection of axons is stimulated, different axons will be stimulated at different stimulus intensities. A weak stimulus will be stimulates will activate only those few axons with low thresholds, whereas stronger stimuli can activate axons with higher thresholds. As the intensity of stimulation increases, more and more axons will become activated.

Explain how and why the depolarization of a neuron strong enough to open Na+ channels can cause a chain reaction resulting in the neuron becoming even more permeable to Na+ than the original depolarization.

When the axon membrane has been depolarized to the threshold level the Na+ gates open allowing Na+ to diffused in. Because the gates for Na+ are voltage regulated, when Na+ enters the membrane and causes additional depolarization it stimulates even more of the voltage regulated gates to open: allowing more Na+ into the cell. This creates a positive feedback loop.

How do cells maintain -70 mV if Na+/K+ Pumps are always working?

Within a membrane there exists both gated K+ channels and leakage K+ channels. The leakage channels are always open to K+. Na+ channels, however, are all gated but are not always shut. Occasionally they will flicker open temporarily allowing some Na+ to escape through and travel into the cell,