Mastering A and P Assignment Unit 2 - Nervous System PDF

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Mastering A and P Assignment: Unit 2 - Nervous System...

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Mastering A and P Assignment: Unit 2 - Nervous System Due: 11:59pm on Friday, January 31, 2020 You will receive no credit for items you complete after the assignment is due. Grading Policy

A&P Flix Quiz: Resting Membrane Potential Watch the animation, then answer the questions.

Part A 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?

Hint 1. Electrical potential difference All cells exhibit an electrical potential difference across their plasma membranes. Only excitable cells can alter this difference by generating action potentials. Thus, an excitable cell, like a neuron, exhibits both a resting and an excited state.

ANSWER:

Positive membrane potential Resting membrane potential (RMP) Threshold potential Action potential

Correct Yes! The resting membrane potential is the baseline potential that can be recorded across the plasma membrane of an excitable cell prior to excitation.

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

Hint 1. Movement of sodium and potassium ions There are several means by which Na+ and K+ can move across plasma membranes. One channel type that occurs in all cells allows Na+ and K+ to passively move across the plasma membrane.

ANSWER:

Sodium-potassium ATPases Ligand-gated channels Voltage-gated channels Leak channels

Correct Yes. Leak channels for Na+ and K+ are ubiquitous, and they allow for the diffusion of these ions across plasma membranes.

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

Hint 1. Separation of charge Electrical charges are unequally distributed across the plasma membrane, creating an electrical potential difference, or voltage, at any two points across the membrane. The two points of interest are (1) just inside the plasma membrane and (2) just outside the plasma membrane. The difference is measured using a voltmeter. The voltmeter has two electrodes: (1) a recording electrode and (2) a reference electrode.

ANSWER:

There is no electrical potential difference between the inside and the outside surfaces of the plasma membrane. The outside surface of the plasma membrane is much more negatively charged than the inside surface. The inside surface of the plasma membrane is much more negatively charged than the outside surface. The inside surface of the plasma membrane is much more positively charged than the outside surface.

Correct Yes! The inside surface of the plasma membrane accumulates more negative charge because of the presence of Na+ and K+ gradients and the selective permeability of the membrane to Na+ and K+.

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

Hint 1. Potassium channels There are several K+ channel types. Some are voltage sensitive. Others are calcium sensitive. Others still allow for the movement of K+ into cells down electrical gradients. Finally, there are certain K+ channels that are always open and available to K+.

ANSWER:

The Na+-K+ pumps transport more K+ into cells than Na+ out of cells. There are many more voltage-gated K+ channels than voltage-gated Na+ channels. Ligand-gated cation channels favor a greater influx of Na+ than K+. There are many more K+ leak channels than Na+ leak channels in the plasma membrane.

Correct Yes! More leak channels translates into more leakiness. Thus the outward flux of K+ is greater than the inward flux of Na+.

Part E 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.

Hint 1. Diffusion across the plasma membrane The phospholipid bilayer is a largely hydrophobic permeability barrier. Small, lipid-soluble substances readily diffuse through it. However, small, water-soluble substances do not. There are two conditions that must be met in order for diffusion of small, water-soluble substances to take place. What are these two conditions?

ANSWER: The presence of concentration gradients and voltage-gated channels The presence of a resting membrane potential and leak channels The presence of concentration gradients and leak channels The presence of concentration gradients and Na+-K+ pumps

Correct Yes! The concentration gradient and the large number of K+ leak channels allow for rather robust K+ diffusion out of a cell. In contrast, the concentration gradient and the relatively few Na+ leak channels allow for much less Na+ diffusion into a cell.

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

Hint 1. Movement of Na+ and K+ Sodium and potassium ions diffuse down their concentration gradients through leak channels. A specific transporter prevents these ions from diffusing to equilibrium. Identify this transporter.

ANSWER: Na+-K+ ATPase Na+ cotransporter Na+ and K+ leaks H+-K+ ATPase

Correct Yes! Also known as the Na+-K+ pump, or simply the pump, this transporter moves three Na+ out of the cell and two K+ into 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.

A&P Flix Quiz: Generation of an Action Potential Watch the animation, then answer the questions.

Part A Where do most action potentials originate?

Hint 1. Parts of a neuron Most input to a neuron is to the cell body and dendrites. If a neuron is excited to threshold, an action potential will be generated in the first part of the axon. Identify this part.

ANSWER:

Cell body Axon terminal Nodes of Ranvier Initial segment

Correct Yes! The first part of the axon is known as the initial segment. The initial segment is adjacent to the tapered end of the cell body, known as the axon hillock.

Part B What opens first in response to a threshold stimulus?

Hint 1. Voltage-gated channels Action potentials occur in areas of the plasma membrane that are densely populated with voltage-gated, or voltage-sensitive, ion channels. Which type of voltage-gated channel is the most rapid to open in response to a threshold stimulus?

ANSWER:

Voltage-gated Na+ channels Ligand-gated cation channels Voltage-gated K+ channels Ligand-gated Cl- channels

Correct Yes! The activation gates of voltage-gated Na+ channels open, and Na+ diffuses into the cytoplasm.

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

Hint 1. Resting membrane potential A resting membrane potential (RMP) is exhibited by all cells, and it is caused by an unequal distribution of ions across the plasma membrane. To measure the RMP, a reference electrode is placed near the outside surface of the plasma membrane and a recording electrode is placed near the inside surface. The recorded value is always negative. Thus, the inside surface of the plasma membrane is polarized to a negative value at rest.

ANSWER:

The membrane potential reaches a threshold value and returns to the resting state. The membrane potential changes to a much more negative value. The membrane potential changes to a less negative (but not a positive) value. The membrane potential changes from a negative value to a positive value.

Correct Yes! The plasma membrane, which was polarized to a negative value at the RMP, depolarizes to a positive value.

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

Hint 1. Resting membrane potential A resting membrane potential (RMP) is exhibited by all cells, and it is caused by an unequal distribution of ions across the plasma membrane. To measure the RMP, a reference electrode is placed near the outside surface of the plasma membrane and a recording electrode is placed near the inside surface. The recorded value is always negative. Thus, the inside surface of the plasma membrane is polarized to a negative value at rest.

ANSWER:

Before the membrane has a chance to reach a positive voltage, it repolarizes to its negative resting value of approximately -70 mV. Once the membrane depolarizes to a peak value of +30 mV, it repolarizes to its negative resting value of -70 mV. Once the membrane depolarizes to a threshold value of approximately -55 mV, it repolarizes to its resting value of -70 mV. As the membrane repolarizes to a negative value, it goes beyond the resting state to a value of -80 mV.

Correct Yes! The plasma membrane was depolarized to a positive value at the peak of the first phase of the action potential. Thus, it must repolarize back to a negative value.

Part E What event triggers the generation of an action potential?

Hint 1. Voltage-gated channels Action potentials occur in areas of the plasma membrane that are densely populated with voltage-gated, or voltage-sensitive, ion channels. This type of channel opens in response to voltage change.

ANSWER:

The membrane potential must hyperpolarize from the resting voltage of -70 mV to the more negative value of -80 mV. The membrane potential must depolarize from the resting voltage of -70 mV to its peak value of +30 mV. The membrane potential must return to its resting value of -70 mV from the hyperpolarized value of -80 mV. The membrane potential must depolarize from the resting voltage of -70 mV to a threshold value of -55 mV.

Correct Yes! This is the minimum value required to open enough voltage-gated Na+ channels so that depolarization is irreversible.

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

Hint 1. Voltage-gated channels Action potentials occur in areas of the plasma membrane that are densely populated with voltage-gated, or voltage-sensitive, ion channels. Threshold stimuli cause conformational changes that open these channels.

ANSWER: Voltage-gated Na+ channels change shape, and their inactivation gates close. Voltage-gated Na+ channels change shape, and their activation gates open. Voltage-gated Ca2+ channels change shape, and their activation gates open. Voltage-gated K+ channels change shape, and their activation gates open.

Correct Yes! The activation gates of voltage-gated Na+ channels open very rapidly in response to threshold stimuli. The activation gates of voltage-gated K+ channels are comparatively slow to open.

Essentials Figure: The Action Potential Neuronal action potentials are integral to the function of the human body. Neuronal action potentials result in the release of chemical signals needed for many physiological functions like skeletal and smooth muscle contraction, and glandular secretions, not to mention the neuronal signaling between neurons. The production of a neuronal action potentials is dependent on the neuron's membrane permeability to specific ions. The following questions will initially test you to see if you understand the concept of permeability. View figure 8.9 in greater detail

Part A - Understanding the Concept of Permeability Which of the following statements best describes the concept of permeability? Choose the best answer.

Hint 1. The plasma membrane The plasma membrane serves as the barrier between the inside of the cell and the outside of the cell. Recall that the plasma membrane consists of phospholipids whose hydrophobic tails create a fatty core that prevents hydrophilic molecules and charged molecules from entering or leaving the cell. Certain proteins embedded within the phospholipid bilayer may serve as channels or carriers that enable charged particles or hydrophilic substances to cross through the plasma membrane. It is considered to be semipermeable, which means that the membrane is permeable to some substances but not others. Hint 2. Rate at which substances move across the plasma membrane To determine the rate at which a substance moves across the plasma membrane, we use Fick's law of diffusion. This law takes into account several different parameters associated with the plasma membrane, including concentration gradients and permeability. While rate of diffusion across a membrane is dependent on permeability, permeability is not dependent on rate.

ANSWER:

the surface area of a cell and its relationship to the diffusion of water the rate at which a molecule crosses the membrane the ability of a molecule to cross the cell membrane whether or not a molecule moves across the cell via passive or active mechanisms

Correct

Part B - Ion Channels and Permeabilities The two cells below are hypothetical cells with a concentration of 100 mOsm of K+ inside the cells and containing only leak channels for K+ within the membrane. Each cell is placed into a different solution containing different concentrations of K+ in the extracellular fluid. Which of the two cells below has a higher permeability to K+ and why? Choose the best answer.

Hint 1. Ions and the plasma membrane The plasma membrane is impermeable to ions due to their charges. In order for an ion to cross the plasma membrane, it would have to travel through a channel. These channels are usually formed by transmembrane proteins that are inserted into the plasma membrane. The transmembrane proteins bypass the lipid core of the bilayer, thus allowing the ions to move through the plasma membrane. Hint 2. Concentration gradients and ion flow While the concentration gradients are important for the rate of ion flow, they will not change permeability. Imagine a cell that has a very, very large concentration of Cl- outside of the cell compared to inside the cell. If this cell does not have any ion channels for the Cl-, then the cell would be impermeable to that ion and no ion would move into the cell. Thus, the concentration gradient becomes irrelevant. Ions need the channels to pass through the plasma membrane, and the more channels that are present, the more ions that can pass through the plasma membrane.

ANSWER:

There is not enough information to tell. A, because it has a greater concentration gradient that B. A and B would have the same amount of permeability. B, because it has more ion leak channels for K+ than A.

Correct

Because our cells have many different ions distributed across the inside and outside of the cell, ion movement is dependent upon the difference in charge between the inside and outside of the cell as well as the concentration of each particles involved. Remember that ions are charged particles, and thus have an attraction for oppositely charged particles and a repulsion to like charged particles. So, one aspect of ion movement across a plasma membrane is attraction to an oppositely charged ion on the other side of the membrane. The term we use for this difference in charge is "electrical gradient." Even when the plasma membrane prevents the movement of ions across it, the ions migrate near to the plasma membrane, attracted to the oppositely charged ions on the other side of the membrane. Ions are also subject to concentration differences. Just like every substance in the body, an ion will move in the direction towards an area of lower concentration, or down a concentration gradient. Together, the electrical gradient and the concentration gradient play a role in determining ion movement across a membrane. To understand membrane potential change, you need to remember how key ions are distributed across our cells: e.g. what ions are more concentrated on the outside of the cell and what ions are more concentrated inside of the cell? The two ions we want to focus on are Na+ and K+. These two ions play a major role in how an excitable cell's membrane potential is formed and how that potential can be manipulated during excitability.

Part C - Understanding the Resting Membrane Potential Which of the following statements best describes the resting membrane potential? Choose the best answer.

Hint 1. What are excitable versus non-excitable cells? Excitable cells are those that are capable of changing their membrane potentials in order to cause the release of a neurotransmitter or contraction of muscle. For this reason, the body's excitable cells are neurons and all types of muscle. Non-excitable cells are all the cells that do not have the ability to change their membrane potentials in order to provide a change and/or communication. Hint 2. What does it mean when something has a potential? A potential occurs when there is a difference between two sides or compartments. There is a potential difference, for example, between the negatively charged side of a water molecule and the positively charged side of the water molecule. What kind of potential do you think excitable cells might be maintaining?

ANSWER:

an osmotic pressure difference that exists between the intracellular and extracellular fluids the differences that exist between excitable cells and non-excitable cells an electrical gradient that exists between the intracellular and extracellular fluids a concentration gradient that exists between the intracellular and extracellular fluids

Correct

Part D - Review of Ion Distribution and Flow Drag and drop the terms below to correctly complete each sentence. Drag the terms on the left to the appropriate blanks on the right to complete the sentences.

Hint 1. Concentration gradients

In order for an ion to passively move across a cell membrane, not only does there have to be a channel for that ion to move, but there has to be a concentration gradient as well. Concentration gradients are a form of potential energy that will allow for an ion (or any substance) to move from an area of its high concentration to an area where its concentration is low. Hint 2. Intracellular fluids vs. Extracellular fluids In chapter 3, we learned that the various compartments have differing concentrations of Na+ and K+ inside and outside of the cell. These differing concentrations are due in part to the actions of the Na+-K+ ATPase pump which moves Na+ and K+ against their gradients. In which direction is Na+ pumped by this pump?

ANSWER:

Reset

1. Sodium (Na+)

ions are more concentrated on the outside

Help

of the cell, this would result in

diffusion of the ion into the cell.

2. Potassium (K+)

ions are more concentrated on the inside

of the cell, this would result in

diffusion of the ion out of the cell.

Correct

When ions move across a plasma membrane, there is a point where the net movement no longer progresses because a balance is reached between the electrical gradient in one direction, and the concentration gradient in the other direction. This point of equilibrium may be calculated by from the concentrations of the ions on either side of a membrane and other factors. The value that is calculated is the electrical potential (difference in charge) of the inside of the cell compared to the outside and is called the equilibrium potential. The membrane potential of a cell is dependent upon the relative concentrations and the relative permeabilities of the all the ions found both within the intracellular fluid and the extracellular fluid. We can say then that the membrane potential reflects the degree of permeability and concentration for each ion that can cross the membrane. Thus, changes to either factor would result in a change membrane potential. There are many different types of channels located within ...