MG Ch 6

A subthreshold stimulus to a neuron

- Depolarizes the local membrane potential briefly before it returns to the resting potential - Does not generate an action potential

Given the following permeabilities, extracellular fluid (ECF) concentrations, and intracellular fluid (ICF) concentrations for Na+, K+ and Cl-, calculate the resting membrane potential of a hypothetical cell: ECF: [Na+] = 145 mM; [K+] = 5 mM; [Cl-] = 100 mM ICF : [Na+] = 4 mM; [K+] = 160 mM; [Cl-] = 17 mM PK+ = 1; PNa+ = 0.05; PCl- = 10

-49 mV

Which best describes the process called "repolarization?"

A neuron is returning from the peak of its action potential toward its resting potential.

Which of these explains why an action potential is able to propagate along the full length of an axon?

Action potentials are regenerated in each patch of membrane by depolarizing local currents arriving from previously-depolarized patches of membrane.

In an experiment with an artificial cell, the equilibrium potentials for ions X+, Y- and Z+ are +50 mV, -50 mV, and +5 mV, respectively. If the cell is equally permeable to ions X+ and Y-, but 10 times more permeable to ion Z+ than to ion Y-, which statement is most likely true?

All three ions have higher concentrations outside the cell than inside, and the resting membrane potential is positive

Which of the following are properties of action potentials but NOT graded potentials?

Are all-or-none Have a refractory period Cannot be summed

Which of the following are TRUE of graded potentials but NOT of action potentials?

Can be summed Amplitude varies with size of stimulus

Which best describes the behavior of chloride ions in excitable cells?

Cl- moves out of the cell in response to the negative membrane potential created by K+ and Na+ movement, setting up a Cl- gradient.

Two fluid-filled experimental chambers are separated by a plasma membrane that has open Cl- channels but no other ion permeability. Into side 1 you put 200 mM NaCl and into side 2 you put 100 mM KCl. Which of the following will occur?

Cl- will flow from side 1 to side 2 and the membrane will develop a membrane potential that is positive on side 1 and negative on side 2.

Which correctly states Ohm's Law?

Current = Potential/Resistance

A cell is floating in an extracellular solution containing 150 mM Na+ and 15 mM K+. If the intracellular concentration of Na+ is 5 mM, the intracellular concentration of K+ is 150 mM, and the cell is permeable ONLY to K+, which of the following correctly lists the equilibrium potentials?

ENa+ = +90 mV; EK+ = -61 mV

Which two of the following readily conduct electrical current?

Extracellular fluid Intracellular fluid

If a positive ion is in higher concentration inside the cell than outside the cell, it will make the resting membrane potential more positive, even if the cell is not permeable to that ion

False

Given the concentration gradients and relative membrane permeabilities for Na+, K+, and Cl-, the resting membrane of a cell can be calculated using the equation

Goldman-Hodgkin-Katz (GHK)

Which 3 of the following are true about action potential conduction in myelinated axons?

It is called called saltatory conduction. It is faster than in unmyelinated axons. Propagation occurs without a decrease in action potential size.

Cell "A" has a resting potential of -70mV. When its membrane potential is next measured it has a membrane potential of -40mV. Which of the following is an accurate statement about the cell during the second measurement?

It is depolarized.

Cell "X" has a resting potential of -70 mV. When its membrane potential is next measured, it has a membrane potential of -80 mV. Which of the following is an accurate statement about the cell during the second measurement?

It is hyperpolarized.

Which of the following ions are higher in concentration within the cytosol compared to extracellularly?

K+

In which of the following will the speed of action potential propagation be fastest?

Large diameter, myelinated fibers

What type of channel is depicted in the figure?

Ligand-gated

What type of channels in the postsynaptic membrane open when neurotransmitters bind to them?

Ligand-gated channels

With regard to neural pathways which best defines the term convergence?

Multiple presynaptic neurons synapse onto a single neuron.

Velocity of action potential propagation is determined by which of the following?

Myelination Axon diameter

Local anesthetics like Novocaine and Xylocaine block sensory signals by blocking voltage-gated channels

Na+

The predominant ions in the extracellular fluid are and the predominant ions in the intracellular fluid are

Na+ and Cl-; K+ and phosphate compounds

The exchange Na+ ions that leak into the cell for K+ ions that had leaked out of the cell, thus maintaining the resting potential

Na+/K+-ATPase pumps

The equation calculates the electrical potential necessary to balance a given ion's concentration gradient across a membrane so that the net flux of the ion is zero

Nernst

The generation of action potentials is prevented by the local anesthetics

Novocaine and Xylocaine

Action potentials tend to travel in only one direction along an axon for which two reasons?

Refractoriness of the region of membrane where an action potential has just occurred prevents a propagating action potential from reversing direction They begin propagating at one end of an axon, and not in the middle

Arrange these events in the order that most logically explains how a cell goes from having no membrane potential to having a steady negative resting membrane potential. Start with the earliest step at the top

The Na+/K+-ATPase pump moves Na+ out of the cell and K+ into the cell, setting up concentration gradients across the membrane. Due to a permeability difference, there a greater net movement of K+ out of the cell than of Na+ into the cell. The membrane potential becomes negative, approaching the K+ equilibrium potential. Equal leaks of Na+ and K+ are offset by the continued activity of the Na+/K+-ATPase pump.

During action potential propagation, when an action potential is occurring in one patch of membrane what induces the neighboring patch of membrane to depolarize to threshold?

The flow of local positive current from the depolarized patch to the neighboring patch of membrane.

Consider an experimental set-up with two fluid-filled chambers that are separated by a plasma membrane with open K+ channels but no other ion permeability. If you were to add 100 mM of NaCl into side 1 and 100 mM of KCl into side 2, what would you expect to happen?

The membrane will develop a membrane potential that is positive on side 1 compared to side 2.

What does the magnitude of the resting membrane potential depend upon?

The permeability of the membrane to each ion type The electrogenic effect of the Na+/K+-ATPase pump The differences in ion concentrations between the intra and extracellular fluid

Which is TRUE regarding the effect of the Na+/K+-ATPase pump on the membrane potential of excitable cells?

The pump has a minor electrogenic effect that directly contributes to the negative value of the resting membrane potential.

Graded potentials can either depolarize or hyperpolarize the membrane potential

True

The length of the refractory periods limit the number of action potentials an excitable membrane can produce in a given period of time

True

Which four of the following are TRUE during the relative refractory period?

Voltage-gated Na+ channels are resetting from the inactivated to the closed position. K+ is continuing to flow out of the cell. Voltage-gated K+ channels are resetting to the closed position. Some voltage-gated Na+ channels are closed while some are still inactivated.

Which is most responsible for the absolute refractory period of a neuron?

Voltage-gated Na+ channels open and then inactivate.

The following statements relate to movement of ions during an action potential. Which are true?

With repeated action potentials, concentration gradients for Na+ and K+ would eventually dissipate without the action of the Na+/K+-ATPase pump. The number of ions that cross the membrane during an action potential is extremely small compared to the total number of ions in a cell.

According to Ohm's Law, an increase in electrical resistance is associated with

a decrease in electrical current

The resting membrane potential is mainly due to

a tiny separation of charges that collect in a thin shell tight against the inner and outer surfaces of the membrane

After the initiation of an action potential in a given region of a neuronal membrane, there is a period of time during which a second action potential cannot be stimulated no matter how strong the stimulus. This is called the period

absolute refractory

Large, rapid, all-or-none depolarizations of neuron membrane polarity are called

action potentials

Synaptic vesicles ready to be released at chemical synapses are at locations known as

active zones

Summation occurs when

additional stimuli cause graded potentials to occur in a neuron before the graded potentials from previous stimuli have died away

Because the voltage-gated K+ channels of an axon close relatively slowly, immediately after the action potential there is a period of time where the membrane is transiently hyperpolarized towards the K+ equilibrium potential; this portion of the action potential is called the

afterhyperpolarization

When voltage-gated K+ channels are slow to close, the membrane potential is between the normal resting potential and the K+ equilibrium potential. This phase of the action potential is known as the phase

afterhyperpolarization

An action potential is either generated because a stimulus is suprathreshold, or is not generated because the stimulus is subthreshold. This property of action potentials is referred to as the property

all or none

Receptor potentials

are graded potentials in afferent neurons

A synapse that transmits information via the release of neurotransmitters into the synaptic cleft between cells is termed a(n) synapse

chemical

A transmembrane concentration gradient for ions in excitable cells is typically produced when they passively move out of the cell in response to the membrane potential set up by the leakage of other ions

chloride

The contributions of Na+, K+ and Cl- to the overall membrane potential are mainly a function of their

concentration gradients and relative permeabilities

When multiple neurons synapse onto a single postsynaptic neuron, the pattern is referred to as

convergence

When a single neuron has collateral axonal branches that synapse with a number of different neurons, it is referred to as "" of neural pathways

divergence

Synapses that consist of gap junctions which allow the local currents from action potentials in one neuron to flow directly across the junction to another neuron are called synapses

electrical

Threshold stimuli

elevate the membrane potential sufficiently to generate an action potential

When the concentration gradient for an ion across a membrane causes a flux that is equal and opposite to the flux caused by the electrical gradient, the membrane is at the for that ion

equilibrium potential

The ability of cells with voltage-gated ion channels to produce electrical signals is called

excitability

The capability of a cell to produce electrical signals through the action of a voltage-gated ion channels is called

excitability

At an the membrane potential of a postsynaptic neuron is brought closer to threshold (depolarized)

excitatory synapse

With regards to the cell, sodium ions are in higher concentration in the fluid, while potassium ions are in higher concentration in the fluid

extracellular intracellular

A large number of ions are required to cause the large fluctuation in membrane potential during an action potential; without the action of the Na+/K+-ATPase pump, the concentration gradients for Na+ and K+ would be significantly altered by a single action potential

false

Graded potentials are "all-or-none," meaning that the change in the membrane potential is the same size regardless of the size of the stimulating event

false

The neuronal membrane is much more permeable to sodium than potassium during rest because there are more ion channels for sodium

false

True or false: If an action potential was started in the middle of an axon it could only propagate in one direction—away from the cell body

false

Membrane potential changes that can vary in amplitude with stimulus strength, decrease in strength with distance, and which have no threshold or refractory period are known as potentials

graded

When ligand-gated channels open and allow a localized flow of ions that changes the membrane potential in a small area, the potential changes are called

graded potentials

If a stimulus causes the inside of a cell to become more negative than its resting potential, the cell has undergone

hyperpolarization

The membrane potential of a postsynaptic neuron is either driven farther from threshold (hyperpolarized) or stabilized at its resting potential at an

inhibitory synapse

As an ion's concentration gradient across the membrane gets (larger or smaller), the absolute value of the equilibrium potential becomes greater

larger

Channels in the membrane of a cell that are always open and passively allow the movement of K+ ions that makes the inside of the cell negative with respect to the outside are called K+ channels

leak

The resting potential of a cell is largely generated by the movement of potassium and sodium down their concentration gradients through ion channels that are always open in the membrane. These channels are called channels

leak

A - gated ion channel is depicted in the figure

ligand

A membrane ion channel operated by the binding of a specific molecule to the channel protein is said to be

ligand gated

The Na+/K+-ATPase pump in the cell membrane

maintains the concentration gradients that ultimately determine the resting potential of the cell moves potassium ions into the cell and sodium ions out of the cell

The equilibrium potential of an ion is the

membrane potential at which ion flux due to the membrane potential is equal and opposite to the flux due to the concentration gradient

When a cell is "hyperpolarized," its membrane potential is

more negative than its resting potential

Divergence occurs in neural pathways when

one neuron makes synapses with a number of different neurons

The gating of Na+ channels during the depolarization of an action potential is an example of feedback, while the gating of K+ channels during the repolarization of an action potential is an example of feedback

positive negative

The is a region of a postsynaptic cell's membrane that incorporates a high concentration of proteins, in particular receptors for neurotransmitter molecules

postsynaptic density

The permeability of a resting plasma membrane is greater for than it is for therefore, a net negative membrane potential develops

potassium; sodium

In afferent neurons, the initial depolarization to threshold that initiates action potentials is achieved by a graded potential called a(n) potential

receptor

The period of time following an action potential during which only stronger than normal stimuli can initiate a second action potential is termed the period of the action potential

relative refractory

The period of time when only a stronger than normal stimulus can produce another action potential in a neuronal membrane is known as the

relative refractory period

When a cell undergoing an action potential is on the way from its peak voltage back to the resting membrane potential, it is described as

repolarization

In the cell, the lipid bilayer of the plasma membrane is an area of high electrical while the intracellular and extracellular fluids are areas of high

resistance; conduction

The term used to describe the type of action potential propagation that regenerates itself at nodes in a myelinated axon is conduction

saltatory

The ions that enter a neuron and cause the depolarization phase of a typical action potential are ions

sodium

Describing an action potential as "all-or-none" means that

stimuli are either suprathreshold (action potential occurs) or subthreshold (no action potential); action potentials are not graded

A stimulus that causes a graded depolarization that is too small to generate an action potential is called a(n) stimulus

subthreshold

When an additional stimulus induces a graded potential before a graded potential from a previous stimulus has died away, it can result in

summation

During the absolute refractory period of a neuronal membrane

the axon cannot undergo a second action potential, no matter how strongly it is stimulated

Limitations on the maximum action potential firing frequency of neurons are determined by

the length of the refractory periods

A stimulus that is just strong enough to trigger the voltage-gated channels of the initial segment of an axon to open and begin the action potential is called a(n) stimulus

threshold

Action potentials in neurons are very brief events, and may repeat at high frequencies-up to several hundred per second

true

In most cells, the Na+/K+-ATPase pump makes a small electrogenic contribution to creating the negative resting membrane potential

true

The larger the concentration gradient across a membrane for a given ion, the greater the absolute value of that ion's equilibrium potential

true

Ion channels that are closed when the cell is at its resting potential but open when the cell's membrane potential changes are categorized as - ion channels

voltage gated

An ion channel that opens in response to a change in the membrane potential of the cell is called a channel

voltage-gated

While K+ ions move through both leakage and voltage-gated channels, Na+ ions move predominantly through during an action potential

voltage-gated channels