Chapter 11 A&P

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 against its electrochemical gradient and K+ is pumped into the cell against its concentration gradient.

Which of the following does NOT describe conditions that occur during an action potential?

Na+ is used to repolarize the membrane.

Which of the following statements is correct regarding the polarization of a neuronal membrane and the formation of a resting membrane potential?

Sodium/Potassium pumps maintain concentration gradients; sodium and potassium move down their concentration gradients through leakage channels

An action potential is self-regenerating because __________.

The Na+‎ diffusing into the axon during the first phase of the action potential creates a depolarizing current that brings the next segment, or node, of the axon to threshold.

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

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.

What opens first in response to a threshold stimulus?

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

What type of event is required for an action potential to be generated?

The axolemma must be depolarized to threshold in order to generate an action potential.

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

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.

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

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

Complete the following sentence. The operation of the Na+−K+Na+−K+ ATPase pump __________.

The pumps move more Na+Na+ than K+K+, which counteracts the effect of the membrane's being more permeable to K+K+ (more K+K+ leak channels than Na+Na+).

Which of the following is a factor that determines the rate of impulse propagation, or conduction velocity, along an axon?

The rate of impulse propagation depends largely on two factors: degree of myelination and axon diameter. The presence of a myelin sheath dramatically increases the rate of impulse (action potential) propagation. Heavily myelinated axons propagate impulses faster than lightly myelinated axons. Also, larger (thicker) axons conduct impulses faster than smaller (thinner) axons.

A postsynaptic potential is a graded potential that is the result of a neurotransmitter released into the synapse between two neurons.

True

Axon diameter and degree of myelination determine nerve impulse conduction velocity.

True

Cell bodies of sensory neurons may be located in ganglia lying outside the central nervous system.

True

During depolarization, the inside of the neuron's membrane becomes less negative.

True

In myelinated axons the voltage-regulated sodium channels are concentrated at the nodes of Ranvier.

True

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

a positive ion is driven into the cell because the inside of the cell is negative compared to the outside of the cell, and Na+ is driven into the cell because the concentration of Na+ is greater outside the cell.

If a postsynaptic cell is stimulated to threshold by temporal summation this implies that ________.

a presynaptic neuron is sending frequent EPSP

The period after an initial stimulus when a neuron is not sensitive to another stimulus is the ________.

absolute refractory period

Bipolar neurons are commonly ________.

found in the retina of the eye

What does the central nervous system use to determine the strength of a stimulus?

frequency of action potentials

Collections of nerve cell bodies outside the central nervous system are called ________.

ganglia

Which of the following neurotransmitters is the principal inhibitory neurotransmitter of the spinal cord?

glycine

An inhibitory postsynaptic potential (IPSP) is associated with ________.

hyperpolarization

When a neurotransmitter like GABA is acting in an inhibitory manner which of the following is likely a result of the GABA acting on the postsynaptic cell?

hyperpolarization of the postsynaptic cell membrane

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

more K+ moves out of the cell than Na+ moves into the cell, helping to establish a negative resting membrane potential.

The interior surface of a neuron's plasma membrane at resting membrane potential will have a ________

negative charge and contains less sodium than outside of the cell

If a postsynaptic neuron is stimulated to threshold by spatial summation this implies that ________.

the postsynaptic cell has many synapses with many presynaptic neurons

Assume you have a membrane with only potassium leakage channels. The RMP is -90mV. Predict the RMP if we add Na+Na+ leakage channels.The most likely RMP value of Na+Na+ is __________.

-70 mV

An action potential is regarded as an example of a positive feedback. Which of the following examples below best illustrates the positive feedback aspect of an action potential?

A threshold stimulus will cause the opening of voltage gated sodium ion channels that will cause further depolarizing stimulus. This stimulus will open still more voltage gated sodium ion channels.

What type of conduction takes place in unmyelinated axons?

An action potential is conducted continuously along an unmyelinated axon from its initial segment to the axon terminals. The term continuous refers to the fact that the action potential is regenerated when voltage-gated Na+‎ channels open in every consecutive segment of the axon, not at nodes of Ranvier.

Trigger zone

Area where nerve impulse is generated.

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

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.

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

Closing of voltage-gated channels is time dependent. Typically, the inactivation gates of voltage-gated Na+‎ channels close about a millisecond after the activation gates open. At the same time, the activation gates of voltage-gated K+‎ channels open.

Cl−Cl− is a common, negatively charged extracellular ion. Predict the effect on the RMP if many Cl−Cl− gated channels are suddenly opened.

Cl−Cl− is negatively charged and has a higher concentration in the ECF. The opening of Cl−Cl− channels would allow more negative charge to attempt to enter the cytoplasm. If the cell's normal RMP were -70mV, it would now become more negative. The farther away the RMP is from zero, either in the positive or in the negative direction, the greater the separation of charges is. This is called hyperpolarization. When the RMP moves closer to 0 mV, depolarization occurs.

The effect of acetylcholine can be stimulating or inhibiting. Which of the following gives the best explanation for why this is so?

Different postsynaptic cells will have different receptors.

Which criterion is used to functionally classify neurons?

Functional classification groups neurons according to the direction in which the nerve impulse travels relative to the central nervous system. Based on this criterion, there are sensory neurons, motor neurons, and interneurons.

If the neuron membrane becomes more permeable to Na+, Na+ will transport across the membrane, causing the cell to depolarize.

In either a graded or action potential, Na+ is transported into the cell faster than during resting potentials. As Na+ enters the cell, the inside of the cell becomes more positive, or depolarized.

Which description of synapses is not correct?

Indirect signaling involves intracellular second messenger molecules, typically via G protein pathways. Direct signaling involves neurotransmitters that bind to ligand-gated ion channels and leads directly to changes to the polarization of the membrane.

In multiple sclerosis, the cells that are the target of an autoimmune attack are the ________

Oligodendrocytes are a type of neuroglial cell that function to form the myelin sheath around the axons of neurons within the central nervous system. These are the glial cells targeted by the immune system in people who have MS.

Receptive region (2)

Plasma membrane exhibits chemically gated ion channels.

Secretory region

Plasma membrane exhibits voltage-gated Ca2+ channels.

Conducting region

Plasma membrane exhibits voltage-gated Na+ and K+ channels.

Local anesthetics block voltage-gated Na+ channels, but they do not block mechanically gated ion channels. Sensory receptors for touch (and pressure) respond to physical deformation of the receptors, resulting in the opening of specific mechanically gated ion channels. Why does injection of a local anesthetic into a finger still cause a loss of the sensation of touch from the finger?

Propagation (spreading) of an action potential from the axon hillock to the axon terminals (synaptic knobs) requires the sequential opening of mechanically and voltage-gated ion channels. When the sequence is interrupted, the message cannot spread to the sensory regions of the central nervous system, causing numbness.

Receptive region

Receives stimuli.

Axon terminal

Release neurotransmitters into the extracellular space

In a lab you are conducting tests with various chemicals and neurotransmitter receptors. You notice that exposed frog muscle cells depolarize when you add the chemical nicotine to the acetylcholine (ACh) receptors. What does this tell us about the relationship between neurotransmitters and their receptors?

The effect of a neurotransmitter is based on the properties of the receptor more than the neurotransmitter.

Where do most action potentials originate?

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.

What component of the reflex arc determines the response to a stimulus?

The integration center receives sensory information (input), determines the proper response, and then signals the appropriate effector(s) to produce the response.

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

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 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 velocity of conduction by two mechanisms. First, myelin insulates the axon, reducing the loss of depolarizing current across the plasma membrane. Second, the myelin insulation allows the voltage across the membrane to change much faster. Because of these two mechanisms, regeneration only needs to happen at the widely spaced nodes of Ranvier, so the action potential appears to jump.

You discover that a new chemical compound interacts with K+ voltage-dependent channels. What would be the effect on a neuron if the chemical came into contact with the axonal membrane?

The neuron would be unable to repolarize.

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

both Na+ and Cl- are in higher concentrations outside the cell.

Which of the following events would be directly affected if a neuron had a mutation that prevented the production of voltage-gated Na+ channels?

depolarization leading to action potentials

Which of the following describes the excitatory postsynaptic potential?

short distance depolarization

A second nerve impulse cannot be generated until ________.

the membrane potential has been reestablished

Saltatory conduction is made possible by ________.

the myelin sheath

During the relative refractory period of an action potential, a larger than normal stimulus is needed to cause another action potential. This is due to the fact that ________.

the voltage gated potassium ion channels remain open long enough to hyperpolarize the axon membrane

Which choice best characterizes K+K+ leakage channels?

transmembrane protein channels that are always open to allow K+K+ to cross the membrane without the additional input of energy

The action potential is caused by permeability changes in the plasma membrane.

true

Which ion channel opens in response to a change in membrane potential and participates in the generation and conduction of action potentials?

voltage-gated channel

Immediately after an action potential has peaked, which of the following channels will open?

voltage-gated potassium channels