Chapter 48: Neurons, Synapses, and Signaling

Why are action potentials conducted usually in one direction along an axon?

The brief refractory period prevents reopening of voltage-gated sodium channels.

If you experimentally increase the concentration of Na+ outside a cell while maintaining other ion concentrations as they were, what would happen to the cell's membrane potential?

The membrane potential would become more negative.

What do we call a membrane potential in which there is no net movement of an ion across a membrane?

an equilibrium potential

If the membrane potential of a neuron decreases, the membrane potential _____.

becomes less negative.

Resting neurons are most permeable to which of the following ions?

K+

Which of these ions is more abundant in the interior of a resting neuron than in the fluid surrounding the neuron?

K+

Which channel is mainly responsible for the resting potential of a neuron?

Potassium leak channel.

Which event triggers the creation of an action potential?

The membrane depolarizes above a certain threshold potential.

Which of the following best explains the observation that a resting neuron membrane, while highly permeable to potassium ions, is not at the equilibrium potential for potassium?

The membrane is also slightly permeable to sodium ions.

Which of the following is the most direct result of depolarizing the presynaptic membrane of an axon terminal?

Voltage-gated calcium channels in the membrane open.

A nerve impulse moves away from a neuron's cell body along _____.

axon

A nerve is a collection of ________.

axons

A neuron's nucleus is located in its _____.

cell body

The information-receiving section of a neuron is its __________.

cell body and dendrites

Suppose a particular neurotransmitter causes an IPSP in postsynaptic cell X and an EPSP in postsynaptic cell Y. A likely explanation is that

cells X and Y express different receptor molecules for this particular neurotransmitter.

Of these choices, neuronal communication between the brain and the muscles of the leg is best conceptualized as __________.

electrical and chemical signaling

Neurotransmitters categorized as inhibitory are expected to ________.

hyperpolarize the membrane

In which of the following locations would we expect to find the release of neurotransmitter molecules?

in a chemical synapse

Which of the following will induce a graded hyperpolarization of a membrane under normal resting conditions?

increasing its permeability to K+

Axons insulated by a(n) _____ are able to conduct impulses faster than those not so insulated.

myelin sheath

What chemical affects neuronal function but is not stored in presynaptic vesicles?

nitric oxide

What would most likely happen if twice as many inhibitory postsynaptic potentials (IPSPs) as excitatory postsynaptic potentials (EPSPs) arrive in close proximity at a postsynaptic neuron?

no action potential

An impulse relayed along a myelinated axon "jumps" from _____ to _____.

node of Ranvier ... node of Ranvier

Pyrethroid insecticides prevent the voltage-gated sodium channels of insects from inactivating. Neurons that were exposed to pyrethroids would ________.

not repolarize during an action potential

Gated ion channels

open and close in response to a stimulus.

The transmission first triggers the _____.

opening of voltage-gated sodium channels and the diffusion of sodium ions into the neuron

An inhibitory postsynaptic potential (IPSP) will occur if a membrane is made more permeable to ________.

potassium ions

The botulinum toxin, which causes botulism, inhibits ________.

presynaptic release of acetylcholine

If you treat a neuron with a drug that increases membrane permeability to Na+, how would you expect the equilibrium potential for Na+ to change?

remain unchanged

Neurons store neurotransmitter molecules in vesicles located within _____.

synaptic terminals

During an action potential

the inside of the neuron becomes positively charged relative to the outside.

The Nernst equation specifies the equilibrium potential for a particular ion. This equilibrium potential is a function of ________.

the ion concentration gradient

Which of the following is the most likely effect of applying the naturally occurring acetylcholinesterase inhibitor onchidal (produced by the mollusc Onchidella binneyi) to a neuromuscular junction?

the muscle cell would receive constant stimulation

Which of the following is a characteristic of an electrical synapse?

gap junctions

You feel something touch your arm, think it might be a fly, and you extend your arm to scare it away. In which order is the information about the touch processed?

sensory input, integration, and motor output

The concentrations of sodium and potassium ions are very different inside and outside a neuron. What contributes to these differences?

sodium-potassium pumps

Excitatory postsynaptic potentials (EPSPs) occurring at multiple synapses on the same postsynaptic neuron can add together through ________.

spatial summation

Which term describes an electrical signal generated by neurons?

Action potential.

In multiple sclerosis, the myelin sheaths around the axons of the brain and spinal cord are damaged and demyelination results. How does multiple sclerosis manifest at the level of the action potential?

Action potentials move more slowly along the axon.

What behavior is observed if the voltage across a neuronal membrane is set to -20 mV?

The sodium channel opens, and Na+ ions flow in.

Which membrane protein maintains the concentration gradients of ions across a neuronal membrane?

The sodium-potassium pump moving Na+ ions out and K+ ions in.

Which of the following is an example of a ligand-gated channel?

acetylcholine receptors at a neuromuscular junction

The rapid reversal of ions across the plasma membrane of a neuron is known as a(n) __________.

action potential

Conduction and refractory periods are typical of ________.

action potentials

Which of these causes the release of neurotransmitter molecules?

an action potential reaching the end of the axon

At a neuromuscular junction, what process releases acetylcholine into the synaptic cleft?

exocytosis

Which of the following is expected in a neuron that is receiving input through tens of thousands of synapses?

highly branched dendrites

For a neuron with an initial membrane potential at -70 mV, what would be the result of an increase in the movement of potassium ions out of that neuron's cytoplasm?

hyperpolarization of the neuron

Tetrodotoxin blocks voltage-gated sodium channels, and ouabain blocks sodium-potassium pumps. If you added both tetrodotoxin and ouabain to a solution containing neural tissue, what responses would you expect?

immediate loss of action potential with gradual shift of resting potential

If a pebble is tossed into a pond, it generates small ripples that decrease in size as the ripples move away from the point where the pebble struck the water. The decreasing size of the ripples as they move away from the initial splash is most like the way that signals spread ________.

in a graded potential

What type of neurons is responsible for the interpretation of sensory input?

interneurons

Action potentials move along axons ________.

more rapidly in myelinated than in unmyelinated axons

Water held behind a dam is similar to what aspect of a neuron?

resting potential

Sodium-potassium pumps

restore the distribution of ions inside and outside a neuron's membrane following an action potential.

A stimulus has opened the voltage-gated sodium channels in an area of a neuron's plasma membrane. As a result, _____ rushes into the neuron and diffuses to adjacent areas; this in turn results in the _____ in the adjacent areas.

sodium ... opening of voltage-gated sodium channels

The space between an axon of one neuron and the dendrite of another neuron is called a(n) _____.

synaptic cleft

What part of a neuron relays signals from one neuron to another neuron or to an effector?

synaptic terminal

Which answer explains why Na+ ions enter the cell when voltage-gated Na+ channels are opened in neurons?

the Na+ concentration is much higher outside the cell than it is inside, and the Na+ ions are attracted to the negatively charged interior

What part of a neuron functions as the integrating center, where the membrane potential reflects the summed effect of all EPSPs and IPSPs?

the axon hillock

A common feature of action potentials is that they

are triggered by a depolarization that reaches threshold.

A neuron has a resting potential of about _____ millivolts.

-70

Which of the following statements about action potentials is correct?

Action potentials propagate towards the synaptic terminal of an axon.

Which of the following will decrease the heart rate of a vertebrate?

acetylcholine

Which of the following explains why one-way synaptic transmission occurs?

Receptors for neurotransmitters are mostly found on the postsynaptic membrane.

The following are events in the transmission of a signal at a chemical synapse. 1. Neurotransmitter binds with receptors associated with the postsynaptic membrane. 2. Calcium ions rush into neuron's cytoplasm. 3. An action potential depolarizes the membrane of the presynaptic axon terminal. 4. The ligand-gated ion channels open. 5. The synaptic vesicles release neurotransmitter into the synaptic cleft. Which sequence of events is correct?

3, 2, 5, 1, 4

Where in the neuron do action potentials begin?

Axon hillock.

True or false? Action potentials travel in only one direction down an axon because potassium channels in the neuron are refractory and cannot be activated for a short time after they open and close.

False Action potentials travel in only one direction down an axon because sodium channels in the neuron are refractory.

True or false? The potential energy of a membrane potential comes solely from the difference in electrical charge across the membrane.

False The potential energy of a membrane potential comes both from the difference in electrical charge and from the concentration gradient of ions across a membrane.

Which of the following is an amino acid that functions as a neurotransmitter at most inhibitory synapses in the brain?

GABA

If a drug mimicked the activity of GABA in the CNS, what general effect on behavior might you expect?

GABA is usually an inhibitory neurotransmitter making it more difficult to trigger an action potential. The neurons become less excitable, thus calming nervous activity.

Which of the following terms describes how a neuronal membrane's potential is altered in the presence of inhibitory signals?

Hyperpolarization

The activity of the sodium-potassium pump results in the movement of which ions across the plasma membrane?

It pumps sodium ions out of the cell and potassium ions into the cell.

The plasma membrane of a neuron has voltage-gated sodium and potassium channels. What is the effect of membrane depolarization on these channels?

Membrane depolarization first opens sodium channels and then opens potassium channels.

Which term describes the difference in electrical charge across a membrane?

Membrane potential

Which structure is not part of a neuron?

Myelin sheath.

Identify the correct statement(s) about the resting membrane potential of a cell.

Potassium (K+) and sodium (Na+) gradients are maintained by active transport in a resting mammalian neuron. Concentration gradients of potassium (K+) and sodium (Na+) across the plasma membrane represent potential energy.

What type of cell makes up the myelin sheath of a motor neuron?

Schwann cells

Which of the following events is the first to occur during an action potential?

Sodium channels open.

How is an action potential propagated down an axon after voltage-gated sodium channels open in a region of the neuron's membrane?

Sodium ions enter the neuron and diffuse to adjacent areas, resulting in the opening of voltage-gated sodium channels farther down the axon.

From what you know about action potentials and synapses, choose two hypotheses for how various anesthetics might block pain.

Some general anesthetics reduce synaptic transmission. Most are lipid soluble and may bind reversibly to the receptor portions of ligand-gated ion channels on the postsynaptic membrane, preventing neurotransmitter uptake. Local anesthetics inhibit sodium channels in nerve cells, preventing transmission of sensations, including pain, from a specific region of the body to the brain.

A researcher uses the chemical inhibitor cyanide to reduce ATP production in a neuron. What would be one effect of preventing ATP production?

The "resting" distribution of potassium and sodium ions would be altered.

What causes an action potential to be conducted along a neuron's axon?

The change in charge difference across the membrane spreads from open sodium channels, causing sodium channels farther along the axon to open.

Which of the following describes the ion channels of a resting neuron?

The channels are open or closed depending on their type, and are specific as to which ion can traverse them.

A neurophysiologist is investigating reflexes in two different animals: a crab and a fish. Action potentials are found to pass more rapidly along the fish's neurons. What is a likely explanation?

The fish's axons are myelinated. The crab's axons are not myelinated.

If you experimentally increase the concentration of K+ inside a cell while maintaining other ion concentrations as they were, what would happen to the cell's membrane potential?

The membrane potential would become more negative.

What happens when a resting neuron's membrane depolarizes?

The neuron's membrane voltage becomes more positive.

Which of the following statements about action potentials in a given neuron is false?

They are propagated down the length of the dendrite.

Acetylcholine released into the junction between a motor neuron and a skeletal muscle binds to a sodium/potassium channel and opens it. This is an example of ________.

a ligand-gated channel

A nerve impulse moves toward a neuron's cell body along _____.

dendrites

In neurons, ____________ receive signals and ____________ transmit signals.

dendrites; axons

In a chemical synapse, where would we find an ionotropic receptor?

the postsynaptic cell

Where are neurotransmitter receptors located?

the postsynaptic membrane

Where are neurotransmitters released in a synapse?

the presynaptic membrane

What is the best indicator of the strength of a signal in a neuron?

the rate at which action potentials are produced

At rest, which of these plays a role in establishing the charge differential across a neuron's plasma membrane?

the sodium-potassium pump moving sodium ions out of the neuron and potassium ions into the neuron

Which of the following describes the cause of the undershoot phase of hyperpolarization?

the sustained opening of voltage-gated potassium channels

When a neuron is at its resting potential

there are more potassium ions inside the neuron than outside.

Which of the following would produce the fastest possible conduction velocity of action potentials?

thick, myelinated axons

Which of the following would most likely occur if you experimentally depolarize the middle of an axon to threshold using an electronic probe?

two action potentials will be initiated and will proceed in opposite directions

After the depolarization phase of an action potential, the resting potential is restored by ________.

voltage-gated potassium channels opening and sodium channels inactivating