Ch 37: Neurons, Synapses, and Signaling

Question 1

The ability of a neuron to receive and transmit information is based on a highly specialized cellular organization. Most of a neuron’s organelles, including its nucleus, are located in the cell body. In a typical neuron, the cell body is studded with highly branched extensions called dendrites (from the Greek _______, tree). Together with the cell body, the dendrites ____ signals from other neurons.

Answer 1

“dendron”
receive

Question 2

A neuron also has a single axon, an extension that transmits signals to other cells. Axons are often much longer than dendrites, and some, such as those that reach from the spinal cord of a giraffe to the muscle cells in its feet, are over a meter long. The cone-shaped base of an axon, called the ____ _______, is typically where signals that travel down the axon are generated. Near its other end, an axon usually divides into many branches.

Answer 2

axon hillock

Question 3

Each branched end of an axon transmits information to another cell at a junction called a synapse. The part of each axon branch that forms this specialized junction is a _____ _____. At most synapses, chemical messengers called neurotransmitters pass information from the transmitting neuron to the receiving cell.

Answer 3

synaptic terminal

Question 4

In describing a synapse, we refer to the transmitting neuron as the ______ cell and the neuron, muscle, or gland cell that receives the signal as the ______ cell.

Answer 4

presynaptic
postsynaptic

Question 5

The neurons of vertebrates and most invertebrates require supporting cells called glial cells, or glia (from a Greek word meaning “glue”). Overall, glia outnumber neurons in the mammalian brain __ __ __-fold.

Answer 5

10- to 50

Question 6

Glia nourish neurons, ________ the axons of neurons, and regulate the extracellular fluid surrounding neurons. In addition, glia sometimes function in replenishing certain groups of neurons and in transmitting information.

Answer 6

insulate

Question 7

Signals within a neuron are ________. Signals sent from one neuron to the next are ________.

Answer 7

electrical

chemical

Question 8

The cone snail’s siphon acts as a sensor, transferring information to neuronal circuits in the snail’s head for processing. If prey is detected, these circuits issue _____ ________—signals that control muscle activity. In this example, motor commands trigger release of a harpoon-like tooth from the proboscis, spearing the prey.

Answer 8

motor commands

Question 9

Most neurons in the brain are motor neurons, interneurons, or sensory neurons?

Answer 9

interneurons

Question 10

We turn now to the essential role of ions in neuronal signaling. In neurons, as in other cells, ions are unequally distributed between the interior of cells and the surrounding fluid. As a result, the inside of a cell is ________ charged relative to the outside. This charge difference, or voltage, across the plasma membrane is called the membrane potential, reflecting the fact that the attraction of opposite charges is a source of ________ energy.

Answer 10

negatively
potential

Question 11

For a resting neuron—one that is not sending a signal—the membrane potential is called the resting potential and is typically between ___ and ___ millivolts (mV).

Answer 11

−60 and −80

Question 12

Potassium ions and sodium ions play an essential role in the formation of the resting potential. These ions each have a concentration gradient across the plasma membrane of a neuron. For most neurons, the concentration of K+ is higher inside the cell, while the concentration of Na+ is higher outside. These gradients are maintained by the ____-______ ____.

Answer 12

sodium-potassium pump.

Question 13

The sodium-potassium pump transports __ Na+ out of the cell for every __ K+ that it transports in. Although this pumping generates a net export of positive charge, the pump acts slowly. The resulting change in the membrane potential is therefore quite small—only a few millivolts. Why, then, is there a membrane potential of -60 to -80 in a resting neuron? The answer lies in ion movement through ion channels, pores formed by clusters of specialized proteins that span the membrane. Ion channels allow ions to _______ back and forth across the membrane. As ions diffuse through channels, they carry with them units of electrical charge. Any resulting net movement of positive or negative charge will generate a membrane potential across the membrane.

Answer 13

3
2
diffuse (passively)

Question 14

Diffusion of K+ through potassium channels that are always open (sometimes called leak channels) is critical for establishing the resting potential. The K+ concentration is 140 millimolar (mM) inside the cell, but only 5 mM outside. The chemical concentration gradient thus favors a net outflow of K+. Furthermore, a resting neuron has many open potassium channels, but very few open _____ channels. Because Na+ and other ions can’t readily cross the membrane, K+ outflow leads to a net negative charge inside the cell. This buildup of ________ charge within the neuron is the major source of the membrane potential.

Answer 14

sodium
negative

Question 15

How does a stimulus alter the membrane potential? Certain ion channels in a neuron, called _____ ___ channels, open or close in response to stimuli. When a gated ion channel opens or closes, it alters the membrane’s permeability to particular ions. The result is a rapid flow of ions across the membrane, altering the membrane potential.

Answer 15

gated ion

Question 16

Sometimes the response to hyperpolarization or depolarization is simply a shift in the membrane potential. This shift, called a graded potential, has a magnitude that varies with the strength of the stimulus: A larger stimulus causes a greater change in the ________ ________. Graded potentials induce a small electrical current that dissipates as it flows along the membrane. Graded potentials thus decay with time and distance from the source.

Answer 16

membrane potential

Question 17

If a depolarization shifts the membrane potential sufficiently, the result is a massive change in membrane voltage called an action potential. Unlike graded potentials, action potentials have a ________ magnitude and can regenerate in adjacent regions of the membrane. Action potentials can therefore spread along axons, making them well suited for transmitting a signal over long distances.

Answer 17

constant

Question 18

Action potentials arise because some of the ion channels in neurons are voltage gated. If a depolarization increases the membrane potential to a level called threshold, the voltage-gated sodium channels open. The resulting flow of Na+ into the neuron results in further depolarization. Because the sodium channels are voltage gated, the increased depolarization causes more ______ _______ to open, leading to an even greater flow of current. The result is a process of positive feedback that triggers a very rapid opening of many voltage-gated sodium channels and the marked temporary change in membrane potential that defines an action potential.

Answer 18

sodium channels

Question 19

A toxin that binds specifically to voltage-gated sodium channels in axons would be expected to prevent…

Answer 19

…. the depolarization phase of the action potential.

Question 20

opening of the __ channels causes depolarization of the cell, which then become _______ (but not closed), and opening of __ channels is what causes repolarization of the cell membrane.

Answer 20

Na+

inactivated

K+

Question 21

The threshold potential of a membrane is the….

Answer 21

….minimum depolarization needed to operate the voltage-gated sodium and potassium channels.

Question 22

An action potential that starts at the axon hillock moves along the axon only toward the synaptic terminals. Why? Immediately behind the traveling zone of depolarization, the sodium channels remain inactivated, making the membrane temporarily ___________ (not responsive) to further input. Consequently, the inward current that _________ the axon membrane ahead of the action potential cannot produce another action potential behind it. This is the reason action potentials do not travel back toward the cell body.

Answer 22

refractory

depolarizes

Question 23

After the refractory period is complete, depolarization of the axon hillock to threshold will trigger a new action potential. In many neurons, action potentials last less than 2 milliseconds (msec), and the ______ _____ can thus reach hundreds of action potentials per second.

Answer 23

firing rate

Question 24

When does the membrane have the greatest permeability to sodium?

Answer 24

During the big upstroke of the action potential.

Question 25

In most cases, action potentials are not transmitted from neurons to other cells. However, information is transmitted, and this transmission occurs at synapses.

The majority of synapses are ________ ________, which rely on the release of a chemical neurotransmitter by the presynaptic neuron to transfer information to the target cell.

Answer 25

chemical synapses

Question 26

While at rest, the presynaptic neuron synthesizes the neurotransmitter at each synaptic terminal, packaging it in multiple membrane-enclosed compartments called _______ ________. When an action potential arrives at the synapse, it depolarizes the plasma membrane at the synaptic terminal, opening voltage-gated channels that allow Ca²⁺ to diffuse in. The Ca²⁺ concentration in the terminal rises, causing synaptic vesicles to fuse with the terminal membrane and release the neurotransmitter.

Answer 26

synaptic vesicles

Question 27

Once released, the neurotransmitter diffuses across the synaptic cleft, the gap that separates the presynaptic neuron from the postsynaptic cell. Diffusion time is very short because the gap is less than 50 nm across. Upon reaching the ________ _________, the neurotransmitter binds to and activates a specific receptor in the membrane, which in turn triggers a response in the postsynaptic cell.

Answer 27

postsynaptic membrane

Question 28

Some synapses are ________ ________, which rely on the movement of electric current rather than a neurotransmitter. In electrical synapses, current flows from one neuron to another via gap junctions. Such synapses, which are less readily modified than chemical synapses, are common in rapid and unvarying neural pathways. For example, electrical synapses associated with the giant axons of squids and lobsters facilitate swift escapes from danger. Electrical synapses are also found in the vertebrate heart and brain.

Answer 28

electrical synapses

Question 29

Vertebrate axons have narrow diameters but can still conduct action potentials at high speed. How is this possible? The evolutionary adaptation that enables fast conduction in vertebrate axons is electrical _________, analogous to the plastic insulation that encases many electrical wires. Insulation causes the depolarizing current associated with an action potential to travel farther along the axon interior, bringing more distant regions to the threshold sooner.

Answer 29

insulation

Question 30

The electrical insulation that surrounds vertebrate axons is called a myelin sheath. Myelin sheaths are produced by glia—___________ in the CNS and Schwann cells in the PNS. During development, these specialized glia wrap axons in many layers of membrane. The membranes forming these layers are mostly lipid, which is a ____ _______ of electrical current and thus a good insulator.

Answer 30

oligodendrocytes

poor conductor

Question 31

A neuron’s nucleus is located in its ____ ____.

Answer 31

cell body

Question 32

______ conduct an impulse from a synapse toward the cell body.

Answer 32

Dendrites

Question 33

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

Answer 33

synaptic terminal

Question 34

A neuron has a resting potential of about _____ millivolts.

Answer 34

-70

Question 35

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

Answer 35

The sodium-potassium pump moves more sodium ions out of the cell than potassium ions into the cell; this net loss of positive ions establishes a charge differential across the plasma membrane.

Question 36

The transmission first triggers the _____.

Answer 36

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

Question 37

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.

Answer 37

sodium

opening of voltage-gated sodium channels

Question 38

Which ion is the cell membrane more permeable to?

Answer 38

potassium

But remember, a potassium channel allows K to diffuse freely across the membrane, but not other ions such as Na+ or Cl-

Question 39

What are ganglia?

Answer 39

Clusters of nerve cell bodies.

Question 40

Integration is specific to the _______ nervous system.

Answer 40

central

Question 41

Which statement is correct?

a. ) a neuron has only a single axon
b. ) axons receive signals from other neurons
c. ) dendrites branch off axons
d. ) The PNS is most closely associated with interneurons.
e. ) none of the above choices is correct

Answer 41

a.) a neuron has only a single axon

Question 42

Which statement is correct?

a. ) The vast majority of neurons in the brain are sensory neurons.
b. ) The nucleus of a neuron is located in the axon hillock.
c. ) Sensory neurons transmit signals to muscle cells.
d. ) Synaptic cells nourish neurons, insulate the axons of neurons, and/or regulate the extracellular fluid surrounding the neurons.
e. ) None of the choices is correct.

Answer 42

e.) None of the choices is correct.

Question 43

Inside the cell, the concentration of ______ is much higher than the concentration of ______.

Answer 43

potassium

sodium

Question 44

True or false?

Ion channels allow ions to cross the cell membrane against their concentration gradients.

Answer 44

False, they do not. They permit diffusion which is passive.

Question 45

Ion channels are ________ permeable.

Answer 45

selectively

Question 46

What would happen if the potassium ion channels in a mammalian cell were blocked?

Answer 46

It would decrease the magnitude of the membrane potential

Question 47

A drug that causes potassium to leak out of a neuron, increasing the positive charge on the outside, would….

Answer 47

…inhibit transmission of nerve signals by the neuron.

The additional positive charge to the outside of the neuron would cause a hyperpolarization of the membrane.

Question 48

Threshold is of great significance in the physiology of neurons because if threshold is not reached….

Answer 48

….positive-feedback depolarization will not occur.

In other words, stimuli that do not depolarize the membrane to the level of the threshold do not generate action potentials.

Question 49

Oligodendrocytes comprise the ___ myelin sheath; Schwann cells comprise the ___ myelin sheath.

Answer 49

CNS

PNS

Question 50

Saltatory conduction is a type of impulse conduction that occurs along ________ axons.

Answer 50

myelinated

Question 51

Acetylcholinesterase is the enzyme that degrades acetylcholine. What effect on nerve transmission would occur following the administration of a chemical that inhibited acetylcholinesterase?

Answer 51

Continuous excitatory postsynaptic potentials would occur in the postsynaptic neuron. Because the neurotransmitter is not degraded, it continues to interact with the receptors on the postsynaptic membrane.

Question 52

Which correctly pairs a type of synapse and a key component of that synapse?

electrical synapse … gap junctions

chemical synapses … gap junctions

electrical synapse … neurotransmitters

junctional synapses … ligand-gated ion channels

None of the choices is correct.

Answer 52

Electrical synapses contain gap junctions.

Question 53

Which is a mechanism by which neurotransmitters can be rapidly cleared from the synaptic cleft?

Neurotransmitters are removed by simple diffusion.

The neurotransmitters are actively transported back into the presynaptic cells to be repackaged into synaptic vesicles.

The neurotransmitters are actively transported into glia to be metabolized as fuel.

Neurotransmitters are removed by enzymes that catalyze hydrolysis of the neurotransmitter.

All of the choices are correct.

Answer 53

All of the choices are correct.

Question 54

The interplay of multiple excitatory and inhibitory inputs most affects what part of a neuron?

Answer 54

The axon hillock – it is the neuron’s integrating center.