chapter 4.5

Question 1

Axodendritic synapses

Answer 1

synapses of axon terminal buttons on dendrites.

Question 2

Dendritic spines

Answer 2

nodules of various shapes that are located on the surfaces of many dendrites. Many axodendritic synapses terminate on these.

Question 3

Axosomatic synapses

Answer 3

synapses of axon terminal buttons on somas (cell bodies).

Question 4

Dendrodendritic synapses

Answer 4

often capable of transmission in either direction.

Question 5

Axoaxonic synapses

Answer 5

can mediate presynaptic facilitation and inhibition. An axoaxonic synapse on, or near, a terminal button can selectively facilitate or inhibit the effects of that button on the postsynaptic neuron.

Question 6

Advantage of presynaptic facilitation and inhibition

Answer 6

is that they can selectively influence one particular synapse rather than the entire presynaptic neuron.

Question 7

Directed synapses

Answer 7

synapses at which the site of neurotransmitter release and the site of neurotransmitter reception are in close proximity.

Question 8

Nondirected synapses

Answer 8

synapses at which the site of release is at some distance from the site of reception.

Question 9

String-of-bead synapses

Answer 9

neurotransmitter molecules are released from a series of varicosities (bulges or swellings) along the axon and its branches and thus are widely dispersed to surrounding targets.

Question 10

Neuropeptides

Answer 10

large neurotransmitters; short amino acid chains comprising between 3 and 36 amino acids; in effect, they are short proteins. Assembled in the cytoplasm, like other proteins, of the cell body on ribosomes; they are then packed in vesicles by the cell body’s Golgi complex and transported by microtubules to the terminal buttons at a rate of about 40 cm a day.

Question 11

Small-molecule neurotransmitters

Answer 11

typically synthesized in the cytoplasm of the terminal button packed in synaptic vesicles by the button’s Golgi complex. Once filled with neurotransmitters, the vesicles are stored in clusters next to the presynaptic membrane.

Question 12

The vesicles that contain neuropeptides

Answer 12

are usually larger than those that contain small-molecule neurotransmitters, and they do not usually congregate as closely to the presynaptic membrane as the other vesicles do.

Question 13

Coexistence

Answer 13

many neurons contain two neurotransmitters.

Question 14

Exocytosis

Answer 14

the process of neurotransmitter release.

Question 15

When a neuron is at rest

Answer 15

synaptic vesicles that contain small-molecule neurotransmitters tend to congregate near sections of the presynaptic membrane that are particularly rich in voltage-activated calcium channels.

Question 16

Voltage-activated calcium channels

Answer 16

when stimulated by action potentials, these channels open, and Ca2+ ions enter the button. The entry of the Ca2+ ions cases synaptic vesicles to fuse with the presynaptic membrane and empty their contents into the synaptic cleft.

Question 17

Small neurotransmitters are typically released in a pulse

Answer 17

each time an action potential triggers a momentary influx of Ca2+ ions through the presynaptic membrane.

Question 18

Neuropeptides are typically released gradually

Answer 18

in response to general increases in the level of intracellular Ca2+ ions, such as might occur during a general increase in the rate of neuron firing.

Question 19

Once released, neurotransmitter molecules

Answer 19

produce signals in postsynaptic neurons by binding to receptors in the postsynaptic membrane.

Question 20

Receptors

Answer 20

a protein that contains binding sites for only particular neurotransmitters; thus, a neurotransmitter can influence only those cells that have receptors for it.

Question 21

Ligand

Answer 21

any molecule that binds to another is referred to as its ligand. Ex. A neurotransmitter is thus said to be a ligand of its receptor.

Question 22

Receptor subtypes

Answer 22

the different types of receptors to which a particular neurotransmitter can bind. The various receptor subtypes for a neurotransmitter are typically located in different brain areas, and they typically respond to the neurotransmitter in different ways.

Question 23

Ionotropic receptors

Answer 23

associated with ligand-activated ion channels. When the neurotransmitter molecule binds to this receptor, the associated ion channel usually opens or closes immediately, thereby inducing an immediate postsynaptic potential.

Question 24

Metabotropic receptors

Answer 24

associated with signal proteins and G proteins (guanosine-triphosphate-sensitive proteins). More prevalent than ionotropic receptors, and their effects are slower to develop, longer-lasting, more diffuse, and more varied. Many different kinds, but each is attached to a serpentine signal protein that winds its way back and forth through the cell membrane seven times. Attached to a protein of the signal poteen outside the neuron; G protein is attached to a portion of the signal protein inside the neuron.

Question 25

EPSPs (depolarizations)

Answer 25

usually occur because the neurotransmitter opens sodium channels, thereby increasing the flow of Na+ ions into the neuron.

Question 26

IPSPs (hyperpolarizations)

Answer 26

usually occur because the neurotransmitter opens potassium channels or chloride channels, thereby increasing the flow of K+ ions out of the neuron or the flow of CI- ions into it.

Question 27

When a neurotransmitter binds to a metabotropic receptor

Answer 27

a subunit of the associated G protein breaks away. Then, depending on the particular G protein, the subunit may move along the inside surface of the membrane and bind to a nearby ion channel, thereby inducing an EPSP or IPSP or it may trigger the synthesis of a chemical called a second messenger. Once created, a second messenger diffuses through the cytoplasm and may influence the activities of the neuron in a variety of ways.

Question 28

first messenger

Answer 28

neurotransmitters

Question 29

autoreceptors

Answer 29

one type of metabotropic receptor; have two unconventional characteristics: they bind to their neuron’s own neurotransmitter molecules; and they are located on the presynaptic, rather than the postsynaptic, membrane. Usual function is to monitor the number of neurotransmitter molecules in the synapse, to reduce subsequent release when the levels are high, and to increase subsequent release when they are low.

Question 30

Small-molecule neurotransmitters tend to be released

Answer 30

into directed synapses and to activate either ionotropic receptors or metabotropic receptors that act directly on the ion channels.

Question 31

Neuropeptides tend to be released

Answer 31

diffusely, and virtually all bind to metabotropic receptors that act through second messengers.

Question 32

The function of small-molecule neurotransmitters appears to be

Answer 32

the transmission of rapid, brief excitatory or inhibitory signals to adjacent cells.

Question 33

The function of neuropeptides appears to be

Answer 33

the transmission of slow, diffuse, and long-lasting signals.

Question 34

Two message-terminating mechanisms

Answer 34

reuptake by transporters and enzymatic degradation.

Question 35

reuptake

Answer 35

more common; the majority of neurotransmitters, once released, are almost immediately drawn back into the presynaptic buttons by transporter molecules.

Question 36

Enzymatic degradation

Answer 36

neurotransmitters are degraded in the synapse by the action of enzymes, which are proteins that stimulate or inhibit biochemical reactions without being affected by them.

Question 37

Acetylcholine

Answer 37

one of the few neurotransmitters for which enzymatic degradation is the main mechanism of synaptic deactivation and is broken down by the enzyme acetylcholinesterase.

Question 38

Once released, neurotransmitter molecules or their breakdown products are

Answer 38

drawn back into the button and recycled, regardless of the mechanism of their deactivation.

Question 39

The vesicles, once they have done their job, are drawn

Answer 39

back into the neuron from the presynaptic membrane and are used to create new vesicles.

Question 40

Astrocytes have been shown

Answer 40

to release chemical transmitters, to contain receptors for neurotransmitters, to conduct signals, and to influence synaptic transmission between neurons.

Question 41

Gap junctions

Answer 41

are narrow spaces between adjacent cells that are bridged by fine, tubular, cytoplasm-filled protein channels, called connexins. Connect the cytoplasm of two adjacent cells, allowing electrical signals and small molecules (ex. second messengers) to pass from one cell to the next. Sometimes called electrical synapses. Transmit signals more rapidly than chemical synapses.

Question 42

Recent research has established that glial cells (particularly astrocytes) and gap junctions

Answer 42

play major roles in brain function.

Question 43

Cerebral gap junctions

Answer 43

occur between all classes of cerebral cells; however, the majority of them seem to occur between cells of like kind.

Question 44

One function of gap junctions

Answer 44

appears to be to synchronize the activities of like cells in a particular area.

Question 45

Astrocytic organization

Answer 45

suggests that they too play a role of synchronizing activities of like cells in a particular area.

Question 46

Unlike neurons, astrocytes are

Answer 46

distributed evenly throughout a particular area, with only one astrocyte per location and little overlap between the projections of adjacent astrocytes. Suggests that each astrocyte coordinates the activities of neurons in its domain, and with as many as 40,000 processes, each astrocyte has a great potential to coordinate activity.

Question 47

Gap junctions on astrocytes

Answer 47

tend to occur at the end of each process, where it comes in contact with processes from adjacent astrocytes.

Question 48

The fact that many astrocytic processes wrap around synapses and are connected to both presynaptic and postsynaptic neurons by gap junctions suggests

Answer 48

that each astrocyte may coordinate the activity of synapses in its domain.

Question 49

Tripartite synapse

Answer 49

the hypothesis that synaptic transmission depends on communication among three cells (presynaptic neuron, postsynaptic neuron, and astrocyte) via gap junctions.