Chapter 3 - Synaptic Transmission

Question 1

Ions

2 types

Answer 1

Ions–electrically charged molecules.
Anions are negatively charged.
Cations are positively charged.

Question 2

Ions are dissolved in _________ fluid, separated from the ___________ fluid by the ______________

Answer 2

Ions are dissolved in intracellular fluid, separated from the extracellular fluid by the cell membrane.

Question 3

Movement down the Axon

Answer 3

Orthodromic

Antidromic – backwards

Question 4

Electrical Synapses Work with No Time Delay
Diagram
Synaptic Transmission:
Sequence of Events:

Answer 4

1. Action potential travels down the axon to the axon terminal.
2. Voltage-gated calcium channels open and calcium ions (Ca2+) enter.
3. Synaptic vesicles fuse with membrane and release transmitter into the cleft.
4. Transmitters bind to postsynaptic receptors and cause an EPSP or IPSP.
5. EPSPs or IPSPs spread toward the postsynaptic axon hillock.
6. Transmitter is inactivated or removed–action is brief.
7. Transmitter may activate presynaptic autoreceptors, decreasing release

Question 5

Ligands

2 types

Answer 5

Ligands fit receptors exactly and activate or block them:

Endogenous ligands–neurotransmitters and hormones

Exogenous ligands–drugs and toxins from outside the body

Question 6

A Nicotinic Acetylcholine Receptor

Answer 6

Some chemicals can fit on cholinergic receptors and block the action of ACh:

Curare and bungarotoxin block ACh receptors–are antagonists

However, muscarine and nicotine mimic ACh and are agonists of the receptor.

Question 7

Receptors control ion channels in two ways:

Answer 7

Ionotropic receptors open when bound by a transmitter (also called a ligand-gated ion channel).

Metabotropic receptors recognize the transmitter but instead activate G-proteins and change intracellular activity.

Question 8

Pinocytosis

Answer 8

is the process of repackaging transmitter into vesicles.

Question 9

Types of synapses:

Answer 9

Axo-dendritic–axon terminal synapses on a dendrite

Axo-axonic–between two axons

Dendro-dendritic–between two dendrites

Retrograde–uses gas to signal presynaptic cell to release transmitter

Question 10

neurophysiology

Answer 10

The study of the life processes within neurons that use electrical and chemical signals.

A neuron at rest is a balance of electrochemical forces.

Question 11

We’ll learn that information flows within a neuron via ________ signals, while information passes between neurons through
_____________ _____________

Answer 11

Electrical

Chemical signals

Question 12

ion

Answer 12

An atom or molecule that has acquired an electrical charge by gaining or losing one or more electrons.

Question 13

anion

Answer 13

A negatively charged ion, such as a protein or chloride ion.

Question 14

intracellular fluid

Answer 14

Also called cytoplasm. The watery solution found within cells.

Question 15

extracellular fluid

Answer 15

The fluid in the spaces between cells (interstitial fluid) and in the vascular system.

Question 16

cell membrane

Answer 16

The lipid bilayer that ensheathes a cell.

Question 17

microelectrode

Answer 17

An especially small electrode used to record electrical potentials from living cells.

Question 18

resting membrane potential

Answer 18

A difference in electrical potential across the membrane of a nerve cell during an inactive period.

Question 19

millivolt (mV)

Answer 19

A thousandth of a volt.

Question 20

Living cells are more negative on the _____ than on the ________

Answer 20

Inside

Outside

Question 21

Of the many ions that a neuron contains, a majority are?

Answer 21

anions

Question 22

Can large protein anions that exit the cell?

Answer 22

no

Question 23

Where are the ions dissolved?

Answer 23

Intercellular fluid

Question 24

What is the measurement of resting membrane potential?

Answer 24

–60 mV (values may range between –50 and –80 mV)

thousandths of a volt, or millivolts (mV)

Question 25

What divides the intercellular and extracellular fluid?

Answer 25

the cell membrane

Question 26

Cell membranes are made up of?

Answer 26

lipid bilayer

Question 27

sorts of specialized proteins in the lipid bilayer?

Answer 27

ion channel

Question 28

Some ion channels stay _____ all the time.

Answer 28

open

Question 29

the cell membrane of a neuron channels that selectively allow to cross the membrane.

Answer 29

potassium ions (K+)

Question 30

What is the symbol for potassium and is it a positive or negative ion.

Answer 30

K+

Question 31

If a cell membrane has selective permeability to potassium it ….

Answer 31

that is, K+ ions can enter or exit the cell fairly freely, while other ions are impeded by the cell membrane

Question 32

The resting potential of the neuron reflects a balancing act between WHAT two opposing forces that drive K+ ions in and out of the neuron?

Answer 32

Diffusion

electrostatic pressure

Question 33

Diffusion

Answer 33

The spontaneous spread of molecules of one substance among molecules of another substance until a uniform concentration is achieved.
the force that causes molecules of a substance to diffuse from regions of high concentration to regions of low concentration.

Question 34

negative polarity

Answer 34

A negative electrical-potential difference relative to a reference electrode.

Question 35

lipid bilayer

Answer 35

The structure of the neuronal cell membrane, which consists of two layers of lipid molecules, within which float various specialized proteins, such as receptors.

Question 36

ion channel

Answer 36

A pore in the cell membrane that permits the passage of certain ions through the membrane when the channels are open.

Question 37

selective permeability

Answer 37

The property of a membrane that allows some substances to pass through, but not others.

Question 38

concentration gradient

Answer 38

Variation of the concentration of a substance within a region.

Question 39

Molecules tend to move down their ____________ ____________until they are evenly distributed.

Answer 39

concentration gradient

Question 40

electrostatic pressure

Answer 40

The propensity of charged molecules or ions to move, via diffusion, toward areas with the opposite charge.

Question 41

Because of electrostatic pressure positively charged ______ are thus ________ to the negatively charged interior of the cell; and conversely, ________ are ________by the cell interior and so tend to exit to the extracellular fluid.

Answer 41

Cations

Attracted

Anions

repelled

Question 42

sodium-potassium pump

Answer 42

The energetically expensive mechanism that pushes sodium ions out of a cell, and potassium ions in.
It pumps three sodium ions (Na+) out of the cell for every two K+ ions pumped in.

Question 43

sodium ion (Na+)

Answer 43

A sodium atom that carries a positive charge because it has lost one electron.

Question 44

What is the symbol of a sodium ion? Is it positive or negatively charged?

Answer 44

Na+

Question 45

In fact, a large fraction of the energy consumed by the brain is used to ….

Answer 45

Maintain the ionic differences across neuronal membranes.

Question 46

How does the sodium-potassium pump causing a net buildup of negative charges inside the cell?

And how does the electrostatic pressure inside the cell compensate to create equilibrium

Answer 46

The sodium-potassium pump causes a buildup of K+ ions inside the cell, but recall that at rest the membrane is much more permeable to K+ ions than Na+ ions. That means K+ ions will tend to leave the interior, down their concentration gradient, causing a net buildup of negative charges inside the cell.
As negative charge builds up inside the cell, it begins to exert electrostatic pressure to pull positively charged K+ ions back inside.

Question 47

Equilibrium

Answer 47

Here, the point at which the movement of ions across the cell membrane is balanced, as the electrostatic pressure pulling ions in one direction is offset by the diffusion force pushing them in the opposite direction.

Question 48

Nernst equation

Answer 48

An equation predicting the voltage needed to just counterbalance the diffusion force pushing an ion across a semipermeable membrane from the side with a high concentration to the side with a low concentration.

Question 49

action potential

Answer 49

The propagated electrical message of a neuron that travels along the axon to the presynaptic axon terminals.

The very brief but large changes in neuronal polarization, which are propagated at high speed along the axon.

(sometimes referred to as a spike because of its shape)

It is a rapid reversal of the membrane potential that momentarily makes the inside of the membrane positive with respect to the outside.

Question 50

Hyperpolarization

Answer 50

An increase in membrane potential (the interior of the neuron becomes even more negative, relative to the outside)

Question 51

So if the neuron already has a resting membrane potential of, say, –60 mV, hyperpolarization does what

Answer 51

makes it even farther from zero, maybe –70 mV

Question 52

Depolarization

Answer 52

A reduction in membrane potential (the interior of the neuron becomes less negative).
In other words, depolarization of a neuron brings its membrane potential closer to zero.

Question 53

What is the reverse of depolarization?

Answer 53

hyperpolarization

Question 54

Capacitance

Answer 54

the distortions at the beginning and end of the neuron’s response are caused by the membrane’s ability to store electricity, known as capacitance

Question 55

The greater the stimulus, the greater the response; so the neuron’s change in potential is called what?

Answer 55

graded response (only with passive graded potentials and not action potentials)

Question 56

local potential

Answer 56

An electrical potential that is initiated by stimulation at a specific site, which is a graded response that spreads passively across the cell membrane, decreasing in strength with time and distance.
Local potentials are graded and diminish over time and distance, also arise at synapses in response to other neurons.

Question 57

The application of depolarizing pulses to the membrane follows the same pattern as for hyperpolarizing stimuli, producing local, graded responses until when?

Answer 57

The stimulus depolarizes the cell to –40 mV or so (the exact value varies slightly among neurons). At this point, known as the threshold, a sudden and brief (0.5–2.0 millisecond [ms]) response the action potential is provoked

Question 58

Threshold

Answer 58

The stimulus intensity that is just adequate to trigger an action potential. (–40 mV or so)
After which , a sudden and brief (0.5–2.0 millisecond [ms]) response the action potential is provoked.

Question 59

How is the action potential unlike the passive graded potentials

Answer 59

the action potential is actively propagated (or regenerated) down the axon, through ionic mechanisms

Question 60

all-or-none property

Answer 60

The fact that the amplitude of the action potential is independent of the magnitude of the stimulus.
Larger depolarizations produce more action potentials, not larger action potentials. In other words, the size (or amplitude) of the action potential is independent of stimulus magnitude.

Question 61

Afterpotential

Answer 61

The positive or negative change in membrane potential that may follow an action potential.
electrical oscillations immediately following the spike; these changes
which are also related to the movement of ions in and out of the cell

Question 62

voltage-gated Na+ channel

Answer 62

A Na+- selective channel that opens or closes in response to changes in the voltage of the local membrane potential; it mediates the action potential.

It is a tubular, membrane-spanning protein, but its central Na+-selective pore is ordinarily closed. When the cell membrane becomes depolarized to threshold levels, the channel’s shape changes, opening the pore to allow Na+ ions through.

the voltage-gated Na+ channel, is really quite a complicated machine. It monitors the axon’s polarity, and at threshold the channel changes its shape to open the pore, shutting down again just a millisecond later. The channel then “remembers” that it was recently open and refuses to open again for a short time. These properties produce and enforce the properties of the action potential.

Question 63

Ionic mechanisms underlie the action potential

Answer 63

the action potential is created by the movement of sodium ions (Na+) into the cell, through channels in the membrane.
At its peak, the action potential approaches the equilibrium potential for Na+ as predicted by the Nernst equation: about +40 mV.
At this point, the concentration gradient pushing Na+ ions into the cell is exactly balanced by the positive charge pushing them out.
The action potential thus involves a rapid shift in membrane properties, switching suddenly from the potassium-dependent resting state to a primarily sodium-dependent active state, and then swiftly returning to the resting state.
This shift is accomplished through the actions of a very special ion channel: the voltage-gated Na+ channel.

Question 64

English neurophysiologists Alan Hodgkin (1914–1998) and Andrew Huxley (1917–2012)

Answer 64

They established that the action potential is created by the move-
ment of sodium ions (Na+) into the cell, through channels in the membrane.
Studied squid axons

Question 65

Why study the giant axon of the squid

Answer 65

More than half a millimeter in diameter, the squid’s giant axon is readily apparent to the naked eye and therefore much better suited to experimentation than mammalian axons.
Microelectrodes can be inserted into a giant axon without greatly altering the properties of the axon; it is even possible to push the intracellular fluid out of the squid axon and replace it with other fluids to study various properties of the action potential.

Question 66

As long as the depolarization is below threshold, Na+ channels remain closed. But when the depolarization reaches threshold …

Answer 66

a few Na+ channels open at first, allowing ions to start entering the neuron, depolarizing the membrane even further and opening still more Na+ channels. Thus, the process accelerates until the barriers are removed and Na+ ions rush in.

Question 67

By the time the voltage-gated Na+ channels close

Answer 67

The membrane potential has approached the sodium equilibrium potential of about +40 mV. Now, positive charges inside the nerve cell push K+ ions out, and voltage-gated K+ channels open, increasing the permeability to K+ even more, so the resting potential is quickly restored.

Question 68

The upper limit to the frequency of action potentials becomes apparent at about …

Answer 68

1200 spikes per second.

Question 69

Refractory

Answer 69

Transiently inactivated or exhausted.
Beyond a certain point, only the first stimulus is able to elicit an action potential. The axonal membrane is said to be refractory (unresponsive) to the second stimulus
The overall length of the refractory phase is what determines a neuron’s maximal rate of firing.

Question 70

absolute refractory phase

Answer 70

A brief period of complete insensitivity to stimuli.

A brief period immediately following the production of an action potential, no amount of stimulation can induce another action potential, because the voltage-gated Na+ channels are either still open or unresponsive

Question 71

relative refractory phase

Answer 71

A period of reduced sensitivity during which only strong stimulation produces an action potential, because K+ ions are still flowing out, so the cell is temporarily hyperpolarized after firing an action
potential

Question 72

What are refractoriness two phases:

Answer 72

absolute refractory phase

relative refractory phase

Question 73

You might wonder if the repeated inrush of Na+ ions would allow them to build up, affecting the cell’s resting potential.

Answer 73

In fact, relatively few Na+ ions need to enter to change the membrane potential (Alle et al., 2009), and the K+ ions quickly restore the resting potential.

Question 74

The cell membrane tends to repel water because…

Answer 74

is made up of fatty molecules

Question 75

Why can’t ions in body fluids easily pass directly through neuronal membranes which are highly selective for particular types of ions?

Answer 75

Because ions in body fluids are usually surrounded by clusters of water molecules

Question 76

Why can minor even minor alteration of channel functioning can cause serious health problems

Answer 76

Given the extreme precision with which these ion channels must operate

Question 77

Channelopathy

Answer 77

A genetic abnormality of ion channels, causing a variety of symptoms.

Is a medical condition in which the form and function of ion
channels is altered as a result of mutation of the genes that encode those channels.

Sodium channelopathy—a problem with sodium channels—is associated with a variety of seizure disorders, as well as heritable muscle diseases and certain types of cardiac ailments.

Question 78

tetrodotoxin (TTX) and saxitoxin

(STX) selectively block

Answer 78

voltage-gated sodium channels,

thereby preventing the production of action potentials; paralysis and death rapidly follow.

Question 79

Tetrodotoxin

Answer 79

A toxin from puffer fish ovaries that blocks the voltage-gated sodium channel, preventing action potential conduction.

Question 80

batrachotoxin,

Answer 80

A toxin, secreted by poison arrow frogs, that selectively interferes with Na+ channels
it forces Na+ channels to stay open, with lethal results.

Question 81

How does the same specificity that makes channel toxins so deadly, makes them useful tools in the laboratory.

Answer 81

In the lab, these toxins have provided important clues about how those channels work

Question 82

Why blocking channels isn’t always a bad thing.

Answer 82

Local anesthetics like lidocaine can be injected into nerves to block voltage-gated sodium channels, stopping action potentials that would otherwise signal pain to the brain.

Question 83

In general, the transmission of action potentials is limited to _______

_______ ________ and ___________ usually have few voltage-gated Na+ channels, so they do not conduct action potentials.

Answer 83

axons

Cell bodies
Dendrites

Question 84

The cell body and dendrites have very different ion channels then the axon. They are stimulated __________ at the __________.

Answer 84

chemically at the synapses

Question 85

once an action potential starts at the ______ ________, the action potential regenerates itself down the length of the axon

Answer 85

axon hillock

Question 86

How action potentials are actively propagated along the axon

Answer 86

another function for which voltage-gated channels are crucial
It is important to understand that the action potential is regenerated along the length of the axon.
the action potential is a spike of depolarizing electrical activity (with a peak of about +40 mV), so it strongly depolarizes the next adjacent axon segment. Because this adjacent segment is similarly covered with voltage-gated Na+ channels, the depolarization immediately creates a new action potential, which in turn depolarizes the next patch of membrane, which generates yet another action potential, and so on all down the length of the axon.
An analogy is the spread of fire along a row of closely spaced match heads in a matchbook. When one match is lit, its heat is enough to ignite the next match and so on along the row.

Question 87

axon hillock

Answer 87

A cone-shaped area from which the axon originates out of the cell body. Functionally, the integration zone of the neuron.

Question 88

Why does the axon normally conducts action potentials in only
one direction—from the axon hillock toward the axon terminals

Answer 88

because as it progresses along the axon, the action potential leaves in its wake a stretch of refractory membrane

also
Propagated activity does not spread from the axon hillock back over the cell body and dendrites, because the membranes there have very few voltage-gated Na+ channels, so they cannot produce an action potential.

Question 89

the size of the action potential is _____ ________ at each point along the axon.

Answer 89

the same

Question 90

conduction velocity

Answer 90

The speed at which an action potential is propagated along the length of an axon (or section of peripheral nerve).

It varies with the diameter of the axon. Larger axons allow the depolarization to spread faster through the interior.

Question 91

nodes of Ranvier

Answer 91

A gap between successive segments of the myelin sheath where the axon membrane is exposed.

small gaps spaced about every millimeter along the axon

Question 92

saltatory conduction

Answer 92

The form of conduction that is characteristic of myelinated axons, in which the action potential jumps from one node of Ranvier to the next.

because the myelin insulation offers considerable resistance to the flow of ionic currents across the membrane, the action potential jumps from node to node.

Question 93

The evolution of rapid saltatory conduction in vertebrates has given them a major behavioral advantage over invertebrates, in which axons

Answer 93

are unmyelinated and mostly small in diameter, and thus slower in conduction.

Question 94

neurotransmitter

Answer 94

Also called synaptic transmitter, chemical transmitter, or simply transmitter. The chemical released from the presynaptic axon terminal that serves as the basis of communication between neurons,

Question 95

Synapses Cause_________, _________ changes in the Postsynaptic Membrane Potential

Answer 95

Graded

Local

Question 96

electrical synapses

Answer 96

Also called gap junction. The region between neurons where the presynaptic and postsynaptic membranes are so close that the action potential can jump to the postsynaptic membrane with-out first being translated into a chemical message.

the presynaptic membrane comes even closer to the postsynaptic membrane than it does at chemical synapses.
At electrical synapses, the facing membranes of the two cells have relatively large channels arranged to allow ions to flow from one neuron directly into the other.
As a consequence, the electrical current that is associated with neural activity in one neuron can flow directly across the gap junction to affect the other neuron.
Transmission at these synapses closely resembles action potential conduction along the axon.
Electrical synapses therefore work with practically no time delay, in contrast to chemical synapses, where the delay is on the order of a millisecond—slow in terms of neurons.
Because of the speed of their transmission, electrical synapses are frequently found in neural circuits that mediate escape behaviors in invertebrates.
They are also found where many fibers must be activated synchronously, as in the system for moving our eyes.
Clinically,
it is suspected that electrical synapses contribute to the spread of synchronized seizure discharges in epilepsy

Question 97

postsynaptic potential

Answer 97

A local potential that is initiated by stimulation at a synapse, can vary in amplitude, and spreads passively across the cell membrane, decreasing in strength with time and distance.
Neurotransmitters released into synapses briefly alter the membrane potential of the postsynaptic cell.

Question 98

excitatory postsynaptic potential (EPSP)

Answer 98

A depolarizing potential in the postsynaptic neuron that is caused by excitatory connections. EPSPs increase the probability that the postsynaptic neuron will fire an action potential.

Question 99

A given neuron, receiving synapses from hundreds of other cells, is subject to _________ or _________ of ____________ ____________. When integrated, this massive array of local potentials determines whether the neuron will ______ _________ and therefore generate an ______ ___________ of its own.

Answer 99

A given neuron, receiving synapses from hundreds of other cells, is subject to hundreds or thousands of postsynaptic potentials. When integrated, this massive array of local potentials determines whether the neuron will reach threshold and therefore generate an action potential of its own.

Question 100

It is important to remember that excitatory and inhibitory neurons get their names from their actions on ________ _______, not from their ________ ___ __________.

Answer 100

It is important to remember that excitatory and inhibitory neurons get their names from their actions on postsynaptic neurons, not from their effects on behavior.

Question 101

Stimulation of an excitatory presynaptic neuron causes it to

Answer 101

produces an all-or-none action potential that spreads to the end of the axon, releasing transmitter.

Question 102

Generally, the _________ _______of many excitatory synapses is needed to elicit an action potential in a ___________ ________.

Answer 102

combined effect

postsynaptic neuron

Question 103

synaptic delay

Answer 103

The brief delay between the arrival of an action potential at the axon terminal and the creation of a postsynaptic potential.

there is a delay: in the fastest cases, the postsynaptic depolarization begins about half a millisecond after the presynaptic action potentials arrive at the axon terminals.

This synaptic delay reflects the time needed for the neurotransmitter to be released and diffuse across the synaptic cleft,

Question 104

inhibitory postsynaptic potential

IPSP

Answer 104

A hyperpolarizing potential in the postsynaptic neuron that is caused by inhibitory connections. IPSPs decrease the probability that the postsynaptic neuron will fire an action potential.

When the inhibitory neuron is stimulated, the postsynaptic effect is an increase of the resting membrane potential. This hyperpolarization moves the cell membrane potential away from threshold—decreasing the probability that the neuron will fire an action potential

Question 105

chloride ion (Cl –)

Answer 105

A chlorine atom that carries a negative charge because it has gained one electron.

Usually IPSPs result from the opening of channels that permit chloride ions (Cl –) to enter the cell. Because Cl– ions are much more concentrated outside the cell than inside (see Figure 3.4), they rush into the cell, making it even more negative.

Question 106

The action potential of an inhibitory presynaptic neuron looks _______ _____ _____ __ ___ excitatory presynaptic neuron; all neurons use the ____ ____ of propagated signal.

Answer 106

The action potential of an inhibitory presynaptic neuron looks exactly like that of the excitatory presynaptic neuron; all neurons use the same kind of propagated signal

Question 107

The nervous system treads a narrow path between overexcitation, which leads to ________, and underexcitation, which leads to _______ and _________.

Answer 107

Seizures

coma and death

Question 108

What determines whether a synapse excites or inhibits the postsynaptic cell?

Answer 108

One factor is the particular neurotransmitter released by the presynaptic cell.

The same neurotransmitter may be excitatory at one synapse and inhibitory at another, depending on the receptors present in the postsynaptic cell.

Transmitter chemicals that can be either depolarizing (excitatory) or hyperpolarizing (inhibitory).

The presynaptic terminals provide excitatory (depolarizing) or inhibitory (hyperpolarizing) stimulation to the postsynaptic cell membrane. If the membrane potential rises (depolarizes) above a threshold level, an action potential is fired.

Question 109

Two types of summation

Answer 109

Spatial

temporal

Question 110

spatial summation

Answer 110

The summation at the axon hillock of postsynaptic potentials from across the cell body. If this summation reaches threshold, an action potential is triggered.

Question 111

temporal summation

Answer 111

The summation of postsynaptic potentials that reach the axon hillock at different times. The closer in time that the potentials occur, the more complete the summation.

Question 112

How do glial cells also play a role in synaptic transmission?

Answer 112

they increase the strength of the postsynaptic potential, overlying the presynaptic terminal and thereby preventing neurotransmitter from leaking out of the synaptic cleft.

Question 113

8 step in Synaptic Transmission

Answer 113

1. The action potential traveling down the axon arrives at the axon terminal.
2. This depolarization opens voltage-gated calcium channels in the membrane of the axon terminal, allowing calcium ions (Ca2+ ) to enter the terminal.
3. The Ca2+ causes synaptic vesicles filled with neurotransmitter to fuse with the presynaptic membrane and rupture, releasing the transmitter molecules into the synaptic cleft.
4. Transmitter molecules cross the cleft to bind to special receptor molecules in the postsynaptic membrane, leading to the opening of ion channels in the postsynaptic membrane.
5. This ion flow creates a local EPSP or IPSP in the postsynaptic neuron.
6. Synaptic transmitter is either (a) inactivated (degraded) by enzymes or (b) removed from the synaptic cleft by transporters, so the transmission is brief and accurately reflects the activity of the presynaptic cell.
7. Synaptic transmitter may also activate presynaptic autoreceptors, regulating future transmitter release.
8. The IPSPs and EPSPs in the postsynaptic cell spread throughout its interior. If the integration of all the EPSPs and IPSPs depolarizes the axon hillock enough, the postsynaptic neuron will fire an action potential of its own. Now let’s examine these steps in detail.

Question 114

calcium ion (Ca 2+)

Answer 114

A calcium atom that carries a double positive charge because it has lost two electrons.

Question 115

exocytosis

Answer 115

The process by which a synaptic vesicle fuses with the presynaptic terminal membrane to release neurotransmitter into the synaptic cleft.
Synaptic vesicles are about 50 nanometers (nm) in diameter and are quite complex structures, with protein machinery to move the vesicles toward the synapse, where they fuse with the presynaptic membrane.

Question 116

ligand

Answer 116

A substance that binds to receptor molecules, such as those at the surface of the cell.

Just as a particular key can open a door, a molecule of the correct shape, called a ligand, can fit into a receptor protein and activate or block it.

Question 117

acetylcholine (ACh)

Answer 117

A neurotransmitter produced and released by parasympathetic postganglionic neurons, by motoneurons, and by neurons throughout the brain.

ACh acts on at least four kinds of cholinergic receptors; nicotinic and muscarinic are the two main kinds.

Question 118

receptor molecule

Answer 118

Also called receptor. A protein that binds and reacts to molecules of a neurotransmitter or hormone.

Question 119

Steps 2 and 3

in Synaptic Transmission

Answer 119

2. This depolarization opens voltage-gated calcium channels in the membrane of the axon terminal, allowing calcium ions (Ca2+ ) to enter the terminal.
3. The Ca2+ causes synaptic vesicles filled with neurotransmitter to fuse with the presynaptic membrane and rupture, releasing the transmitter molecules into the synaptic cleft.

When an action potential reaches a presynaptic terminal, it opens voltage-gated calcium channels that allow an influx of calcium ions (Ca 2+), rather than K+ or Na+, into the axon terminal. These Ca2+ ions activate enzymes that cause vesicles near the presynaptic membrane to fuse with the membrane and discharge their contents into the synaptic cleft. The higher the frequency of action potentials arriving at the terminal, the greater the influx of Ca2+, and the more vesicles that dump transmitter into the synapse. Most synaptic delay is caused by the time needed for Ca2+ to enter the terminal. Both the diffusion of the transmitter across the cleft and the interaction of transmitter molecules with their receptors also take some time.

Question 120

The rate of production of transmitter is governed by …

Answer 120

The rate of production of transmitter is governed by enzymes that are manufactured in the neuronal cell body and transported down the axons to the terminals. Intense activity of the neuron reduces the number of available vesicles, but soon more vesicles are produced to replace those that were discharged.

Question 121

Explain how ACh can function as either an inhibitory or an excitatory neurotransmitter, at different synapses.

Answer 121

Each nicotinic ACh receptor consists of five subunits. The two ligand-binding sites normally bind ACh molecules, but they also bind exogenous ligands like nicotine and other nicotinic drugs.

Question 122

endogenous ligand

Answer 122

Any substance, produced within the body, that selectively binds to the type of receptor that is under study.

Question 123

exogenous ligand

Answer 123

Any substance, originating from outside the body, that selectively binds to the type of receptor that is under study.

Question 124

curare

Answer 124

An alkaloid neurotoxin that causes paralysis by blocking acetylcholine receptors in muscle.
(exogenous ligand)

Question 125

bungarotoxin

Answer 125

A neurotoxin, isolated from the venom of the banded krait, that selectively blocks acetylcholine receptors.
(exogenous ligand)

Question 126

agonist

Answer 126

A molecule, usually a drug, that binds a receptor molecule and initiates a response like that of another molecule, usually a neurotransmitter.

Examples
carine and nicotine

Question 127

antagonist

Answer 127

A molecule, usually a drug, that interferes with or prevents the action of a transmitter.

Question 128

cholinergic

Answer 128

Referring to cells that use acetylcholine as their synaptic transmitter.

Question 129

Various chemicals can fit onto receptor proteins and ______ the entrance of the key.

Answer 129

Block

Question 130

Neurotransmitters and hormones made inside the body are examples of __________ ligands; drugs and toxins from outside the body are _________ ligands.

Answer 130

Endogenous

exogenous

Question 131

Some exogenous ligands

Answer 131

curare

bungarotoxin

Question 132

Lock and Key analogy

Answer 132

The action of a key in a lock is a good analogy for the action of a transmitter on a receptor protein. Just as a particular key can open a door, a molecule of the correct shape, called a ligand, can fit into a receptor protein and activate or block it

The lock-and-key analogy is strengthened by the observation that various chemicals can fit onto receptor proteins and block the entrance of the key. (endogenous ligands and exogenous ligands)

Just as there are master keys that fit many different locks, there are submaster keys that fit a certain group of locks, and keys that fit only a single lock. Similarly, each chemical transmitter binds to several different receptor molecules.

In the case of receptor-selective drugs, though, we have to think of keys (drug molecules) trying to insert themselves into all the locks (receptor molecules) in the neighborhood; each such key fits into only a particular subset of the locks. Once the drug (the key) binds to the receptor (the lock), it alters the activity of the receptor, activating it or blocking it. But the binding is usually temporary, and when the drug or transmitter breaks away from the receptor, the receptor returns to its unbound shape and functioning.

Question 133

Nicotinic cholinergic receptors

Answer 133

found at synapses on muscles and in autonomic ganglia; it is the blockade of these receptors that is responsible for the paralysis caused by curare and bungarotoxin (see Box 3.1). Most nicotinic cholinergic synapses are excitatory, but there are also inhibitory nicotinic synapses
acetylcholine receptor

Question 134

up-regulation

Answer 134

A compensatory increase in receptor availability at the synapses of a neuron.

Question 135

down-regulation

Answer 135

A compensatory reduction in receptor availability at the synapses of a neuron.

Question 136

ionotropic receptor

Answer 136

A receptor protein that includes an ion channel that is opened when the receptor is bound by an agonist.

A directly control an ion channel. When bound by the transmitter, the ion channel opens and ions flow across the membrane. (Ionotropic receptors are also known as chemically gated ion channels, or ligand-gated ion channels.)

Question 137

ligand-gated ion channel

Answer 137

Also known as chemically gated ion channel. An ion channel that opens or closes in response to the pres-ence of a particular chemical.

Question 138

metabotropic receptor

Answer 138

A receptor protein that does not contain an ion channel but may, when activated, use a G protein system to alter the functioning of the postsynaptic cell.

Recognize the synaptic transmitter, but they do not directly control ion channels. Instead, they activate molecules known as G proteins.

Question 139

G proteins

Answer 139

A class of proteins that reside next to the intracellular portion of a receptor and that are activated when the receptor binds an appropriate ligand on the extracellular surface.

Are proteins that bind the compounds guanosine diphosphate (GDP), guanosine triphosphate (GTP), and other guanine nucleotides. Sometimes the G protein itself acts to open ion channels, but in other cases the G protein activates another, internal chemical signal to affect ion channels.

About 80% of the known neurotransmitters and hormones activate cellular signal mechanisms through receptors coupled to G proteins, so this coupling device is very important (Birnbaumer et al., 1990). The G protein is located on the inner side of the neuronal membrane. When a transmitter molecule binds to a receptor that is coupled to a G protein, parts of the G protein complex separate from each other.

Question 140

second messenger

Answer 140

A slow-acting substance in the postsynaptic cell that amplifies the effects of synaptic activity and signals synaptic activity within the postsynaptic cell.

If we think of the neurotransmitter as the first, external messenger arriving at the receptor on the cell’s surface, then the next chemical signal, activated inside the cell, is a second messenger. Several different second messengers—such as cyclic adenosine monophosphate (cyclic AMP), diacylglycerol, or arachidonic acid—amplify the effect of the first messenger and can initiate processes that lead to changes in electrical potential at the membrane. An important feature of second-messenger systems is their ability to amplify and prolong the synaptic signals that a neuron receives.

Question 141

In general, an increase in receptor numbers is referred to as ___________, and a process that decreases receptor density is called
_____________of that receptor type.

Answer 141

up-regulation

down-regulation

Question 142

The recognition of transmitter molecules by receptor molecules controls the opening of ion channels in two different ways.

Answer 142

Ionotropic receptors

Metabotropic receptors

Question 143

When a chemical transmitter such as ACh is released into the synaptic cleft, its post-synaptic action is not only prompt but usually very brief as well. This brevity ensures that the message is repeated faithfully. Accurate timing of synaptic transmission is necessary in …

Answer 143

many neural systems—for example, to drive the rapid cycles of muscle contraction and relaxation essential to many coordinated behaviors.

Question 144

The prompt cessation of transmitter effects is achieved in one of two ways

Answer 144

1. Degradation. Transmitter can be rapidly broken down and thus inactivated by a special enzyme—a process known as degradation
2. Reuptake. Alternatively, transmitter molecules may be rapidly cleared from the synaptic cleft by being taken up into the presynaptic terminal—a process known as reuptake.

Question 145

degradation

Answer 145

The chemical breakdown of a neurotransmitter into inactive metabolites.

Degradation. Transmitter can be rapidly broken down and thus inactivated by a special enzyme—a process known as degradation For example, the enzyme that inactivates ACh is acetylcholinesterase (AChE). AChE breaks down ACh very rapidly into choline and acetic acid, and these products are recycled (at least in part) to make more ACh in the axon terminal. AChE is found especially at synapses, but also elsewhere in the nervous system. Thus, if any ACh escapes from a synapse where it is released, it is unlikely to reach other synapses intact, where it could start false messages.

Question 146

reuptake

Answer 146

The process by which released synaptic transmitter molecules are taken up and reused by the presynaptic neuron, thus stopping synaptic activity.

Reuptake. Alternatively, transmitter molecules may be rapidly cleared from the synaptic cleft by being taken up into the presynaptic terminal—a process known as reuptake. Norepinephrine, dopamine, and serotonin are examples of transmitters whose activity is terminated mainly by reuptake. In these cases, special receptors for the transmitter, called transporters, are located on the presynaptic axon terminal and bring the transmitter back inside. Once taken up into the presynaptic terminal, transmitter molecules may be repackaged into newly formed synaptic vesicles. Malfunction of reuptake mechanisms has been suspected to cause some kinds of mental illness, such as depression.

Question 147

transporters

Answer 147

Specialized receptors in the presynaptic membrane that recognize transmitter molecules and return them to the presynaptic neuron for reuse.

Question 148

autoreceptor

Answer 148

A receptor for a synaptic transmitter that is located in the presynaptic membrane, telling the axon terminal how much transmitter has been released.

Some neurotransmitter molecules never make it to the postsynaptic membrane. They may bind to receptors on the presynaptic membrane, a so-called autoreceptor. Through autoreceptors, the presynaptic cell is informed about the net concentration of transmitter in the synaptic cleft and may regulate future neurotransmitter release to adjust that concentration.

Question 149

axo-dendritic

Answer 149

Referring to a synapse in which a presynaptic axon terminal synapses onto a dendrite of the postsynaptic neuron, either via a dendritic spine or directly onto the dendrite itself.

Question 150

axo-somatic

Answer 150

Referring to a synapse in which a presynaptic axon terminal synapses onto the cell body (soma) of the postsynaptic neuron.

Question 151

4 Different Types of Synaptic Connections

Answer 151

1. Most synapses are formed by an axon stimulating a dendrite (Axo-dendritic),
2. but axons also sometimes synapse upon cell bodies (Axo-somatic)
3. or even other axons (Axo-axonic)
4. And in some instances, specialized dendrites synapse upon other dendrites (Dendro-dendritic).

Question 152

axo-axonic

Answer 152

Referring to a synapse in which a presynaptic axon terminal synapses onto another axon’s terminal.

Question 153

retrograde synapse

Answer 153

A synapse in which a signal (usually a gas neurotransmitter) flows
from the postsynaptic neuron to the presynaptic neuron, thus counter to the usual direction of synaptic communication.

Transmission starts with classic axo-dendritic synaptic activity, but the postsynaptic cell subsequently releases a gas neurotransmitter, which signals the presynaptic cell to release more transmitter.

Question 154

dendro-dendritic

Answer 154

Referring to a type of synapse in which a synaptic connection forms between the dendrites of two neurons.

allowing coordination of their activities

Question 155

ectopic transmission

Answer 155

Cell-cell communication based on release of neurotransmitter in
regions outside traditional synapses.

Occurs between many neurons; in this mode of transmission, the location of transmitter release and the sites at which the transmitter acts are both well outside the conventional boundaries of nearby synapses

Question 156

varicosity

Answer 156

The axonal swelling from which neurotransmitter diffuses in a nondirected synapse.

And throughout the brain are found axons with regular swellings, called varicosities, along their length; like a drip-irrigation system, these nondirected synapses steadily release neurotransmitter to broadly affect surrounding areas.

Question 157

nondirected synapse

Answer 157

A type of synapse in which the presynaptic and postsynaptic cells
are not in close apposition; instead, neurotransmitter is released by axonal varicosities and diffuses away to affect wide regions of tissue.

Question 158

neural chain

Answer 158

A simple kind of neural circuit in which neurons are attached linearly, end-to-end.

Question 159

Circuits of Neurons: two kinds of processes

Answer 159

1. analog-like signals that vary in strength (such as graded potentials at synapses)
2. and digital-like, all-or-none signals (such as action potentials) that vary in frequency.

Question 160

Neurons and Synapses Make Circuits

Answer 160

The nervous system comprises many different types of neural circuits to accomplish basic functions in cognition, emotion, and action—all the categories of behavior and experience.

Question 161

knee jerk reflex

Answer 161

A variant of the stretch reflex in which stretching of the tendon beneath the knee leads to an upward kick of the leg.

For example, the basic circuit for the stretch reflex, such as the knee jerk reflex, consists of a sensory neuron, a motor neuron, and a single synapse where the sensory neuron communicates with the motor neuron. Note that this reflex is extremely rapid: only about 40 ms elapse between the stimulus and the initiation of the response.

Question 162

Three factors account for this rapidity of the knee jerk reflex:

Answer 162

Electrical circuits can represent signals in either analog or digital ways— that is, in terms of continuously varying values or in terms of integers.
Neurons similarly feature two kinds of processes: analog-like signals that vary in strength (such as graded potentials at synapses)
digital-like, all-or-none signals (such as action potentials) that vary in frequency.

Question 163

Two other features that are common to many kinds of neural circuits:

Answer 163

convergence and divergence

Question 164

divergence

Answer 164

The phenomenon of neural connections in which one cell sends signals to many other cells.

Question 165

convergence

Answer 165

The phenomenon of neural connections in which many cells send signals to a single cell.

Question 166

electroencephalogram (EEG)

Answer 166

A recording of gross electrical activity of the brain recorded from large electrodes placed on the scalp.

Question 167

epilepsy

Answer 167

A brain disorder marked by major sudden changes in the electrophysiological state of the brain that are referred to as seizures.

Question 168

seizure

Answer 168

An epileptic episode.

Generalized seizures are characterized by loss of consciousness and symmetrical involvement of body musculature.

Seizures are an unfortunate manifestation of the electrical character of the nervous system. Because of the extensive connections among its nerve cells, the brain can generate massive waves of intense nerve cell activity that seem to involve almost the entire brain. In the normal, active brain, electrical activity tends to be desynchronized; that is, different brain regions carry on their functions more or less independently. In contrast, a seizure features widespread synchronization of electrical activity: broad swaths of the brain start firing in simultaneous waves of excitation,

Many abnormalities of the brain, such as trauma, injury, or metabolic problems, can predispose brain tissue to produce synchronized epileptiform activity, which can easily spread.

Question 169

grand mal seizure

Answer 169

A type of generalized epileptic seizure in which nerve cells fire in high-frequency bursts.

Abnormal EEG activity is evident all over the brain. The person loses consciousness and makes characteristic movements: an enduring toniccontraction of the muscles for 1 or 2 minutes, followed by jerky, rhythmic clonic contractions and relaxations. Minutes or hours of confusion and sleep follow the seizure.

Question 170

petit mal seizure

Answer 170

Also called an absence attack. A seizure that is characterized by a spike-and-wave EEG and often involves a loss of awareness and inability to recall events surrounding the seizure.

are a more subtle variant of generalized seizures, in which the
characteristic spike-and-wave EEG activity is evident for 5–15 seconds at a time, sometimes occurring many times per day. The person is unaware of the environment during these periods, and later cannot recall events that occurred during the petit mal episode. Behaviorally, the person does not show unusual muscle activity, except for a cessation of ongoing activity and sustained staring.

Question 171

“spike-and-wave” pattern

of brain activity

Answer 171

a seizure features widespread synchronization of electrical activity: broad swaths of the brain start firing in simultaneous waves of excitation, which are evident in the EEGs as an abnormal “spike-and-wave” pattern of brain activity.

Question 172

complex partial seizure

Answer 172

In epilepsy, a type of seizure that doesn’t involve the entire brain and therefore can cause a wide variety of symptoms.

Do not involve the entire brain and thus can produce a wide variety of symptoms, often preceded by an unusual sensation, or aura.

Question 173

Three types of seizures

Answer 173

grand mal seizure
petit mal seizure
complex partial seizure

Question 174

aura

Answer 174

In epilepsy, the unusual sensations or premonition that may precede the beginning of a seizure.

Question 175

kindling

Answer 175

A method of experimentally inducing an epileptic seizure by repeatedly stimulating a brain region.

In other words, the kindling stimulations somehow change the tissue and make it more epilepsy-prone. Interestingly, after years of epilepsy some human patients develop multiple foci for the initiation of seizures, perhaps because of a kindling process

Question 176

event-related potential (ERP)

Answer 176

Also called evoked potential. Averaged EEG recordings measuring brain responses to repeated presentations of a stimulus. Components of the ERP tend to be reliable because the background noise of the cortex has been averaged out.

Gross potential changes evoked by discrete sensory stimuli, such as light flashes or clicks.

Question 177

Controlling seizures

Answer 177

Many seizure disorders can be effectively controlled with the aid of antiepileptic drugs. Although these drugs have a wide variety of different targets, they have in common a tendency to selectively modulate the excitability of neurons, either by counteracting problems with ionic balance or by promoting inhibitory processes

Question 178

Optogenetics

Answer 178

The use of genetic tools to induce neurons to become sensitive to light, such that experimenters can excite or inhibit the cell by stimulating it with light.

uses genetic tools to insert light-sensitive ion channels into neurons
so that stimulating the brain with light, delivered by fiber-optic cables, can excite or inhibit those targeted neurons

Question 179

channelrhodopsin

Answer 179

A protein that, in response to light of the proper wavelength, opens a channel to admit sodium ions, which results in excitation of neurons.

The first opsin that was studied, channelrhodopsin, responds to blue light by allowing Na+ ions to enter the cell, depolarizing it

Question 180

Halorhodopsin

Answer 180

A protein that, in response to light of the proper wavelength, opens a channel to admit chloride ions, which results in inhibition of neurons.

which when stimulated by yellow light, pumps Cl – ions
into the cell, hyperpolarizing it

Question 181

calcium ion (Ca 2+

Answer 181

A calcium atom that carries a double positive charge because it has lost two electrons.

Question 182

exocytosis

Answer 182

The process by which a synaptic vesicle fuses with the presynaptic terminal membrane to release neurotransmitter into the synaptic cleft.

Question 183

ligand

Answer 183

A substance that binds to receptor molecules, such as those at the surface of the cell.

Question 184

acetylcholine (ACh)

Answer 184

A neurotransmitter produced and released by parasympathetic postganglionic neurons, by motoneurons, and by neurons throughout the brain.

Question 185

receptor molecule

Answer 185

Also called receptor. A protein that binds and reacts to molecules of a neurotransmitter or hormone.

Question 186

the sequence and timing of events in the knee jerk reflex

Answer 186

Tap on patellar tendon stimulates stretch receptor in quadriceps muscle and starts chain of events.
Action potentials are triggered when threshold receptor potential reaches initial segment of sensory neuron.
Action potentials speed along large sensory neuron at about 100 m/s.
Action potentials in axon terminal cause release of synaptic transmitter glutamate. About 0.5 ms later, excitatory postsynaptic potential (EPSP) appears in motor neuron.
EPSP spreads passively to axon hillock, where it triggers action potentials.
Action potentials speed down large motor axon at about 100 m/s.
Action potentials reach neuromuscular junctions. ACh is released as the neurotransmitter.
Neuromuscular junction potential starts about 0.5 ms after arrival of presynaptic action potential. Action potentials are generated in the muscle fibers, which contract and cause leg to kick, about 40 ms after the hammer tap.