Chapter 12 Part 4

Question 1

what are action potentials?

Answer 1

propagated changes in the transmembrane potential that affect an entire excitable membrane

Question 2

voltage gated sodium channels are most abundant where?

Answer 2

on the axon, its branches, and synaptic terminals

Question 3

what is the FIRST STEP in generating an action potential

Answer 3

the opening of voltage-gated sodium channels at one site (usually the initial segment of the axon)

Question 4

the action potential is propagated (spread) along the length of the axon, eventually reaching….

Answer 4

the synaptic terminals

Question 5

what does threshold mean? what is the typical threshold for an axon?

Answer 5

threshold is the transmembrane potential at which an action potential begins

average threshold for an axon is typically -60mV to -55mV

Question 6

is the threshold for an axon is -60mV, what would be the depolarization?

Answer 6

10mV

Question 7

will a stimulus that shifts the resting membrane potential from -70mV to -62mV produce an action potential?

Answer 7

no – only a graded depolarization

Question 8

what causes the depolarization of the initial segment of the axon?

Answer 8

local currents resulting from the graded potential of the axon hillock

Question 9

explain the “all or nothing” principle

Answer 9

as long as the depolarizing stimulus exceeds threshold, the properties of the action potential remain the same.
the force of depolarization doesnt matter. action potential remains the same regardless as long as threshold is reached

Question 10

list the 4 steps of the generation of an action potential

Answer 10

1. depolarization to threshold – this opens voltage-gated sodium channels
2. activation of sodium channels and rapid depolarization – sodium ions rush into the cell and the transmembrane potential is now positive
3. inactivation of sodium channels and activation of potassium channels – as the transmembrane potential approaches +30mV, sodium channels are inactivated and voltage gated potassium channels open, causing potassium ions to leave the cell. repolarization now begins
4. closing of potassium channels – they begin closing as the membrane reaches the normal resting potential of about -70mV. Until ALL of these potassium channels have closed, potassium continues to leave the cell, producing a brief HYPERPOLARIZATION (-90mV)

Question 11

as the voltage gated potassium channels close, what is happening to the transmembrane potential?

Answer 11

the transmembrane potential is returning back to normal resting levels. the action potential is now over

Question 12

the resting potential depends on ___ channels
the graded potential depends on ___ channels
the action potential depends on ____ channels

Answer 12

resting – leak channels
graded – chemically gated
action – voltage gated

Question 13

WHAT brings an area of excitable membrane to threshold?

Answer 13

graded depolarization of axon hillock that is large enough to open voltage-gated sodium channels

Question 14

what is the refractory period

Answer 14

the time in which the membrane cannot respond to further stimulation from the moment the voltage-gated sodium channels open at threshold until sodium channel inactivation ends, because all the voltage gated sodium channels are either already open or inactivated

Question 15

what are the 2 parts of the refractory period and how long do they last

Answer 15

part 1 = absolute refractory period. last 0.1msec-1msec (the smaller the axon diameter, the longer the duration)

part 2 = relative refractory period. begins when sodium channels regain their normal resting conditions and continues until the transmembrane potential stabilizes at resting levels

Question 16

can another action potential occur during the relative refractory period? explain

Answer 16

yes
the depolarization, however, requires a larger than normal stimulus because…

-the local current must deliver enough Na+ to counteract the exit of K+ ions (through the voltage gated potassium channels)

-the membrane is hyperpolarized to some degree through most of the relative refractory period (due to the exit of K+ ions until every channel is closed)

Question 17

in an action potential, depolarization results from the influx of Na+ ions and repolarization involves the loss of K+.
What returns these ion concentrations to prestimulation levels? Is it essential? explain

Answer 17

the sodium potassium exchange pump

it is NOT essential. The number of ions involved in a single action potential is insignificant compared to the total number of ions inside and outside of the cell

Question 18

explain when the sodium-potassium exchange pump is ESSENTIAL

Answer 18

a maximally stimulated neuron can generate action potentials 1000/second

this is when the exchange pump is needed

Question 19

explain how the sodium-potassium exchange pump works

Answer 19

each time the pump brings 2 potassium ions in and 3 sodium ions out, one molecule of ATP is broken down into ADP.

the transmembrane protein of the pump is sodium-potassium ATPase which gets the energy to pump ions by splitting a phosphate group from a molecule of ATP, forming ADP

Question 20

action potentials may travel along an axon by ____ or ____

Answer 20

continuous propagation or saltatory propagatio

Question 21

explain what continuous propagation is

Answer 21

continuous propagation occurs in UNMYELINATED axons.

an action potential moves across the membrane in a series of tiny steps.
the local current depolarizes adjacent portions of the membrane and continues in a chain reaction

Question 22

explain how continuous propagation does not move backwards

Answer 22

because the previous segment of the axon is still in the absolute refractory period and cannot respond again to a stimulus

Question 23

continuous propagation along an unmyelinated axon occurs at a speed of….

Answer 23

1 meter/second (2 miles an hour)

Question 24

explain saltatory propagation

Answer 24

occurs in myelinated axons of the PNS and CNS. much faster than continuous propagation

when an action potential appears at the initial segment of a myelinated axon, the local current SKIPS the myelinated internodes and depolarizes the closest node.
“jumps” from one node to another instead of moving in a series of tiny steps (continuous)

Question 25

which uses less energy --- continuous or saltatory propagation?

Answer 25

saltatory. because less surface area is involved and fewer sodium ions must be pumped out of the cytoplasm (and into the cell)

Question 26

the larger the diameter, the ___ the speed of the action potential

Answer 26

greater (faster)

Question 27

classify axons into 3 groups according to the relationships between diameter, myelination, and propagation speed

Answer 27

Type A fibers = largest myelinated axons. (4-20micro meters) carry action potentials 120m/sec or 268mph Type B fibers = smaller myelinated axons. diameter (2-4 micrometers) 18m/sec or 40mph Type C fibers UNMYELINATED less than 2 micrometers in diameter. 1m/sec or 2mph

Question 28

what is the FUNCTION of type A fibers

Answer 28

type A fibers carry sensory information about position, balance, and delicate touch and pressure senses FROM THE SKIN SURFACE TO THE CNS

Question 29

what is the FUNCTION of types B and C fibers

Answer 29

carry information to the CNS and carry instructions to smooth muscle, cardiac muscle, glands, and other peripheral effectors

Question 30

why isnt every axon in the nervous system large and myelinated?

Answer 30

physically impossible -- too large

Question 31

action potentials can also be called

Answer 31

nerve impulses

Question 32

to be effective, an action potential must not only be propagated along an axon, but also.....

Answer 32

be transferred in some way to another cell (transfer takes place at synapses)

Question 33

a synapse may be _____, involving DIRECT PHYSICAL CONTACT between the cells, or it may be ______, involving a neurotransmitter

Answer 33

electrical synapse -- direct physical contact chemical ---- involving a neurotransmitter

Question 34

at electrical synapses, the presynaptic and postsynaptic membranes are locked together at.....

Answer 34

gap junction

Question 35

the lipid portions of opposing membranes at a synapse are held in position by binding between....

Answer 35

integral membrane proteins called CONNEXONS

Question 36

What do connexons do?

Answer 36

form pores that permit ions to pass between cells

Question 37

explain the advantage of the 2 cells being linked in this way (gap junctions, binding between connexons)

Answer 37

changes in the transmembrane potential of one cell produce local currents that affect the other cell as if the 2 shared a common membrane. therefore, an electrical synapse propagates action potentials quickly and efficiently from 1 cell to the next

Question 38

are electrical synapses common?

Answer 38

NO -- they are extremely rare in both the PNS and CNS. they occur in some areas of the brain, the eye, and in at least 1 pair of PNS ganglia

Question 39

what is the most abundant type of synapse

Answer 39

a chemical synapse

Question 40

neurotransmitters are either classified as ___ neurotransmitters or ____ neurotransmitters

Answer 40

excitatory or inhibitory

Question 41

explain the difference between excitatory and inhibitory neurotransmitters

Answer 41

excitatory neurotransmitters -- cause depolarization and promote the generation of action potentials inhibitory neurotransmitters -- cause hyperpolarization and suppress the generation of action potentials

Question 42

in which synapse is an action potential ALWAYS propagated to the next cell --- chemical or electrical?

Answer 42

electrical

Question 43

explain how an action potential MAY OR MAY NOT be propagated to the next cell at a chemical synapse

Answer 43

-an action potential may or may not release enough neurotransmitter to bring the postsynaptic neuron to threshold -other factors may intervene and make the postsynaptic cell more or less sensitive to arriving stimuli

Question 44

the effect of a neurotransmitter on the postsynaptic membrane depends on ____ NOT ____ give a specific example

Answer 44

effect of a neurotransmitter on a postsynaptic membrane depends on THE PROPERTIES OF THE RECEPTOR, NOT the neurotransmitter itself ACh typically causes depolarization in the postsynaptic membrane, but ACh released at neuromuscular junctions in the heart has an inhibitory effect

Question 45

Synapses that release ACh (acetylcholine) are known as......

Answer 45

cholinergic synapses

Question 46

the ___ junction is an example of a cholinergic synapse

Answer 46

neuromuscular

Question 47

name 4 areas in which ACh is released

Answer 47

1. ALL neuromuscular junctions involved with skeletal muscle fibers 2. at many synapses in the CNS 3. at ALL neuron-neuron synapses in the PNS 4. ALL neuromuscular and neuroglandular junctions in parasympathetic divisions of the ANS

Question 48

describe the events in the functioning of a cholinergic synapse

Answer 48

1. action potential arrives and depolarizes the synaptic terminal 2. extracellular calcium enters the synaptic terminal. this triggers the exocytosis of ACh 3. ACh binds to receptors and depolarizes the postsynaptic membrane 4. ACh is removed by acetylcholinesterase (AChE)

Question 49

what exactly is ACh binding to on the postsynaptic membrane?

Answer 49

CHEMICALLY GATED SODIUM ION CHANNELS. This will initiate an action potential if threshold is reached at the initial segment

Question 50

the depolarization of the synaptic terminal causes calcium (Ca2+) ions to rush in. What do they rush in through?

Answer 50

voltage gated calcium channels

Question 51

the depolarization of the postsynaptic membrane due to the influx of sodium ions through chemical-gated channels is a ___ potential. explain

Answer 51

a GRADED potential the greater the amount of ACh released at the presynaptic membrane, the greater the number of open channels in the postynaptic membrane, THUS, THE LARGER DEPOLARIZATION

Question 52

Where does AChE reside?

Answer 52

in the synaptic cleft and in the postsynaptic membrane

Question 53

around what percent of acetylcholine released at the presynaptic membrane is broken down before it reaches receptors at the postsynaptic membrane?

Answer 53

around half is broken down

Question 54

AChE breaks down molecules of ACh by ____ into ___ and ____

Answer 54

by hydrolysis into acetate and choline

Question 55

how is more ACh synthesized?

Answer 55

the choline resulting from the breakdown of ACh by AChE is actively absorbed by the synaptic terminal and used to synthesize more ACh also acetate is used too and is provided by CoA (coenzyme A)

Question 56

the acetate that results from the hydrolysis reaction by AChE goes where?

Answer 56

it can be absorbed and metabolized by the postsynaptic cell or by other cells and tissues

Question 57

what is synaptic delay and how long is it

Answer 57

0.2-0.5msec occurs between the arrival of an action potential (depolarization) at the synaptic terminal and the effect on the postsynaptic membrane

Question 58

most of the synaptic delay is contributed by...

Answer 58

calcium influx and the RELEASE of neurotransmitter (not its diffusion across the cleft)

Question 59

the fastest reflexes have how many synapses?

Answer 59

ONE the more synapses, the more synaptic delay

Question 60

when does ACh release from the presynaptic cell cease?

Answer 60

when active transport mechanisms quickly remove calcium from the synaptic terminal

Question 61

is the synaptic terminal totally dependent on the ACh synthesized in the cell body of the neuron?

Answer 61

NO ACh is made from recycled choline and acetate

Question 62

when under intensive stimulation, resynthesis and transport mechanisms may not be able to keep up with the demand for neurotransmitter. What is the term for this?

Answer 62

synaptic fatigue

Question 63

during synaptic fatigue, the synapse weakens until....

Answer 63

Acetylcholine has been replenished

Question 64

name some important neurotransmitters

Answer 64

norepinephrine dopamine serotonin GABA (gamma-aminobutyric)

Question 65

if the neurons that produce ______ are damaged or destroyed, the result can be the characteristic stiffness or rigidity that is Parkinson's disease (this is a _____ effect)

Answer 65

DOPAMINE inhibitory

Question 66

Explain the excitatory effect of dopamine

Answer 66

cocaine inhibits the removal of dopamine from synapses in specific areas of the brain. This rise in dopamine concentration creates a "high"

Question 67

inadequate ____ production causes severe chronic depression

Answer 67

serotonin

Question 68

GABA has an ___ effect

Answer 68

inhibitory

Question 69

what do SSRI's do? (selective serotonin reuptake inhibitors)

Answer 69

inhibit the reabsorption of serotonin by synaptic terminals, increasing the concentration of serotonin

Question 70

what are 4 major CATEGORIES of neurotransmitters

Answer 70

-biogenic amines -amino acids -neuropeptides -dissolved gasses

Question 71

what are adrenergic synapses?

Answer 71

synapses that release norepinephrine

Question 72

norepinephrine has a __ effect

Answer 72

excitatory, depolarizing effect

Question 73

norepinephrine (NE) is widely distributed where>?

Answer 73

in the brain and portions of the ANS

Question 74

Name 2 "dissolved gases" neurotransmitters

Answer 74

nitric oxide (NO) carbon monoxide (CO)

Question 75

Nitric oxide is generated by synaptic terminals that innervate ________

Answer 75

smooth muscle in the walls of blood vessels of the PNS, and at synapses in several regions of the brain

Question 76

where is CO generated

Answer 76

similar to NO, at specialized synaptic terminals in the brain

Question 77

what are neuromodulators?

Answer 77

active compounds that are released from synaptic terminals along with neurotransmitters. they alter the rate of neurotransmitter release by the presynaptic cell OR change the post synaptic cell's response to neurotransmitters

Question 78

neuromodulators are typically _____

Answer 78

neuropeptides (small peptide chains synthesized and released by the synaptic terminal)

Question 79

explain how neuromodulators act

Answer 79

they bind to receptors in the pre or post synaptic membranes and activate cytoplasmic enzymes