Action Potentials Human Physiology Chapter 7

Front 1

what two items have the ability to alter their membrane potentials in response to stimulation? (two tissues)

Back 1

Muscles and nervous tissue

Front 2

why do cells have a negative resting potential?

Back 2

the membranes potential changes if membranes permeability to specific ions is altered (usually increased) Now the ion flows down its concentration gradient = ion current

Front 3

what happens when a postivie charge flows into the cell?

Back 3

If positive charges flow into a cell, the potential inside becomes less negative, i.e., more positive = depolarization

Front 4

What happens when positive charge goes out of cell?

Back 4

If the positive charges flow back out of the cell, the potential inside the cell becomes more negative = repolarization

Front 5

what happens when positive charge flows out of resting cell or negative charges flow in?

Back 5

If positive charges flow out of a resting cell (or negative charges flow in), the inside of the cell will be more negative than RMP = hyperpolarization

Front 6

why do voltage gated channels open?

Back 6

Voltage gated (VG) channels open in response to change in membrane potential...Gated channels are part of proteins that comprise the channel.
• Can be open or closed in response to change.
– 2 types of channels for K+:
• 1 always open.
• 1 closed in resting cell.
– Channel for Na+:
• Always closed in resting cells though some Na+ does leak into the cells.

Front 7

what does stimulus do to threshold?

Back 7

Stimulus causes depolarization to threshold

Front 8

what happens when VG Na+ channels open?

Back 8

Electrochemical gradient inward.
• + feedback loop.
– Rapid reversal in membrane potential from –70 to + 30 mV.
– VG Na+ channels become inactivated

Front 9

what happens when VG K+ channels open?

Back 9

Electrochemical gradient outward.
– - feedback loop.
– Restore original RMP

Front 10

Membrane Permeabilities

Back 10

AP is produced by an increase in Na+ permeability.
• After short delay, increase in K+ permeability

Front 11

Depolarization and repolarization occur via_______________

Back 11

diffusion....does not use active transport

Front 12

what happens once AP is completed? does it require atp?

Back 12

Once AP completed, Na+/K+ ATPase pump extrudes Na+, and recovers K+. (Requires ATP.)

Front 13

why does amplitude not become more positive that +30mV?

Back 13

Amplitude does not normally become more positive than + 30 mV because VG Na+ channels close quickly and VG K+ channels open.
– Duration is the same, only open for a fixed period of time

Front 14

what is recruitment

Back 14

Recruitment:
– Stronger stimuli can activate more axons with a higher threshold.

Front 15

what is an Absolute refractory period?

Back 15

Axon membrane is incapable of producing another AP due to Na+ gated VG channels not opening

http://www.youtube.com/watch?v=Gsf9IB-wQdU
http://www.youtube.com/watch?v=r2kvUbP_rw4

Front 16

what is Relative refractory period?

Back 16

VG ion channel shape alters at the molecular level.
– VG K+ channels are open.
– Axon membrane can produce another action potential, but requires stronger stimulus

Front 17

Cable Properties of Neurons

Back 17

Ability of neuron to transmit charge through cytoplasm

Front 18

Cable Properties of Neurons

Back 18

Axon cable properties are poor:
– High internal resistance.
– Many charges leak out of the axon through membrane.
• An AP does not travel down the entire axon.
• Each AP is a stimulus to produce another AP in the next region of membrane with VG channels.

Front 19

Conduction in an Unmyelinated Axon

Back 19

Cable spread of depolarization with influx of Na+ depolarizes the adjacent region membrane, propagating the AP.
• Conduction rate is slow.
• Occurs in 1 direction; previous region is in its refractory period.
• The “WAVE”

Front 20

Conduction in Myelinated Axon

Back 20

Myelin prevents movement of Na+ and K+ through the membrane.Interruption in myelin (Nodes of Ranvier) contain VG Na+ and K+ channels.

Front 21

where do AP occur

Back 21

AP occurs only at the nodes.
– AP at 1 node depolarizes membrane to reach threshold at next node.
• Saltatory conduction (leaps).
– Fast rate of conduction.
• Axon diameter also influences speed of conduction
Table 7.3

Front 22

Synapse

Back 22

Functional connection between a neuron (presynaptic) and another cell (postsynaptic – either another neuron or an effector cell).

Front 23

There are chemical and electrical synapses.

Back 23

Synaptic transmission at chemical synapses is via neurotransmitters (NT).
– Electrical synapses are rare in the nervous system

Front 24

Electrical Synapse

Back 24

Impulses can be regenerated without interruption in adjacent cells.

Front 25

Electrical Synapse

Back 25

Depolarization flows from presynaptic into postsynaptic cell through channels called gap junctions

Front 26

gap junctions

Back 26

Adjacent cells electrically coupled through a channel.
– Formed by connexin proteins
– Found in smooth and cardiac muscles, brain, and glial cells

Front 27

Chemical Synapse

Back 27

Synaptic cleft separates terminal bouton of presynaptic from postsynaptic cell

Front 28

Chemical Synapse

Back 28

NTs are in synaptic vesicles

Front 29

Chemical Synapse

Back 29

Axon of first (presynaptic) to second (postsynaptic) neuron.
• Synaptic vesicles fuse with bouton (presynaptic) membrane; release NT by exocytosis
• Amount of NT released depends upon frequency of APs

Front 30

Synaptic Transmission

Back 30

APs travel down axon to depolarize bouton

Front 31

Synaptic Transmission

Back 31

Open VG Ca2+ channels in bouton
– Ca2+ enters bouton by electrochemical gradient
• Ca2+ binds to synaptotagmin (protein sensor)
• Triggers exocytosis of synaptic vesicles; release of NTs by exocytosis

Front 32

Neurotransmitter Release

Back 32

Action potentials reach the axon terminal

Front 33

Neurotransmitter Release

Back 33

Ca2+ enters axon terminal via voltage gated channels

Front 34

Neurotransmitter Release

Back 34

Ca2+ binds to sensor protein (snaptotagmin) in cytoplasm

Front 35

Neurotransmitter Release

Back 35

• Ca2+ -protein complex stimulates fusion and exocytosis of neurotransmitter..Neurotransmitter is released from the vesicles into synapse

Front 36

Neurotransmitter Release

Back 36

Neurotranmitter diffuses across cleft
– Binds to receptor proteins on postsynaptic membrane
• Opening chemically-regulated ion channels
– Depolarizing channels cause EPSPs (excitatory postsynaptic potentials)
– Hyperpolarizing channels cause IPSPs (inhibitory postsynaptic potentials)
– These affect VG channels in postsynaptic cell
• EPSPs and IPSPs summate
• If MP in postsynaptic cell reaches threshold at the axon hillock, a new AP is generated
– Axon hillock has many VG channels and is site where APs are normally initiated

Front 37

axon hillock

Back 37

If MP in postsynaptic cell reaches threshold at the axon hillock, a new AP is generated
– Axon hillock has many VG channels and is site where APs are normally initiated

Front 38

net diffusion occurs in what two stages...what is an action potential

Back 38

1-Na+ moves into axon 2. k+ MOVES OUT..FLOW OF IONS AND CHANGE IN MEMBRANE POTENTIAL..CALLED AN ACTION POTENTIAL

Front 39

Define depolarization, repolarization, and hyperpolarization.

Back 39

If positive charges flow into a cell, the potential inside
becomes less negative, i.e., more positive = depolarization.................If the positive charges flow back out of the cell, the potential
inside the cell becomes more negative = repolarization....• If positive charges flow out of a resting cell (or negative
charges flow in), the inside of the cell will be more negative
than RMP = hyperpolarizatio

Front 40

Explain the actions of voltage-regulated Na+
and K+
channels
and describe the events that occur during the production of an
action potential.

Back 40

2 types of channels for K+
:
• 1 always open.
• 1 closed in resting cell.
– Channel for Na+
:
• Always closed in resting cells though some Na+
does leak into the
cells?????

Front 41

Describe how the permeability of the axon membrane to Na+
and K+
is regulated and how changes in permeability to these
ions affect the membrane potential.

Back 41

???

Front 42

Draw and describe the sequence of events that are occurring in a
voltage-gated ion channel.

Back 42

???

Front 43

Describe the properties of action potentials and explain the
significance of the all-or-none law and the refractory periods.

Back 43

Relative refractory period:
– VG ion channel shape alters at the molecular level.
– VG K+
channels are open.
– Axon membrane can produce another action potential, but
requires stronger stimulus.

Front 44

Explain the all-or-none law of action potentials and describe the
effect of increased stimulus strength on action potential
production. How do the refractory periods affect the frequency
of action potential production?

Back 44

......

Front 45

Distinguish between absolute refractory period and relative
refractory period

Back 45

Absolute refractory period:
– Axon membrane is incapable of producing another AP.

Relative refractory period:
– VG ion channel shape alters at the molecular level.
– VG K+
channels are open.
– Axon membrane can produce another action potential, but
requires stronger stimulus.

Front 46

Describe what is meant by cable properties of an axon.

Back 46

Ability of neuron to transmit charge through cytoplasm.
• Axon cable properties are poor:
– High internal resistance.
– Many charges leak out of the axon through membrane.
• An AP does not travel down the entire axon.
• Each AP is a stimulus to produce another AP in the next
region of membrane with VG channels.

Front 47

Explain how action potentials are (re)generated along
myelinated and unmyelinated axons.

Back 47

Conduction in an Unmyelinated Axon
• Cable spread of depolarization with influx of Na+
depolarizes
the adjacent region membrane, propagating the AP.
• Conduction rate is slow.
• Occurs in 1 direction; previous region is in its refractory
period.
• The “WAVE”........................Conduction in Myelinated Axon
• Myelin prevents movement of Na+
and K+
through the
membrane.

Front 48

Draw and identify the conduction of action potentials in an
unmyelinated axon. (Figure 7.19.)

Back 48

..

Front 49

Describe how action potentials are conducted by
unmyelinated nerve fibers. Why is salutatory conduction in
myelinated fibers more rapid?

Back 49

Saltatory conduction (leaps).
– Fast rate of conduction. • Axon diameter also influences speed of conduction..The cytoplasm of an axon is electrically conduction and because myelin inhibits charge leakage through the membrane, depolarization at one node of Ranvier is sufficient to elevate the voltage at a neighboring node to the threshold for action potential initiation.

Front 50

Draw and identify the conduction of action potentials in a
myelinated axon.

Back 50

...

Front 51

How does the diameter of a nerve affect its conduction
velocity and function?

Back 51

The conduction of action potentials along an axon behaves exactly the same way- larger diameter axons have a greater total volume for charges to flow through and are correspondingly meet less resistance from the membrane.

Front 52

Describe the events that occur in the interval between the
electrical excitation of an axon and the release of
neurotransmitter. (Figure 7.23.)

Back 52

...

Front 53

Describe the structure, locations, and functions of gap
junctions.

Back 53

Depolarization flows from presynaptic into postsynaptic cell
through channels called gap junctions
– Adjacent cells electrically coupled through a channel.
– Formed by connexin proteins
– Found in smooth and cardiac muscles, brain, and glial cells

Front 54

Describe the location of neurotransmitters within an axon and
explain the relationship between presynaptic axon activity and
the amount of neurotransmitters released.

Back 54

..

Front 55

Describe the sequence of events at the terminal boutons from
the time of the arrival of action potentials until the release of
neurotransmitters from presynaptic axons.

Back 55

..

Front 56

Distinguish between voltage-regulated and chemically
regulated ion channels.

Back 56

Ligand-gated ion channels are cell membrane receptors that are activated by neurotransmitters and this causes the opening of the ion channel causing depolarization of the cell. Voltage gated ion channels are ion channels also present on the membrane but are activated by a change in voltage across the membrane and this will cause the ion channel to open and will start depolarization of the cell membrane. Often both type are present in some neurons and cardiac myocytes. When the ligand-gated ion channels are activated by a neurotransmitter, like norepinephrine these channels will open and change the permeability of the membrane to sodium. This change in voltage across the cell membrane will open the voltage dependent ion channels opning more sodium channels and causing the depolarization of the membrane.

Front 57

What is an EPSP?

Back 57

Depolarizing channels cause EPSPs (excitatory postsynaptic
potentials)

Front 58

What is an IPSP?

Back 58

Hyperpolarizing channels cause IPSPs (inhibitory postsynaptic
potentials)

Front 59

Distinguish between EPSPs and IPSPs.

Back 59

...