Action Potential

Front 1

How are action potentials generated?

Back 1

They are generated by special types of voltage gated ion channels embedded in a cells plasma membrane.

Front 2

Regarding voltage gated ion channels, are the gates always shut to ions?

Back 2

No. These channels are shut when the membrane potential is near the resting potential of the cell, but rapidly begin to open if the membrane potential increases to a precisely defined threshold value.

Front 3

What happens when the channels open?

Back 3

They permit an inward flow of sodium ions, which alters the electrochemical gradient, which in turn produces a further rise in the membrane potential. This results in the opening of more channels, producing a greater electric current.

Front 4

What is the result regarding the rapid influx of sodium ions?

Back 4

This causes the polarity of the plasma membrane to reverse, and the ion channels then rapidly inactivate.

Front 5

What is happening as the sodium channels close?

Back 5

Sodium ions can no longer enter the neuron, and they are actively transported out of the plasma membrane.

Front 6

Once sodium ions can no longer enter the neuron, what other ion channels begin to open?

Back 6

Potassium channels are then activated and there is an outward current of potassium ions, returning the electrochemical gradient to the resting state.

Front 7

What is afterhyperpolarization?

Back 7

After an action potential has occurred, there is a transient negative shift, called the afterhyperpolarization or refractory period, due to additional potassium currents.

Front 8

What is the function of the afterhyperpolarization?

Back 8

This mechanism prevents an action potential from traveling back the way it just came.

Front 9

In animal cells there are two primary types of action potentials(APs). What are they?

Back 9

1. One type is generated by voltage gated sodium channels.
The other by voltage gated calcium channels.

Front 10

Are the sodium and calcium potentials equal in depolarization porential?

Back 10

No. The sodium based potentials usually last for under one millesecond, whereas calcium based potentials may last for 100 milleseconds or longer.

Front 11

Give an example of a calcium based potential.

Back 11

In cardiac muscle cells, an initial fast sodium spike provides a primer to provoke the rapid onset of a calcium spike, which results in muscle contraction.

Front 12

It is said, that animal cells and other types of cells function as batteries regarding action potential. How is this statement justified?

Back 12

They act as batteries in the sense that they maintain a voltage difference between the interior and the exterior of the cell, with the interior being the negative pole of the battery.

Front 13

How is the voltage of a cell usually measured?

Back 13

It is usually measured in millivolts, or thousands of a volt. The typical voltage for the animal cell is app. -70mV, app. one fifteenth of a volt.

Front 14

What is meant by up and down cycle?

Back 14

In some cells, the voltages sometimes show very rapid up and down fluctuations that have a stereotyped form: These up and down cycles are known as action potentials.

Front 15

Do all up and down cycles have the same duration?

Back 15

In brain cells of animals, the entire up and down cycle may take place in less than a thousandth of a secod. In other types of cells, the cycle may last for several seconds.

Front 16

What is meant by membrane potential?

Back 16

All cells in animal body tissues are electrically polarized: in other words, they maintain a voltage difference across the cells plasma membrane. This is membrane potential.

Front 17

What is the impetus for electrical polarization?

Back 17

It results from a complex interplay between protein structures embedded in the membrane called ion pumps and ion channels.

Front 18

Are ion channels always alike?

Back 18

No. In neurons, the types of ion channels in the membrane usually vary across different parts of the cell, giving the dendrites, axons, and cell body different electrical properties.

Front 19

Regarding the previous answer, can one say: " some parts of the membrane of a neuron may be excitable( capable of generating action potentials), whereas others are not".

Back 19

Yes.

Front 20

Which is usually considered the most excitable part of the neuron?

Back 20

It is usually the axon hillock(the point where the axon leaves the cell body), but the axon and cell body are also excitable in most cases.

Front 21

Each excitable patch of membrane has 2 important levels of membrane potential. What are they?

Back 21

The resting potential-which is the value the membrane potential maintains as long as nothing perturbs the cell:
Threshold potential- considered the higher potential.

Front 22

At the axon hillock of a typical neuron, whatis the resting potential?

Back 22

Itis about -70mV. The threshold potetial is around -55mV.
Synaptic inputs to a neuron cause the membrane to depolarize or hyperpolarize(they cause the membrane potential to rise or fall.

Front 23

When are action potentials exactly triggered?

Back 23

They are triggered when enough depolarization accumulates to bring the membrane potential up to threshold. When the AP is triggered, the membrane potential abruptly shoots upward often reaching as high as +100mV, then abruptly shoots back downward, often ending below the resting level, where it remains for some period of time.

Front 24

A voltage gated ion channel is a cluster of proteins embedded in the membrane that has 3 key properties: What are they?

Back 24

1. It is capable of assuming more than one conformation
2.At least, one of the conformations creates a channel through the membrane that is permeable to specific types of ions:
3.The transitions between conformations is influenced by the membrane potential.

Front 25

Several types of channels that are capable of producing the positive feedback necessary to generate an action potential exist. What are they?

Back 25

Voltage gated Na+ channels are responsible for the fast action potentials involved in nerve conduction.
Slower action potentials in muscle cells and some types of neurons are generated by voltage gated calcium channels. These all have different temporal dynamics.l

Front 26

The membrane potential controls the state of the ion channels. True/False

Back 26

This is true, but the state of the ion channels controls the membrane potential as well.

Front 27

What are the most intensively studied type of voltage dependent ion channels?

Back 27

The sodium channels involved in fast nerve conduction. These are sometimes referred to as Hodgkin-Huxley sodium channels because they were first characterized by these researchers. They are also scripted as Na(v) channeles. The V stands for voltage.

Front 28

An Na(v) channel has three possible states. What are they called?

Back 28

Deactivated
Activated
Inactivated

Front 29

The Na(v) channel is permeable only to sodium ions when it is in the activated state. True/false

Back 29

True. When the membrane potential is low, the channel spends most of its time in the closed(deactivated) state.
If the membrane potential is raised above a certain level, the channel shows increased probability of transitioning to the activated(open) state. The higher the membrane potential, the greater the probability of activation.

Front 30

Once a channel has activated, it will eventually transition to the inactivated(closed) state. True/False

Back 30

True.
During an action potential, most channels of this type go through a cycle-
Deactivated---Activated---Inactivated---Deactivated.

Front 31

As the membrane potential is increased, what happens to sodium channels?

Back 31

The Na+ channels open, permitting the entry of Na+ ions into the cell.
This is followed by the opening of potassium ion channels that permit the exit of K+ ions from the cell.

Front 32

What actually causes a depolarization spike?

Back 32

The inward flow of Na+ ions increases the concentration of positively charged cations in the cell and causes depolarization, where the potential of the cell is higher than the cell's resting potential.

Front 33

The efflux of potassium ions decreases the membrane potential or hyperpolarizes the cell. true/false

Back 33

True. For small voltage increases from rest, the K+ current exceeds the sodium current and the voltage returns to its normal resting value, typically, -70mV.

Front 34

The ions exchanged during an action potential, make a negligible change in the interior and the exterior ionic concentrations. true/Faslse

Back 34

True. The few ions that do cross are pumped out again by the continuous action of the sodium potassium pump, which with other ion transporters, maintains the normal ratio of ion concentrations across the membrane.

Front 35

Calcium cations and chloride anions are involved in which types of APs?

Back 35

Calcium in the cardiac action potential.
Chloride in the single cell alga,Acetabularia.

Front 36

Although APs are located locally on patches of excitable membrane, the resulting currents can trigger APs on neighboring stretches of membrane, precipitating a domino-like propagation. True/False

Back 36

True.

Front 37

Are myelinated sections of neurons excitable?

Back 37

No. They do not produce APs and the signal is propagated passively as electronic potential.

Front 38

What are Nodes of Ranvier?

Back 38

Regularly spaced unmyelinated patches, called the Nodes of Ranvier, generate APs to boost the signal. This is known as Saltatory Conduction.
This type of signal propagation provides a favorable tradeoff of signal velocity and axon diameter.

Front 39

Depolarization axon terminals, in general triggers what?

Back 39

Neurotransmitters into the synaptic cleft.
Also, backpropagating APs have been recorded in the dendrites of pyramidal neurons, which are ubiquitous in the neocortex. These may play a role in spike timing dependent plasticity.

Front 40

What is a neuron?

Back 40

Neurons are electrically excitable cells composed, in general of one or more dendrites, a single soma, a single axon and one or more axon terminals.

Front 41

The dendrite is one of the 2 types of synapses, the other being the axon tyerminal boutons. True/False

Back 41

True. Dendrites form protrusions in response to the axon terminal boutons.

Front 42

What are the dendritic protrusions?

Back 42

These protrusions or spines, are designed to capture, the neurotransmitters released by the presynaptic neuron. They have a high concentration of ligand activated channels. It is here where synapses from 2 neurons communicate with one another.

Front 43

These spines or protrusions have a thin neck, connecting a bulbous protrusion to the main dendrite. is there as reason for this?

Back 43

This ensures that changes occurring inside the spine are less likely to affect the neighboring spines.

Front 44

The soma houses the nucleus, which acts as the regulator for the neuron. True/false

Back 44

True.The surface of the soma is populated by voltage activated ion channels.

Front 45

What is rather unique about the axon hillock?

Back 45

It has a very high concentration of voltage gated sodium channels. It is the spike initiation zone for APs.

Front 46

Immediately after the axcon hillock is the__________________.

Back 46

Axon. This travels away from the soma. The axon is insulated by a myelinated sheath.

Front 47

What is myelin composed of?

Back 47

It is composed of either Schwann cells(peripheral nervous system) or Oligodendrocytes, in the CNS. types of glial cells.

Front 48

What is the function of glial cells?

Back 48

They communicate and provide important biochemical support to neurons. They are not involved with transmission.

Front 49

What is another function of the myelin sheath?

Back 49

Myelin wraps multiple times around the axonal segment, forming a thick fatty layer that prevents ions from entering or escaping the axon. It also prevents significant signal decay as well as ensuring faster signal speed.

Front 50

The regularly spaced patches of membrane, which have no insulation can be considered as little axon hillocks. True/false

Back 50

True.

Front 51

How are most APs initiated?

Back 51

By an excitatory postsynaptic potential from a presynaptic neuron.

Front 52

What is the result when neurotransmitters are released by the the presynaptic neuron?

Back 52

They bind to receptors on the postsynaptic cell, and this opens various types of ion channels.

Front 53

If the binding increases the voltage(depolarizes the membrane), the synapse is excitatory. True/False

Back 53

True. if the binding decreases the voltage(hyperpolarizes the membrane), it is inhibitory.
More typically, the ecitatory potentials from several synapses must work together at nearly the same time to provoke a new action potential.

Front 54

Due to the direct connection between excitable cells in the form of gap junctions, can action potentials be transmitted directly from one cell to the next in either direction?

Back 54

Yes Rectifying channels ensure that APs move only in one direction.

Front 55

Are APs considered to be all or none signals?

Back 55

Yes. The amplitude of an AP is independent of the amount of current that produced it. Larger currents do not create larger action potentials. They occur either fully, or not at all.

Front 56

In sensory neurons, an external signal such as pressure, temperature, light or sound is coupled with the opening and closing of ion channels. True/False

Back 56

True.

Front 57

Give relevant examples of sensory neurons.

Back 57

The olfactory receptor neuron and Meissner's corpuscle which are critical for the sense of smell, and touch respectively.

Front 58

Do all sensory neurons convert their external signals into APs?

Back 58

No.Some do not even possess an axon. Instead they may convert the signal into the release of a neurotransmitter, or into continuous graded potentials, either of which may stimulate subsequent neurons into firing an AP.

Front 59

Give an example of the above explanation.

Back 59

In the human and animal ear, hair cells convert the incoming sound into the opening and closing of mechanically gated ion channels which may cause neurotransmitter molecules to be released.

Front 60

Explain what happens in the mammalian retina.

Back 60

The retina, the initial photoreceptor cells and the next layer of cells(comprising bipolar cells and horizontal cells) do not produce APs; only some amacrine cells and the third layer, the ganglion cells produce APs, when they travel up the optic nerve.

Front 61

The course of the action potential can be divided into 5 parts. What are they?

Back 61

1. Rising phase
2. Peak phase
3. falling phase
4.Undershoot phase
5.Refractory phase

Front 62

What is the rising phase

Back 62

The membrane potential depolarizes(becomes more positive).

Front 63

What is the peak phase?

Back 63

The point at which depolarization stops. The membrane potential reaches its maximum.

Front 64

What is the Falling phase?

Back 64

During this stage,the membrane potential hyperpolarizes(becomes more negative).

Front 65

What is the Undershoot phase?

Back 65

This is the point during which the membrane potential becomes temporarily more negatively charged than when at rest..

Front 66

What is the Refractory phase?

Back 66

The time during which a subsequent action potential is impossible or difficult to fire.This may overlap with other phases.

Front 67

It is said that the course of the action potential is determined by two coupled effects. What is meant by this?

Back 67

First, voltage sensitive ion channels open and close in response to changes in the membrane voltage. This changes the membranes permeability to ions.
Second,according to the Goldman equation, this change in permeability changes in the equilibrium potential potential(Em) and thus the membrane voltage (Vm).

Front 68

A typical AP begins where on the neuron?

Back 68

The axon hillock. This occurs with a sufficiently strong depolarization(a stimulus that increases). this depolarization is often caused by the injection of extra sodium cations into the cell. These cations can come from a variety of sources, such as chemical synapses, sensory neurons or pacemaker potentials.

Front 69

For a neuron at rest, there is a high concentration of __________ and __________ ions in the ECF, compared to the ICF while there is a high concentration of __ions in the ICF compared to the ECF.

Back 69

Sodium/Chlorine/Potassium

Front 70

With respect to the above question, this concentration gradient along with K+ leak channels present on the membrane of the neuron causes an efflux of what?

Back 70

Potassium ions making the rest of the potential close to Ek=-75mV.
The depolarization opens both the Na+ and K+ channels in the membrane allowing the ions to flow into and out of the axon respectively.

Front 71

What is the consequence if the depolarization is small, increasing from -70mV to -60mV?

Back 71

The outward potassium current overwhelms the inward sodium current and the membrane repolarizes back to its normal resting potential around -70mV.

Front 72

What is the result if the depolarization is large enough?

Back 72

The inward sodium current increases more than the outward potassium current and a runaway condition(positive feedback) results. The more inward current there is the more Vm increases which inturn further increases the inward current.

Front 73

What is the critical threshold voltage for this runaway condition?

Back 73

It is usually around -45mV. But, this also depends on the recent activity of the axon.

Front 74

A membrane that has just fired an AP cannot fire another one for what reason?

Back 74

This is because the ion channels have not yet returned to their usual state.(Absolute refractory period)

Front 75

Explain relative refractory period.

Back 75

At longer times, after some, but not all of the ion channels have recovered, the axon can be stimulated to produce another action potential, but only with a much stronger depolarization, e.g.-30mV.
The period which action potentials are unusually difficult to evoke is called the Relative refractory period.

Front 76

What is myelin?

Back 76

It is a multi lamellar membrane that enwraps the axon in segments separated by intervals known as nodes of Ranvier.

Front 77

What produces myelin?

Back 77

It is produced by specialized cells. Schwann cells in the PNS and by oligodendrocytes exclusively in the CNS.

Front 78

What is another function of myelin sheath/

Back 78

It reduces membrane capacitance and increases membrane resistance in the inter node intervals, thus permitting a fast saltatory movement of action potentials from node to node.

Front 79

Myelin prevents ions from entering or leaving the axon along myelinated segments of the axon. True/False

Back 79

True. myelination increases the conduction velocity of APs and makes them more energy efficient.

Front 80

What is the mean conduction of an AP?

Back 80

Whether saltatory or not, it ranges from 1 m/s to over 100m/s and in general increases with axonal diameter.

Front 81

Can action potentials propagate through the membrane in myelinated segments of the axon?

Back 81

No. The current is carried by the cytoplasm, which is sufficient to depolarize the first or second subsequent node of Ranvier.

Front 82

The ionic current from an AP at one node of Ranvier provokes another AP at the next node. True/False

Back 82

True. This hopping of the AP from node to node is called saltatory conduction.

Front 83

In general, APs that reach the synaptic knobs cause a neurotransmitter to be released into the synaptic cleft. True/False

Back 83

True. Neurotransmittrs are small molecules that may open ion channels in the postsynaptic cell.

Front 84

What happens with the arrival of the AP at the terminal?

Back 84

It opens voltage sensitive calcium channels in the presynaptic membrane; the influx of Ca+ causes vesicles filled with neurotransmitter to migrate to the cells surface and release their contents into the synaptic cleft.

Front 85

can one say, regarding the above mechanism, that this complex process is inhibited by the neurotoxins, tetanospasmin and botulinum toxin?

Back 85

Yes.

Front 86

Do all synapses employ a neurotransmitter?

Back 86

No. When an action potential reaches such a synapse, the ionic currents flowing into the presynaptic cell can cross the barrier of the 2 cell membranes and enter the post synaptic cell through pores. These pores are called connexons.

Front 87

What is the advantage of connexons?

Back 87

Electrical synapses allow for faster transmission because they do not require the slow diffusion or neurotransmitters across the synaptic cleft.

Front 88

What is a neuromuscular junction?

Back 88

The axon of a motor neuron terminates on a muscle fiber. The released neurotransmitter is acetylcholine, which binds to the acetylcholine receptor in the sarcolemma of the muscle fiber.

Front 89

Does the acetylcholine remain bound?

Back 89

No. It dissociates and is hydrolyzed by the enzyme, acetylcholinesterase, located in the synapse.

Front 90

What are some poisons that can inactivate acetylcholinesterase to prevent this control?

Back 90

Sarin, tabun, Diazinon and Malathion.

Front 91

How does the cardiac action potential differ from the neuronal AP?

Back 91

The cardiac potential has an extended plateau. The membrane is held at a high voltage for a few hundred milliseconds prior to being repolarized by the potassium current as usual.

Front 92

What causes this cardiac plateau?

Back 92

It is due to the action of slower calcium channels opening and holding the membrane voltage near their equilibrium potential even after the sodium channels have inactivated.

Front 93

List several antiarrhythmia drugs that act on the cardiac potential.

Back 93

Quinidine, Lidocaine, beta blockers and Verapamil.