Lesson 5 (6) Propagated Signaling: The Action Potential​

What do nerve cells generate and for what purpose?

a variety of electrical signals that are used to transmit and store information.

What is the mechanism or force of electrical signaling in nerve cells?

The mechanism is the unequal distribution of ions across the membrane and the movement of these ions across cell membranes at particular times.

What is the negative potential generated by neurons called, and how is it generated?

The resting membrane potential (RMP), generated by pumping sodium ions out of the cell and potassium ions into the cell.

How can the Resting Membrane Potential (RMP) be measured?

by recording the voltage inside the neuron relative to the outside using a microelectrode.

Do all neurons have the same exact RMP value?

No, the RMP varies according to the neuron being examined; not all neurons have the same exact RMP value.

What is the typical range of the Resting Membrane Potential (RMP)?

The RMP typically ranges from -40 to -90 mV, depending upon the type of neuron.

What are "all-or-none" signals used by neurons for long-distance transmission, and what are their other names?

Action potentials (APs), also known as impulses or spikes, are all-or-none signals used by neurons to transmit signals over long distances, typically down axons. (recall, axons are not good electrical conductors!)​

How do special channels contribute to neural signaling based on sensory messages?

They transduce sensory messages into various electrical signals.

*Such potentials control the rate of firing down the axon or the rate of neurotransmitter release​

What type of signals are Receptor potentials (RPs), and how are they generated?

They are graded signals generated by activating sensory neurons.

Where do Synaptic potentials (SPs) occur, and what type of potentials are they?

They are graded potentials that occur between neurons, such as those found in complex neural circuits in both the CNS and PNS.

Where is the intensity of a sensory stimulus encoded?

in the amplitude of the receptor potential.

Describe the conversion of an analog signal from a receptor potential into a digital signal for neural transmission.

The analog signal (the amplitude of the receptor potential) is converted into a digital signal—a pulse code—in which the intensity of the stimulus is proportional to the frequency of action potentials.

In a laboratory setting, what type of changes in membrane potential can an electrical current generate?

Passive electrical responses.

What is the term for when a current delivered to a membrane makes it more negative?

The membrane is hyperpolarized.

When a current of opposite polarity is delivered, making the membrane less negative or more positive, what is this process called?

The membrane is depolarized.

What happens if an injected current depolarizes the membrane to a critical point more positive than the resting potential (threshold)?

An active electrical response in the form of an action potential (AP) will occur.

What is an Action Potential (AP)?

An AP is an active electrical response (lasting about 1 ms) to a change in membrane potential, specifically a change away from the Resting Membrane Potential (RMP).

How does the membrane potential change during an Action Potential (AP)?

It changes from a negative value to a less negative, often positive value.

What determines the amplitude of an Action Potential (AP)?

The amplitude of the AP is independent of the magnitude of the current injected or the current used to evoke the AP.

Why do larger currents not elicit larger Action Potentials (APs)?

Larger currents do not elicit larger APs because an AP is an all-or-none event.

What happens to Action Potentials (APs) when the injected current or stimulus is increased?

Increasing the current (or stimulus) leads to more APs being produced.

How is the intensity of a stimulus encoded in neural transmission?

The intensity of a stimulus is encoded by the frequency of APs, not in their amplitude.

How is an action potential (AP) brought about?

By the sequential opening of Na^+ and K^+ channels.

Although depolarization is a type of graded response, how does each increment of depolarization affect voltage-gated Na⁺ channels?

They increases the number of voltage-gated Na⁺ channels that open.

What is the relationship between the opening of voltage-gated Na⁺ channels and membrane depolarization?

The more voltage-gated Na⁺ channels that open, the more the membrane is depolarized (as g_Na increases, so does I_Na+).

What happens at threshold regarding the membrane potential?

There is a sudden reversal of the membrane potential.

How can voltage-gated ion channels be modulated?

By intracellular Ca⁺² concentrations (which enhances the probability of opening of calcium activated voltage-sensitivity K⁺ channels) and by neurotransmitters and second-messenger pathways.

Why does the trigger zone at the axon initial segment have a low threshold for action potential generation? (lower than other areas of the neuron)

Because it has a high density of Na⁺channels.

What critical role do voltage-gated ion channels play in the transformation of graded potentials?

They play a critical role in transforming graded synaptic and receptor potentials into the all-or-none action potentials.

Is all electrical activity in neurons characterized by an all-or-none response?

No, not all electrical activity is characterized by an all-or-none response.

What kind of membrane potentials do hyperpolarizing currents produce?

Hyperpolarizing currents produce negative (downward) membrane potentials.

What type of membrane potentials do depolarizing currents produce?

Depolarizing currents produce positive (upward) membrane potentials.

What happens when two largest depolarizing currents are applied to a neuron?

The two largest depolarizing currents evoke identical action potentials.

How does electrotonic conduction (e.g., of a hyperpolarizing response) differ from the propagation of an action potential along an axon?

A hyperpolarizing response spreads along the axon via electrotonic conduction, which contrasts with an action potential's propagation at constant velocity and amplitude.

What does action potential (AP) conduction require?​

both active and passive current flow.

How can action potentials travel long distances despite the poor passive properties of neurons?

Action potentials can travel long distances because the depolarization at one segment of the membrane triggers the opening of sodium channels in the next segment, actively regenerating the signal along the axon.

What happens when sodium channels open due to membrane depolarization?

the actively moving inward current then spreads passively along the axon, depolarizing the adjacent region (the “passively” moving current need not Na+, but a shuttling of charge)​. This process repeats at the next segment of membrane (that is, “adjacent membrane”)​

Why doesn’t the action potential travel backward?

because of the refractory period caused by K⁺ channel opening and Na⁺ channel inactivation.

What does the refractory period ensure during an action potential?

polarized propagation of the action potential from the point of initiation! (because it prevents it from traveling backward)

How do axons compare to electrical wires in terms of electrical conductivity?

Axons are poor electrical conductors compared to electrical wires.

Can both axons and electrical wires passively conduct electricity?

Yes, both axons and electrical wires can passively conduct electricity, but the electrical properties of neurons compare poorly with those of electrical wires.

Why does passive current flow dissipate quickly over distance in axons?

Because axons are poor conductors of electric current, any passive current flow will dissipate quickly over distance.

What is the key difference of active conduction compared to passive conduction?

In active conduction (D), the injected current is large enough to produce an action potential.

• Subthreshold current yields only passiveconduction

• A subthreshold current yieldsa subthreshold change inmembrane potential​

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• As the current spreads alongthe axon, the amplitudedecreases with increasingdistance ​

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• This decrease in amplitude ormembrane potential is due tocurrent leaking out along theaxonal membrane ​

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• This leakiness of the axonalmembrane prevents effectiveconduction of electrical signalsin all but the shortest axons​

What type of conduction does a subthreshold current yield?

Only passive conduction.

What kind of change in membrane potential does a subthreshold current produce?

A subthreshold change in membrane potential.

As a subthreshold current spreads along the axon, what happens to its amplitude?

The amplitude decreases with increasing distance.

The decrease in amplitude or membrane potential is due to

Current leaking out along the axonal membrane.

What is the consequence of the leakiness of the axonal membrane for effective electrical signal conduction?

It prevents effective conduction of electrical signals in all but the shortest axons.

What type of conduction does a suprathreshold current yield?

an active conduction, which is regenerative.

What specific response does a suprathreshold current produce in membrane potential?

it yields an active response in membrane potential in the form of an action potential.

As a suprathreshold current spreads along the axon, what happens to its amplitude?

The amplitude remains constant with increasing distance.

How does this type of conduction (suprathreshold/active) deal with the leakiness of long axons?

This type of conduction reduces and circumvents the inherent leakiness associated with long axons.

In an unmyelinated axon, what happens to the current in inactive zones located far from the site of activation (the active zone)?

The current is not strong enough to change the membrane potential (V_m) to threshold. That is, the current leaks out along the entire axon as it spreads passively down the axon​

How does myelination affect current flow in the inactive zones of an axon?

In a myelinated axon, inactive zones are insulated, conserving enough current to change the membrane potential (V_m) of the next node to threshold.

What property of myelin prevents current leakage at the internodes in a myelinated axon?

The high resistance of myelin prevents current leakage at the internodes.

Here in this diagram, the density or amount of membrane current is represented by the thickness of the arrows​

How is the rapid spread of Action Potentials (APs) along the internodes of a myelinated axon achieved?

APs spread rapidly along the internodes because of the low capacitance of the myelin sheath.

What effect does crossing the node of Ranvier have on an Action Potential (AP), and what does this enable?

The AP slows down as it crosses the high capacitance node of Ranvier (bare nodal membrane), which enables the AP to jump down the axon from node to node.

Under what condition does an Action Potential (AP) slow down when moving from a myelinated region to an unmyelinated region?

The AP is slowed by the fact that inward current may be too small to depolarize the demyelinated membrane to threshold.

What is Conduction Velocity (CV)?

It is the rate or speed at which an AP moves or propagates along an axon.

What contributes to the speed at which an action potential (AP) can propagate along the axon?

Both active and passive current flows contribute to the speed.

What largely controls the propagation of an action potential (AP), and what is its role?

Propagation of the AP is largely controlled by electrotonic conduction, which serves as a rate-limiting factor, and hence requires a rapid propagation of the AP.

How can the passive current flow, which affects AP propagation, be improved?

This passive current flow can be improved by (a) increasing the diameter of the axon (decreasing internal resistance) or (b) by reducing leakage of current along the axon by insulating the axonal membrane with myelin.

Between increasing axon diameter and myelination, which has a greater effect on Conduction Velocity (CV)?

Myelination has a much greater effect on CV than does increasing axon diameter alone. This is why CV is much faster in a myelinated axon versus a nonmyelinated axon of the same diameter. This is because the rate at which depolarization spreads along the axon is determined by axial resistance (ra) and the capacitance per unit length of the axon (cm), where the rate varies inversely with the product ra cm (however, myelination decreases this product more so than does an equal increase in diameter).

Where are voltage-gated sodium channels highly concentrated?

At the nodes of Ranvier.

What are the nodes of Ranvier?

Areas where the current can flow in and out of the axon.

What do voltage-gated sodium channels at the nodes of Ranvier ensure?

The "active" regeneration of passive current from one node to the next.

What primary method do myelinated axons use to increase Conduction Velocity (CV)?

Saltatory conduction.

In saltatory conduction, where does the mechanism of Action Potential (AP) generation occur?

At the nodes of Ranvier.

What is the key advantage of saltatory conduction regarding AP regeneration?

It allows the AP to jump along the axon, so it doesn't have to be regenerated at every segment.

In saltatory conduction, how does an AP generated in one segment influence the next segment?

It supplies depolarizing current to the adjacent axonal membrane, causing it to depolarize gradually toward threshold.

What is the characteristic movement of an Action Potential (AP) during saltatory conduction?

The AP actually jumps from one node to the next node.

What effect does myelination have on conduction velocity along an axon?

Myelination greatly increases conduction velocity.

What is the typical range for Conduction Velocities (CVs) in an unmyelinated axon?

0.5 to 10 m/s.

How fast can Conduction Velocities (CVs) be in a myelinated axon?

They can be as fast as 150 m/s.

How does myelin contribute to electrical signaling in an axon?

Myelin acts as an electrical insulator, preventing the leakage of current along the axon.

How does the Action Potential (AP) move down the axon and what does it require?

The movement of the AP down the axon must be regenerated at each segment or part of the axon.

Describe the process of depolarization as an AP moves along the axon.

Depolarization of one segment of the axon depolarizes the next segment and so on, as current moves through both the axoplasm and the ECF.

What are the two ways in which Action Potential (AP) regeneration can occur?

This regeneration can occur by either continuous or saltatory conduction.