Action Potentials

Week 3

What is neural communication dependent on?

Changes in the resting membrane potential (RMP).

What does it mean for a membrane to be polarized?

The membrane potential is other than 0 mV.

What is depolarization?

Membrane potential becomes less negative/positive than the RMP

What is repolarization?

The phase where the membrane potential returns to the RMP (approximately -70 mV).

What is hyperpolarization?

When the membrane potential becomes more negative than the RMP (a greater magnitude of negativity).

What are the primary characteristics of an action potential?

Rapid changes in the resting membrane potential (RMP) that last only a few milliseconds.

 How does the electrical charge shift inside the cell during the reversal of polarization in an action potential?

The inside of the cell becomes positive relative to the outside.

What is required to initiate an action potential, and what is its approximate value?

Achieving a threshold potential, which is approximately -50 mV.

What law do action potentials follow, and what are its three key characteristics?

They follow the "All-or-None" law. The characteristics are:

• Constant amplitude

• Constant duration

• Current doesn't die out

 Does stimulus strength affect the amplitude or duration of an action potential?

Stimulus strength does not change the amplitude or duration of the action potential

Which scientists discovered how action potentials spread down an axon, and what specimen did they study?

Alan Hodgkin and Andrew Huxley studied this by looking at a giant squid axon.

What type of channels did Hodgkin and Huxley find in the giant squid axon, and how do they work?

They found voltage-gated ion channels. They are activated by a change in membrane potential and allow ions to move through.

What everyday analogy can be used to describe the conduction (spread) of an action potential?

It can be compared to a snapshot of dominos falling, where each domino is in a different position at that moment in time.

What does each electrode record during action potential conduction, and what does it show?

Each electrode records an electrical "snapshot". This shows that each patch of membrane is in a different electrical phase at the same moment in time.

What is the direction of action potential conduction in an axon?

It is unidirectional (one way), traveling from the trigger zone to the end of the axon.

What directly causes changes in the membrane potential?

Changes in ion permeability (P) across the cell membrane.

What occurs immediately after the membrane potential reaches the threshold potential (TP)?

A rapid depolarization event that drives the membrane potential up to +30 mV.

What happens immediately after the membrane potential reaches +30 mV?

A rapid repolarization back toward the resting potential (RP) occurs.

What is the equilibrium potential for Sodium (Na+)?

+60 mV.

What is the equilibrium potential for Potassium (K+)?

-90 mV.

What is the typical shift in membrane potential (MP) from resting state to the threshold potential (TP)?

A slow increase from -70 mV to -50 mV.

What molecular mechanism causes the cell to reverse polarity during depolarization?

The opening of voltage-gated sodium channels, which causes a rapid influx of sodium ions into the cell.

What happens to sodium permeability almost immediately after the rapid influx?

There is a rapid decrease in sodium permeability.

What stops the movement of the membrane potential (MP) in a positive direction?

A decrease in sodium permeability

What drives the membrane potential (MP) back down past the threshold to its resting state?

An increase in potassium permeability

What event occurs right before the resting membrane potential (MP) is fully reestablished?

A hyperpolarization event

What is contiguous conduction?

The movement of an action potential along an unmyelinated axon in one direction.

In what direction do sodium Na+ ions diffuse during conduction?

They diffuse in all directions.

How is one-way conduction achieved along the axon?

the refractory period temporarily locks sodium channels behind the action potential

What is the Refractory Period?

a brief recovery time after a response or stimulation

What are the two gates of the voltage-gated Na+ channel?

• Gate 1: Fast gate (activation gate)

• Gate 2: Slow gate (inactivation gate)

What causes the initial increase and subsequent decrease in sodium permeability during an action potential?

• Increase: Opening of gate 1 (fast gate)

• Decrease: Closing of gate 2 (slow gate)

How do both Na+ gates respond when threshold voltage is reached?

Both gates respond at the same time: the fast gate opens immediately, while the slow gate begins to close.

What are the 3 states of the Na+ channel based on its 2 gates?

1. Closed but ready to open (Resting potential)

2. Open (Threshold to peak)

3. Closed/Inactivated (Peak to resting)

What are the voltage ranges for the 3 states of the Na+ channel?

• Closed but ready: -70 mV

• Open: -50 mV to +30 mV

• Closed/Inactivated: +30 mV to -70 mV

What is the state of the membrane when the Na+ channel is closed from peak to resting potential +30 mV to -70 mV?

The membrane is refractory (cannot easily fire another action potential).

What is the Absolute Refractory Period?

The period during which an action potential is actively occurring and another cannot be triggered.

What is the Relative Refractory Period?

The period when channels are closed but capable of reopening if given a strong enough stimulus

What role does potassium K+ permeability play in an action potential?

An increase in K+ permeability leads to repolarization of the membrane.

What are the key features of voltage-gated potassium K+ channels?

They have one gate and are slow to open/close.

How does the movement of potassium K+ affect the cell during repolarization?

Potassium leaves the cell, which helps restore the negative resting membrane potential.

What is saltatory conduction?

The "jumping" propagation of an action potential along a myelinated axon

What channels are located at the Nodes of Ranvier?

Voltage-gated sodium Na+ and potassium K+ channels.

Where does the reversal of membrane potential occur during saltatory conduction?

Only at the nodes—the areas NOT covered by the myelin sheath.

How does the electrical signal travel underneath the myelin sheath?

It propagates passively through the cytoplasm, though the signal can dissipate over distance

What are the two primary propagation traits of an action potential?

They are self-propagating and self-regenerating.

What type of conduction occurs in unmyelinated axons?

Continuous (contiguous) conduction along the entire length of the axon.

What type of conduction occurs in myelinated axons, and why?

Saltatory conduction, because the action potential jumps between nodes due to voltage-gated channels opening/closing at the nodes of Ranvier.

What two factors directly influence the conduction velocity of an action potential?

1. Myelination

2. Axon diameter