Chapter 12.5 Action Potentials

Local current

The movement of positive charges parallel to the inner and outer surfaces of the membrane

Fifth step of continuous propagation

When the sodium ions enter the second segment are spreading laterally, a graded depolarization brings the membrane in segment three to threshold. The cycle is repeated.

Where does an action potential start?

Axon hillock. Only occurs at one site. Depolarizes neighboring sites to threshold level.

Third step of saltatory propagation

The first segment is in repolarization (refractory) state. An action potential is created at the second node of ranvier.

Are action potentials created in the soma?

No, because there is not a sufficient amount of voltage-gated sodium ion channels. They are spread out, so we will not have a coordinated depolarization of the plasma membrane.

Are action potentials created on the dendrites?

No, because they do not have voltage-gated sodium ion channels, and voltage-gated potassium channels.

Threshold

The level of depolarization we need to open the voltage-gated sodium ion channels. The action potential starts here.

Second step of continuous propagation

The membrane potential becomes positive (depolarizes). Occurs for a short amount of time at the action potential's peak. It depolarizes to +30mV.

Refractory period

The period of time where a neuron cannot produce another action potential.

Frequency coding

the coding of stimulus intensity by the frequency of action potentials in a neuron, in which a stronger depolarizing stimulus above threshold causes the action potential frequency to increase

The fourth step of creating an action potential

Sodium channel inactivation occurs when the voltage-gated sodium channels close. Only happens when the membrane potential approaches +30mV. The large amount (influx) of sodium ions causes an action potential to travel down to the synapse. At the same time, the voltage-gated potassium ion channels open. These channels are opened to balance the cell's electrical gradient. These ions move out of the cell, which brings the membrane potential, back to resting level. The cell's charge decreases and depolarization starts. Is referred to as inactivation of sodium ion channels and activation of potassium ion channels starts repolarization.

Where do action potentials spread out?

Spread out along the plasma membrane's surface without decreasing in strength.

First step of continuous propagation

The action potential is created at the initial segment.

The first step of creating an action potential

The axolemma, the axon's cell membrane, has voltage-gated sodium ion and potassium ion channels. These channels are closed when the axolemma is at resting membrane potential. Is at -70mV. Is referred to resting membrane potential. The ions continue to move through leak channels.

What lets the body know the urgency of a stimulus?

The frequency of action potentials. Only occurs if action potentials are sent with the same strength.

Action potential and messages

The message (action potential) is given from one location to another location by many steps. The message is repeated in many steps.

Saltatory propagation

The movement of an action potential along a myelinated axon. Is faster than continuous propagation. The action potential must jump between each node of ranvier during propagation. This occurs because myelin limits the movement of ions across the axolemma.

Relative refractory period

The period of time following an action potential, when it is possible, but difficult, for the neuron to fire a second action potential, due to the fact that the membrane is further from threshold potential (are in hyperpolarization).

The third step of creating an action potential

The sodium ions come into the cell membrane because of the large electrochemical gradient. The membrane potential increases, which results in depolarization (becomes more positive; moves toward 0mV). The sodium ions spread throughout the other areas of the cell. Refers to activation of sodium ion channels and rapid depolarization. Sodium will continue to depolarize all parts of the cell, if the stimulus is strong enough. If the stimulus is not strong enough, sodium will depolarize a small part of the cell and an graded potential develops.

Propagation

The spread of the action potential down an axon, caused by successive changes in electrical charge along the length of the axon's membrane.

The sixth step of creating an action potential

The voltage-gated potassium ion channels close. Too many potassium ions leave the cell, so the cell goes into relative refractory period. The sodium potassium pump and leak channels helps the membrane potential to return to resting membrane potential.

The fifth step of creating an action potential

The voltage-gated sodium ion channels stay inactive until the plasma membrane has repolarized, close to threshold level. Now, these channels stay closed, but are able to open. Occurs during absolute refractory period. The voltage-gated potassium ions start to close when the membrane reaches to the normal resting membrane potential. The potassium ions continue to leave the cell until all of the potassium channels close. Causes a short hyperolarization period. Is due to a lag in the total closing of voltage-gated potassium ion channels. Is in the refractory period. Is referred to time lag in closing all potassium ions channels leads to temporary hyper polarization.

Propagation and axolemma

Think of the axolemma as a series of neighboring segments.

Where does the resting membrane potential occur?

Throughout the neuron.

Action potentials movie analogy

You are in line for the movie theater, and the usher said 15 minutes until the movie starts. You would tell the next person in line and have them repeat the message to the next person. Similar to how an action potential spreads.

Fourth step of continuous propagation

In the first segment, an action potential occurs. In the second segment, depolarization occurs.

Absolute refractory period

In this period, the neuron cannot respond to more stimulation. no matter how strong. The length of time when the voltage-gated sodium ions channels open at threshold until the sodium ion channel inactivation ends. Cannot respond to more stimulation because the channels are open or closed. The first part of the refractory period.

Saltare

Jumping.

What are action potentials?

Nerve Impulses. Electrical impulses that are carried along the lengths of the axons. Always the same regardless of the stimulus. Constantly depolarizing.

Analogy for Action Potentials

No action occurs when you just press the toilet handle. Resting membrane potential. You press the handle, until the water starts to flow. Threshold is reached. The amount of water that is released is independent of how hard or fast you pressed the handle. All-or-none principle. Cannot flush the toliet again, until the tank refills.

Continuous propagation

One type of travel method of when an action potential travels along the surface of unmyelinated axons. There is depolarization for each part of the axon. Is slower than saltatory propagation.

Axon hillock

Only have voltage-gated sodium ion channels. Does not have voltage-gated potassium ion channels. Cannot create an action potential here. If these channels open, we can have a significant amount of depolarization.

All-or-none principle

Refers to the fact that the action potential in the axon occurs either full-blown or not at all. An action potential will either be sent or won't be sent at all. A strong, half, and weak action potential does not exist.

Excitable membrane

A cell membrane that regulates the movement of ions so that an electrical signal can be generated.

Graded depolarization

A depolarization which has not reached threshold. A stimulus that changes the resting membrane potential from -70mV to -62mV.

The second step of creating an action potential

A graded depolarization (stimulus) causes the voltage-gated sodium ion channels to open. The channels opening only occurs at threshold. Usually, happens at the initial segment of the axon. Is at -60mV. The stimulus arrives at the dendrites. Is referred to the depolarization to threshold.

What does a graded potential do to create an action potential?

A graded potential has to depolarize the axolemma to a certain level to stimulate an action potential (threshold).

Second step of saltatory propagation

A local current creates a graded depolarization that brings the axolemma at the neighboring node of ranvier to threshold. Is between -55mV to -60mV.

Third step of continuous propagation

A local current forms when the sodium ions enter the initial segment. These ions move away from the open voltage-gated channels. The graded depolarization brings the axolemma in the second segment to threshold.

Fourth step of saltatory propagation

A local current produces a graded depolarization that brings the third node of ranvier to threshold. The cycle continues.

Neurons and graded potentials

All neurons get information in the form of graded potentials on the soma, and dendrites.

First step of saltatory propagation

An action potential is developed at the initial segment. Is at +30mV

Action potential

An electrical signal that is produced by a graded potential. Travels along the axon's surface to the synapse. Spread changes in the membrane potential that affect the whole excitable membrane, once initiated. Does not diminish as it moves away from the site of stimulation. Also known as nerve impulses.

Voltage-gated sodium channels

Are mostly found on the axon, and synaptic knobs.

The threshold for a normal axon

Between -60mV and -55mV. Corresponds to a depolarization of 10mV to 15mV.

Action potential and movement

Everytime a local current is developed, the action potential only moves forward because the previous segment is still in absolute refractory period.

What must happen before an action potential starts?

Depolarization to threshold must happen first.

How does the action potential travel?

Travels by continuous propagation or saltatory propagation.