3.5 - Action Potential

Question 1

How do neurons communicate with one another?

(Interneuronal communication)

Answer 1

Intraneuronal communication is via “action potentials”

Question 2

How do neurons communicate within a single neuron?

(Intraneuronal communication)

Answer 2

Interneuronal communication is via neurotransmitter release into the synapse

Question 3

What method of communication are action potentials?

Answer 3

Electrical

Question 4

What method of communication are neurotransmitters?

Answer 4

Neurotransmitters are chemical and bind to receptors, causing cellular changes

Question 5

What is typically the charge inside AND outside of a neuron?

Answer 5

The inside of a neuron is more NEGATIVE and the outside of a neuron is more POSITIVE

This is because of the Na/K Pump

Question 6

What are the three types of channels which allow neurons to send information?

Answer 6

• Ligand-gated
• Voltage-gated (unnecessary for this class)
• Mechanically-gated (unnecessary for this class)

Question 7

What is a ligand-gated channel?

Answer 7

A ligand-gated channel is a protein channel that opens or closes when a signaling molecule (typically a neurotransmitter from another neuron) binds to it

• Also called ionotropic receptors

Question 8

What is typically the voltage inside of a neuron?

Answer 8

The inside of a neuron is normally at -70mV

Question 9

What happens when a stimulus allows Na+ to enter the cell?

Answer 9

If a stimulus allows Na+ to enter the cell, the voltage will become positive

• This also allows K+ to leave the cell

Question 10

What happens when enough K+ leaves the cell?

Answer 10

Eventually enough K+ leaves the cell to cause the inside to go back to being negative

• In fact, loss of K+ from the cell is so much that voltage will go lower than -70mV (ex: -90), “overshooting” the desired voltage

Question 11

What is the minimum voltage threshold that must be reached in order to activate the action potential?

Answer 11

-55mV

Question 12

When do Na+ channels start opening?

Answer 12

Some Na+ channels open when there is a stimulus (like a neurotransmitter from another neuron)

All Na+ channels open when -55mV is reached

Question 13

Why can action potential only go in one direction?

Answer 13

Think of an action potential like a wave moving along a string. Once the wave passes a point, that area needs some time to settle before it can handle another wave. In a similar way, after an action potential occurs in a part of a nerve cell, that part needs a break before it can handle another action potential. This helps the nerve signal move in one direction, like a domino effect.

Question 14

What happens to the space in front of the ligand-gated channels in a neuron?

Answer 14

The space AHEAD of these channels DO get activated, allowing the electrical signal to move in one direction along the axon

• The dominos keep falling forward, pushing the signal along

Question 15

What are the 5 (6) stages of action potential?

Answer 15

0. Resting potential
1. Depolarization to threshold
2. Rapid depolarization
3. End of depolarization, start of repolarization
4. End of repolarization, start of hyperpolarization
5. Return to resting potential

Question 16

What happens during resting potential?

Answer 16

Neuron at REST
- Resting membrane potential at -70mV; being maintained by active Na+/K+ pump
- All gated channels closed (gated Na+ and gated K+)

Question 17

What happens during depolarization to threshold?

Answer 17

Local/graded potential triggers action potential
- Stimulus opens CHEMICAL/ MECHANICAL gated channel in dendrites, lets in Na+
- Membrane potential starts to move towards ZERO: If it doesn’t reach -55mV, system resets. If it hits THRESHOLD at -55mV, gated Na+ channels in axon hillock OPEN, action potential STARTS

Question 18

What happens during rapid depolarization?

Answer 18

Depolarization spreads
- As Na+ enters cell more channels open, more depolarization
- Membrane potential hits ZERO and rises
- This continues until +30mV is reached

Question 19

What happens during end of depolarization/start of repolarization?

Answer 19

Depolarization stops/Repolarization starts At +30mV:
- Voltage gated Na+ channels INACTIVATE (no more Na+ can enter)
- Voltage gated K+ channels OPEN
(K+ starts to leave the cell, loss of + charge starts to DROP membrane potential → REPOLARIZATION)

Question 20

What happens during end of repolarization/start of hyperpolarization?

Answer 20

Repolarization leads to hyperpolarization
- Open voltage gated K+ channels continue to allow escape of K+
- Membrane potential drops to -70mV and then OVERSHOOTS until it hits -90mV → HYPERPOLARIZATION (voltage gated K+ channels INACTIVATE, voltage gated Na+ channels RESET to closed)

Question 21

What happens during the return to resting potential?

Answer 21

Reset to RMP of -70mV
- Action of Na+/K+ pump pushes Na+ back out and pulls K+ back in to re-establish resting membrane potential
- All voltage gated channels now closed, ready for next round

Question 22

How does the message travel during action potential if an axon is UNMYELINATED?

Answer 22

If an axon is UNMYELINATED depolarization/ repolarization wave travels along the entire length of the axon patch by patch

• Called continuous propagation
  ** Speed ~1m/s (slow)
  *** Seen in short axons

Question 23

How does the message travel during action potential if an axon is MYELINATED?

Answer 23

In a myelinated axon, the ion current during an action potential at one node of Ranvier spreads along the interior of the axon to the next node, triggering an action potential there. The action potential thus jumps from node to node as it propagates along the axon

• Called saltatory propragation (jumping conduction)
  ** Signal leaps down axon at ~18-140m/s
  *** Seen in all long axons (sensory and motor)

Question 24

What is summation?

Answer 24

When a neuron gets activated (to start its action potential), it needs a certain amount of stimulation

This stimulation is called “summation” since you must get a minimum level of voltage change

Question 25

What are the 2 forms of summation?

Answer 25

• Spatial summation
• Temporal summation

Question 26

What is spatial summation?

Answer 26

Spatial summation is when multiple neurons send a message and additively provide enough activation (like having many people shovel dirt to fill a hole)

Question 27

What is temporal summation?

Answer 27

Temporal summation is when one neuron sends a message continuously over a period of time (like having one person shovel a lot to fill the hole)

Question 28

What happens when the signal has reached the end of the axon?

Answer 28

At the end of the axon, the cell will transmit the signal to neighbouring neurons through the synapse

Question 29

What are the two different forms of synapse?

Answer 29

• Electrical synapse
• Chemical synapse

Question 30

What is an electrical synapse?

Answer 30

An electrical synapse is when a neuron directly sends ions into another neuron, to initiate its action potential

• Extremely quick way to send information between adjacent neurons
  ** Downside is they are not well-regulated, since you don’t have different “gated” channels

Question 31

What is a chemical synapse?

Answer 31

A chemical synapse is where neurotransmitters are released from one axon to bind the receptors on others neurons’ dendrites

• When an action potential reaches the axon terminal, it causes voltage-gated Calcium channels to open, allowing Ca2+ to enter into the cell
  ** In-flow of calcium allows vesicles to fuse will the cell membrane to release neurotransmitters (exocytosis)

Question 32

What are the steps of synaptic transmission, and how does it work?

Answer 32

Action Potential Travels Down Axon:
- Imagine a tiny electric wave (action potential) traveling down a long wire (axon) in your brain.
Synaptic Vesicles Release

Neurotransmitter:
- When the wave reaches the end (axon termini), it triggers the release of little messenger balloons (synaptic vesicles) filled with chemicals (neurotransmitters).

Neurotransmitter Causes Depolarization:
- The balloons burst, releasing chemicals into a gap (synaptic cleft). These chemicals float across the gap and open tiny gates (Na+ channels) in the next wire (postsynaptic membrane).
This opening of gates makes the next wire a bit electric (depolarizes it).

If Enough Electricity, New Action Potential:
- If the electric charge is strong enough (hits -55 mV), it triggers a new electric wave (action potential) in the next wire.

Deactivation and Reset:
- The chemicals are taken back or broken down to stop the electric effect.
- Everything gets ready for the next electric message. More messenger balloons (neurotransmitters) are made for the next round.