Nerve & Muscle - Action Potential

Question 1

What are action potentials?

Answer 1

The basis of neuronal communication. They are electrical impulses passing along axons

Question 2

Axons are biologically…

Answer 2

active so the information sent down the axon is amplified to move along the full length of the axon

Question 3

What are voltage gated sodium channels designed for?

Answer 3

So that the signal is brief

Question 4

What is the action of the voltage gated sodium channel at resting membrane potential (-70mV)?

Answer 4

The activation gate is closed and the inactivation gate is open so the channel is closed

Question 5

What is the action of the voltage gated sodium channel at the threshold (-60mV)?

Answer 5

Activation and inactivation gates are both open so the channel is open and sodium is able to enter

Question 6

What is the action of the voltage gated sodium channel at +30mV?

Answer 6

The inactivation gate closes so that the channel becomes inactive

Question 7

What occurs at the axon hillock?

Answer 7

The decision point

Question 8

What happens at the decision point?

Answer 8

If enough signal for an action potential is received, the sodium channels in the initial segment of the axon will open.

Question 9

What happens before the action potential reaches the initial segment?

Answer 9

A graded potential spreads through the axon hillock

Question 10

What is AP propagation?

Answer 10

When voltage-gated sodium channels open at the axon initial segment

Question 11

What is step 1?

Answer 11

Depolarisation to threshold

Question 12

What happens during depolarisation to threshold?

Answer 12

A local change in membrane potential occurs sufficient to depolarise the cell and open voltage gated sodium channels

Question 13

What is step 2?

Answer 13

Activation of sodium ion channels and rapid depolarisation

Question 14

What happens during activation of sodium ion channels and rapid depolarisation?

Answer 14

Voltage gated sodium channels open causing rapid depolarisation

Question 15

What is step 3?

Answer 15

Inactivation of sodium channels and activation of potassium ion channels

Question 16

What happens during inactivation of sodium channels and activation of potassium ion channels?

Answer 16

The voltage gated sodium channels become inactive by closing the inactivation gate and the voltage gated potassium channels open. This causes repolarisation of the membrane which occurs slower than the depolarisation.

Question 17

What is step 4?

Answer 17

Potassium ion channels close

Question 18

What happens when potassium ion channels close?

Answer 18

Membrane potential returns to resting and both the voltage gated sodium and potassium channels close. However, a slight hyperpolarization occurs before resting membrane potential is reached because of the slow response time of the voltage gated potassium channel

Question 19

What is the absolute refractory period?

Answer 19

The period when a new action potential cannot be received

Question 20

Where does the absolute refractory period start and finish?

Answer 20

Begins when the threshold membrane potential is reached and ends around the middle of repolarisation

Question 21

What is the relative refractory period?

Answer 21

The period when a new action potential can be received

Question 22

Where does the relative refractory period begin and end?

Answer 22

Begins around the middle of the repolarisation and ends when the resting membrane potential has been restored

Question 23

What is in step one of action potential propagation of an unmyelinated axon?

Answer 23

As an action potential develops at the initial segment, the membrane potential at this site depolarises to 30mV

Question 24

What is in step two of action potential propagation of an unmyelinated axon?

Answer 24

A the sodium ions entering spread away from the voltage gated channels, a graded depolarisation quickly brings the membrane in segment 2 to threshold

Question 25

What is in step three of action potential propagation of an unmyelinated axon?

Answer 25

An action potential develops in segment 2. The initial segment begins repolarisation and is refractory

Question 26

What is in step four of action potential propagation of an unmyelinated axon?

Answer 26

As the sodium ions entering at segment 2 spread laterally, a graded depolarisation quickly brings the membrane in section 3 to a threshold

Question 27

What does conduction velocity in an axon depend on?

Answer 27

The size of the organism

Question 28

How does the axon conduct if the organism is small?

Answer 28

Slowly, for example approximately 0.5m/s in a fly

Question 29

How does the axon conduct if the organism is large?

Answer 29

Fast, for example approximately 50-70m/s in mammals`

Question 30

How does nature improve action potential conduction velocity?

Answer 30

Myelination of axons

Question 31

What happens in step one of action potential propagation of myelinated axons?

Answer 31

An action potential develops at the initial segment. Then the action potential jumps from node to node to node, skipping the myelinated areas

Question 32

What happens in step two of action potential propagation of myelinated axons?

Answer 32

A local current produces a graded depolarisation that brings the axolemma at node 1 to threshold

Question 33

What is the axolemma?

Answer 33

The plasma membrane of an axon

Question 34

What happens in step three of action potential propagation of myelinated axons?

Answer 34

An action potential develops at node 1. The initial segment begins depolarisation and is now refractory

Question 35

What happens in step four of action potential propagation of myelinated axons?

Answer 35

A local current produces a graded depolarisation the brings the axolemma at node 2 to threshold

Question 36

What do myelinated areas act as?

Answer 36

Excellent conductors

Question 37

Where does chemical synaptic transmission occur?

Answer 37

At the plasma membrane of the post-synaptic neuron

Question 38

What happens first at the pore during chemical synaptic transmission?

Answer 38

ACh binds the the specific binding site in the subunit

Question 39

When is the pore closed?

Answer 39

When there is no ACh bound to the subunit of the pore

Question 40

When is the pore open?

Answer 40

When there is ACh bound to the subunit of the pore

Question 41

What passes through the pore?

Answer 41

Sodium until it closes and then waits for another ACh to bind so that it can reopen

Question 42

Where is the post-synaptic neuron?

Answer 42

At the receiving end of the action potential

Question 43

What does the pre-synaptic neuron do?

Answer 43

Release the action potential

Question 44

What do regulatory neurons do?

Answer 44

Regulate the pre-synaptic neuron

Question 45

Neurons can be both...

Answer 45

pre and post synaptic at the sometime if they are in a sequence

Question 46

What is calcium?

Answer 46

A cation

Question 47

How much calcium is in the ECF?

Answer 47

2-2.5mM (1.3mM in the fluid but the rest is bound to things such as surfaces and proteins)

Question 48

What is calcium important for?

Answer 48

Not just bones but also nervous system health and function

Question 49

What is the first step of chemical synaptic transmission?

Answer 49

Action potential triggers the opening of the voltage-gated calcium channels into the axon terminal

Question 50

What happens after the voltage-gated calcium channels are opened?

Answer 50

Calcium ions diffuse into the axon terminal and trigger synaptic vesicles to release ACh by exocytosis. This is also helped by proteins

Question 51

What happens once the ACh has been released?

Answer 51

ACh diffuses across the synaptic cleft, binds to ACh gated sodium ion channels and produces a graded depolarization

Question 52

What happens once the graded depolarisation has occurred?

Answer 52

The depolarisation ends as ACh is broken down into acetate and choline boy AChe

Question 53

What happens after ACh is broken down?

Answer 53

The axon terminal reabsorbs choline from the synaptic cleft and uses it to synthesise new molecules of ACh

Question 54

Explain depolarization

Answer 54

Occurs when the chemical stimulus opens sodium ion channels. When the stimulus is removed and excess sodium ions are transported out of the cytosol repolarization occurs

Question 55

What is another name for depolarisation?

Answer 55

Excitatory Post Synaptic Potential (EPSP)

Question 56

Explain hyperpolarization

Answer 56

Occurs when the chemical stimulus opens potassium ion channels. When the stimulus is removed repolarization occurs

Question 57

What is another name for hyperpolarization?

Answer 57

Inhibitory Post Synaptic Potential (IPSP)

Question 58

Where do EPSP's always reach the threshold?

Answer 58

At the neuromuscular junction

Question 59

How can EPSP's reach the threshold if they don't individually?

Answer 59

If the EPSP doesn't meet the threshold but another one arrives at the same time they may sum to reach the threshold and cause an action potential

Question 60

What happens if an EPSP and an IPSP arrive at the same time?

Answer 60

There may be no action potential produced

Question 61

How would synaptic transmission be described at the NMJ?

Answer 61

Excitatory and absolute

Question 62

Why is there no axon hillock at the NMJ?

Answer 62

Because the EPSP always exceeds the threshold and therefore no IPSP/EPSP summation occurs

Question 63

What is temporal summation?

Answer 63

Summation in time

Question 64

Explain temporal summation

Answer 64

If the first stimulus doesn't fade away before the second stimulus arrives the two will add to hopefully reach the threshold for causing an action potential

Question 65

How can temporal summation be maximised?

Answer 65

The less time between the two stimuli arriving, the greater the summation and likelihood of reaching the threshold

Question 66

What is spatial summation?

Answer 66

Summation in space

Question 67

Explain spatial summation

Answer 67

If stimuli arrive close enough together at the same time they will add