chapter 13 p2

Question 1

After the sensory receptor has detected a change in the environment…

Answer 1

an impulse is sent along the neuron by temporarily changing the voltage (potential difference) across the axon’s membrane.
As a result, the axon membrane switches between two states - a resting potential and an action potential.

Question 2

Resting Potential

Answer 2

When a neurone is not transmitting an impulse, the potential difference across its membrane (difference in charge between the inside and outside of the axon) is known as a resting potential.
In this state, the outside of the membrane is more positively charged than the inside of the axon.
The membrane is said to be polarised as there is a potential difference across it - It is normally about -70 mV.

Question 3

Channel Proteins and Ion Movement at resting potential

Answer 3

• The resting potential occurs as a result of the movement of sodium and potassium ions across the axon membrane.
• The phospholipid bilayer prevents these ions from diffusing across the membrane and, therefore, they have to be transported via channel proteins.
• Some of these channels are gated - they must be opened to allow specific ions to pass through them.
• Other channels remain open all of the time allowing sodium and potassium ions to simply diffuse through them.

Question 4

axon membrane at resting potential diagram

Answer 4

Question 5

the following events result in the creation of a resting potential:
Sodium ions (Na+) are actively…

Answer 5

• transported out of the axon whereas potassium ions (K) are actively transported into the axon by a specific intrinsic protein known as the sodium-potassium pump.
• However, their movement is not equal; For every three sodium ions that are pumped out, two potassium ions are pumped in.
• As a result there are more sodium ions outside the membrane than inside the axon cytoplasm, whereas there are more potassium ions inside the cytoplasm than outside the axon.
• Therefore, sodium ions diffuse back into the axon down its electrochemical gradient (this is the name given to a concentration gradient of ions), whereas potassium ions diffuse out of the axon.

Question 6

the following events result in the creation of a resting potential: However, most of the ‘gated’ sodium ion channels are closed, preventing the ….

Answer 6

movement of sodium ions, whereas many potassium ion channels are open, thus allowing potassium ions to diffuse out of the axon.
Therefore, there are more positively charged ions outside the axon than inside the cell.
This creates the resting potential across the membrane of -70 mV, with the inside negative relative to the outside.

Question 7

Generation of Action Potential:

Answer 7

When a stimulus is detected by a sensory receptor, the energy of the stimulus temporarily reverses the charges on the axon membrane.
As a result the potential difference across the membrane rapidly changes and becomes positively charged at approximately +40m V.
This is known as depolarisation - a change in potential difference from negative to positive.

Question 8

Repolarization and Return to Resting State:

Answer 8

As the impulse passes repolarisation then occurs - a change in potential difference from positive back to negative.
The neurone returns to its resting potential.
An action potential occurs when protein channels in the axon membrane change shape as a result of the change of voltage across its membrane.
The change in protein shape results in the channel opening or closing. These channels are known as voltage-gated ion channels.

Question 9

change of potential across axon membrane during an axon potential diagram

Answer 9

Question 10

The numbers on the graph correspond to the sequence of events that take place during an action potential:
1

Answer 10

The neurone has a resting potential - it is not transmitting an impulse.
Some potassium ion channels are open (mainly those that are not voltage-gated) but sodium voltage-gated ion channels are closed.

Question 11

The numbers on the graph correspond to the sequence of events that take place during an action potential:
2

Answer 11

The energy of the stimulus triggers some sodium voltage-gated ion channels to open, making the membrane more permeable to sodium ions.
Sodium ions therefore diffuse into the axon down their electrochemical gradient.
This makes the inside of the neurone less negative.

Question 12

The numbers on the graph correspond to the sequence of events that take place during an action potential:
3

Answer 12

This change in charge causes more sodium ion channels to open, allowing more sodium ions to diffuse into the axon.
This is an example of positive feedback.

Question 13

The numbers on the graph correspond to the sequence of events that take place during an action potential:
4

Answer 13

When the potential difference reaches approximately +40 mV the voltage-gated sodium ion channels close and voltage-gated potassium ion channels open.
Sodium ions can no longer enter the axon, but the membrane is now more permeable to potassium ions.

Question 14

The numbers on the graph correspond to the sequence of events that take place during an action potential:
5

Answer 14

Potassium ions diffuse out of the axon down their electrochemical gradient. This reduces the charge, resulting in the inside of the axon becoming more negative than the outside.

Question 15

The numbers on the graph correspond to the sequence of events that take place during an action potential:
6

Answer 15

Initially, lots of potassium ions diffuse out of the axon, resulting in the inside of the axon becoming more negative (relative to the outside) than in its normal resting state.
This is known as hyperpolarisation.
The voltage-gated potassium channels now close.
The sodium-potassium pump causes sodium ions to move out of the cell, and potassium ions to move in.
The axon returns to its resting potential - it is now repolarised.

Question 16

Propagation of Action Potentials:

Answer 16

A nerve impulse is an action potential that starts at one end of the neurone and is propagated along the axon to the other end of the neurone.
The initial stimulus causes a change in the sensory receptor which triggers an action potential in the sensory receptor, so the first region of the axon membrane is depolarised.
This acts as a stimulus for the depolarisation of the next region of the membrane.
The process continues along the length of the axon forming a wave of depolarisation.

Question 17

Sodium Ion Movement and Repolarization:
Propagation of action potentials:

Answer 17

Once sodium ions are inside the axon, they are attracted by the negative charge ahead and the concentration gradient to diffuse further along inside the axon, triggering the depolarisation of the next section.
The region of the membrane which has been depolarised as the action potential passed along now undergoes repolarisation to return to its resting potential.

Question 18

Role of Refractory Period

Answer 18

• After an action potential there is a short period of time when the axon cannot be excited again, this is known as the refractory period.
• During this time, the voltage-gated sodium ion channels remain closed, preventing the movement of sodium ions into the axon.
• A refractory period is important because it prevents the propagation of an action potential backwards along the axon as well as forwards.
• The refractory period makes sure action potentials are unidirectional.
• It also ensures that action potentials do not overlap and occur as discrete impulses.

Question 19

Propagation of action potentials diagram

Answer 19

Question 20

change of channel proteins in axon during action potentials

Answer 20

Question 21

Saltatory conduction:
Myelination and Axonal Depolarisation:

Answer 21

Myelinated axons transfer electrical impulses much faster than non-myelinated axons.
This is because depolarisation of the axon membrane can only occur at the nodes of Ranvier where no myelin is present.
Here the sodium ions can pass through the protein channels in the membrane.

Question 22

Saltatory Conduction Mechanism

Answer 22

Longer localised circuits therefore arise between adjacent nodes.
The action potential then jumps’ from one node to another in a process known as saltatory conduction.
This is much faster than a wave of depolarisation along the whole length of the axon membrane.

Question 23

Energy Efficiency in Saltatory Conduction

Answer 23

Every time channels open and ions move it takes time, so reducing the number of places where this happens speeds up the action potential transmission.
Long-term, saltatory conduction is also more energy efficient.
Repolarisation uses ATP in the sodium pump, so by reducing the amount of repolarisation needed, saltatory conduction makes the conduction of impulses more efficient.

Question 24

Saltatory Conduction Mechanism
diagram

Answer 24

Question 25

Apart from myelination, two other factors affect the speed at which an action potential travels:

Answer 25

Axon diameter - the bigger the axon diameter, the faster the impulse is transmitted.
This is because there is less resistance to the flow of ions in the cytoplasm, compared with those in a smaller axon.

Temperature - the higher the temperature, the faster the nerve impulse.
This is because ions diffuse faster at higher temperatures.
However, this generally only occurs up to about 40°C as higher temperatures cause the proteins (such as the sodium-potassium pump) to become denatured.

Question 26

All-or-nothing principle:
p1

Answer 26

Nerve impulses are said to be all-or-nothing responses.
A certain level of stimulus, the threshold value, always triggers a response.
If this threshold is reached an action potential will always be created.
No matter how large the stimulus is, the same sized action potential will always be triggered.
If the threshold is not reached, no action potential will be triggered.

Question 27

All-or-nothing principle:
p2

Answer 27

The size of the stimulus, however, does affect the number of action potentials that are generated in a given time.
The larger the stimulus the more frequently the action potentials are generated.
The effect of the size of the stimulus on the frequency of nerve impulses can be seen in Figure 6.

Question 28

Measuring action potentials

Answer 28

The presence and frequency of action potentials can be recorded using an oscilloscope.

The diagram below shows some sample data collected in this manner, showing two action potentials: