Nerve impulses (A-level only)

Question 1

Neurones

Answer 1

Cells that transmit information from receptors to effectors

Question 2

3 main types of neurones

Answer 2

Sensory
Motor
Relay

Question 3

Sensory neurone

Answer 3

Sensory neurones carry nervous impulses from receptors (e.g. pressure receptors) into the Central Nervous Systems (CNS).

Question 4

Motor neurone

Answer 4

Motor neurones carry impulses from the CNS to effector organs (e.g. muscles or glands).

Question 5

Relay neurone

Answer 5

Relay neurones are intermediate neurones.

Relay neurones receive impulses from a sensory neurone and relay them to motor neurones.

Question 6

Basic stucture of neurone

Answer 6

Neurones can be myelinated or non-myelinated but all neurones have the following basic structures:

Dendrites - carries nervous impulses towards a cell body.

Axons - carries nervous impulses away from the cell body.

Cell body - where the nucleus is normally located.

Question 7

Myelinated motor neurone

Answer 7

Motor neurones in vertebrates are usually myelinated.

Schwann cells are wrapped around the axon of the neurone.

These cells form the myelin sheath.

Gaps between adjacent Schwann cells are called nodes of Ranvier.

Question 8

Potential difference

Answer 8

At resting state there is a difference in charge across the neurone membrane: the inside of the neurone is more negatively charged than outside.

This is because there are more positive ions outside the cell than inside.

The difference in charge is called a potential difference.

Question 9

Sodium-potassium pumps

Answer 9

The resting potential is maintained by sodium-potassium pumps in the neurone membrane.

Three Na+ ions are actively transported out of the neurone by the pumps for every two K+ ions that are transported in.

This leads to a build-up of positive ions outside the cell.

Question 10

Potassium ion channels

Answer 10

There are potassium ion channels in the neurone membrane.

This means it is permeable to K+ ions.

When K+ ions are transported into neurones, they can diffuse back out.

The neurone membrane is also impermeable to Na+ ions so the ions cannot diffuse back into the cell after they have been transported out.

Question 11

Resting potential

Answer 11

Together the action of sodium-potassium pumps and potassium ion channels leads to a potential difference across the neurone membrane.

This potential difference is called the resting potential.

The neurone is said to be polarised.

Resting potential is about −70mV.

Question 12

Repolarisation

Answer 12

When a resting neurone is stimulated, its membrane experiences a change in potential difference.

This change is called repolarisation.

Question 13

Stages of repolarisation

Answer 13

Stimulation
Depolarisation
All-or-nothing
Repolarisation
Hyperpolarisation
Resting potential

Question 14

Stimulation

Answer 14

Na+ ion channels in the cell membrane open when a neurone is stimulated.

Na+ ions flood into the neurone.

The potential difference across the membrane changes to become more positive inside the neurone.

Question 15

Depolarisation

Answer 15

If the potential difference increases above the threshold value (about −55mV) then the membrane will become depolarised.

More sodium channels open and there is a sharp increase in potential difference to about +30mV.

Question 16

All-or-nothing

Answer 16

Depolarisation is an all-or-nothing response.

If the potential difference reaches the threshold, depolarisation will always take place and the change in potential difference will always be the same.

If the stimulus is stronger, action potentials will be produced more frequently but their size will not increase.

Question 17

Repolarisation

Answer 17

After the neurone membrane has depolarised to +30mV, the sodium ion channels close and potassium ion channels open.

K+ ions are transported back out of the neurone and the potential difference becomes more negative.

This is called repolarisation.

Question 18

Hyperpolarisation

Answer 18

There is a short period after repolarisation of a neurone where the potential difference becomes slightly more negative than the resting potential.

This is called hyperpolarisation.

Hyperpolarisation prevents the neurone from being restimulated instantly.

This is called the refractory period.

Question 19

Resting potential

Answer 19

After the refractory period, the potassium ion channels close and the membrane returns to its resting potential.

The process where a neurone is depolarised and returns to resting potential is called an action potential.

Question 20

4 stages of an action potential

Answer 20

Sodium ions
Sodium ion channels
Wave of depolarisation
Refractory period

Question 21

Sodium ions

Answer 21

When an action potential is generated, there are more Na+ ions inside the neurone than outside.

Some of these Na+ ions diffuse sideways along the neurone axon.

Question 22

Sodium ion channels

Answer 22

The presence of Na+ ions creates a change in potential difference further along the neurone membrane.

If this reaches the threshold value, sodium ion channels at this part of the membrane open.

Na+ ions diffuse into the neurone.

This part of the neurone now becomes depolarised.

Question 23

Wave of depolarisation

Answer 23

Na+ diffuse all along the neurone in this way.

Depolarisation takes place along the neurone membrane.

This creates a wave of depolarisation.

Question 24

Refractory period

Answer 24

The period of hyperpolarisation in an action potential is called the refractory period.

The ion channels are recovering during the refractory period.

This means an action potential cannot be stimulated again instantly.

This ensures that the wave of depolarisation travels in one direction.

Question 25

Factors that speed up the transmission of nerve impulses include:

Answer 25

Myelination
Temperature
Axon diameter

Question 26

Myelination

Answer 26

Schwann cells wrap around the axon of neurones to create a myelin sheath.

The myelin sheath acts as an electrical insulator because it is impermeable to ions (Na+ and K+).

Depolarisation and action potentials cannot occur at the myelinated parts of the axon and can only occur in the gaps between (the nodes of Ranvier).

The nervous impulse jumps from one node to the next.

This is called saltatory conduction.

Question 27

Temperture

Answer 27

An increase in temperature increases kinetic energy.

Ions move across the membrane more rapidly when they have more kinetic energy.

Question 28

Axon diameter

Answer 28

Giant axons are found in the giant squid and allow it to have a rapid escape response.

Greater axon diameter means there is a greater surface area for the movement of ions across the cell membrane.