3.6.2.1 Nerve impulses (A-level only)

Question 1

How is the resting potential maintained across a neurone cell membrane?

Answer 1

1. Sodium-potassium pumps move sodium ions out of the neurone but membrane is impermeable to Na+ so they can’t diffuse back in
   => creates a Na+ electrochemical gradient as there are more positive Na+ ions outside of cell than inside.
2. K+ also moved into neurone by pump, but the membrane is permeable to K+ so they diffuse back out via K+ ion channels.
3. => outside of the cell is more positively charged compared to the outside.

Question 2

How does a stimulus trigger an action potential?

Answer 2

1. Stimulus - excites neurone cell membrane, causing Na+ ion channels to open —> membrane becomes more permeable to Na+ - diffuse down electrochemical gradient => inside of neurone now less negative.
2. Depolarisation - If potential difference reaches threshold (-55mV), more Na+ ion channels open —> more Na+ diffuse rapidly into the neurone.
3. Repolarisation - at a potential difference of around +30mV, the Na+ channels close and K+ channels open - membrane more permeable to K+ which diffuse out of the neurone down the K+ concentration gradient => membrane begins to return to resting potential.
4. Hyperpolarisation - K+ channels are slow to close - slight overshoot where too many K+ diffuse out of neurone => pd becomes more negative than resting potential.
5. Resting potential - the ion channels are reset. Na+/K+ pump returns membrane to its resting potential and maintains it until membrane excited by another stimulus.

Question 3

What is the refractory period?

Answer 3

= Period in which cell membrane cannot be excited again immediately after an action potential —> because ion channels are recovering and can’t be made to open —> Na+ channels closed during repolarisation and K+ channels closed after hyperpolarisation.

Question 4

What is the significance of the refractory period?

Answer 4

Acts as a time delay between one action potential and the next:

1. Action potentials don’t overlap, but pass along as discrete (separate) impulses.
2. Limit to frequency at which the nerve impulses can be transmitted.
3. Action potentials are unidirectional (only travel in one direction).

Question 5

Draw a graph of potential difference across membrane (y) vs time (x). Label the 5 stages.

Answer 5

1. Stimulus just as pd increases.
2. Depolarisation on steep +ive gradient.
3. Repolarisation on steep -ve gradient.
4. Hyperpolarisation on dip.
5. Resting potential when back to normal level.

Question 6

How does the action potential travel along the neurone?

Answer 6

1. Some Na+ that enter the neurone diffuse sideways.
2. Na+ channels in the next region of the neurone open as a result and Na+ diffuse into that part
3. Causing a wave of depolarization to travel along the neurone.
4. Wave moves away from the parts of the membrane in the refractory period as these parts can’t fire an action potential.

Question 7

Comment on the all or nothing nature of action potentials.

Answer 7

1. Once the threshold level is reached, an action potential will always fire with the same change in voltage, no matter how big the stimulus is.
2. If the threshold isn’t reached an action potential won’t fire => all or nothing nature.
3. Bigger stimulus won’t cause a bigger action potential, but it will cause them to fire more frequently.

Question 8

How does myelination affect the speed of conduction of an action potential?

Answer 8

Myelination = having a myelin sheath - electrical insulator on exterior of neurones.
In the PNS, sheath made of Schwann cells.
1. Between Schwann cells are patches of bare membrane - Nodes of Ranvier - where Na+ ion channels are concentrated.
2. In myelinated neurones, depolarization only happens at nodes, where Na+ can get through membrane.
3. Neurone’s cytoplasm conducts enough electrical charge to depolarise the next node - impulse can jump from node to node.
=> salutatory conduction - very fast.
BUT
4. In non-myelinated neurones, the impulse travels as a wave along the whole length of the axon membrane - depolarisation along whole length of the membrane.
5. Still fast, but slower than salutatory conduction.

Question 9

How does the axon diameter affect the speed of conduction of an action potential?

Answer 9

1. Action potentials are conducted more quickly along axons with bigger diameters as less resistance to ion flow than in cytoplasm of smaller ion.
2. With less resistance, depolarisation reaches other parts of the neurone cell membrane more quickly.

Question 10

How does temperature affect the speed of conduction of an action potential?

Answer 10

1. Speed of conduction increases as the temperature increases, as ions diffuse more quickly.
2. Speed only increases up to around 40C - after that the proteins begin to denature and the speed decreases.