I/E: Neurones

Question 1

Describe the resting state of a neurone:

• outside?
• inside?

Why is this?

Answer 1

Membrane is polarised - outside is positively charged compared to the inside.

More positive ions outside the cell than inside.

Question 2

What is the resting potential of a neurone?

Answer 2

-70mV

Question 3

What is the resting potential of a neurone created and maintained by?

Answer 3

Sodium-potassium pumps and potassium ion channels.

Question 4

What does the sodium-potassium pump transport?

Answer 4

Use active transport to move 3 sodium ions out of the neurone for every 2 potassium ions moved in.

Requires ATP.

Question 5

What do potassium ion channels do?

Answer 5

Allow facilitated diffusion of potassium ions out of the neurone, down their concentration gradient.

Question 6

Describe how the resting potential of a neurone is created:

Answer 6

• Sodium-potassium pump moves sodium ions out of neurone, but membrane isn’t permeable to sodium ions, so they can’t diffuse back in.
• Sodium ion electrochemical gradient is created as there are more positive sodium ions outside than inside the cell.
• SPP also moves potassium ions in to the neurone, but membrane is permeable to potassium ions so they diffuse out again.
• Makes outside of the cell positively charged compared to the inside.

Question 7

Describe the permeability of the neurone membrane to sodium ions and potassium ions:

Answer 7

Sodium ions = not permeable

Potassium ions = permeable

Question 8

What does a stimulus trigger the opening of?

Answer 8

Sodium ion channels

Question 9

Describe the 5 events of an action potential:

Answer 9

1. Stimulus excites neurone cell membrane, opening sodium ion channels.
2. Depolarisation - if the potential differences reaches the threshold (-55mV).
3. ​Repolarisation - +30mV sodium ion channels close and potassium ion channels open.
4. ​Hyperpolarisation - K ions are slow to close.
5. Resting potential - ion channels are reset

Question 10

Refractory period

Answer 10

The period of time after an action potential where the neurone cell membrane can’t be excited again.

Question 11

Why does the refractory period exist?

Answer 11

Ion channels are recovering and can’t be made to open.

Na channels are closed during repolarisation and K ion channels are closed during hyperpolarisation.

Question 12

How does an action potential move along the neurone?

Answer 12

As a wave of depolarisation.

• Some Na ions enter the neurone sideways.
• Na ion channels in the next region of the neurone open and Na ions diffuse into that part.
• Causes a wave of depolarisation to travel along the neurone.
• Wave moves away from parts of the membrane in refractory period because these parts can’t fire an action potential.

Question 13

Give three characteristics of a neurone that result from the refractory period:

Answer 13

• Action potentials don’t overlap and are discrete impulses.
• Limited frequency at which nerve impulses can be transmitted.
• Unidirectional.

Question 14

Explain what is meant by the ‘all-or-nothing’ principle:

Answer 14

• Once the threshold is reached, an action potential will always fire with the same change in voltage, no matter how big the stimulus is.
• If the threshold is never reached, an action potential won’t fire.

A bigger stimulus won’t cause a bigger action potential, but it will cause them to fire more frequently.

Question 15

What 3 factors affect the speed of conduction of action potentials?

Answer 15

• Myelination
• Axon diameter
• Temperature

Question 16

How does axon diameter affect the speed of conduction of action potentials?

Answer 16

• Quicker along axons with bigger diameters because there’s less resistance to the flow of ions than in the cytoplasm of a smaller axon.
• So, depolarisation reches other parts of the neurone cell membrane quicker.

Question 17

How does temperature affect the speed of conduction of action potentials?

Answer 17

• Speed of conduction increases as the temperature increases too, because ions diffuse faster.
• Speed only incraeses up to around 40^oC - after this proteins begin to denature and speed decreases.

Question 18

How does myelination affect the speed of conduction of action potentials?

Answer 18

• Myelination increases the speed of conduction, as the impulse jumps from node to node (saltatory conduction).

Question 19

What type of cell makes up the myelin sheath in the PNS?

Answer 19

Schwann cell

Question 20

Describe the structure of a myelin sheath:

Answer 20

• Between Schwann cells are tiny patches of bare membrane - nodes of Ranvier.
• Na ion channels are concentrated at the nodes.

Question 21

Nodes of Ranvier

Answer 21

Tiny patches of bare membrane on the myelin sheath of a neurone, where sodium ion channels are concentrated.

Question 22

How does an action potential travel along a myelinated neurone?

Answer 22

• Depolarisation only happens at the nodes of Ranvier.
• Cytoplasm of neurone conducts enough electrical charge to depolarise the next node.

Saltator conduction.

Question 23

How does an action potential travel along a non-myelinated neurone?

Answer 23

Impulse travels as a wave along the whole length of the axon membrane - this is slower than saltatory conduction.

Question 24

Describe the first step of an action potential:

Answer 24

• Stimulus excites neurone cell membrane, opening sodium ion channels.
• Membrane becomes more permeable to sodium, so Na diffuses into the neurone down the Na ion electrochemical gradient.
• Inside of neurone is less negative.

Question 25

Describe the second step of an action potential:

Answer 25

• Depolarisation - if the potential differences reaches the threshold (-55mV).
• More Na ion channels open, so more Na rapidly diffuses into neurone.

Question 26

Describe the thrid step of an action potential:

Answer 26

• ​​Repolarisation
• +30mV sodium ion channels close and potassium ion channels open.
• Membrane is more permable to K so K ions diffuse out of neurone, down the concentration gradient (starts membrane back to its resting potential).

Question 27

Describe the fourth step of an action potential:

Answer 27

• Hyperpolarisation - K ions are slow to close.
• Slight ‘overshoot’ where too many K ions diffuse out of neurone.
• PD becomes more negative than resting potential.

Question 28

Describe the fifth and last step of an action potential:

Answer 28

• Resting potential - ion channels are reset.
• Sodium-potassium pump returns the membrane to its resting potential and maintains it until it is excited by another stimulus.