Nervous System

Question 1

Nerve impulse

Answer 1

• the change in electrical charge that moves along a neurone in response to a stimulus

Question 2

Why does the phospholipid bilayer of the axon p,as a membrane prevent Na+ and K+ ions diffusing across it?

Answer 2

• intrinsic proteins called gated ion channels in the membrane can be opened or closed to allow Na+ and K+ ions to move in or out of the axon

Question 3

What does leaking of the channels cause?

Answer 3

• Na+ ions and K+ ions to leak out down their concentration gradients

Question 4

Which ion leaks in?

Answer 4

• Na+

Question 5

Which ion leaks out?

Answer 5

• K+

Question 6

The membrane is 100x more permeable to potassium K+ ions. What effect would that have on the charged inside or outside of the axon?

Answer 6

• outside becomes more positively charged and inside of axon more negatively charged
• creates a large electrical gradient as well as a chemical gradient
• as more K+ ions diffuse out, inside becomes more negative and try to move back in again, reaching equilibrium

Question 7

Sodium potassium pump

Answer 7

• intrinsic protein that uses ATP to actively transport 2K+ ions into the axon and transport 3Na+ ions out of the axon (more positively charged ions moved out of the axon)
• combined with leakage, results in an overall negative charge inside the axon (-70mv resting potential)

Question 8

Resting potential

Answer 8

• the electrical potential across the plasma membrane of a cell that is not conducting an impulse

Question 9

From resting potential to action potential (1)

Answer 9

• action potential occurs when a neurone sends information down axon
• at resting potential (-70mv) some K+ voltage gated channels are open and Na+ voltage gated channels are closed

Question 10

Depolarisation (2)

Answer 10

-Stimulus causes the membrane at one part of the neurone to increase in permeability to Na+.
- Sodium voltage-gated channels open and Na+ ions enter the axon down their electrochemical gradient by diffusion.
- This causes resting potential to move towards 0mV. When stimulated therefore, the membrane potential is briefly depolarised.

Question 11

Depolarisation pt 2 (3)

Answer 11

• When depolarisation reaches –30mV (stimulus to threshold value), more Na+ channels open (for 0.5 ms).
• This causes even more Na+ ions to rush in by diffusion- inside the cell becomes more positively charged

Question 12

Peak of the action potential (4)

Answer 12

• Once the action potential of around +40mV has been established, the voltage gates on the Na+ channels close.
• This causes K+ ion channels to begin to open.

Question 13

Repolarisation and hyperpolarisation (5)

Answer 13

• K+ rush out down their electrochemical gradient 🡪 making the inside the cell more negative. Since this restores the original polarity, it is called repolarisation.
• There is a slight ‘undershoot’ in the movement of K+ meaning that the inside of the axon actually becomes more negative than usual. This is called hyperpolarisation. When the K+ gated channels close, this triggers a wave of depolarisation in adjacent channels – this is how impulse spreads. The Na+/K+ pump restores the resting potential in this part of the axon.

Question 14

Direction of travel of action potential (depolarising)

Answer 14

• depolarising, resting, resting

Question 15

Direction of travel of action potential (repolarising)

Answer 15

• repolarising, depolarising, resting

Question 16

Direction of travel of action potential (resting)

Answer 16

• resting, repolarising, depolarising

Question 17

Why do myelinated neurones conduct impulses faster than unmyelinated ones?

Answer 17

• action potential only occurs at the nodes of ranvier
• nerve impulse jumps from node to node (saltatory conduction)
• so action potential does not need to travel along the whole length of the axon

Question 18

What is another factor that influences the speed of impulse?

Answer 18

• diameter of axon
• greater the diameter, greater the speed of conductance due to less leakage of ions (more ions stay inside the axon which helps preserve the strength of the action potential as it moves)

Question 19

Why do squid have giant axons?

Answer 19

• faster speed of impulse and conduction
• due to less leakage of ions (more ions stay inside the axon which helps preserve the strength of the action potential as it moves)

Question 20

The three factors affecting speed of a nerve impulse

Answer 20

1. Myelin sheath
2. Diameter of axon
3. Temperature

Question 21

How does temperature affect the rate of facilitated diffusion of ions, and in turn, the speed of impulses?

Answer 21

• The higher the temperature, the greater the rate of diffusion of ions, so action potentials occur faster – the speed of nerve impulses increases.

Question 22

Respiration is controlled by enzymes. Explain fully how a temperature well beyond the optimum could lead to nerve impulses failing to be conducted at all.

Answer 22

• Respiration provides the ATP required for active transport of ions – if enzymes involved in respiration are denatured, the sodium-potassium pump would be unable to maintain resting potential.
• A too high temperature could also denature the pump itself and plasma proteins. This would affect movement of ions into and out of the axon, and therefore an action potential being generated

Question 23

All or nothing principle

Answer 23

• action potential can only be set up if threshold value is met for a given stimulus

Question 24

How to measure a larger stimulus

Answer 24

• determine the frequency of impulses
• larger stimulus will generate more impulses in a given time

Question 25

Absolute refractory period

Answer 25

• the moment sodium channels open, neurone is not able to respond to any other stimulus so action potential happens without any other

Question 26

Relative refractory period

Answer 26

• moment the sodium channels start closing
• no action potential can start until resting potential reached again

Question 27

Benefits of refractory periods

Answer 27

1. Impulses seen as separate
2. You can limit how many impulses you can send
3. Ensure single direction propagation of the impulse