6. Organisms Respond to change in environment

Question 1

Explain the effect of myelination on the rate of nerve impulse conduction. (2 marks)

Answer 1

1. myelin insulates
2. saltatory conduction

Question 2

Give one reason why a speed would be an underestimate of
the speed of transmission of an impulse along a neurone. (1 mark)

Answer 2

diffusion across synapse

Question 3

Damage to the myelin sheaths of neurones can lead to problems controlling the
contraction of muscles.
Suggest one reason why. (2 marks)

Answer 3

1. action potentials travel more slowly
2. so delay in muscle contraction

Question 4

A myelinated axon conducts impulses faster than a non-myelinated axon.
Explain this difference. (3 marks)

Answer 4

1.(In myelinated) action potential only at node(s);
2.(In myelinated, nerve impulse) jumps from node to node in saltatory conduction
3.(In myelinated) action potential does not travel along entire length

Question 5

Describe how calcium ions are involved in synaptic transmission. (2 marks)

Answer 5

1. (Nerve impulse causes) Ca 2+ to enter presynaptic neurone/membrane;
2. (Ca 2+ entry) causes fusion of vesicles with presynaptic membrane

Question 6

It is important that a neurotransmitter such as serotonin is transported back out of synapses. Explain why. (2 marks)

Answer 6

1. (If not removed) keeps binding (to receptors);
2. Keeps causing action potentials

Question 7

Sodium and potassium ions can only cross the axon membrane through proteins.
Explain why. (2 marks)

Answer 7

1. can not pass through phospholipid bilayer;
2. because water soluble