Action potentials

Question 1

The plasma membrane of the neurone is more permeable to potassium ions than to sodium ions. Give the evidence from the diagram that supports this observation.

Answer 1

fewer protein B molecules, which transport sodium ions / more protein A molecules, which transport potassium ions;

Question 2

Explain how a resting potential is maintained in a neurone

Answer 2

membrane relatively impermeable / less permeable to sodium ions / gated channels are closed / fewer channels;
sodium ions pumped / actively transported out;
by sodium ion carrier / intrinsic proteins;
inside negative compared to outside / 3 sodium ions out for two potassium ions in;

Question 3

Describe how the release of acetylcholine into a neuromuscular junction causes the cell membrane of a muscle fibre to depolarise.

Answer 3

movement by diffusion;
binding to receptors on (post-synaptic) membrane;
causing sodium channels to open / sodium ions to move in to muscle (cell);

Question 4

The cobra is a very poisonous snake. The molecular structure of cobra toxin is similar to the molecular structure of acetylcholine. The toxin permanently prevents muscle contraction.

Answer 4

toxin binds to / competes for / blocks the acetylcholine receptors; acetylcholine can not depolarise the membrane / the toxin does not cause depolarisation;

Question 5

The insecticide DFP combines with the active site of the enzyme acetylcholinesterase. The muscles stay contracted until the insecticide is lost from the neuromuscular junction.

Answer 5

acetylcholinesterase is unable to breakdown acetylcholine; acetylcholine still available to depolarise the membrane / generate action potentials in the membrane

Question 6

Suggest two advantages of simple reflexes.

Answer 6

1. Rapid
2. Protect against damage to body tissues;
3. Do not have to be learnt;

Question 7

The nerve pathway shown in the diagram may be regarded as a simple reflex arc. Use the diagram to explain why.

Answer 7

Only 3 neurones / nerve cells (in reflex arc)

Question 8

Damage to the myelin sheath of neurones can cause muscular paralysis (lines 2–4).
Explain how.

Answer 8

(Refers to) saltatory conduction
Depolarisation occurs along whole length (of axon);
(Nerve) impulses slowed/stopped;
(Refers to) neuromuscular junction

Question 9

Guillain–Barré syndrome is a rare disease in which the immune system damages the myelin sheath of neurones.

Answer 9

Slower/fewer impulse(s) along sympathetic/parasympathetic (pathway/neurones);(Impulses) from cardiac centre
OR
(Impulses) from medulla;
To SAN;

Question 10

The first successful drug trial to reduce concentrations of huntingtin in the brain used
single-stranded DNA molecules (lines 13–14).
Suggest and explain how this drug could cause a reduction in the concentration of the protein huntingtin.

Answer 10

1. It/DNA is complementary to (m)RNA;
   Accept (transcription) results in complementary (m)RNA.
   Ignore miRNA/siRNA/transcriptional factors.
2. Binds to mRNA (for huntingtin);
3. Prevents translation;

Question 11

Suggest and explain one way epigenetics may affect the age when symptoms of Huntington’s disease start.

Answer 11

1. (Increased) methylation of DNA/gene/allele;.
2. Inhibits/prevents transcription;
   OR
3. Decreased methylation of DNA/gene/allele; 4. Stimulates/allows transcription;
   OR
4. Decreased acetylation of histone(s);
5. Inhibits transcription;

Question 12

Describe how the change shown in the diagram occurs when an action potential is produced. (inside of neurone starts to become positive)

Answer 12

sodium gates or channels open / increase in permeability of axon membrane to sodium ion; sodium ions enter axon;

Question 13

Explain what causes the conduction of impulses along a non-myelinated axon to be slower than along a myelinated axon.

Answer 13

non-myelinated – next section of membrane depolarised / whole membrane;
myelinated – depolarisation / ion movement only at nodes; impulse jumps from node to node / saltatory conduction;

Question 14

When a neurone transmits a series of impulses, its rate of oxygen consumption increases. Explain why.

Answer 14

(More) respiration;
(More) energy supplied / (more) ATP supplied;
For active transport of ions / ‘sodium (-potassium) pump’ / pumping out sodium ions / for neurotransmitter synthesis / for vesicle movement;

Question 15

A taser is a device used by the police to arrest violent suspects. It fires electrical impulses very similar to action potentials into a suspect. The frequency of the impulses is between 15 and 20 per second.
(i) Suggest the effect a taser has on a suspect’s muscles.

Answer 15

Causes them to contract; And relax; Rapidly/twitch;

Question 16

Tasers with frequencies of between 40 and 80 per second are not used, because they are considered too dangerous. Suggest how they might be dangerous to a suspect.

Answer 16

Cause continuous muscle contraction;
Accept a reasonable suggestion of harm ‒ linked
to muscle contraction
At high force;
Causing failure to breathe/heart stops pumping/ damage to bones or joints;

Question 17

Dopamine is a neurotransmitter. Production of too much dopamine is associated with schizophrenia. A drug used to treat schizophrenia binds to dopamine receptors in synapses. This binding does not lead to the formation of an action potential.

Suggest why binding of the drug does not lead to production of an action potential.

Answer 17

(Binding) does not lead to opening of sodium ion channels;

(So) no depolarisation / threshold not reached / sodium ions do not diffuse in;

Question 18

Other than temperature, give two factors that affect the speed of nerve impulse conduction.

Answer 18

1. Myelination / saltatory conduction;
   Accept reference to presence of nodes of Ranvier
2. Axon diameter;

Question 19

Describe how the resting potential is established in an axon by the movement of ions across the membrane.

Answer 19

active transport / pump of Na+ out of axon;

diffusion of K+ out of axon / little diffusion of Na+ into the axon

Question 20

Sodium and potassium ions can only cross the axon membrane through proteins. Explain why.

Answer 20

can not pass through phospholipid bilayer;

because water soluble / not lipid soluble / charged / hydrophilic / hydrated;

Question 21

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

Answer 21

1. Action potentials travel more slowly / don’t travel;
   Accept: fewer / no saltatory movement of potentials
2. So delay in muscle contraction / muscles don’t contract / muscles contract slow(er);

Question 22

Cannabinoid receptors are found in the pre-synaptic membrane of neuromuscular junctions. When a cannabinoid binds to its receptor, it closes calcium ion channels.
Suggest how cannabinoids could prevent muscle contraction.

Answer 22

Prevents influx of calcium ions (into pre-synaptic membrane);

(Synaptic) vesicles don’t fuse with membrane / vesicles don’t release neurotransmitter;
Accept vesicles don’t release acetylcholine

Neurotransmitter does not diffuse across synapse / does not bind to receptors (on post-synaptic membrane);

No action potential / depolarisation (of post-synaptic membrane) / sodium (ion) channels do not open / prevents influx of sodium ions.

Question 23

Cannabinoids include substances found in cannabis that can enter brain tissue. Scientists are developing artificial cannabinoids that can enter neuromuscular junctions but cannot enter brain tissue.
Suggest why these artificial cannabinoids would be better to use than cannabis when treating someone with MS.

Answer 23

They won’t affect synapses in brain;
They won’t cause problems with the brain’s function / won’t damage brain;
Accept: suitable named problem e.g. hallucination
Ignore: unqualified references to ‘side effects’
Accept: reference to addiction / harm of smoking (cannabis)
(So only the) muscle / neuromuscular junctions treated / affected.

Question 24

After exercise, some ATP is used to re-establish the resting potential in axons. Explain how the resting potential is re-established.

Answer 24

Pump / active transport / transport against concentration gradient; Of sodium from axon / sodium out / of potassium in;