Nervous Conditions

Question 1

During an action potential the permeability of the axon membrane to sodium ions increases slowly and then increases rapidly. Explain these changes.

Answer 1

(Ion) channel proteins open;

Sodium ions enter in;
Changes membrane potential/makes inside of axon less negative/positive/depolarisation/reaches threshold;
More channels open/positive feedback;

Question 2

During an action potential, the membrane potential rises to +40 mV and then falls. Explain the fall in membrane potential

Answer 2

Potassium ion channels open;
Potassium ions move out;
Sodium ion channels close;

Question 3

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

Answer 3

Pump/active transport/transport against concentration gradient;
Of 3 sodium ions out of the axon for every 2 potassium ions into the axon;

Question 4

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

Answer 4

active transport / pump of Na+ out of axon;
Active transport of K+ into the axon;
The axon membrane is more permeable to K+ than Na+ so there is diffusion of K+ out of axon but very little Na+ diffusion into the axon;

Question 5

Explain how an action potential passes along a motor neurone

Answer 5

Depolarisation of axon membrane/influx of Na+ establishes local
currents;
Change permeability to Na+ /open Na+ gates of adjoining region;
Adjoining region depolarises / influx of Na+ ;

This process repeated along axon / self propagation;

Correct reference to/description of saltatory conduction;

Question 6

Fewer action potentials occur along a myelinated axon than along an unmyelinated axon of the same length. Explain why.

Answer 6

Myelin insulates axon / ions can only pass through (plasma membrane
of axon) at gaps in myelin sheath;
(Gaps in sheath are called) nodes of Ranvier;

Question 7

Above a certain strength stimulus, the refractory period makes it impossible for information about the differences in the stimulus strength to reach the brain. Explain why.

Answer 7

1. (Refractory period) leads to discrete / separate nerve impulses / time when another nerve impulse can’t happen;
   OR
   (Refractory period) limits number of impulses per second / frequency of nerve impulses;
2. When maximum frequency reached / exceeded, no further increase in information / all (higher) strength stimuli seem the same;

Question 8

Explain how the release of acetylcholine at an excitatory synapse reduces the membrane potential of the postsynaptic membrane.

Answer 8

Binds to receptor/proteins;
and opens Na+ channels;

Na+ enter and make membrane potential less negative/depolarised

Question 9

Describe the sequence of events which allows information to pass from one neurone to the next neurone across a cholinergic synapse.

Answer 9

1. (impulse causes) calcium ions/Ca2+ channels to open
2. Ca2+ diffuse into axon;
3. vesicles move towards and then fuse with (presynaptic) membrane;
4. acetylcholine (released);
5. (acetylcholine) diffuses across synaptic cleft/synapse;
6. binds with receptors on (postsynaptic) membrane;
7. sodium ions/Na+ enter (postsynaptic) neurone;
8. depolarisation of (postsynaptic) membrane;
9. if above threshold nerve impulse/action potential produced

Question 10

Serotonin diffuses across the synaptic gap and binds to a receptor on the post-synaptic membrane. Describe how this causes depolarisation of the post-synaptic membrane

Answer 10

1. Causes sodium ion channels to open;

2. Sodium ions enter (cell and cause depolarisation);

Question 11

Explain what causes transmission at a synapse to occur in only one direction

Answer 11

(Vesicles containing) neurotransmitter only in presynaptic membrane/
neurone;
receptor/proteins only in postsynaptic membrane/neurone;
so neurotransmitter diffuses down concentration gradient;

Question 12

GABA is a neurotransmitter which can inhibit the production of action potentials by interacting with K+ and Cl- channels. an action potential is less likely if GABA is released at the same time. Explain why

Answer 12

GABA opens K+ and Cl– channels;
K+ passes out and Cl– passes in;
Membrane potential more negative/hyperpolarised;
Requires increased stimulation/must open more Na+ channels / allow more Na+ to enter;
To reach threshold;

Question 13

Give five differences between a cholinergic synapse and a neuromuscular junction

Answer 13

neurone to neurone and neurone to muscle;

action potential in neurone and no action potential in muscle/
sarcolemma;

no summation in muscle;
muscle response always excitatory (never inhibitory);

some neuromuscular junctions have different neurotransmitters;

Question 14

How can inhibition of acetylcholinesterase result in muscles being permanently contracted?

Answer 14

1. Acetylcholine not broken down / stays bound to receptor;
2. Na+ ions (continue to) enter / (continued) depolarisation / Na+ channels
(kept) open / action potentials / impulses fired (continuously);
3. muscles stay contracted / cannot relax;