1. How is a resting potential maintained in an axon ?
Answer
The resting potential in an axon is maintained by the sodium–potassium pump, which actively transports 3 Na+ ions out of the axon for every 2 K+ ions brought in. Additionally, more K+ ions leak out of the axon than Na+ ions leak in. This results in a greater concentration of positive ions outside the membrane than inside. At rest, both sodium and potassium ion channels remain closed, helping to preserve the resting potential.
2. Describe the structure of a myelinated motor neurone.
Answer
A myelinated motor neurone has its cell body located at one end, with short dendrites that receive impulses and a long axon that transmits them. The axon is surrounded by a myelin sheath, which is formed by Schwann cells wrapping around it. Between adjacent Schwann cells are tiny gaps called nodes of Ranvier, which play a key role in saltatory conduction.
3. Describe temporal and spatial summation and explain how they are different.
Answer
Summation is when several impulses are needed to set up an action potential in the post-synaptic neurone.
Temporal summation is when several impulses on the same neurone in quick succession contribute to an action potential on the post-synaptic neurone.
Spatial summation is when several neurones at the same time contribute to an action potential of a single postsynaptic neurone.
4. What is meant by "unidirectionality" of synapses ? What causes this ?
Answer
Transmission across a synapse only happens in one direction from pre-synaptic neurone to post-synaptic neurone, because transmitter substance only produced in pre-synaptic neurone and receptors only present in post-synaptic neurone.
5. Name three factors that affect the speed at which action potentials can pass along an axon.
Answer
Myelination, axon diameter and temperature.
6. The figure below shows the changes in membrane potential as well as changes in membrane permeability during an action potential. Lines Y and Z represent the permeability of the membrane to different ions. Name these ions.
Answer
Y = sodium ions,
7. Describe and explain the changes that have occurred to the membrane after 1.0 milliseconds.
Answer
Voltage-gated Na+ channels open, allowing Na+ to enter the axon. Then K+ channels open, causing K+ to leave the axon, which leads the membrane potential to fall.
8. Explain the importance of the changes occurring from 1.4 milliseconds.
Answer
Hyperpolarization has occurred. During this time, Na+ gated channels in this region cannot reopen, which ensures that action potentials remain discrete and unidirectional.
9. Scientists have found that certain types of brain tumours form connections with neurones in the same way that two neurones at an excitatory synapse do. The connection appears to stimulate tumour growth. Scientists have found that some tumours have a large number of AMPA receptors. When stimulated by the neurotransmitter glutamate, these receptors cause depolarisation of the tumour membrane that leads to increased growth. Suggest how AMPA receptors respond to glutamate but not other neurotransmitters
Answer
AMPA receptors have specific shape (a tertiary structure) which only glutamate will fit.
10. AMPA receptors in healthy brain cells are different from those in tumour cells. Suggest how this information might enable scientists to develop a drug to stop the growth of these tumours.
Answer
Receptors have different tertiary structures. The drug is complementary only to receptors on tumour cells, so it will attach to and kill only tumour cells.
11. Myasthenia gravis (MG) is a neuromuscular disease causing weakness of skeletal muscles. The symptoms are due to the production of antibodies that inhibit the acetylcholine receptors on the post-synaptic membrane. One of the most common treatments for MG is to take a drug called acetylcholinesterase inhibitors. Suggest how the antibodies produced by an MG sufferer result in the reduction of post-synaptic depolarisation.
Answer
The antibodies have a specific tertiary structure that allows them to bind to acetylcholine receptors. This prevents sodium ion channels from opening and stops depolarization.
12. Explain how acetylcholinesterase inhibitors are useful in treating the symptoms of MG.
Answer
The drug inhibits the enzyme acetylcholinesterase, so acetylcholine (ACh) is not broken down. As a result, ACh remains in the synapse and bound to receptor molecules in the sarcolemma. This gives it more time to act on the membrane and cause depolarization, and so the muscle stays contracted longer.
13. People with MG often find their symptoms are worse after a period of prolonged activity, and are better after they have had a rest. Suggest why.
Answer
During vigorous exercise, muscles need to contract. For contraction to occur, the membrane must be depolarized by acetylcholine binding to its receptors, which in this case cannot happen.