There are many mitochondria located in the presynaptic knobs of neuromuscular junctions and in the sarcomeres of muscle fibres of striated muscle. Explain the need for many mitochondria in the presynaptic knobs and in the sarcomeres.

1. (a)There are many mitochondria located in the presynaptic knobs of neuromuscular junctions and in the sarcomeres of muscle fibres of striated muscle. Explain the need for many mitochondria in the presynaptic knobs and in the sarcomeres.[4]
2. (b)Fig. 7.1 is a diagram of synapses between a postsynaptic neurone and two presynaptic neurones, X and Y. [Figure 7.1] • Neurone X releases the neurotransmitter acetylcholine. • Neurone Y releases the neurotransmitter glutamate. • Both neurotransmitters bind to channel proteins in the membrane of the postsynaptic neurone. • Acetylcholine binding results in an influx (entry) of sodium ions. • Glutamate binding results in an influx of chloride ions. A student made three statements: 1. When only neurone X is stimulated, an action potential will occur in the postsynaptic neurone. 2. When only neurone Y is stimulated, an action potential will occur in the postsynaptic neurone. 3. When neurone X and neurone Y are stimulated at the same time, an action potential will not occur in the postsynaptic neurone. Explain whether or not you agree with these statements.[3]
3. (c)Multiple sclerosis is a condition in which the myelin sheath breaks down in some neurones. Suggest the effect of multiple sclerosis on the transmission of action potentials in the affected neurones.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.