6.2 Nervous Coordination (A Level only)

Figure 1 below shows how axon diameter affects the speed of impulse conduction in several different types of neurone.

Figure 1

Describe and explain what Figure 1 shows about the effect of axon diameter on conduction speed.

Calculate how many times faster the conduction speed is for a myelinated fish neurone than a non-myelinated fish neurone with a diameter of 10μm. 

Explain why conduction speed in a myelinated neurone is so much faster than in an unmyelinated neurone.

Due to their large diameter, squid axons have always been popular in neuroscience research. They were instrumental in the early discovery of how action potentials occur, used alongside a piece of equipment called a ‘voltage clamp’. A voltage clamp allows scientists to set and maintain the membrane potential of a neurone.

Suggest why voltage clamps were so essential in research on action potentials. 

Serotonin is a neurotransmitter found in the brain. Low serotonin levels are thought to contribute to symptoms of clinical depression, and one commonly used treatment for depression involves a group of drugs called SSRIs.

Figure 1 below shows a serotonin synapse in the brain both before (left) and after (right) treatment with SSRIs.

Figure 1

Using the information provided in Figure 1, suggest how SSRIs can be effective in the treatment of depression.

Figure 2 below shows the chemical structures of serotonin and an SSRI called Citalopram.  Citalopram is commonly prescribed due to its limited side effects, but it does sometimes cause some nausea and sleep disturbance during the first few weeks.

 Figure 2

Use Figure 2 to explain how citalopram can have the effect shown in Figure 1 above while also having some side effects.

Although the use of SSRIs has increased significantly in recent years, there is still controversy over their effectiveness in the treatment of depression, along with some concern surrounding the difficulty that can be involved in coming off medication at the end of treatment.  One group of researchers analysed the results of around 300 studies involving treatment for depression, and some of their results are shown in Figure 3 below.

Figure 3

A placebo is a pill, for example a sugar pill, that is identical in appearance to an SSRI but that contains no active chemical ingredients. 

Explain the role of a placebo in a study of this type. 

State what conclusions about the effectiveness of SSRIs might be drawn from Figure 3.

Figure 1 shows the effect of applying different electrical stimuli to a neurone on its  membrane potential. 

Figure 1

Describe how the response to the changing stimulus differs between the negative stimulus in trace 1 and the positive stimulus in trace 2.

Suggest an explanation for the changes in membrane potential shown in trace 1 in Figure 1.

Figure 2 shows the effect of changing the type of stimulus on the transmission of a nerve impulse.

Figure 2

Suggest an explanation for the difference between the transmission of the impulses shown in traces 3 and 4 of Figure 2.

Tetrodotoxin (TTX) is a toxin found in pufferfish.  Its interaction with sodium channels is shown in Figure 3 below. 

Figure 3

Explain how TTX will affect nerve impulses and suggest how  consumption of contaminated puffer fish might affect humans.

Figure 1 below shows junctions between 4 presynaptic neurones and one postsynaptic neurone.  The effects of these 4 synapses on the membrane potential of the postsynaptic neurone can be seen in the graph.

Figure 1

The point marked X on Figure 1 represents the resulting membrane potential when presynaptic inputs 1 and 4 are provided at the same time. Suggest why no change in membrane potential is produced at point X

Explain how presynaptic inputs 2 and 3 can result in an action potential being generated at points Y and Z on Figure 1. 

Parasympathetic control of heart rate involves cholinergic synapses.  Figure 2 below shows a section of the postsynaptic cell surface membrane of a cardiac cholinergic synapse.

Figure 2

Suggest how acetylcholine acts as an inhibitory neurotransmitter in this synapse.

The sympathetic control of heart rate can be regulated by treatment with a class of drugs called beta blockers.  People who suffer from anxiety or high blood pressure are sometimes prescribed these drugs to help control their heart rate.  Suggest how beta blockers might function to achieve this.  

Describe the mechanism which allows information to pass across a cholinergic synapse.

State three differences between a neuromuscular junction and a cholinergic synapse.

Explain the difference between spatial summation and temporal summation.

Epilepsy can occur due to increased neuronal activity in the brain. One particular form of epilepsy is caused by insufficient GABA. GABA is broken down on the postsynaptic membrane by the enzyme GABA transaminase. Substance Y is a new drug being tested as a form of treatment for this particular type of epilepsy. Substance Y has a very similar molecular structure to GABA. Suggest how substance Y will work in treating this form of epilepsy.

Dopamine is an excitatory neurotransmitter that plays a vital role in areas of the brain responsible for muscle control. It is transported back out of the synaptic cleft by a transporter protein located within the presynaptic membrane. Dopamine diffuses across the synaptic gap and binds to a receptor on the postsynaptic membrane. Describe how this results in the depolarisation of the postsynaptic membrane.

Explain why it is essential that neurotransmitters like dopamine are transported back out of synapses.

Researchers investigated the effect of cocaine on the movement of rats. They measured the amount of movement of three groups of rats, A, B and C. Group A, were not given cocaine, group B, were given cocaine and group C, were mutant rats who had reduced dopamine transporter function and were given cocaine. 

Figure 1 below shows the results.

Figure 1

The researchers concluded that cocaine affects movement in animals by binding to the dopamine transporters and preventing the re-uptake of dopamine into the presynaptic membrane. Evaluate this conclusion.

Cocaine-induced heart attacks are particularly common in younger people. It has been estimated that cocaine is the cause of nearly 30% of heart attacks in people under 45. Cocaine also affects the reuptake of neurotransmitter noradrenaline, causing an increased concentration of noradrenaline in synapses/neuromuscular junctions. Suggest how this could lead to a heart attack.

Researchers were studying the effect of different stimulation frequencies on the production of action potentials by a single neurone.

Figure 1 below shows a recording of the action potentials generated when the frequency of stimulation was 155 per second. At this specific frequency, each stimulus is able to produce one action potential.

Figure 1

The time required for the completion of one action potential is x. Calculate the value of x. Give your answer to the nearest microsecond. Show your working.

Figure 2 shows the results when the stimulation frequency was 220 per second.

Figure 2

Not every stimulus generated an action potential. Explain why.

Figure 3 shows the level of force generated by a specific muscle when it is stimulated
by nerve impulses of different frequencies.

Figure 3

A taser is an electrical device used by some law officials to detain or stop violent suspects. It fires electrical impulses that are very similar to action potentials into an individual. The frequency of the impulses used by a taser is usually between 16 and 22 per second. Suggest the effect this would have on the muscles of an individual.

Suggest why tasers with frequencies of 50+ per second are not used.

A non-myelinated axon conducts impulses slower than a myelinated axon. Explain why.

Scientist investigated the relationship between myelin content present in brain tissue and the different types of dementia. There are over 5 forms of dementia and all forms of dementia involves a severe loss of mental ability.

The scientists measured the mean amount of myelin content in brain tissue samples from 3 different groups:

• A control group of 15 people without dementia

• 22 people with vascular dementia (VD)

• 18 people with Alzheimer's dementia (AD)

Their results are shown in Figure 1 below. The vertical bars represent standard errors.

Figure 1

A statistical test was used to compare the results for AD and VD. A value of P = 0.041was obtained. What does this result show about the difference between the means for AD and LD?

A research assistant who reviewed this study concluded that there was a relationship between the concentration of myelin present in an individual’s brain and the likelihood of whether or not they had dementia. Evaluate this conclusion.

Other than the presence of myelin, name two other factors that affect the speed of impulse conduction.