Compare and contrast the changes in membrane potential that occur during the cardiac action potential in part a) with the changes that occur during a regular action potential.
Note that no credit will be given for describing the difference already noted in part a).
A group of heart conditions known as arrhythmias involves problems with the co-ordination or speed of the heart beat. Potassium channel blockers are a group of drugs used to treat some types of arrhythmia.
i)
Sketch on the graph in part a) the effect that you would expect a potassium channel blocker to have on the cardiac action potential.
[1 mark]
ii)
Suggest how potassium channel blockers have the effect predicted in part i).
A study was carried out into the impact of a node of Ranvier length on nerve transmission. The graph below shows the relationship between the node of Ranvier length and the number of sodium ion channels at each node.
A student concluded from the graph that there will be more sodium ion channels at longer nodes of Ranvier.
The study looked at nodes of Ranvier length in mice and found that an individual mouse could have some axons with longer nodes of Ranvier and some axons with short nodes of Ranvier.
Suggest why it might be advantageous to have axons with varying nodes of Ranvier length.
In an extension to the investigation shown in part c), electrodes were inserted into a fully myelinated axon of the same diameter before applying a positive stimulus at the stimulating electrode. Electrodes 1, 2, and 4 were inserted into sections of the axon surrounded by myelin while electrode 3 was inserted into a node of Ranvier.
Suggest, each with a reason, two ways in which the results would look different to those in the bottom graph shown in part c).
Serotonin is a neurotransmitter found in the brain. Low serotonin levels are thought to contribute to symptoms of clinical depression. One commonly used treatment for depression involves a group of drugs called SSRIs.
The diagram below shows a serotonin synapse in the brain both before (left) and after (right) treatment with SSRIs.
Suggest how SSRIs might be effective at treating the symptoms of depression.
The image below shows the chemical structures of serotonin and an SSRI called Citalopram. Citalopram is commonly prescribed for depression due to its limited side effects, but it does sometimes cause some nausea and sleep disturbance during the first few weeks.
Use the image to suggest how Citalopram can have the effect shown in part a) 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 withdrawal symptoms at the end of treatment. One group of researchers analysed the results of around 300 studies involving treatment for depression; some of their results are shown in the graph below.
The placebo in this case was a pill identical in appearance to an SSRI but containing no active chemical ingredients.
Suggest the role of a placebo in a study of this type.
State what can be concluded about the effectiveness of SSRIs in the treatment of depression from the information provided in part c). Note that the error bars in the graph represent standard deviation.
