HBS202 - Advanced Systems Physiology A Excitable Tissues And Toxins - Case Study

Assignment Task

Purpose

Case studies are analyses of situations or problems to be solved. They are used to communicate challenges and recommendations, in order to drive change. In this assessment, you will apply theories taught in Module 1 to a real work business scenario, helping you develop your analysis, problem-solving, and writing skills.

The Case Study

There are many different types of ion channels within nerve and skeletal muscle cells. This case study is going to look at the effects of targeting some of these ion channels, using venom from several species of Australian animals. Due to millions of years of evolutionary pressure (i.e. the quicker you can immobilise your prey the more likely you are to catch it, and the less likely it is to fight back!), animal venoms often contain neurotoxins with very specific and potent effects on particular ion channels

In the laboratory, we can examine neuromuscular activity using the chick biventer cervicis nerve-muscle preparation. The better cervicis muscle is located in the back of the neck and helps a chick hold its head up. In this preparation, the biventer muscle and the nerve that stimulates it are isolated and placed into an organ bath. Using electrodes, we are able to electrically stimulate the nerve to produce an action potential and then measure the contraction of the muscle using a force transducer. In this experiment the nerve has been stimulated and, in addition to measuring the muscle contractions, the amount of ACh released has been measured. The following venoms have been used:

1. A spider venom that contains a toxin that forms pores in the cell membrane of neurons, allowing the influx of Ca2+ into the nerve terminal

2. An octopus venom that contains a toxin which blocks voltage-gated Na+ channels

3. A snake venom that contains a toxin that prevents ACh binding to nAChRs on muscle fibers

The amount of ACh released during a single stimulation (i.e. one action potential) was measured in the presence of each venom, as well as in the absence of any venom (i.e. the control measurement). To provide more accurate results, the experiment was conducted 6 times. The following data were collected (units are mM of ACh per action potential):

Your Task

You are not required to investigate the specific species to be able to answer the following questions. However, you may need to conduct independent research, refer to your subject lecture notes, and use credible scientific sources to formulate your responses.

1. Draw a graph of the data above (using Microsoft Excel) including appropriate axis labels. Instructions on how to do this can be found on the LMS.

2. In addition to measuring muscle contraction and neurotransmitter release, it is also possible to use electrodes to measure the conduction of action potentials along a motor neuron. IMPORTANT – remember we are artificially generating an action potential in the motor neuron, so the only site of synaptic transmission is the NMJ. Given the mechanism of action of these venom (as indicated above), predict what effect each of them would have had on the conduction of action potentials along the motor neuron in this experiment. 

3. What effect do you predict each of these venoms will have on muscle contractions of the better muscle in response to motor nerve stimulation? You must justify your answer including the necessary physiological mechanisms.

4. Some snake venoms contain a toxin that blocks the opening of voltage-gated K+ channels on neurons. What effect would this venom have on the following parameters?

a. The profile of the neuronal action potential

b. ACh release at the NMJ

c. Subsequent contraction of the chick biventer muscle 

5. Every venomous creature produces a slightly different venom which targets different receptors and ion channels. Would you expect a venom that targets muscarinic ACh receptors to have an effect on the chick biventer preparation? 

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