AQA A2 level Biology - MS Barnes' Biology 12



Neural toxins and poisons

Aims

This worksheet aims to allow you to investigate nervous transmission by considering the effects of various toxins and poisons on the nervous system. You will need knowledge of neuron structure and synapses.

Stopping nervous transmission

Many drugs and poisons affect the nervous system. This worksheet looks at some of these and considers how they affect nervous transmission. Before looking at the effects of these substances, a clear definition of the difference between a poison and a toxin needs to be decided, even though both can ultimately have the same outcome – death. A poison is any substance that causes injury, illness or death because of its molecular action within the organism. A toxin is a natural substance that can kill quickly, even in small quantities.

Toxins

Nervous transmission relies on the passage of an action potential along neurons. This impulse must then also cross synapses as it jumps from one neuron to the next. There are many places that this impulse could be stopped or interfered with.

Botulinum toxin (botox), one of the most powerful toxins known to man, is produced by the bacterium Clostridium botulinum. It blocks nervous transmission at synapses by preventing the vesicles containing the neurotransmitter from fusing with the presynaptic membrane. This causes muscles to be in a constant state of relaxation. However, we have been able to use its effects on the nervous system for our own benefit, in beauty treatments and for therapeutic uses. For example, in people with turrets syndrome, quick and harsh movements are made involuntarily. Botox injections into the site of the muscle can help prevent unwanted nervous transmissions to the muscle.

Another member of the Clostridium genus is Clostridium tetani, which is the bacterium that causes tetanus. It produces the toxin tetanospasmin, which has the opposite effect of botulinum, causing the muscles to go into spasm. This occurs because the tetanospasmin prevents the release of GABA (a neurotransmitter used to naturally block transmission).This causes overactivity in the muscles.

A very different toxin, called tetradotoxin (TTX), is produced by pufferfish (amongst others). It differs from the two already considered, as it does not affect the synapses, but instead affects the sodium channels on the surface of neurons. It binds to the channel blocking the movement of sodium ions. In Japan, pufferfish are considered a delicacy, but due to the danger of eating it, chefs have to have a special licence to prepare it.

Poisons

Having looked at toxins, we can now look at poisons. Lead is an example of a poison that affects the nervous system. One of the noticeable differences is the quantity needed to cause significant damage (which is much larger than any of the toxins) and the timescale it can take for symptoms to appear (probably because it is unusual for lead to be ingested in the quantities which would cause immediate problems). Lead acts as a competitive inhibitor to calcium (used to signal the arrival of an electrical signal at the postsynaptic membrane) at ion channels on cell membranes: the ion channels move lead into the neuron rather than calcium, which can affect both nerves and muscles directly by blocking the release of neurotransmitters from vesicles. Alternatively, it can also cause these calcium channels to take up more lead, to an amount that can cause the release of neurotransmitters without an action potential.

Another poison is atropine, which is derived from the plant Deadly Nightshade (Atropa belladonna). Atropine affects the parasympathetic nervous system, blocking the receptors for the neurotransmitter acetylcholine. Deadly nightshade lives up to its name with just a single leaf enough to be fatal if eaten.

Questions

1. Explain the importance of the sodium ions in the formation and transmission of an action potential.

2. Describe the role of vesicles at a synapse. Where a neurotransmitter is prevented from fusing with the presynaptic membrane at a synapse, the transmission of an impulse is halted. Explain why it is halted.

3. Use your knowledge of the structure of a neurone to explain why many neural toxins and poisons work at the synapses, rather than along the length of the neurone.

4. Explain how competitive inhibition by lead on calcium for a place in the ion channels on the presynaptic membrane could disrupt nervous transmission.

5. Suggest how the effect of atropine on the parasympathetic nervous system could lead to death.

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Topic number 10.7

Transmission across a synapse

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