Nerve Impulses and Neurotoxins Quiz

Quiz Answer Key and Fun Facts

1. In its unexcited state, the cell membrane exists at what is called a resting potential. What does this mean?

Answer: There is a separation of charge maintained across the membrane

The membrane of a cell is made of a phospholipid bilayer and several proteins which are embedded in this bilayer. When discussing nerve potentials, it is appropriate to refer to the membrane in terms of electrical circuitry. Either side of the membrane, there are varying concentrations of both positive and negative ions, giving a difference in charge (a voltage) across the membrane.

The phospholipid bilayer is impermeable to such ions and so maintains this voltage (i.e. it behaves as a capacitor).

The proteins may open upon stimulation (either by a ligand or a change in the voltage) and allow ions (charge) to flow. These proteins are therefore the electrical switches in this analogy.

2. In order to transmit a signal, ions cross the membrane, resulting in the so-called depolarisation of the membrane. How do most of these ions cross the membrane?

Answer: Via ion channels

Ion channels can be categorised according to several traits. For example, they can be classed by whether they allow the flow of sodium, or calcium, or chloride, etc. They can be classified on the basis of how they are opened - do they open upon binding specific ligands? Do they open when the potential difference across the cell membrane changes? Or are they always open? Ion channels can also be categorised by how many transmembrane domains they have (i.e. how many times they pass through the cell membrane). Those with five transmembrane domains (pentamers) tend to be ligand-gated, whereas those with six transmembrane domains (hexamers) tend to be voltage-gated.

3. Depolarisation allows a signal to be transmitted, but before a second signal can be sent, repolarisation must occur. What does repolarisation mean?

Answer: Restoring the cell membrane's resting potential

At the resting potential, the inside of the cell is negative relative to the outside. Depolarisation occurs when the correct ion channels open and allow positive ions to flood into the cell, causing the inside to now be positive relative to the outside. Following this, there is an effort to repolarise the cell (i.e. to make the inside more negative and the outside more positive).

This is partly done by the ATPase pump, which pumps three sodium ions out and two potassium ions into the cell, with a net effect of making the outside more positive.

In addition, potassium channels open, allowing the potassium inside the cell to move out. This latter action actually causes hyperpolarisation, meaning the inside of the cell is "too negative". This means that for a short period of time, a second nerve impulse can only be sent if the stimulus is far greater than normal.

This allows us to distinguish between different intensities of stimulation.

4. Many ions are involved in the depolarisation and repolarisation of cell membranes, and are thus important in the transmission of the resulting signals. Which of these ions is least important for this?

Answer: Iron

Sodium, potassium, chloride, and calcium ions are regarded as the most important in determining the potential difference across cell membranes, with sodium and potassium being the main two ion species which change in concentration across the membrane, therefore accounting for things like depolarisation and repolarisation.

These ions all have an associated charge, as well as a certain concentration inside and outside of the cell. The direction in which the ion will move across the plasma membrane (if given the chance) can be calculated using either the Nernst equation, or the Goldman-Hodgkin-Katz equation (the latter takes into account the concentrations of other ion species).

5. The movement of ions allows nerve impulses to be transmitted along neurones, but ions cannot jump from one neurone to the next when there is a junction. Instead, our bodies use chemical messengers called neurotransmitters to cross these junctions. What is the more common name for these junctions?

Answer: Synapses

Synapses provide points of regulation for nerve potentials. Along a neurone, each impulse is the same intensity. At a synapse, it can be amplified, dampened, or otherwise modified. It is for this reason that toxins tend to exert their effects at synapses.

When a nerve impulse reaches the end of one neurone (called the pre-synaptic bulb), there is an influx of calcium into the bulb, which causes the bulb to release neurotransmitter molecules. These molecules diffuse across the synapse to the post-synaptic bulb (the second neurone) where it binds to receptors/ion channels, which then lead to the propagation of the nerve impulse.

6. Many toxins affect how our nerve impulses are transmitted by interfering with these junctions. For example, the botulism toxin (the active ingredient of Botox) prevents vesicles which are carrying neurotransmitter molecules from fusing with the membrane and releasing its contents. How does the botulism toxin do this?

Answer: By cleaving SNARE proteins

As mentioned in the II of the previous question, synaptic transmission is dependent on the fusion of neurotransmitter-containing vesicles at the pre-synaptic bulb, thus releasing the neurotransmitter into the synapse. The fusion of these vesicles with the pre-synaptic membrane is dependent on proteins called SNAREs, which are found on both the vesicle and the membrane, and which intertwine, pulling the two entities together and forcing the membranes to fuse.

7. Toxins may inhibit specific ion channels, thus preventing the passage of ions. For example, tetrodotoxin (from puffer fish) inhibits sodium channels. What effect is this likely to have?

Answer: Inhibition of nerve impulses

By inhibiting sodium channels, tetrodotoxin prevents sodium ions from flowing into the cell and causing a depolarisation of the membrane. This is the toxin responsible for the deaths of people who eat fugu that has been incorrectly prepared. With a hint of toxin in this dish, however, you get a characteristic numb, tingly sensation on the tongue. (I was not brave - or rich - enough to try fugu when I visited Japan).

8. Studying toxins has been useful in determining the role of various proteins. For example, tetraethylammonium (TEA) binds to and inhibits a protein which is responsible for moving potassium ions OUT of the cell after an action potential has occurred. What effect is this likely to have?

Answer: Delayed repolarisation

Despite the inside of the cell being negative (relative to the outside), the concentration of positive potassium ions is much higher inside than outside. The passage of potassium ions is tightly regulated. After sodium ions have flooded the cell and have caused an action potential, voltage-dependent potassium channels open, allowing potassium ions to move out of the cell, therefore repolarising the cell. TEA blocks this potassium channel and therefore prevents repolarisation and, hence, prevents the possibility of more nerve impulses being fired.

9. The black mamba, as seen in "Kill Bill", can bite you and paralyse you in minutes. One reason for this is a group of neurotoxins called fasciculins which inhibit acetylcholinestease (AChE). Why does this cause paralysis?

Answer: AChE (when inhibited) cannot breakdown acetylcholine which therefore continues to induce nerve impulses

In normal circumstances, acetylcholine is released from the pre-synaptic membrane and diffuses across the synaptic cleft before binding to the post-synaptic membrane. Specifically, acetylcholine may bind to nicotinic acetylcholine receptors. This pentameric ion channel opens when bound to two acetylcholine molecules and allows and influx of positively charged ions which is coupled to an action potential.

10. Toxins from deadly animals such as sea snakes and black widow spiders can offer medical benefits and perhaps save lives.

Answer: True

As well as using toxins to study the roles of various proteins in our body, and therefore improving our understanding of things like nerve impulses, toxins can be used to directly benefit some patients. For example, digitalis, a toxin obtained from the foxglove plant can be used to treat patients with congestive heart failure. Rather than affecting ion channels (like some of the other examples discussed thus far), digitalis inhibits the ATPase pump which usually pumps sodium ions out of the cell.

This leads to an increased duration of repolarisation and a stronger and slower heartbeat. Modified toxins may also be useful as painkillers. For example, the king cobra toxin-derived drug hannalgesin is said to be 200 times more effective than morphine in killing pain.

Source: Author doublemm

This quiz was reviewed by FunTrivia editor WesleyCrusher before going online.
Any errors found in FunTrivia content are routinely corrected through our feedback system.

This quiz will first look at how nerve impulses are transmitted in our bodies, before looking at various toxins, and how and why they affect us in the ways they do. Enjoy!

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