Question

A Draw graphs showing the effect on action potentials in a cell following effective doses of each of the listed neurotoxins. Assume that the cell is normally brought to threshold by an electrical stimulus applied to it, so that any abnormality is due to the toxin. Precise values for voltage and duration are not important, just a general trend in how the action potential may be different from normal 1. puffer fish poison (blocks voltage-gated sodium channel activation) (25 Points) 2. tetraethylammonium (blocks voltage-gated potassium channels) (25 Points) Multiple sclerosis (MS) is one of the better knon diseases resulting from demyelination of axons (in MS, only CNS axons are affected) Some of the earliest symptoms of the disease are difficulty in focusing the eyes, such as in reading, and difficulty in maintaining balance, and frequently not being able to make adjustments in posture to avoid falling. How do these symptoms "fit" with what you know about nerve impulses and myelin sheaths? (25 Points) Explain the processes that lead to the exocytosis of neurotransmitter from a presynaptic cell. Which components are recycled? Which ion is important in triggering exocytosis? (25 Points)

Answer 1

An action potential is a rapid change in potential difference that traverses down an axon to the synaptic bout. The action potential is characterized by three distinct phases.

1. Depolarization: At the resting state, the membrane of the neuron has a negative potential of almost -70 mV. This is followed by passive Na+ ion influx, which leads to the conversion of membrane potential to a more positive value. After it reaches about -40 mV, the voltage-gated Na+ channels open up leading to a rapid influx of Na+ ions and the membrane is said to be depolarized. This continues up to a membrane potential of about +40 mV.

2. Repolarization: This is followed by the closing of Na+ channels and the opening of voltage-gated K+ channels, which leads to rapid efflux of K+ ions until the potential drops below the resting membrane potential up to -90 mV.

3. Hyperpolarization: This phase is called hyperpolarization. Eventually, the potential restores back to normal.

This is summarized in the figure below:

Potential Difference Depolarization Repolarization Time Hyperpolarization

The Puffer toxin is a voltage-gated sodium channel blocker. Hence it will inhibit the depolarization phase itself and there will be no development of action potential at all.

The tetraethylammonium blocks the voltage-gated potassium channels which are activated during the repolarization phase. Thus this will prevent the cells from repolarizing and the membrane potential will decrease to normal very slowly due to leaky channels. Thus, the action potential will be present for a much longer time.