Question

Draw graphs showing the effects on action potentials in a postsynaptic cell of effective doses for each of the listed toxins. Assume that the cell is normally brought to threshold by the stimuli applied to its inputs, so that any abnormality is due to the toxin.

1. curare (prevents receptor from binding neurotransmitter)

2. botulinum toxin (prevents neurotransmitter release)

Answer 1

Curare is classified as a neuromuscular blocking agent. It is an example of a non depolarizing muscle relaxant that blocks the nicotinic acetylcholine receptor(nAchR) , one of the two types of acetylcholine (ACh) receptors at the neuro muscular junction.Curare affects the stimulus ( excitatory post synaptic potential, EPSP) which normally leads to the initiation of the muscle action potential.An animal that is poisoned with curare will asphyxiate because the process of neuro muscular transmission at respiratory muscles will be blocked.

In simple words, Curare allows the action potential to form but cannot pass between nerve to muscle.Curare binds to acetylcholine binding sites on post synaptic cell membrane which prevents acetylcholine from acting.Below is the image of a graphical representation of a normal action potential.

Below is the image for graphical representation of action potential affected by curare.The graphical representation indicates the lower rate of action potential taking place due to the curare toxin that prevents acetylcholine from acting.

2. Botulinum neurotoxin A is the most toxic protein known.They enter nerve terminals and decrease the number of acetylcholine containing vesicles released into the synaptic cleft,without affecting acetylcholine synthesis or action potential conduction in the motor nerve. It acts by binding presynaptically to high affinity recognition sites on nerve terminals and decreasing the release of acetylcholine , causing a neuro muscular blocking affect.It entirely blocks transmitter release from motor nerve conduction and causes an end plate potential.Below graph determines the end plate potential after affected by botulinum toxin.

End plate potentials in mV. A& C are weakend end potentials in a muscle to elicit an action potential.The end plate potential C is is caused by botulinum toxin that decreases end plate releases of acetylcholine, too weak to elicit a muscle action potential.

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