10. Draw and/or describe Otto Loewi’s experiment that demonstrated the principle
ID: 179959 • Letter: 1
Question
10. Draw and/or describe Otto Loewi’s experiment that demonstrated the principle of chemical synaptic transmission using frog hearts (8 points total). Then describe what would have happened if someone added:
A) A muscarinic receptor agonist to the saline before stimulating the vagus nerve
B) A muscarinic receptor antagonist to the saline before stimulating the vagus nerve
C) Acetyl cholinesterase to the saline before stimulating the vagus nerve
D) Norepinephrine to the saline immediately after stimulating the vagus nerve
Explanation / Answer
Loewi investigated how vital organs respond to chemical and electrical stimulation. He also established their relative dependence on epinephrine for proper function. Consequently, he learnt how nerve impulses are transmitted by chemical messengers. The first chemical neurotransmitter that he identified was acetylcholine.
Before Loewi's experiments, it was not clear whether signalling across the synapse was electrical or chemical. Loewi's famous experiment answered this question. According to Loewi, the idea for his key experiment came to him in his sleep.
He dissected out of frogs two beating hearts: one with the vagus nerve which controls heart rate attached, the other heart on its own. Both hearts were bathed in a saline solution (i.e. Ringer's solution). By electrically stimulating the vagus nerve, Loewi made the first heart beat slower. Then, Loewi took some of the liquid bathing the first heart and applied it to the second heart.
The application of the liquid made the second heart also beat slower, proving that some soluble chemical released by the vagus nerve was controlling the heart rate. He called the unknown chemical Vagusstoff. It was later found that this chemical was Acetylcholine
Muscarinic receptor agonist is an agent that activates the activity of the muscarinic acetylcholine receptor.
Muscarinic receptor antagonists bind to muscarinic receptors, thereby preventing ACh from binding to and activating the receptor. By blocking the actions of ACh, muscarinic receptor antagonists very effectively block the effects of vagal nerve activity on the heart. By doing so, they increase heart rate and conduction velocity.
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