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Understanding Synapses and Neurotransmitters

Sep 14, 2024

Lecture Notes on Synapses

Introduction

  • Transmission of information in neurons is primarily electrical.
  • Synapses also involve electrical processes but are mainly chemical.

Historical Context

  • Sherrington:

    • Identified the existence of synapses in the early 20th century.
    • Concluded that synaptic transmission is slower than axonal transmission, believing it was electrical.
    • Faced opposition to his conclusions from other scientists.
  • T.R. Eliot (1905):

    • Conducted studies on the effects of adrenaline in different organs.
    • Proposed sympathetic nervous system works by releasing adrenaline or similar chemicals.
    • His findings were largely ignored due to Sherrington's reputation.
  • Otto Loewy (1920):

    • Inspired by a dream, he aimed to prove neurons communicate through chemicals.
    • Conducted experiments with frog hearts to support his hypothesis.

Loewy's Experiments

  • Dissected frogs and placed their hearts in fluid containers.

  • Vagus Nerve Stimulation:

    • Slowed the heart rate of the first frog’s heart.
    • Transferred fluid to the second heart, observing a similar decrease in heart rate.
  • Stimulating another nerve:

    • Increased heart rate in the first frog’s heart.
    • Transferred the fluid again, observing the same increase in the second heart.

Conclusion from Experiments

  • Loewy concluded that changes in heart rate were due to chemical molecules transferred via fluid, not electrical ions.

Neurotransmitter Molecules

  • Chemicals released at synapses affecting postsynaptic neurons are called neurotransmitters.
  • Approximately 100 neurotransmitters identified, mainly amino acids.
    • Main Types:
      • Amino Acids: Glutamate, GABA, Glycine.
      • Others: Acetylcholine, Serotonin, Dopamine, Norepinephrine, Epinephrine.
      • Peptides: Endorphins, Substance P.
      • Gaseous: Nitric oxide.

Formation of Neurotransmitters

  • Raw materials primarily obtained from food:
    • Acetylcholine: Formed from acetate (from vinegar, lemon) and choline (from milk, eggs, nuts).
    • Serotonin: Derived from tryptophan (found in soybeans).
    • Dopamine, Epinephrine, Norepinephrine: Formed from phenylalanine and tyrosine (found in high-protein foods).

Storage of Neurotransmitters

  • After synthesis, neurotransmitters are stored in synaptic vesicles at the presynaptic terminals.

Visualizing Synapses

  • Image A: Electron micrograph showing presynaptic terminal and postsynaptic dendritic spine, with synaptic gap highlighted.
  • Image B: Diagram showing presynaptic terminals and cell body of postsynaptic neuron.

Conclusion

  • Discussion on synapses will continue in the next lecture, focusing on events that facilitate neuronal communication.