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.