Lecture Notes on Synapses

Transmission of Information in Neurons

• Information transmission along a neuron is primarily an electrical process.
• At synapses, the primary event is a chemical process, not electrical.
• Discovery of chemical reactions at synapses occurred in the early 20th century.

Historical Background

• Sherrington:
  • Identified the existence of synapses.
  • Concluded different physiological processes at synapses slowed down information transmission compared to axons.
  • Initially assumed synaptic transmission was electrical.
• T.R. Elliott (1905):
  • Conducted experiments with adrenaline on organs (heart, stomach, pupils).
  • Proposed that the sympathetic nervous system functions by releasing chemicals like adrenaline.
  • Ideas were largely ignored due to Sherrington's influence.
• Otto Loewi (1920):
  • Inspired by Elliott's ideas.
  • Conducted an experiment (inspired by a dream) proving chemical communication between neurons.
  • Used frog hearts to demonstrate chemicals affect heart rate, supporting the chemical transmission hypothesis.
  • Concluded neurotransmitters are responsible for synaptic transmission.

Neurotransmitters

• Chemicals released at synapses affecting postsynaptic neurons.
• Approximately 100 molecules function as neurotransmitters.
• Most are amino acids; exceptions exist, such as nitric oxide (a gas).
• Common Neurotransmitters:
  • Amino Acids: Glutamate, GABA, Glycine.
  • Others related to amino acids: Acetylcholine, Serotonin, Dopamine, Norepinephrine, Epinephrine.
  • Peptides: Endorphins, Substance P.
  • Gases: Nitric oxide.

Synthesis and Storage of Neurotransmitters

• Raw materials for neurotransmitters are derived from dietary amino acids.
• Examples:
  • Acetylcholine: Formed from acetate (vinegar, lemon) and choline (milk, eggs, nuts).
  • Serotonin: Formed from tryptophan (found in soybeans).
  • Dopamine, Epinephrine, Norepinephrine: Derived from phenylalanine and tyrosine (high protein foods).
• Neurotransmitters are stored in synaptic vesicles at presynaptic terminals.

Visualizing Synapses

• Synapses can be visualized using electron micrographs.
• Images Explained:
  • Image A: Shows presynaptic terminal and postsynaptic dendritic spine with synaptic gap.
  • Image B: Highlights presynaptic terminals in blue and postsynaptic neuron cell body in orange.

Next Steps

• Upcoming discussions will cover the sequence of events at synapses facilitating neuronal communication.