Synaptic Transmission and Plasticity

May 28, 2024

Module Two Lecture: Synapses, Synaptic Transmission, and Synaptic Plasticity

Recap of Module One

  • Action potentials
  • Receptors
  • Types of chemical synapses
    • Ionotropic (ligand-gated ion channels)
    • Metabotropic (G protein-coupled receptors)

Module Two Overview

  • Focus on synapses, synaptic transmission, and synaptic plasticity

Types of Synapses

1. Chemical Synapses

  • Chemical synapses featured in module one
  • Neurotransmitter removal from the synapse
  • Neuroplasticity (synaptic plasticity and learning)
  • Anterograde (presynaptic to postsynaptic) and retrograde communication (reverse direction)

2. Electrical Synapses

  • Also known as gap junctions
  • Formation: two pores forming a tunnel between cell membranes
  • Ions (charge carriers) flow bi-directionally and rapidly
  • Made of connexins (6 subunits = 1 connexon)
  • Location: cortex, hippocampus, neurons, neuroglial interactions (e.g., astrocytes, oligodendrocytes)
  • Simple messages (depolarizing or hyperpolarizing currents)

3. Neuromuscular Junction (NMJ)

  • Specialized chemical synapse involving motor neurons and muscle cells
  • Neurotransmitter: acetylcholine (ACH)
  • Causes muscle contraction (excitation-contraction coupling)
  • Enzymatic breakdown of acetylcholine essential for quick synaptic transmission (acetylcholinesterase)

4. Volume Transmission

  • Slow and non-deliberate form of neurotransmission
  • Cerebrospinal fluid (CSF) carries neurotransmitters
  • Location: brain ventricles and spinal cord
  • Efficacy is low but requires minimal energy

Neurotransmitter Categories

1. Amino Acids

  • Simple and most abundant in brain
  • Examples: glutamate (excitatory), GABA (inhibitory), glycine (inhibitory in spinal cord)

2. Amines

  • Involved in more complex signaling
  • Examples: dopamine, acetylcholine, serotonin, histamine

3. Peptides

  • Chains of amino acids
  • Examples: enkephalins, endorphins (pain relief), dynorphin

Synthesis of Neurotransmitters

Peptides

  • Precursor peptide synthesized in the Soma
  • ER → Golgi: precursor peptide + active peptide
  • Stored in secretory granules

Amines and Amino Acids

  • Synthesis at terminal end
  • Enzymes cleave and combine molecules into neurotransmitters
  • Stored in synaptic vesicles

Neurotransmitter Release Process

  • Review of exocytosis: snare complex, synaptotagmin, calcium involvement
  • Reuptake: endocytosis, Transporters, clathrin-mediated endocytosis
  • Diffusion and Volume Transmission

Reuptake and Synaptic Termination

  • Endocytosis: dynamin helps pinch off the membrane, formation of vesicles
  • Methods of termination: diffusion, reuptake, enzymatic destruction (especially at NMJ), desensitization
  • Application Example: SSRIs in depression (block serotonin reuptake)
  • Enzyme specificity at NMJ (acetylcholinesterase)

Synaptic Plasticity

  • Plasticity: ability of synapses to strengthen/weaken
  • Learning: acquisition of new information
  • Hebbian Theory: neurons that fire together wire together
    • Hebbian synapses: strengthening connections through simultaneous activation

Long-Term Potentiation (LTP)

  • LTP: high-frequency stimulation creating lasting enhancement
  • Experiment: Shaffer collaterals in hippocampus, potentiation of electrical signals
  • Key Players: glutamate, AMPA receptors, NMDA receptors

Mechanism of LTP

  • Depolarization: AMPA receptor activation, Mg2+ removal from NMDA receptor
  • Calcium influx, activating protein kinase C, CAM Kinase II
  • Retrograde signaling: nitric oxide increases glutamate release
  • Increased number of AMPA receptors

Long-Term Depression (LTD)

  • LTD: low-frequency, out-of-sync activation, weakening connections
  • Key Players: lesser NMDA receptor activity, protein phosphatases (remove AMPA receptors)

Differences Between LTP and LTD

  • LTP: protein kinases, more calcium, stronger connections, synaptic strengthening
  • LTD: protein phosphatases, less calcium, weakened connections, synaptic weakening

Summary

  • Types of synapses: chemical, electrical, neuromuscular junction, volume transmission
  • Neurotransmitters: categories (amino acids, amines, peptides)
  • Synaptic processes: synthesis, release, removal/reuptake
  • Synaptic plasticity: mechanisms of LTP and LTD, importance for learning