Lecture on Synapses

Overview of Synapses

• Synapses are functional units that mediate the transfer of information between neurons or from neurons to effector cells (e.g., muscle cells, glandular cells).
• Presynaptic Neuron: Conducts impulses towards the synapse, releases chemicals.
• Postsynaptic Neuron: Receives information, transmits electro-signals away from the synapse, and can be a neuron, muscle, or glandular cell.
• Neurons can function as both presynaptic and postsynaptic in a chain.

Structure and Types of Synapses

• Axon Terminals: Expanded tips of axons where synapses occur, filled with synaptic vesicles containing neurotransmitters.
• Synapse Locations:
  • Axodendritic: Between axon terminals and dendrites of another neuron.
  • Axosomatic: Between axon terminals and soma (cell body) of another neuron.
  • Other Types: Axoaxonic, dendrodendritic, somatodendritic (less common).

Classes of Synapses

• Chemical Synapses:

  • Utilize neurotransmitters for communication.
  • Can occur at various locations like axosomatic or axodendritic.
  • More steps involved allowing for regulation and modification.

• Electrical Synapses:

  • Use gap junctions for direct ion flow between cells.
  • Faster than chemical synapses but less regulatable.
  • Found in specific brain regions and embryonic tissue.

Information Transmission Steps

1. Action potential travels down axon.
2. Triggers voltage-gated calcium channels at axon terminal.
3. Calcium influx leads to synaptic vesicle fusion with the axon membrane.
4. Neurotransmitter release into synaptic cleft.
5. Neurotransmitter binds to postsynaptic receptors.
6. Ion channels open causing graded potentials (excitatory or inhibitory).

Termination of Neurotransmitter Effects

• Reuptake: Neurotransmitters reabsorbed by presynaptic neuron.
• Degradation: Enzymatic breakdown, e.g., acetylcholinesterase.
• Diffusion: Neurotransmitters diffuse away from synaptic cleft.

Key Concepts

• Synaptic Cleft: Small space between presynaptic and postsynaptic neurons.
• Synaptic Delay: Time for neurotransmitter release and binding, affecting transmission speed.
• Graded Potentials:
  • Excitatory: Depolarize the cell, increasing action potential likelihood.
  • Inhibitory: Hyperpolarize the cell, decreasing action potential likelihood.

Comparison: Chemical vs. Electrical Synapses

• Chemical Synapses:

  • Slower with more steps but highly regulatable.
  • Modification possible through neurotransmitter/receptor amounts and activity.

• Electrical Synapses:

  • Faster due to direct ion flow.
  • Limited in regulation and modification, simple structure.

Applications and Importance

• Reflexes: Influence speed based on synaptic complexity.
  • Monosynaptic Reflexes: Faster due to single synapse.
  • Polysynaptic Reflexes: Slower, involve multiple synapses.
• Brain Regions: Electrical synapses in rapid response areas like the hippocampus and for eye movements.

Conclusion

• Synapses are crucial for neuron communication.
• Understanding synapse types and mechanics is key for comprehending the nervous system's functioning.