Understanding Synapses

Introduction to Synapses

• Synapses are anatomical structures that facilitate communication between cells.
• Not limited to neurons; other cell types, like cardiac muscle cells, can also form synapses.

Types of Synapses

• Electrical Synapse

• Definition: Involves gap junctions that allow ions and small molecules to pass freely between adjacent cells.

• Gap Junctions: Special channels that enable rapid communication, allowing molecules like sodium, potassium, or calcium to flow between cells.

• Speed: Very fast due to direct connection, with no synaptic delay.

• Chemical Synapse

• Definition: Involves release of neurotransmitters from presynaptic cells to postsynaptic cells.

• Components:

  • Presynaptic terminal: Where neurotransmitters are released.
  • Synaptic vesicles: Store neurotransmitters.
  • Synaptic cleft: Gap between presynaptic and postsynaptic membranes.
  • Receptors: Located on the postsynaptic membrane that bind neurotransmitters.

• Process of Communication:

  1. Neuron generates an action potential.
  2. Action potentials travel down the axon to presynaptic terminals.
  3. Arrival of the action potential causes voltage-gated calcium channels to open and calcium enters the presynaptic terminal.
  4. Increased calcium concentration triggers synaptic vesicles to fuse with the presynaptic membrane (exocytosis).
  5. Neurotransmitters are released into the synaptic cleft.
  6. Neurotransmitters bind to receptors on postsynaptic membrane, resulting in changes to the postsynaptic cell's membrane potential (EPSP or IPSP).
  7. If neurotransmitter action is completed, neurotransmitters can be reuptaken into the presynaptic terminal.

Postsynaptic Potentials

• EPSP (Excitatory Postsynaptic Potential):
• Depolarization of the postsynaptic membrane, making it more positive.
• IPSP (Inhibitory Postsynaptic Potential):
• Hyperpolarization of the postsynaptic membrane, making it more negative.

Summation of Potentials

• Definition: Integration of EPSPs and IPSPs determines whether the postsynaptic neuron will generate an action potential.
• Types of Summation:
• Spatial Summation: Simultaneous stimulation from multiple synapses.
• Temporal Summation: Rapid stimulation from the same synapse.

Synaptic Plasticity

• Definition: Ability of synapses to change in strength and efficacy over time, which is crucial for learning and memory.
• Examples:
• Increased release of neurotransmitters from presynaptic terminals.
• Changes in number of receptors on postsynaptic membrane (more receptors = greater sensitivity).
• Alterations in the number of synapses (addition or pruning of synapses).
• All changes are not random; they are regulated processes.

Conclusion

• Synaptic transmission and plasticity are fundamental to understanding how neurons communicate, how networks function, and the basis of learning and memory.
• Importance of regulating neurotransmitter release and receptor activation to maintain healthy neural function.