10 Minute Neuroscience: Synapses

Introduction

• Focus on synapses: specialized areas for neuron communication.
• Discussion includes components of a synapse, communication methods, and types of synapses.

What is a Synapse?

• A synapse allows neurons to communicate.
• Most neurons have thousands of synapses, contributing to the brain's capabilities.
• Estimates of synapses in the human brain: hundreds of trillions to over two quadrillion.
• Enables formation of neural circuits for tasks ranging from simple to complex functions.

Types of Synapses

Chemical Synapses

• More common than electrical synapses.
• Rely on neurotransmitters released from the axon terminals of one neuron to communicate with the dendrites of another (axodendritic synapses).
• Synaptic Cleft:
  • Space (20-40 nm wide) separating the presynaptic and postsynaptic neurons.
  • Presynaptic neuron sends signals; postsynaptic neuron receives them.

Electrical Synapses

• Less common in adults; more prevalent in embryos and invertebrates.
• Communicate through gap junctions (2-4 nm wide) connecting pre- and postsynaptic neurons.
• Allow ions to flow directly between neurons for fast, bidirectional communication.

Mechanism of Communication

Chemical Synapse Process

1. Action Potentials:

   • Electrical signals travel down the axon of the presynaptic neuron.

2. Synaptic Vesicles:

   • Vesicles containing neurotransmitters mobilize at the axon terminal.
   • Release neurotransmitters into the synaptic cleft via exocytosis.

3. Receptor Interaction:

   • Neurotransmitters bind to receptors on the postsynaptic neuron, affecting its likelihood to fire an action potential.

4. Removal of Neurotransmitters:

   • Essential to prevent overstimulation of receptors.
   • Methods of removal include:
     • Diffusion: Some neurotransmitters float away.
     • Enzymatic Degradation: Enzymes like acetylcholinesterase break down neurotransmitters.
     • Reuptake: Transport proteins in the presynaptic neuron recycle neurotransmitters for reuse.

Importance of Reuptake

• Target for medications (e.g., SSRIs) that increase neurotransmitter levels in the synaptic cleft to treat depression.

Comparison of Synapse Types

• Chemical Synapses:

  • Amplify signals; a weak current can release many neurotransmitters.

• Electrical Synapses:

  • Provide instantaneous communication and synchronize activity among neurons (e.g., brainstem neurons regulating breathing).

Conclusion

• Summary of the key points about synapses and their significance in neural communication.