Physiology of Synapses

Introduction

• The human brain contains approximately 86 billion neurons.
• Neurons communicate via electrochemical signals at synapses.
• Synapses consist of a presynaptic and postsynaptic terminal.
  • Presynaptic terminal: located at the end of an axon, converts electrical signals into chemical signals.
  • Postsynaptic terminal: contains receptors that receive neurotransmitters.
• The type of neurotransmitter and receptor determines the quality of information transmission.

Cellular Level

• Neurons: Composed of a cell body, axons, and dendrites.
  • Cell Body: Contains the nucleus and is the site of neurotransmitter synthesis.
  • Dendrites: Receive incoming signals and relay them to the cell body.
  • Axons: Transmit signals away from the cell body.
• Synapse Types:
  • Axodendritic: Excitatory, connection between axon and dendrite.
  • Axosomatic: Inhibitory, connection between axon and cell body.
  • Axoaxonic: Regulatory, affects neurotransmitter release.
• Chemical synapses involve neurotransmitter release; electrical synapses use gap junctions.

Development

• Neurons develop from the embryonic neural tube.
• Neurons differentiate based on size, with motor neurons developing first.
• Glial cells assist in neuron differentiation and neurotransmitter reuptake.

Mechanism

Types of Synapses

• Chemical Synapses: Involve neurotransmitter release, synaptic delay of 0.5 to 1 ms.
• Electrical Synapses: Use connexins for direct current passage, shorter synaptic delay.

Neurotransmitters

• Acetylcholine (ACh): Excitatory, found at neuromuscular junctions.
• Norepinephrine (NE): In sympathetic nervous system.
• Dopamine (DA): Inhibitory, associated with Parkinson's disease.
• Serotonin (5-HT): Regulatory, affects mood states.

Synaptic Transmission Steps

1. Synthesis: Neurotransmitters synthesized and transported in vesicles.
2. Release: Triggered by action potential and calcium influx.
3. Receptor Activation: Binds to receptors on the postsynaptic neuron.
4. Signal Termination: Involves reuptake, enzymatic destruction, or diffusion.

Signal Termination Methods

• Reuptake: Small-molecule neurotransmitters reabsorbed by presynaptic neuron or glial cells.
• Enzymatic Destruction: Breakdown by enzymes like Monoamine Oxidases and COMT.
• Diffusion: Neurotransmitter diffuses away from synapse.

Clinical Significance

• Myasthenia Gravis: Autoimmune blockade of ACh receptors leads to muscle weakness.
• Lambert-Eaton Syndrome: Autoimmune disruption of calcium channels prevents neurotransmitter release.
• Botulism: Toxin prevents ACh release, causing paralysis.
• Tetanus: Toxin inhibits inhibitory neurotransmitters, causing spastic paralysis.

Important Enzymes

• Monoamine Oxidases (MAO): Break down serotonin, norepinephrine, dopamine.
• Catechol-O-Methyltransferase (COMT): Degrades catecholamines like dopamine.

These notes capture essential information on synaptic function, types, development, and clinical relevance, making them a useful study guide.