15.06 Synapses

Structure of a Synapse

• Synaptic Cleft: A small 20 nm gap between two neurones where signal transmission occurs.
• Synapse Components:
  • Presynaptic Neurone: Neurone ending at the synapse, containing vesicles with neurotransmitter.
  • Postsynaptic Neurone: Neurone on the opposite side, with receptor proteins on its membrane.
  • Neurotransmitter: Chemical substance that transmits impulses across the synaptic cleft (e.g., acetylcholine).

Mechanism of Synaptic Transmission

1.Arrival of Action Potential:

• An action potential reaches the presynaptic neurone’s terminal, depolarizing the membrane.

2.Calcium Ion Influx:

• Depolarization opens voltage-gated calcium channels, allowing Ca²⁺ ions to enter the presynaptic neurone due to a steep electrochemical gradient.

3.Vesicle Fusion and Neurotransmitter Release:

• Ca²⁺ ions cause vesicles containing acetylcholine (ACh) to fuse with the presynaptic membrane, releasing ACh into the synaptic cleft via exocytosis.

4.Diffusion Across Cleft:

• ACh molecules diffuse rapidly (within 0.5 ms) across the cleft to the postsynaptic membrane.

5.Receptor Binding and Depolarization:

• ACh binds to ligand-gated ion channels on the postsynaptic membrane, causing these channels to open and allowing Na⁺ to enter.
• Na⁺ Influx: Depolarizes the postsynaptic membrane; if threshold potential is reached, it triggers an action potential.

6.Enzyme Breakdown of ACh:

• Acetylcholinesterase in the synaptic cleft breaks down ACh into acetate and choline to prevent continuous stimulation.

7.Recycling:

• Choline is reabsorbed by the presynaptic neurone, where it recombines with acetyl coenzyme A to form ACh, ready for the next impulse.

Types of Synapses and Examples

• Cholinergic Synapses: Use ACh as the neurotransmitter, found in the central and peripheral nervous systems.
• Neuromuscular Junction: Specialized synapse between a motor neurone and muscle fibre, leading to muscle contraction.

Functions and Advantages of Synapses

• One-Way Transmission: Ensures impulses travel in one direction only, as neurotransmitter release occurs only from the presynaptic side.
• Signal Integration and Pathway Interconnection:
  • Divergence: One neurone can stimulate multiple postsynaptic neurones, allowing one signal to affect multiple effectors (e.g., in reflex responses).
  • Convergence: Multiple neurones can stimulate a single postsynaptic neurone, allowing integration of information from various sources, crucial for complex processing in the brain.
• Modulation of Impulse Strength:
  • Synapses allow the brain to prioritize and filter signals, enhancing decision-making and coordinated responses.

Synaptic Fatigue

• Synaptic Fatigue: If action potentials arrive continuously, synapses can temporarily deplete their neurotransmitter supply, leading to a temporary inability to transmit signals.

Summary Diagram of Synaptic Transmission

• Acetylcholinesterase breaks down ACh, recycling components back to the presynaptic neurone.
• Action potential arrives at the presynaptic terminal.
• Calcium channels open, allowing Ca²⁺ to enter.
• Synaptic vesicles release neurotransmitter (ACh) by exocytosis.
• ACh diffuses across the synaptic cleft.
• ACh binds to receptors on the postsynaptic membrane, opening Na⁺ channels.
• Depolarization occurs, possibly triggering an action potential.

Key Terms

• Synapse: Junction between two neurones, including the presynaptic and postsynaptic membranes and synaptic cleft.
• Synaptic Cleft: Gap between two neurones where neurotransmitter is released and diffuses across.
• Neurotransmitter: Chemical that transmits nerve impulses across synapses (e.g., acetylcholine, noradrenaline).
• Presynaptic Neurone: The neurone releasing neurotransmitter into the synapse.
• Postsynaptic Neurone: The neurone receiving neurotransmitter, triggering depolarization.
• Acetylcholinesterase: Enzyme that breaks down acetylcholine in the synaptic cleft.
• Neuromuscular Junction: Synapse between a motor neurone and muscle, leading to muscle contraction.
• Ligand-Gated Channel: Protein channel on the postsynaptic membrane that opens in response to neurotransmitter binding.