Cholinergic Synapses | Cambridge (CIE) A Level Biology Revision Notes 2023

Overview

• Synapses are the junctions where two neurones meet, separated by a small gap known as the synaptic cleft.
• The combination of the ends of two neurones and the synaptic cleft forms a synapse.

Synaptic Transmission: Basic Mechanism

• Electrical impulses cannot cross synapses. Instead, chemical messengers called neurotransmitters are released.
• When an electrical impulse reaches the end of the axon on the presynaptic neurone, neurotransmitters are released from vesicles at the presynaptic membrane.
• These neurotransmitters diffuse across the synaptic cleft and bind temporarily to receptor molecules on the postsynaptic membrane.
• This binding stimulates the postsynaptic neurone to generate an electrical impulse that travels down its axon.
• The neurotransmitters are then destroyed or recycled to prevent repeated impulses.

Detailed Mechanism of Synaptic Transmission

• Over 40 different neurotransmitters exist; a key one is acetylcholine (ACh).
• Synapses using ACh are termed cholinergic synapses.
• Steps:
  1. Action potential arrives at the presynaptic membrane, causing depolarisation.
  2. Voltage-gated calcium ion channels open, allowing calcium ions to diffuse into the presynaptic neurone.
  3. This influx stimulates ACh-containing vesicles to release ACh into the synaptic cleft.
  4. ACh diffuses across the cleft and binds to receptor proteins on the postsynaptic membrane.
  5. This causes a conformational change, opening channels that allow sodium ions to enter the postsynaptic neurone, leading to depolarisation.
  6. To prevent continuous depolarisation, ACh is broken down by the enzyme acetylcholinesterase into acetate and choline.
  7. Choline is reabsorbed into the presynaptic membrane, reacts with acetyl coenzyme A to reform ACh, which is repackaged into vesicles.
• This process takes approximately 5-10 ms.

Key Points

• Neurotransmitter Recycling: Prevents indefinite stimulation and helps maintain synaptic function.
• Speed: The entire synaptic transmission is rapid, ensuring quick response times in neural communication.

Additional Information

• Understanding synaptic transmission is crucial for comprehending how the nervous system communicates and responds to stimuli.

Note: These notes are based on resources provided by Save My Exams and aligned with the Cambridge (CIE) A Level Biology curriculum.