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Lesson Starter

Learn about the structure of a cholinergic synapse and understand the role of neurotransmission in the transmission of action potentials. Explore the process of synaptic transmission and the function of acetylcholinesterase. Answer textbook questions.

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Lesson Starter

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  1. Lesson Starter • What is the function of Schwan cells in a neurone? • What is meant by a saltatory conduction? • What three factors speed up the rate of conduction of action potentials? • Produce myelin • When electrical impulses “jump” from one node of Ranvier to the next • Temperature, diameter of axon and presence of myelin

  2. Nerve Junctions A2 Biology Miss Tagore

  3. Learning Objectives • Describe with the aid of diagrams, the structure of a cholinergic synapse; • Outline the role of neurotransmission in the transmission of action potentials.

  4. The Structure of a Cholinergic Synapse • Synapses are the junctions between two or more neurones. • Here, neurones are able to signal to the next neurone in the sequence. • The synaptic cleft the gap between two neurons and is only 20nm wide. • Action potentials cannot cross the gap between two neurons so instead they release chemicals (transmitter substances) that diffuse across the cleft to the postsynaptic neurone. • Once here, a new action potential is generated. • Synapses that use acetylcholine as the neurotransmitter are called cholinergic synapses

  5. The Synaptic Knob • The presynaptic neuron ends in a swelling called the synaptic knob. This knob contains a number of specialised features: • Many mitochondria (active transport, therefore ATP required) • Large amount of smooth endoplasmic reticulum • Vesicles containing acetylcholine (the neurotransmitter) • Voltage-gated calcium ion channels in the membrane.

  6. The Postsynaptic Membrane • There are specialised sodium ion channels in the post synaptic membrane that respond to acetylcholine (the transmitter substance) • The ion channels consist of 5 polypeptides, two of which have special receptor sites specific to acetylcholine. • The receptor sites and acetylcholine fit together as they have complementary surfaces. • When acetylcholine binds to the sites, the sodium channels open.

  7. Transmission across the synapse • Action potential arrives at the synaptic knob • The voltage-gated calcium ion channels open • Calcium ions diffuse into the synaptic knob • The calcium ions cause the synaptic vesicles to move to and fuse with the presynaptic membrane • Acetylcholine is released by exocytosis • Acetylcholine molecules diffuse across the cleft • Acetylcholine molecules bind to the receptor sites on the sodium ion channels in the postsynaptic membrane • Sodium ions diffuse across the postsynaptic membrane into the postsynaptic neurone • A generator potential or excitatory postsynaptic potential (EPSP) is created • If sufficient generator potentials combine then the potential across the postsynaptic membrane reaches the threshold potential • A new action potential is created in the postsynaptic neurone. Once an action potential is achieved it will pass down the postsynaptic neurone.

  8. The role of acetylcholinesterase • Acetylcholinesteraseis an enzyme found in the synaptic cleft. • Its role is to hydrolyse acetylcholine to ethanoic acid and choline. • This stops transmission of signals so that the synapse does not continue to produce action potentials in the postsynaptic neurone. • The products of this degradation reaction are recycled • Ethanoic acid + choline + ATP -> Acetylcholine • Recycled acetylecholine is stored in synaptic vesicles for future use.

  9. What to do • Answer questions 1-3 on page 19 of the textbook

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