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

Nerve Junctions A2 Biology Miss Tagore

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.

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

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.

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.

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.

The role of acetylcholinesterase Acetylcholinesterase is 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.

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