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.

5. 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

7. 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.

9. 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.

11. 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.

12. 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.

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