Download Nerve Junctions

Lesson Starter
1. What is the function of Schwan cells in a neurone?
2. What is meant by a saltatory conduction?
3. What three factors speed up the rate of conduction of action potentials?
1. Produce myelin
2. When electrical impulses “jump” from one node of Ranvier to the next
3. 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
1.
2.
3.
4.
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
5. Acetylcholine is released by exocytosis
6. Acetylcholine molecules diffuse across the cleft
7. Acetylcholine molecules bind to the receptor sites on the sodium ion
channels in the postsynaptic membrane
8. Sodium ions diffuse across the postsynaptic membrane into the
postsynaptic neurone
9. A generator potential or excitatory postsynaptic potential (EPSP) is
created
10. If sufficient generator potentials combine then the potential across the
postsynaptic membrane reaches the threshold potential
11. 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