13.5 Synapses Flashcards
What is a synapse?
The area which connects 2 neurones
What chemicals are used to transmit impulses across a synapse?
Neurotransmitters
What are the 6 key structures of a synapse?
- Synaptic cleft
- Presynaptic neuron
- Postsynaptic neuron
- Synaptic knob
- Synaptic vesicles
- Neurotransmitter receptors
Describe what the synaptic cleft is.
Gap which separates the axon of one neuron from the dendrite of another neuron
20-30nm across.
Describe what the presynaptic neuron is.
Neuron along which impulse has arrived
Describe what the postsynaptic neuron is.
Neuron that receives the neurotransmitter.
Describe what the synaptic knob is.
The swollen end of a presynaptic neuron
What does the synaptic knob contain?
Lots of mitochondria and endoplasmic reticulum to enable it to make neurotransmitters.
Describe what the synaptic vesicle is.
Vesicles containing neurotransmitters.
How do synaptic vesicles release their contents into the synaptic cleft?
By fusing with the presynaptic membrane.
What happens when a synaptic vesicle fuses with the presynaptic membrane?
The vesicle releases its contents into the synaptic cleft.
Describe what neurotransmitter receptors are.
Receptors which neurotransmitters bind to in the postsynaptic membrane.
What are the 2 types of neurotransmitters?
- Excitatory
2. Inhibitory
What is the function of excitatory neurotransmitters?
Cause depolarisation of the postsynaptic neuron.
If the threshold is reached in the postsynaptic membrane, an action potential is triggered.
What is one example of an excitatory neurotransmitter?
Acetylcholine.
What is the function of inhibitory neurotransmitters?
Cause hyperpolarisation of postsynaptic membrane.
This prevents an action potential being triggered.
What is one example of an inhibitory neurotransmitter?
Gamma-aminobutyric acid (GABA), found in parts of brain
What are the 8 stages in synaptic transmission?
- AP reaches end of presynaptic neuron
- Depolarisation of presynaptic membrane = Ca ion channels open
- Ca ions diffuse into the presynaptic knob
- This causes synaptic vesicles to fuse w/ presynaptic membrane. Neurotransmitter is released into the synaptic cleft by exocytosis.
- Neurotransmitter diffuses across synaptic cleft and binds w/ its specific receptor molecule on postsynaptic membrane
- This causes Na ion channels to open
- Na ions diffuse into the postsynaptic neuron
- This triggers an AP and impulse is propagated along the postsynaptic neurone
What happens after a neurotransmitter has triggered an action potential?
- The neurotransmitter must be removed from the postsynaptic neurone so the stimulus isn’t maintained
- The neurotransmitter is broken down by enzymes and the products are absorbed into the presynaptic knob
What neurotransmitter do cholinergic synapses use?
Acetylcholine
Where are cholinergic synapses found?
Neuromuscular junctions
What is a neuromuscular junction?
Where a motor neurone meets with a muscle cell (effector)
How does transmission across cholinergic synapses work? 6 stages.
- Arrival of AP at presynaptic neurone causes Ca ion channels to open. Ca ions enter synaptic knob.
- Influx of Ca ions causes synaptic vesicles to fuse w/ presynaptic membrane, releasing acetylcholine into synaptic cleft.
- Acetylcholine fuses w/ receptor sites on Na ion channel in postsynaptic membrane. This causes Na ion channels to open, allowing Na ions to rapidly diffuse along a conc. gradient
- Influx of Na ions generates new AP in postsynaptic neurone
- Acetylcholinesterase hydrolyses ACE into choline and ethanoic acid, which diffuses back across the synaptic cleft into the presynaptic neurone.
- ATP released by mitochondria is used to recombine choline+ethanoic acid into ACE, stored for future use. Na ion channels close.
What enzyme hydrolyses acetylcholine?
Acetylcholinesterase
