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What is a synapse?
The junction that occurs between two neurones
How do impulses cross the synaptic gap?
Through chemical transmitters
What is the most common neurotransmitter?
What are synapses containing acetylcholine called?
What is the first stage of synaptic transmission across cholinergic synapses?
An action potential arrives at the synapse, depolarising the synaptic knob
What is the second stage of synaptic transmission across cholinergic synapses?
- Protein channels specific to calcium ions open
- Calcium ions diffuse into the synaptic knob
What is the third stage of synaptic transmission across cholinergic synapses?
Synaptic vesicles carrying acetylcholine fuse with the pre-synaptic membrane and release the neurotransmitter into the synaptic cleft
What is the fourth stage of synaptic transmission across cholinergic synapses?
The acetylcholine diffuses across the cleft and binds with specific protein receptors on the post synaptic membrane due to their complementary tertiary structure
What is the fith stage of synaptic transmission across cholinergic synapses?
- Protein channels specific to sodium ions open in the post synaptic membrane
- This allows sodium ions to diffuse into the neurone
- This causes depolarisation of the post synaptic membrane
What is the sixth stage of synaptic transmission across cholinergic synapses?
If depolarisation exceeds the threshold value an action potential is initiated
What is the seventh stage of synaptic transmission across cholinergic synapses?
- Acetylcholine in the cleft is broken down by the enzyme, acetylcholinesterase
- The products are reabsorbed by the synaptic knob and the acetylcholine is re-synthesised
- This prevents the acetylcholine from remaining in the cleft and continuing to produce action potentials in the adjoining neurone
What is the neuromuscular junction?
The synapse between the motor neurone and muscle
How is the impulse transmitted across the neuromuscular junction?
The same sequence of events as in the cholinergic synapse except that the post synaptic membrane is the muscle fibre (sarcolemma) and its depolarisation leads to a sequence of events resulting in muscle contraction
How are synapses unidirectional?
- Vesicles are only present in the presynaptic neurone
- Synapses can only pass impulses in one direction
What requirements must be met for the potential difference across the synapse to be changed?
The threshold value must be reached
What may result from a low frequency of action potentials sent from the pre synaptic neurone?
Sufficient neurotransmitters are not released to lead to an action potential in the post synaptic membrane
What is spatial summation?
A number of pre synaptic neurones converge providing a combined amount of neurotransmitter capable of exceeding threshold value
What is temporal summation?
A higher frequency of action potentials along a single pre synaptic neurone may release enough neurotransmitter to exceed threshold value
To where do neurotransmitters bind in inhibitory synapses?
The protein channel specific to negatively charged chloride ions
What is the result of neurotransmitter binding in inhibitory synapses?
- The diffusion of chloride ions into the post synaptic membrane makes the inside of the neurone more negative
- This makes it less likely for threshold value to be reached and action potentials to be produced
How may drugs alter synaptic transmission?
- Some have a similar shape to the transmitter and bind to the protein receptors on the post synaptic membrane causing sodium channels to open producing action potentials
- Some have a similar shape to the transmitter and block the protein receptors on the post synaptic membrane preventing the neurotransmitter from binding and action potentials being generated
- Some inhibit acetycholinesterase or inhibit the uptake of the transmitter so the transmitter remains in the cleft and continues to produce action potentials
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