Synapses
- Created by: unicorn_rainbow
- Created on: 09-03-16 10:56
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- Synapses
- Transmission
- A junction between a neurone and the next neurone or effector cell
- The tiny gap between the cells of a synapse is called the synaptic cleft
- The presynaptic neurone has a swelling called the synaptic knob
- This contains synaptic vesicles filled with chemicals called neurotransmitters
- When an action potential reaches the end of a neurone, it causes neurotransmitters to be released into the synaptic cleft
- They diffuse across to the postsynaptic membrane and bind to specific receptors
- Because the receptors are only on the postsynaptic membranes, synapses make sure impulses are unidirectional- they only travel in one direction
- Neurotransmitters are removed from the cleft so the response doesn't keep happening
- Electrical impulses
- Electrical synapses in vertebrates contain many gap junctions
- Membrane proteins form tiny pores linking the two cells allowing movement of ions between them
- The potential difference and action potential can be transferred directly from one cell to another
- Electrical synapses allow for the very fast transmission of impulses from one neurone to anoher
- They are used for rapid signalling within the CNS
- They have many limitations and the chemical synapse is still regarded as the main method of communication between neurons
- Drugs
- Stimulation
- Nicotine: mimics the action of the acetylcholine on post synaptic membranes
- Stimulates the nervous system
- Strychnine + serine: inactivates cholinesterase at the post-synaptic membrane, preventing the breakdown of acetylcholine
- Enhances and prolongs the effects of acetylcholine as it is no longer destroyed, meaning nerves fire continuously
- Nicotine: mimics the action of the acetylcholine on post synaptic membranes
- Inhibition
- Curare: Interferes with the action of acetylcholine and stops the depolarisation of the post-synaptic membrane
- Causes paralysis as the muscles can no longer be stimulated by the nerevs
- Botulinus toxin: affects the presynaptic membrane and stops the release of acetylcholine
- Prevents transmission of impulses across synapses, preventing the nervous system from working
- Curare: Interferes with the action of acetylcholine and stops the depolarisation of the post-synaptic membrane
- Stimulation
- Electron micrographs
- Showed that the synaptic cleft is too wide for an impulse the size of an action potential to 'jump' across it (transmission chemical)
- Shows that after a nerve has been strongly stimulated, the neurone has a lack of vesicles (it has become accommodated)
- Normal transmission across the synapse results from the release of transmitter from a small number of vesicles
- Drugs and poisons can interfere with the working of synapses
- Substances that interfere with the formation of acetylcholine, stops it being released, prevents it reacting with synaptic receptors or reduces the rate its broken down will have a major impact on the nervous communication system of the body
- Sequence of events
- 1. An action potential arrives
- 2. The membrane depolarises. Calcium ion channels open. Calcium ions enter the neurone
- 3. Calcium ions cause synaptic vesicles containing neurotransmitter to fuse with the presynaptic membrane
- 4. Neurotransmitter is released into the synaptic cleft
- 5. Neurotransmitter binds with receptors on the postsynaptic membrane. Cation channels open. Sodium ions flow through the channels
- 6. The membrane depolarises and initiates an action potential
- 7. When released the neurotransmitter will be taken up across the presynaptic membrane (whole or after being broken down), or it can diffuse away and be broken down
- 6. The membrane depolarises and initiates an action potential
- 5. Neurotransmitter binds with receptors on the postsynaptic membrane. Cation channels open. Sodium ions flow through the channels
- 4. Neurotransmitter is released into the synaptic cleft
- 3. Calcium ions cause synaptic vesicles containing neurotransmitter to fuse with the presynaptic membrane
- 2. The membrane depolarises. Calcium ion channels open. Calcium ions enter the neurone
- 1. An action potential arrives
- Transmission
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