Synaptic Transmission
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- Created by: Rachelezy
- Created on: 17-02-20 17:20
Structure of a Synapse
- Synaptic Cleft - Gap prevents electrical impulses from passing directly from one neurone to another. Contains enzymes that break down the neurotransmitter
- Information travels from the pre-synaptic neurone to the post-synaptic neurone
- Post-synaptic Neurone - Membrane has neurotransmitter receptors. These are specialised ion channels that open or close when a neurotransmitter binds to a specific receptor site
- Synaptic Knob - Filled with vesicles of neurotransmitters (e.g. acetylcholine) which are chemicals that stimulate the post-synaptic neurone. Also consists of voltage-gated calcium channels that detect when an action potential arrives at the end of the axon
- Vesicles of Neurotransmitters - made in the smooth endoplasmic reticulum (SER)
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Neuromuscular Junctions
Point at which a motor neurone meets a skeletal muscle fibre
- At rest, vesicles containing acetylcholine present in presynaptic motor neurone
- Action potential reaches NJ, voltage-gated calcium ion channels open, causing calcium ions to diffuse into the neurone
- Calcium ions cause synaptic vessel to fuse with the presynaptic membrane and release acetylcholine into the synapse
- Diffuses across synapse and binds with receptors on muscle cell membrane, results in muscle contraction
- When action potential stops, the acetylcholine is broken down by ethanoic acid and choline by acetylcholinesterase (enzyme)
- Ethanoic acid and choline diffuse back into presynaptic neurone
- Once returned, ethanoic acid and choline are recombined into acetylcholine (using energy provided by mitochondria in the presynaptic region)
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Events involved in transmission across a cholinerg
- Action potential arrives at the pre-synaptic membrane of an axon
- Causes calcium ion channels to open
- Calcium ions rapidly move by facilitated diffusion into the synaptic knob (because Ca2+ is higher in cleft than in the knob)
- When Ca2+ enters, some vesicles move to pre-synaptic membrane and fuse with it, releasing acetylcholine into the synaptic cleft
- Diffuses across short distance to post-synaptic membrane
- Post-synaptic membrane has receptor proteins to which acetylcholine molecules attach to. (Binding sites complementary to acetylcholine)
- Each receptor protein located next to a sodium ion channel. When acetylcholine binds, changes shape and pushing sodium channel open
- If enough sodium ions enter, and the threshold is exceeded, an action potential is triggered
- Travels along the neurone as an impulse until it reaches the synapses at the other end.
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Aspects of the neurone that ensures that impulses
Unidirectional - travels in one direction
- Hyperpolarisation - inactive ion channels
- Neurotransmitters only present in vesicles in the pre-synaptic neurone
- Dendrites - IN (only at one end)
- Axon Terminal - EXIT (only at one end)
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Summation
Temporal (TIme):
- Effects of several impulses are added together in a short space of time
- If several impulses reach the synaptic knob in quick succession, enough acetylcholine is released for the membrane potential to reach its threshold value
- This causes sodium ion channels to open and produces an action potential
Spatial:
- Motor neurones release enough neurotransmitters to exceed the threshold value which triggers an action potential
- Several impulses arrive simultaneously at different synaptic knobs stimulating the cell body
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Inhibition
- Reduces membrane potential
- Hyperpolarises membrane (action potential cannot be created)
- Helps to enable specific pathways to be stimulated while prevenitng random impulses all over the body
Excitatory Post-synaptic Potential - Post-synaptic membrane is depolarised. May or may not reach its threshold (goes towards the threshold) (Excitatory synapse)
Inhibitory Post-synaptic Potential - Post-synaptic membrane is hyperpolarised away from its threshold (Inhibitory synapse) - Chlorine ions enter
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Effects of drugs on synapses
Curare:
- Blocks the effects of acetylcholine by blocking nicotinic cholinergic receptors at neuromuscular junctions, so muscle cells cannot be stimulated
- Results in the muscle being paralysed (Post-synaptic neurone cannot generate an action potential
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Neuromuscular Junctions VS Cholinergic Synapses
Excitatory or Inhibitory?
- NJ - Only excitatory CS - Can be both
What does this structure link?
- NJ - Neurones to muscles CS - Neurone to neurone OR Neurone to other effector
Structure's relation to action potentials?
- NJ - Action potential ends here CS - Another a.p. is generated along a different neurone
What type of neurone is involved?
- NJ - Motor neurones CS - Intermediate, motor and sensory neurones
Where does acetylcholine bind?
- NJ - Receptors on membrane of muscle fibre CS - Receptors on membrane of post-synaptic membrane
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