Synaptic Transmission

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  • Created by: LBCW0502
  • Created on: 25-09-18 18:10
What are dendrites?
Branches which form the dendritic tree. They receive information from other neurons
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What is the function of the cell body (soma)?
Contains the nucleus and drives maintenance and metabolism. Integrates information received by the dendrites
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Where is the action potential initiated in the neuron?
At the axon hillock (all or none)
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Describe features of the axon
Variable length and conducts action potentials. Axon can be insulated by myelin or may not. Insulated axons conduct more quickly than uninsulated axon (saltatory conduction)
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What are synapses?
The junction between neurons which allows communication with other cells. This can be chemical (predominant) or electrical
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What is the skeletal neuromuscular junction?
Motor nerves with their cell bodies in the CNS communicate with skeletal muscle to control movement
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Describe characteristics of a chemical synapse
Pre-synaptic neuron (contains neurotransmitters in synaptic vesicles, synaptic bouton), synaptic cleft (distinct synaptic delay), post-synaptic cell (contain receptors). Transmission is unidirectional
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For any chemical synapse there must be which three things? (1)
A chemical that acts as a neurotransmitter (synthesised/stored in pre-synaptic cell and activates receptors on post-synaptic cell, defines nature of signal, excitatory or inhibitory).
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For any chemical synapse there must be which three things? (2)
Mechanism to trigger release of neurotransmitter into synaptic cleft at an appropriate time
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For any chemical synapse there must be which three things? (3)
A mechanism for removing the neurotransmitter so that the synaptic signal can be turned off
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Whenever you come across a chemical synapse, you must consider what (2) ?
What is the neurotransmitter? What receptors does it act upon?
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What are the five key points at which drugs might interfere with the process of chemical synaptic transmission?
Synthesis, storage, release, exocytosis and removal
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What are the two main types of chemical neurotransmitters in the NS?
Small organic molecules and peptides
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Give examples of small organic molecules
ACh, NE, dopamine, seretonin, glutamate, GABA, ATP
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Give examples of peptides
Substance P, calcitonin gene related peptide (CGRP), neuropeptide Y, enkephalin
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A given neuron releases how many types of neurotransmitter?
Usually one but there are examples of co-transmitters e.g. ATP and NE released from some nerves in the PNS
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Neurotransmitters are produced in the pre-synaptic nerve terminals and defined by what?
Synthetic pathways
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Give an example of how synthetic enzymes can be a target by drugs to alter neurotransmission
Inhibitors of tyrosine hydroxylase are used to inhibit NE synthesis in some forms of cancer (tyrosine - tyrosine hydroxylase - L-dopa - dopa carboxylase - dopamine - dopamine beta hydroxylase - NE)
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ACh is stored in what kind of vesicles?
Small, round clear vesicles
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NE is stored in what kind of vesicles?
Small, dense core vesicles
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Why is the storage of neurotransmitters in vesicles important (2)?
Protects the neurotransmitter from enzymes present in cell cytoplasm that would destroy it. It is a releasable form of transmitter, released by the process of exocytosis
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Give an example of how drugs interfere with storage of neurotransmitters
Amphetamine displaces NE from its vesicles
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Describe the process of exocytosis (1)
Presynaptic terminal becomes depolarised. Voltage-gated Ca2+ channels open. Influx of Ca2+ ions. Vesicles fuse with presynaptic membrane and release neurotransmitters which diffuse across synaptic cleft.
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Describe the process of exocytosis (2)
Loops and hoops, vesicles docked/primed by synaptobrevin interacting with syntaxin and SNAP-25. Ca2+ binds to synaptotagmin
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Give an example of how drugs interfere with the release process
Botulinum toxin destroys SNAP-25 which inhibits the release of ACh
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Are all neurotransmitters agonists or antagonists?
Agonists
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What are the two main classes of receptors?
Ionotropic and metabotropic
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Describe features of ionotropic receptors
Receptor-operated ion channels composed of several sub-units. Mediate fast synaptic potentials. E.g. nicotinic AChR, NMDA and AMPA glutamate receptors, 5HT3 receptor, GABA A receptor
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Describe features of metabotropic receptors
G-protein coupled with characteristic heptahelical structure. Mediate slower synaptic transmission. E.g. Muscarinic AChR, metabotropic glutamate receptors (mglu1-mglu8), all 5HT receptors (except 5HT3), GABAB receptors, all dopamine receptors (D1-D5)
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Whenever you come across an ion channel in physiology, which two questions must be considered?
What goes through the ion channel (selectively permeable) and what opens the ion channel (open under certain conditions)
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Describe the structure and function of ionotropic receptors (1)
Ion channel within multiprotein complex. Channel normally closed but opens when neurotransmitter binds to receptor
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Describe the structure and function of ionotropic receptors (2)
Depending upon which ions flow through the channel (selective permeability) the effect of the neurotransmitter will either depolarise or hyperpolarise the post synaptic membrane
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What is the effect of depolarisation on the post synaptic cell?
Makes the cell more likely to fire an action potential - excitatory post synaptic potential (EPSP)
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Hyperpolarisation inhibits what?
The target cell - inhibitory post synaptic potential (IPSP)
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Describe the structure and function of metabotropic receptors (1)
Activation of receptor triggers cascade of biochemical/metabolic reactions within the post-synaptic cell (e.g. activation/inhibition of enzymes, modulation of ion channel activity, gene transcription)
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Describe the structure and function of metabotropic receptors (2)
The first protein in the cascade is a trimeric GTP-binding protein (G-protein).
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Describe the structure and function of metabotropic receptors (3)
Receptors are called G-Protein Coupled Receptors (GPCRs) - have single proteins that go back and forth across membrane several times (heptahelical) with N-terminal outside cell and C-terminal inside cell (where it binds to G-protein)
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What are post synaptic potentials?
Changes in membrane potential caused by neurotransmitters activating receptors
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What is the function of post synaptic potentials?
Move the membrane potential towards (excitatory/depolarisation) or away from (inhibitory/hyperpolarisation) the threshold for firing an action potential). Resting at -70 Mv, threshold at -55 mV, see graphs/diagrams
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Describe features of an action potential (5)
Propagated along the axon, all or nothing, transient (2-5 msec), co-ordinated opening and closing of voltage-gated Na and K channels, followed by distinct refractory period
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Describe features of a synaptic potential (5) - 1
Regulate firing of action potential, graded (more receptors activated/bigger synaptic potential), peak in 10-100 msec depending on type of receptor,
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Describe features of a synaptic potential (5) - 2
Caused by opening of channels controlled by neurotransmitter receptors, end when neurotransmitter diffuses from receptor and is removed from synapse
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What are the three ways neurotransmitters can be removed from the synapse?
Simple diffusion, being taken up into nearby cells (uptake), being destroyed by extracellular enzymes (e.g. ACh destroyed by AChE, NE is removed by uptake)
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Give an example of how drugs can interfere with neurotransmitter removal
AChE inhibitors are used to treat Alzheimer's disease whilst Selective Seretonin Reuptake Inhibitors (SSRIs) are used to treat depression
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Describe features of electrical synapses
Glial to glial cell communication (rare in neuronal signalling), no signally specificity, narrow synaptic cleft (2 nm), mediate by direct interactions between gap junction proteins (connexins), bidirectional, fast (<1 msec synaptic delay)
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What is the function of the cell body (soma)?

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Where is the action potential initiated in the neuron?

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Card 4

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Describe features of the axon

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Card 5

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