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36 Cards in this Set
- Front
- Back
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Signalling in nervous system |
Binary code, either on or off |
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Neuron signalling |
May have to travel long distance, along nerve axons from 1 place to another Involves rapid changes in the membrane potential |
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The firing of a neurons refers to? |
rapid depolarization by opening of voltage-activated (cations/opearated) channel |
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Voltage activated (cation/operated) channels |
cation-selective pore opens when Vm changes |
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Types of channels |
Voltage gated Intracellular ligand gated Extracellular ligand gated Stress activated
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Initial depolarisation |
Initial change in membrane potential (depolarisation) -> reaches the threshold -> large voltage channels open -> AP generated |
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Graded response |
Opening of small capacitance cation receptor channel (NAchr at neuromuscular junction)
Graded response can summate
Different amplitude
If the sum up of the graded potential reaches a threshold lv of depolarisation, the voltage activated Na channel opens -> AP generate
The stimulus is proportional to the AP |
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Why is AP generation short-lived? |
Because K channel soon opens -> reverse the change in Vm |
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Depolarisation |
Opening of voltage gated Na channel after the graded potential reached the threshold |
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Repolarisation |
closure of voltage gated Na channel Opening of voltage gated K channel |
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Hyperpolarisation |
Voltage gated channel remain open after the potential reaches resting level => refractory period |
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Refractory period |
After the potential reached the resting level, the voltage gated channel remains open so another AP cannot be generated |
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Permeability of Na and K during depolarisation |
Na permeability rapidly increases K permeability gradually increases |
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Permeability of Na, K during repolarisation |
K permeability rapidly increases, and once opened, remained for a long time (refractory period) Na permeability rapidly decreases. |
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If the Na pump is blocked by ouabain, what happened? |
The magnitude of AP will decrease progressively until the membrane Na gradient decreases to the point where AP fail
Many still be able to be generated |
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Blocking voltage activated Na channel by tetrodotoxin (TTX) |
Abolished AP -> no graded post synaptic potential |
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Why does AP not summate?
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Because they demonstrate refractoriness AP code info.by frequency, not amplitude
The frequency limit imposed by the refractory period due to the inactivation of voltage activated Na channel
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Propagation of AP along axon |
At rest: the membrane is polarised Depolarisation due to AP sets up local circuit currents in both direction In backward direction, the Na channel are in refractory phase -> another AP can't be generated -> AP moves forward |
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Factors affecting the speed of AP conductance |
Myelination, Temperature, axon diameter |
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What is it about axon diameter that could affect the speed of AP conductance? |
membrane has high capacitance compared with cytosol The fatter the axon, the lower the resistance length ways, the faster the conductance |
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What is it about myelination that could affect the speed of AP conductance? |
insulating the axon membrane with myelin is more efficient to increase AP conductance velocity than increasing the diameter |
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Oligodendocytes |
Wrap multiple CNS axons with myelin and Schwan cells |
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Where are the ion channels on the axon? |
Nodes of Ranvier
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Saltatory conductance |
As ions channels are located on the node of Ranvier therefore the AP jumps from node to node |
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Mammalian conduction velocity |
Require the lowest threshold: A alpha, largest threshold: C fibre Largest diameter: A alpha, smallest diameter: C Fastest conduction velocity: A alpha, slowest conduction velocity: C fibre |
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Mechanism of action of Local Anaesthetics |
Block the conductance of APs in sensory nerves by blocking the voltage activated Na channel from the inside |
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Properties of Local anaesthetic |
It needs to diffuse through axon membrane so it must be uncharged at plasma pH
Charged molecules block the channel |
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Unionised and ionised drugs and their effects |
Unionised drugs are more readily to pass through the lipid cell membrane => reach the target more quickly
Ionised drugs are not readily passed back through the membrane => trapped => exerts the effect (binds to voltage gated Na channel |
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Differential sensitivity to Local anaesthetics |
myelinated and thicker axons are more difficult for LAs to penetrate C fibre carries pain signal -> easier to block than motor nerve A fibre because it is unmyelinated and thin LAs reduce pain at lower dose, but if overdose, they affect other sensation and motor function because they affect other nerve |
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Application of LAs: surface anaesthesia |
sprays on mucus membrane, cornea drops Lidocaine, tetracaine Not generally effective for skin but EMLA produce LA in abt 1 hr |
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EMLA |
eutectic mixture of lidocaine and prilocaine |
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LAs application: Infiltration |
injection to the tissues -> reaches nerve Most LAs Minor surgery Adrenaline might be added, vasoconstrictor -> prevent diffusion away from site) Not for finger and toes -> risk of ischaemia
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LAs application: I.V regional |
Injected distal -> pressure cuff (stop blood flow) Limb surgery Prilocaine, lidocaine Danger of systemic toxicity if release the cuff prematurely |
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LAs application: nerves block |
injection closes to nerve trunks -> reduces sensation peripherally Surgery, dentistry |
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LAs application: spinal |
Injection to subarachnoid space (containing CSF) Depress spinal roots and cord Surgery: abdomen, pelvis, legs Glucose is added to increase the density -> limit spread due to tilting patient during the operation Risk of CVS effect, repiratory depression -> spread to brain -> avoid tilting Post-op urinary retention due to block of pelvic autonomic outflow
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LAs application: epidural |
injection to epidural space blocking spiral roots Lidocaine, bupivacaine Spinal applications, painless childbirth |
