BIOC32H3 Study Guide - Midterm Guide: Synapsin, Axon Terminal, Synaptobrevin

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Published on 12 Nov 2019
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BIOC32 Term Test 2 Review
Week 6
Excitation
Excitation: depolarization, which requires a net influx of positive charges into the cell
o Ex. 3Na+/2K+ exchanger
o Ex. 3Na+/Ca2+ exchanger
Chemical Synapses
Synaptic Vesicles
When an action potential arrives at the axon terminal, voltage-gated Ca2+ channels open
and Ca2+ enters the axon terminal, which triggers neurotransmitter release
o K+ channels are activated and hyperpolarization will occur, causing the Ca2+
channels to close
o This will cause the amount of neurotransmitter release to decrease
If you block this Ca2+/K+ exchanger, this hyperpolarization can be
prevented and neurotransmitters can continue to be released
Synaptic vesicles usually build up in presynaptic terminals in unidirectional synapses
o Bidirectional synapses have synaptic vesicles in both terminals
Both terminals should be able to release neurotransmitters into the
synaptic cleft
There should be receptors for the neurotransmitters on the membranes of
both terminals so nerve impulses can be transmitted in both directions
At chemical synapses, EPSPs are broth about by the binding of neurotransmitters to Na+
or Ca2+ channel-linked receptors
o Neurotransmitters released from presynaptic neurons
o Reserve pool synaptic vesicles are tethered to the actin cytoskeleton via synapsin
which prevents their contents from being released
Not released until link between actin and the synaptic vessel is broken
(usually by synapsin phosphorylation)
o If actin is depolymerized in another way, this will cause the release of the
neurotransmitters and cause the excitation of postsynaptic neuron
A larger synaptic vesicle diameter means a larger amount of neurotransmitter release
A smaller synaptic cleft means a higher concentration of neurotransmitters
SNARE Proteins
Synaptobrevin (v-snare)
o Associated with vesicles
o Clathrin is involved in the endocytosis of synaptic vesicular membrane from the
pre-synaptic membrane after exocytosis
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o Since Synaptobrevin is associated with vesicles, clathrin should have a high
affinity for it so it can locate the vesicular membranes within the presynaptic
membrane to facilitate endocytosis
Botulism
o Botulinum toxin degrades SNARE proteins prevents neurotransmitters from
being released
o Prevent acetylcholine release prevent muscle contraction
Used for lip fillers can’t contract the muscles in the lips and face
becomes less expressive
Ca2+ binds to synaptotagmin synaptotagmin develops an affinity for
phosphatidylserine
o If this is electrostatically mediated, then phosphatidylserine is probably negatively
charged (which is why synaptotagmin is only attracted to it once it is bound to
positively charged Ca2+)
o Synaptotagmin is located inside the cell, so phosphatidylserine is probably in the
inner leaflet of the plasma membrane where it can access synaptotagmin
Electrical Synapses
Electrical synapses are characterized by a lack of synaptic delay
o An excitatory stimulus arriving at the presynaptic terminal would lead to an
immediate depolarization of the post-synaptic membrane (duration would depend
on decremental transmission i.e. membrane and cytosolic resistance)
o As long as the depolarization persists, the post-synaptic cell will respond to it
if the excitatory stimulus arriving at the presynaptic terminal leads to a
delayed inhibition (hyperpolarization) via GABA release and receptor
binding, the rate that the cell returns to resting potential will be faster
responds to excitation input for a short duration
GABA would enhance synchrony
o Blocking GABA decrease synchrony
Electrical synapses are found in most reflex arcs because they have no synaptic delay
o Rapid nervous communication
o No reflexive response, then all nervous communication has ceased (i.e. with
anaesthetic)
Neural Tube Guiding Cues
Motor neurons emerge from the ventral region of the spinal cord
o Motor neurons should be induced by Shh, a guiding cue produced in the ventral
regions of the developing neural tube
o Shh provides information to developing neurons about the ventral position within
the neural tube
Gap Junctions
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Connexin is the protein that comprises gap junctions
Gap junctions allow for the synchronous action of cells
o Hormone-producing cells need to have a synchronous action in order to release
hormones in pulses to make them effective
o Loss-of-mutation in connexin genes: release of sex hormones in a continuous
fashion delayed onset of puberty
Week 7
Membrane Vesicle Recycling
Endosomes in axon terminals are largely formed by bulk endocytosis
o This is considered a fast mechanism of endocytosis
o Should be common during periods of high action potential frequency
Synaptic vesicle exocytosis and neurotransmitter release would be high
and would necessitate more vesicle recycling
Neurotransmitters
Acetylcholine is the principle neurotransmitter of the parasympathetic nervous system
Epinephrine is the principle neurotransmitter of the sympathetic nervous system
o Neurons that innervate areas of the digestive tract and lower urinary tract should
produce only one of these, not both at the same time
GABA
Most synapses are excitatory
o Take SSRI to increase serotonin, GABA WILL increase to offset the excitatory
state caused by serotonin
o GABA should increase with any increased excitatory state
E.g. citalopram
Increases brain serotonin effect
o Either a serotonin receptor agonist or a serotonin reuptake
transporter inhibitor
Dopamine
Dopamine is the major neurotransmitter of the brain’s reward system
Cocaine inhibits the dopamine reuptake transporter
o Increase in synaptic dopamine concentration after dopaminergic neurons are
activated
o Get a decrease in dopamine receptor levels (brain’s response to dopamine release
back to normal levels
o Cocaine can also act as a local anaesthetic!
It has an aromatic ring, terminal amine, and ester linkage in between
Glutamate
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