Synapses are a fundamental part of neural pathways as they regulate decisionmaking in terms of exciting or inhibiting the postsynaptic neurons Review Action potentials AP reach terminal bud of the presynaptic neuron ID: 261329
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Communication via Synapses
Synapses are a fundamental part of neural pathways as they regulate decision-making in terms of exciting or inhibiting the post-synaptic neurons.
Review:
Action potentials (AP) reach terminal bud of the pre-synaptic neuron.
Neurotransmitters (NT), chemical messengers, diffuse across the synapse to bind with receptors on the post-synaptic membrane.
Neurotransmitters
are:
Excitatory
, which means they excite the post-synaptic neuron (contributing to
depolarisation
and propagation of the AP.
OR:
Inhibitory
,
hyperpolarising
the post-synaptic neuron and preventing AP. Slide3
Neurotransmitters
can be
excitatory
or
inhibitory
Neurotransmitters
(NT) are proteins
diffuse across the synaptic cleftbind with a receptor on the post-synaptic neuron.Whether or not the post-synaptic neuron propagates the action potential depends on: Which NT diffuses acrossWhich receptors they bind toWhich ions flow in/out of the post-synaptic neuronWhether or not depolarisation reaches threshold
-70mv
0mv
time
threshold
resting
depolarisation
hyperpolarisation
Excitatory NTs cause
depolarisation
e.g.
ACh
, dopamine
NT binds, Na+ channels open, Na+ rushes in
Membrane potential
depolarises
, AP propagated
Inhibitory NTs cause
hyperpolarisation
e.g. GABA, dopamine (on different pathways)
NT binds to receptor
K+ channels open, K+ rushes out
OR
Cl
- channels open,
Cl
- rushes in
Membrane potential become more negative
Action potential is prevented from propagatingSlide4
This is a useful example of an inhibitory neurotransmitter.
http://
www.youtube.com
/
watch?v
=-pfG6yHAQ5USlide5
Decision-making in the Central Nervous System (CNS)
axon
synapse
axon hillock
action potential
The axons of
many
pre-synaptic neurons feed into the dendrites of one post-synaptic neurons via synapses.
The ‘decision’ whether or not to propagate
the action potential along the axon of the
post-synaptic neuron takes place
in a region of the cell body
called the
axon hillock
.
This is achieved through
summation
of the incoming impulses. If the total
impulse reaches
threshold
,
the post-synaptic neuron
depolarises
and
the action potential is propagated.
If the sum does not reach threshold, the AP is not propagated.
There are two main methods of summation:
temporal
and
spatial
.
Slide6
Decision-making in the Central Nervous System (CNS)
Temporal summation
Action potentials arrive in rapid succession
Depolarisation
in the axon hillock is summative
If it reaches threshold before
repolarisation
, the AP is propagated.
Spatial summation
Action potentials arrive simultaneously from multiple sources
Some neurotransmitters are excitatory (increasing
depolarisation)
Other NTs are inhibitory (hyperpolarising). Summation in the axon hillock is summative.
If it reaches threshold the AP is propagated. Slide7
How do psychoactive drugs affect the brain?
Before thinking about how drugs affect the synapses, be sure you understand how they work and are reset.
Some NTs have a normal excitatory function
Other NTs have a normal inhibitory function
In general, psychoactive drugs can:
Increase or
decrease the release of NTs (e.g. THC – cannabis)
Breakdown re-uptake proteins which are responsible for returned used components of NTs to the pre-synaptic neuron (ready to use again)Block re-uptake proteins (e.g.cocaine)Mimic or block NTs, binding to the receptors on post-synaptic membranesInhibit production of new NTsSlide8Slide9
Excitatory drugs
increase post-synaptic transmission
Examples:
Nicotine, amphetamines,
cocaine
What is the effect of cocaine?
Normal:
Dopamine acts as excitatory NTDopamine is re-uptaken by pumps on the pre-synaptic membrane. With Cocaine:Cocaine blocks re-uptake pumpsDopamine remains in synaptic cleft
More dopamine continues to be releasedSummative increase in post-synaptic transmissionEffects on mood:
Dopamine is involved in reward pathways, enhancing feelings of pleasureLonger-lasting feelings as dopamine is not re-uptaken
Effects on behaviour:
feelings of euphoriaincreased energy and alertnesshighly addictive
association with depression as body reduces production of own dopamine over timeSlide10
Inhibitory drugs
decrease
post-synaptic transmission
Examples:
Alcohol,
benzodiazapines
, THCWhat is the effect of tetrahydrocannibol (THC)? Normal: Dopamine release is moderated (inhibited) by GABAWith THC:THC mimics cannabinoids and inhibits
GABA release by binding to cannabinoid receptorsGABA cannot inhibit dopamine releaseMore dopamine is released
Effects on mood: Dopamine is involved in reward pathways, enhancing feelings of pleasure
Not as extreme release of dopamine as with cocaine, but still higher than normalEffects on behaviour
:intoxicationhunger
memory impairmentpotential dependencySlide11Slide12
http://
www.youtube.com
/
watch?v
=bCChf2WHNE4Slide13Slide14Slide15Slide16