Communication via Synapses
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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. - PowerPoint PPT Presentation
Transcript of Communication via Synapses
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.
Neurotransmitters can be excitatory or inhibitoryNeurotransmitters (NT) are proteins • diffuse across the synaptic cleft• bind with a receptor on the post-synaptic neuron.
Whether or not the post-synaptic neuron propagates the action potential depends on: • Which NT diffuses across• Which receptors they bind to• Which ions flow in/out of the post-synaptic neuron• Whether 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 propagating
This is a useful example of an inhibitory neurotransmitter.
http://www.youtube.com/watch?v=-pfG6yHAQ5U
Decision-making in the Central Nervous System (CNS)
axon
synapse
axon hillock
actio
n po
tenti
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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 summationof 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.
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.
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: 1. Increase or decrease the release of NTs (e.g.
THC – cannabis)2. Breakdown re-uptake proteins which are
responsible for returned used components of NTs to the pre-synaptic neuron (ready to use again)
3. Block re-uptake proteins (e.g.cocaine)4. Mimic or block NTs, binding to the receptors
on post-synaptic membranes5. Inhibit production of new NTs
Excitatory drugs increase post-synaptic transmission
Examples: • Nicotine, amphetamines, cocaine
What is the effect of cocaine? Normal: • Dopamine acts as excitatory NT• Dopamine is re-uptaken by pumps on the pre-
synaptic membrane.
With Cocaine:• Cocaine blocks re-uptake pumps• Dopamine remains in synaptic cleft• More dopamine continues to be released• Summative increase in post-synaptic transmission
Effects on mood: • Dopamine is involved in reward pathways,
enhancing feelings of pleasure• Longer-lasting feelings as dopamine is not re-
uptaken
Effects on behaviour:• feelings of euphoria• increased energy and alertness• highly addictive• association with depression as body reduces
production of own dopamine over time
Inhibitory drugs decrease post-synaptic transmission
Examples: • Alcohol, benzodiazapines, THC
What is the effect of tetrahydrocannibol (THC)? Normal: • Dopamine release is moderated (inhibited) by GABA
With THC:• THC mimics cannabinoids and inhibits GABA release
by binding to cannabinoid receptors• GABA cannot inhibit dopamine release• More 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 normal
Effects on behaviour:• intoxication• hunger• memory impairment• potential dependency
http://www.youtube.com/watch?v=bCChf2WHNE4
