2. Assessment Statements Obj. E4.1 1 E4.2 3 E4.3 E4.4
State that some neurotransmitters excite postsynaptic transmission and others inhibit postsynaptic transmission. 1
E4.2
Explain how decision-making in the CNS can result from the interaction between the activities excitatory and inhibitory presynapatic neurons at synapses. 3
E4.3
Explain how psychoactive drugs affect the brain and personality by either increasing or decreasing postsynaptic transmission.
E4.4
List three examples of excitatory and three examples of inhibitory psychoactive drugs.
Excitatory: nicotine, cocaine, amphetamines
Inhibitory: benzodiazepines, alcohol, tetrahydrocannabinol (THC)
E4.5
Explain the effects of cocaine and THC in terms of their action at synapses in the brain.
E4.6
Discuss the causes of addiction, including genetic predisposition, social factors and dopamine secretion.
Be sure you have a solid understanding of action potentials and synapses as you work through this subtopic.
Command terms:
Assessment statements from: Online IB Biology Subject Guide

3. 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.
Work through this tutorial:

4. Neurotransmitters can be excitatory or inhibitory
Neurotransmitters (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
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
0mv
depolarisation
threshold
resting
-70mv
hyperpolarisation
time

5. This is a useful example of an inhibitory neurotransmitter.
This is a useful example of an inhibitory neurotransmitter.

6. Decision-making in the Central Nervous System (CNS)
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.
action potential
axon
synapse
axon hillock
Diagram adapted from:

7. Decision-making in the Central Nervous System (CNS)
Test out temporal and spatial summation:
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.
Diagram adapted from:

8. 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 membranes
Inhibit production of new NTs
Work through the excellent animations and explanations from

10. 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

11. 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

13. A scale of harm for drugs
Which dots represent:
tobacco?
alcohol?
heroin?
cocaine?
THC/ cannabis?
ecstasy?
Drag the red dot to point you think represents alcohol.
How can drugs cause physical harm?
How can drug use lead to addiction (dependency)?
What factors contribute to development of addiction?
ActivePrompt link: or

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