E.4.1 State that some pre-synaptic neurons excite pos-synaptic transmission and others inhibit post-synaptic
transmission.
E.4.2 Explain how decision-making in the CNS can result from the interaction between the
activities of excitatory and inhibitory pre-synaptic neurones at synapses.
1. Neurones form synaptic junctions with the cell body of other neurones.
2. A post synaptic neurone can have many pre-synaptic neurones forming synaptic junctions with it.
3. Pre synaptic neurones depolarise (excitatory) or hyperpolarise (inhibitory) the post synaptic membrane
locally.
4. The sum of their effects takes place at the axon hillock
The thing that decides if an impulse is passed on to create further neural activity is the overall summation of
excitatory and inhibitory input to the post synaptic membrane.
Spatial Summation:
In which different pre-synaptic neurones ‘add together’ their local depolarisation's to reach threshold at the
axon hillock
The diagram of the summation of
the depolarising effects of three
different excitatory pre-synaptic
neurones.
1. Having reached the
threshold there would be a new
action potential in the post
synaptic neurone.
2. If sub threshold there
would be no action potential or
response from the central nervous
system.
Temporal Summation:
A pre-synaptic neurones depolarised the post synaptic membrane with a number of rapid stimulations.
The same pre-synaptic has a high frequency of action potentials arriving at the synapse. This sends a number of
waves of neurotransmitter to depolarise the post synaptic membrane.
1.
The waves of depolarisation
are added together to
reach threshold
2. Notice that (a) the
wave of depolarisation
comes from one neurone
only
Question:
What kind of stimuli would
cause spatial or temporal
summation?
What happens if the depolarisation of the post-synaptic membrane is sub-threshold?
Other scenarios:
Any one or more of the pre-synaptic membranes could be inhibitory and would hyperpolarise the post synaptic
potential therefore failing to reach threshold, no action potential is produced, hence inhibitory.
In the brain a single neurone can form thousands of synaptic connections in all sorts of combinations of
inhibitory and excitatory synapses.
The brain is composed of perhaps 3 x109 of neurones, the networking potential is then the number of neurones
( x109) x number of connections of each neurone (x104) = 1013. Even if these figures are wrong by a factor of
ten, a hundred or a thousand, imagine how long it would take to draw out all possible connections, perhaps you
can calculate it! The brain remains one of the greatest challenges to Biology, perhaps one that will ultimately
never be fully resolved.
E.4.3 Explain how psychoactive drugs affect the brain and personality by either increasing or
decreasing post-synaptic transmission.
The post-synaptic membrane contains receptors for the normal neurotransmitters. Normally when the neuro
transmitter attaches to these receptors it either:
1. Depolarise the post synaptic neurone resulting in an action potential
2. Hyperpolarise the post synaptic neurone resulting in no action potential
A drug can therefore change post synaptic output by:
1. Blocking or enhancing the depolarising neurotransmitter
2. Blocking or enhancing the hyperpolarise neurotransmitter
This provides a complex of possible effects. The important issue however is to consider the type of receptor in
the post synaptic membrane and its associated normal effect on the mind.
1. Psychoactive drugs are those that alter the state of mind by changing (stopping or enhancing ) the
activity at the post synaptic neurone.
2. Psychoactive drugs characteristically create either calm or excite moods.
NB: There are other ways to change transmission at the synaptic cleft but they involve the Pre-synaptic
membrane and the enzymes that recycle neurotransmitter to the pre-synaptic membrane.
E.4.4 List three examples of excitatory and three examples of inhibitory psychoactive drugs.
E.4.5 Explain the effects of THC and cocaine in terms of their action at synapses in the brain.
View MOUSE PARTY for an excellent presentation on cocaine and THC
Psycho-active drugs:
Our understanding of how cocaine and THC have their effects as mood changing psyco-active drugs demands a
little knowledge of the reward pathways.
Certain behaviours produce a pleasuable mood enhancing sensation. One of the causes of this natural 'high'
mood is the secretion of the neurotransmitter dopamine. Dopamine receptors are found in the post synaptic
membrane which when activited depolaris the post synaptic neurone in regions of the brain associated with a
feeling of pleasure.
Cocaine
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the drug attaches to the presynaptic dopamine pumps
The recycling of dopamine is blocked and it remains in the synaptic cleft
There is now a constant stimulation of the post-synaptic neurone (by the dopamine in the synaptic cleft)
Cocaine results in post synaptic excitement of cholinergic synapses which are associated will elevated levels of
activity.
Cocaine is described as creating a mood of euphoria.
Individuals show greater alertness / energetic behaviour/ very talkative
Tetrahydrocannabinol (THC):
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THC mimics the neuro-receptor anandamide.
THC bonds to the post synaptic membrane for anandamide called the cannabinol receptor.
The effect is to hyperpolarise the post synaptic neurone.
THC is therefore a inhibitory psychoactive drug
Cannabinol receptors are found at synapses in regions of the brain associated with short term
memory, coordination and appetite.
THC users report feeling mellow and calm.
E.4.6 Discuss the causes of addiction, including genetic predisposition, social factors and
dopamine secretion
Psychoactive drugs are often associated with ‘addictive behaviour’. Addiction includes habitual behaviour in
which the drug taker:
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Craves the use of the drug
Cannot exercise self control in limiting the taking of the drug
Habituation means that increasing quantities must be taken to achieve the same mental
conditions.
Show side effects of talking the drug and withdrawal behaviour if unable to satisfy the craving.
Other issues in addiction: