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E4: Neurotransmitters & Synapses
Pp 481 - 488
Assessment Statements
 E.4.1 State that some presynaptic neurons excite





postsynaptic transmission and others inhibit postsynaptic
transmission.
E.4.2 Explain how decision-making in the CNS can result
from the interaction between the activities of excitatory and
inhibitory presynaptic neurons at synapses.
E.4.3 Explain how psychoactive drugs affect the brain and
personality by either increasing or decreasing postsynaptic
transmission.
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.
E.4.6 Discuss the causes of addiction, including genetic
predisposition, social factors and dopamine secretion.
Synaptic transmission
 Some Pre-synaptic neurons
excite postsynaptic
transmission
 They are called excitatory
pre-synaptic neurons
 Some Pre-synaptic neurons
inhibit postsynaptic
transmission
 They are called inhibitory
pre-synaptic neurons
Excitatory and Inhibitory synapses
Excitatory Presynaptic neurons
Inhibitory Presynaptic neurons
 Depolarises the
postsynaptic
membrane
 Hyperpolarises the
postsynaptic
membrane
 Influx of Na+ ions
 Influx of Cl- or loss of
K+ ions
 Possibly reaching
threshold
 Makes it more difficult
to reach threshold
 Generation of new
 Lowers the resting
action potential
 E.g.
Acetylcholine
potential
 E.g.
Gama-aminobutryic
acid (GABA)
 An action potential
in post-synaptic
neurone will only
occur if the
potential difference
at the axon hillock
(specialized part of
the cell body of a
neuron that
connects to the
axon)rises above
the threshold value
Decision-making in the CNS can result from the
interaction between the activities of excitatory
and inhibitory pre-synaptic neurons at synapses.
Neurones form synaptic
junctions with the cell
body of other neurones
Pre-
Pre-
Synapse
Pre-
Post-
 A post synaptic neurone
can have many presynaptic neurones forming
synaptic junctions with it.
 Pre synaptic neurones
depolarise (excitatory) or
hyperpolarise (inhibitory)
the post synaptic
membrane locally.
 The sum of their effects
takes place at the axon
hillock
Pre-synaptic
Post-Synaptic Potentials (PSPs) can be:
Excitatory (EPSP)(+);
or
Inhibitory (IPSP) (-).
Pre-synaptic
Pre-synaptic
+
-
Synapse
+
Post-synaptic
Pre-synapt
The EPSP must build up in the postsynaptic neuron to
the threshold level to allow the formation of an action
potential.
-
+
+
Synapse
E.g. this Neuron needs
a 2 more “+” than “-”
before it can generate
an action potential.
+
Neurons can integrate signals from other neurons:
1) through spatial summation (different pre-synaptic
neurones ‘add together’ their local depolarisation's to
reach threshold at the axon hillock)
+
Synapse
+
+
 Summation of the
depolarising effects of three
different excitatory presynaptic neurones i.e.
a+b+c
 Having reached the
threshold (T), a new action
potential is generated in
the post synaptic neurone
 If the summation of the 3 is
below threshold there
would be no action
potential or response from
the central nervous system
2) through temporal summation where a number
of waves neurotransmitter from the same presynaptic neurone depolarises the post synaptic
membrane (i.e. waves of depolarisation are added
together to reach threshold).
+
++ + +
+
Synapse
Pre-synaptic neurones
depolarises the post synaptic
membrane with a number of
rapid stimulations.
 The same pre-synaptic
neurone(a)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.
 Waves of depolarisation
(from one neurone) are
added together to reach
threshold
Decision-making in the CNS
 synapses are the sites of decision-making in the brain
 a post-synaptic neuron's membrane potential is the
summation of input from pre-synaptic neurons
 EPSPs depolarize post-synaptic neurons while
 IPSPs hyper-polarize post-synaptic neurons
 if the post-synaptic neuron reaches threshold
potential at its axon hillock, it will produce an action
potential
 pre-synaptic neurons can vary in the frequency, but
not intensity of their input, since action potentials are
"all-or-none“ rule
Psychoactive drug:
Marijuana
Amphetamine
Alcohol
A Psychoactive drug is
chemical substances
that acts upon the CNS
altering the state of
mind by changing
(stopping or enhancing)
the activity at the post
synaptic neuron.
It result in changes
in perception, mood,
consciousness,
cognition, and
behaviour.
How psychoactive drugs affect the brain and
personality:
 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
neurotransmitter attaches to these receptors it either:
 Depolarise the post synaptic neuron resulting in an action
potential
 Hyperpolarise the post synaptic neuron resulting in no action
potential
 A drug can therefore change post synaptic output by:
 Blocking or enhancing the depolarising neurotransmitter
 Blocking or enhancing the hyperpolarise neurotransmitter
Major Neurotransmitters in the Body
Neurotransmitter
Role in the Body
 Acetylcholine (excitatory)
 used by the spinal cord
 Dopamine (inhibitory)
neurons to control muscles and
by many neurons in the brain
to regulate memory.
 produces feelings of pleasure
when released by the brain, it
has multiple functions
depending on where in the
brain it acts.
 Gamma-amino butyric acid
 The most common inhibitory
(GABA) (inhibitory)
neurotransmitter in the brain.
Examples of excitatory &
inhibitory psychoactive drugs:
Excitatory psychoactive Inhibitory psychoactive
drugs;
drugs;
 nicotine,
 benzodiazepines,
 cocaine ,
 alcohol ,
 amphetamines.
 tetrahydrocannabinol
(THC).
Tetrahydrocannabinol (THC):
 Marijuana/ganja is the
buds and leaves of the
Cannabis sativa plant.
 Marijuana contains more
than 400 chemicals,
including
tetrahydrocannabinol
(THC), the plant's main
psychoactive chemical.
 Cannabinoid receptors are
activated by a
neurotransmitter called
anandamide.
 Anandamide belongs to a
group of chemicals called
cannabinoids.
 THC is also a cannabinoid
chemical.
 THC mimics the actions of
anandamide, meaning that
THC binds with cannabinoid
receptors and activates
neurons, which causes adverse
effects on the mind and body.
Effects of Tetrahydrocannabinol (THC)
 THC affects brain’s short-term memory
 THC affects motor coordination & appetite
 THC increases heart rate and raises levels of anxiety
 THC contains cancer-causing chemicals similar to those in
tobacco
 THC affects higher order thinking
 THC users report feeling mellow and calm
 THC mimics the neuro-receptor anandamide
 THC binds to the post synaptic membrane receptor s for
anandamide called Cannabinoid Receptor
 THC effect is to hyperpolarise the post synaptic neuron
inhibiting generation of an action potential
Cocaine:
 Cocaine:- a powdered drug
that is made from the leaves of
the coca plant grown primarily
in South America.
 Cocaine has been in use for
centuries, many generations of
South American Indians have
chewed its leaves to give them
strength and energy.
 The drug is nowadays taken in
by either snorting, injecting or
smoking.
 Dopamine is released by cells
of the nervous system during
pleasurable activities such as
eating or having sex.
 Once released, dopamine
travels across a synapse, and
binds to a receptor on a postsynaptic neuron.
 This sends a signal to that
Cocaine interferes
nerve cell, which produces a
with a chemical
good
feeling.
messenger in the
 Under normal conditions, once
brain called
the dopamine sends that signal
dopamine, which is
it is reabsorbed by the neuron
involved in the body's
that released it
pleasure response.
Effects of Cocaine:
 cocaine is an excitatory psychoactive drugs that
increases synaptic transmission
 on post synaptic membrane receptor, it prevents the
removal of acetylcholine
 on pre-synaptic membrane, it prevents the recycling of
acetylcholine
 both effects maintain the stimulation of the post
synaptic membrane
 cocaine also blocks the recycling of Dopamine at the
Pre-synaptic membrane, making dopamine to remain
active in the synapse
 dopamine is associated with a feeling of pleasure , thus
cocaine creates a mood of euphoria
Drug addiction:
 Psychoactive drugs often cause
addictive behaviour i.e. an individual
persists in use of drugs despite
problems related to their use
 habitual behaviour associated with
addiction includes:
 craving for the use of the drug;
 lack of self control in limiting the
taking of the drug;
 increasing quantities must be taken to
achieve the same mental conditions;
 development of withdrawal behaviour
if unable to satisfy the craving for the
drug.
Causes of addiction:
(i) Genetic
predisposition;
 Some people are more
vulnerable to drug
addiction than others,
 studies indicates that
genetic factors have
some influence on
addiction,
 alcoholism, especially,
tends to run in certain
families
(ii) Social
factors;
 various social factors
correlate positively with
addiction, including:
 cultural traditions;
 peer pressure;
 Poverty;
 social deprivation;
 traumatic life
experiences;
 mental health
problems; etc
(iii) Dopamine
secretion;
 many addictive drugs are
excitatory at dopaminergic
synapses “reward pathway” giving
a feel good effect/pleasure
 addiction results from
dopaminergic synapses
responding to regular drug use
 tolerance to a drug , leads to
increased dosage to produce the
desired effect
 withdrawal is caused by normal
levels of dopamine failing to
produce pleasure hence craving
for drugs
STOP DRUG ADDICTION!
Any Questions on Drugs?
Thank
You!
A Drug-Free World: We Can Do It!
Self Assessment Questions (SAQs)
 Distinguish between
 List 3 examples of
presynaptic excitatory &
inhibitory neurons. [2]
 Explain the process of
decision-making in the
CNS.
[3]
 Explain how
psychoactive drugs affect
the brain and personality
of an individual.
[3]
excitatory & 3 examples
of inhibitory
psychoactive drugs. [2]
 Explain the effects of
THC and cocaine on the
brain.
[4]
 Explain the effects of
cocaine and cocaine on
the brain.
[4]
 Discuss the causes of
addiction.
[6]
Sources:
 http://www.mun.ca/biology/desmid/brian/BIOL2060
/BIOL2060-13/1322.jpg
 http://health.howstuffworks.com/marijuana.htm
 http://health.howstuffworks.com/crack3.htm
 http://intranet.canacad.ac.jp:3445/BiologyIBHL2/5044
 http://click4biology.info/c4b/E/E4.htm
 http://en.wikipedia.org/wiki/Substance_dependence
 http://www.google.co.in/images
 http://www.google.co.in/images?hl=en&gbv=2&q=sto
 http://www.daretoact.net/