Transcript E4 - Neurotransmitters and Synapses - IBDPBiology-Dnl

E4: Neurotransmitters and
Synapses
DnlNjuguna ~ DAIS
Excitatory and Inhibitory synapses
Some Pre-synaptic neurons excite
postsynaptic transmission
(excitatory pre-synaptic neurons)
and
others inhibit postsynaptic transmission
(inhibitory pre-synaptic neurons )
Excitatory Presynaptic neurons
 Depolarises the
postsynaptic
membrane
 Influx of Na+ ions
 Possibly reaching
threshold
 Generation of new
action potential
 E.g.
Acetylcholine
Inhibitory Presynaptic neurons
 Hyperpolarises the
postsynaptic
membrane
 Influx of Cl- or loss of
K+ ions
 Makes it more difficult
to reach threshold
 Lowers the resting
potential
 E.g.
Gama-aminobutryic
acid (GABA)
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
Is chemical substances
that acts upon the CNS
altering the state of
mind by changing
(stopping or enhancing)
the activity at the post
synaptic neurone.
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 neurone resulting in an action
potential
 Hyperpolarise the post synaptic neurone 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-aminobutyric acid
 The most common inhibitory
(GABA) (inihibitory)
neurotransmitter in the brain.
Examples of excitatory & inhibitory
psychoactive drugs:
Excitatory psychoactive Inhibitory psychoactive
drugs;
drugs;
 nicotine,
 benzodiazepines,
 cocaine ,
 alcohol ,
 amphetamines.
 tetrahydrocannabinol
(THC).
Effects of THC in terms of its action at
synapses in the brain.
 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
tobaco
 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 neurone
inhibiting generation of an action potential
Effects of Cocaine in terms of its
action at synapses in the brain:
 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 neurone.
 This sends a signal to that
Cocaine interferes
nerve cell, which produces a
with a chemical
good
feeling.
Under
normal
messenger in the
conditions, once the dopamine
brain called
sends that signal it is
dopamine, which 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!
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/