Transcript Neuro-transmitters

NEUROPSYCHOLOGY
Dr. Malcolm Hughes
Watson defined psychology as the “science of the mind” and in that
respect, the subject of psychology has focused on a range of topics
which reflect the manner in which human subjects behave.
Attempts to explain behaviour in terms of brain mechanisms is also
expressed by the term “biological psychology”.
The subject is sub-divided into disciplines which include:
a) Neuro-anatomy
b) Psycho-genetics
c) Psycho-endocrinology
d) Psycho-neuro-immunology
e) Psychopharmacology
“Biological psychology” is defined as explaining behaviour in
biological terms – considerable emphasis in placed on the brain
structure at a cellular level and the associated processes e.g.
chemical mechanisms.
Now recognised that chemical energy within the brain can be
converted into various kinds of brain activity; conversely, brain
activity, e.g. cognitive processes and emotions, can instigate
changes in the brain’s chemistry resulting in other mental and
behavioural responses.
Biological Explanations of Human Behaviour
Example: some psychologists would advocate that anger in an
experience that merely reflects a pattern of activity within a specific
area of the brain, heightened by certain hormones.
A further suggestion is that the brain is “programmed” by natural
selection to enable such experiences to happen. This implies a
“psychogenic” influence.
Within these areas of study, the understanding of the brain and its
constituent parts have led to substantial contributions to the relationship
between brain and behaviour.
Included among these areas are the physiological mechanisms
associated with the central nervous system.
An overall understanding of the anatomy of the brain structure
reflects the diversity of the individual’s mental processes and
behaviour. Some structures can highlight these effects quite
significantly. Among such structures are:
a) The thalamus
b) The hypothalamus
c) The hippocampus and amygdala
d) The corpus callosum
e) The cerebral hemispheres
The midline structures of a human brain.
From the biological standpoint, there are two types of likely explanation:
1. Factors that “force” a behaviour to occur e.g. sweating,
constriction of the pupil of the eye, knee-jerk reflex.
2. Factors that “enable” a behaviour to occur e.g. social or
competitive motivation, “choice” of aggression, sexual motivation.
Therefore, theories would suggest that human behaviour is a product of
several different, but possibly integrated forces.
The Mind-Brain Relationship
Problem of definition – what is meant by the “Mind”? Various interpretations
1. MATERIALISTS: view the brain as a machine in which the consciousness
was irrelevant to its functioning.
2. DUALISTS: consider the mind to exist independently of the brain, but
could exact some control over it. Mind and body are separate
but interactive.
3. IDENTITY POSITION: the mind is viewed as being the same as the brain
except that it is described in different terms.
Neuro-transmitters
These are chemicals released at the synapse separating two
sections along a nerve fibre. It is now recognised that a large
number of chemicals function as neurotransmitters within the
brain.
The three major categories of neurotransmitters are:
a) Biogenic amines (contain an amine (NH2 group)
b) Amino acids – derived from the breakdown of proteins
c) Peptides
Each neurone has the capacity to synthesise its neurotransmitters
from precursor molecules that reach the cell by way of the blood,
derived originally from foods that the individual ate.
Many neurotransmitters are synthesised both in the cell body and
in the terminal, close to the point of release.
Under normal circumstances, the brain maintains fairly constant
levels of each neurotransmitter, even during periods of fasting.
Cell body
Synthesis of
neurotransmitter,
formation of vesicles
Vesicles without
neurotransmitter are
transported back to cell body
Transport of neuro-transmitter
down axon
Reuptake of neurotransmitter to be
recycled
Separation of neurotransmitter molecules
from receptors
Release of
neurotransmitter
Postsynaptic neuron
Interaction with receptor
exciting or inhibiting
postsynaptic neuron
The major events in
transmission at a
synapse.
Diagram of a synapse: the
end of the presynaptic
axon swells to form the
terminal which releases
the neurotransmitters.
NEUROTRANSMITTERS
Biogenic amines
Amino acids
(contain NH2 gp.) (glutamate, G.A.B.A.)
Acetylcholine
Peptides
(endorphins)
Others
(adenosine)
Monoamines
Serotonin
Catecholamines
Dopamine
Norepinephrine
Epinephrine
(Nor-adrenaline)
(Adrenaline)
The above diagram illustrates the different types of neurotransmitters
and their synthesis pathway.
Drug effects on Pre-synaptic Neurones
Certain drugs exert their effects by altering the synthesis of a
neurotransmitter e.g. A.M.P.T. (α – methyl para-tyrosine) – has the
effect of blocking the synthesis of:
Dopamine
Adrenaline
Nor-adrenaline
These effects are due to the similarity in the chemical structure of
tyrosine to A.M.P.T.
A.M.P.T. blocks the enzyme from attaching to tyrosine, therefore very
little tyrosine is converted into DOPA.
Amphetamines have the capacity to release neurotransmitters from
storage in the pre-synamptic neurones:
Results in:
a) increase in the release of adrenaline and noradrenaline.
b) increases the alertness and arousal of the
individual.
After some hours, the individual can enter a “rebound” state of
depression.
This is due to the brain’s inability to re-synthesise new nor-adrenaline
in sufficient quantities to replace that released as a result of
amphetamine intake.
Effects of caffeine
Adenosine (a neurotransmitter) acts at many pre-synaptic receptors to
inhibit the release of excitatory transmitters e.g. glutamate.
Caffeine can block the effects of adenosine (Silinsky, 1989) – this
results in an increase in the release of glutamate.
Therefore the nervous system becomes stimulated.
Effects on Post-synaptic Receptors
Some drugs have similar chemical structures to the neurotransmitter; therefore both substances can fit into the same
receptor site.
e.g. acetylcholine has certain synapses referred to as “nicotinic
synapses” (are naturally stimulated by the drug nicotine).
Therefore, nicotine acts as an acetylcholine agonist.
Has the effects of:
increasing heart rate
increasing cerebral cortex activity
Other drugs are capable of slotting into receptor sites but do not
necessarily stimulate it:
e.g. Haloperidol – blocks the DOPA synapses. Is thus defined as a
DOPA antagonist.
N.B. this is now take as evidence that dopamine activity in excess of
normal, contributes to schizophrenia.
Neuro-anatomical and neuro-chemical influences:
These can impact on a diverse range of cognitive, emotional and
behavioural outcomes.
1. At a cognitive level, there are implications in terms of thought
processes, memory and learning and the way the individual
can perceive.
2. In emotional terms, the impact of stress can bring about
changes in the neuro-endocrine system. Can in turn impact on
other elements of the brain’s biochemistry and the immune
system, thereby affecting both mental and physical health.
3. Behavioural outcomes, often associated with psychogenetic
conditions and physical trauma to the brain, including lesions.
Therefore, in conclusion, it is important to recognise that the
brain is a complex organ in terms of its function together with the
way it can itself can be profoundly affected by events occurring
elsewhere in the body:
e.g. biochemical changes or electrolyte imbalance.
In a similar manner, the brain can also be influenced by factors
external to the body:
e.g. events perceived by the individual, intensity of light
Thus, when considering the processes which reflect on the overall
functioning of the human individual, it is vital to remember that
even the slightest changes can bring about alterations in how that
individual may react emotionally and the nature of their behaviour,
both at a psychological and physical level.