Transcript NEUROPSYCHOLOGY 3. The Neurobiology of Emotional

NEUROPSYCHOLOGY 3.
The Neurobiology of Emotional
Behaviours and Stress
Dr. Malcolm Hughes
Of all the terms that psychologists commonly use, ‘emotion’ may be the most
difficult to define.
Even though the term ‘emotion’ is poorly defined, the behaviours generally
associated with it e.g. escape and attack, are too important to ignore.
When we observe people who have become “emotional” their behaviour is
undeniably different from usual – often, such behaviours are vigorous and
persistent (Thompson, 1980).
Role of the Autonomic Nervous System (A.N.S.) in Emotional
Behaviours
Two parts of the A.N.S. govern behaviour:
a) the sympathetic nervous system (S.N.S.) – prepares the body for
intense, vigorous activity.
b) the parasympathetic nervous system (P.N.S.) – increases
digestion and other processes associated with relaxation.
These systems are active at all times, although at times, one may be more
active than the other.
Arousal of the S.N.S. and P.N.S.
The S.N.S. is activated, not by stimuli themselves but rather by the way an
individual interprets those stimuli.
e.g.
One group of boys given a task and told it was a test –
responded with increased heart rates;
Second group given the same task but told it was a game –
reacted with decreased heart rates (Darley & Katz, 1973).
In a study by Malcuit (1973), people who received inescapable shocks and
knew the shocks were inescapable had decreased heart rates – a typical
parasympathetic response to uncontrolled distress.
Those who were misled into thinking they could escape the shocks had
increased heart rates.
Emotions, Autonomic Responses and Health
Today there is little doubt that stressful experiences and thoughts of despair
can increase the risk of many kinds of illness and disease.
Conversely, social support, sense of humour and other positive emotions can
prolong survival or improve the quality of like for those with serious illnesses.
Appreciating the psychological factors in health is relatively new in Western
medicine – trend more towards behavioural medicine or medical
psychology:
- such areas focus on diet, drinking and smoking habits, exercise and other
behaviours on people’s health, including variations in emotional states.
The A.N.S. and Psychosomatic Illness
Individual’s A.N.S. responsiveness is highly consistent over time but will vary from
one person to another. Some show stronger and quicker sympathetic responses –
such individuals also tend to:
show more emotional expression
are generally more gregarious, more impulsive and less patient.
Those with a highly responsive S.M.S. also tend to be vulnerable to heart
disease and other medical disorders.
In such psychosomatic illnesses the probability of getting the disease or
recovering from it depends largely on the psychological make-up of the
individual e.g. personality and/or experiences.
The role of the A.N.S. in Ulcer formation
Ulcers can form in various ways, but those who experience work-related stress
are believed to be particularly vulnerable.
Proved in animal studies during the 1950’s and 1960’s – the “executive” and
“passive” monkey experiments (Brady et al, 1958; Foltz & Millett, 1964).
Evidence indicates that ulcers do not form during stress periods, but usually
during periods following stress (Desiderato, et al, 1974).
The period of stress greatly activates the S.N.S.
During the less stressful periods, the P.N.S. rebounds, releasing an excess of
digestive juices that damage the lining of the stomach and intestines.
Digestive secretions are not the full explanation – during periods of stress and
the period immediately following, the stomach makes intense contractions;
- Has the effect of breaking up the mucus lining of the stomach, thereby
exposing more of the stomach wall to the digestive secretions (Garrick, 1990).
The Impact of Chronic Stress on the Immune System
The body’s response to chronic stress differs from its response to transient
episodes of stress; there is a different set of responses to more severe and
prolonged periods of stress.
Stressors excite both the S.N.S. and an axis comprising the hypothalamus,
pituitary gland and adrenal cortex.
The hypothalamus induces the anterior pituitary gland to secrete
adrenocortico-tropic hormone (A.C.T.H.). This in turn stimulates the secretion
of cortisol and other steroid hormones.
Cortisol has the effect of elevating blood sugar and enhancing metabolism; in
the medium to long-term, this causes a shift away from the synthesis of proteins,
including those proteins necessary for the immune system.
Thus, in the long term, this causes a weakening of the immune system making
the individual vulnerable to a variety of illness conditions.
The hypothalamus /
anterior pituitary /
adrenal cortex axis
Pathways for the effect of emotional stress through the anterior and posterior
hypothalamus (Stoll, 1983).
Now recognised that the nervous system has considerable control over
the immune system – psychoneuroimmunology (O’Leary, 1990,
Kiecolt-Glaser, 1993, 1998).
Certain prolonged stressful events can release endorphins - decrease
pain but suppress blood levels of natural killer (N.K.) cells (which naturally
attach to tumour cells and cells infected with viruses).
If N.K.cell levels decrease, the individual is more vulnerable to infection; if
they develop a tumour, the tumour grows more rapidly.
The relationship between stress, the immune system and health change in
humans remains partly unclear e.g. most rat tumours are caused by viruses,
while the vast majority of human ones are not.
Also, depression often correlates with impaired responses to the immune
system (Weisse, 1992), yet chronically depressed individuals have virtually
normal life expectancies (Stein et al, 1991).
Still unclear as to whether particular types of stress or stressful
life episodes have a more adverse effect than others.
The Limbic System and Emotional States
Emotional behaviour dependant upon structures within the limbic system –
include the:
- hypothalamus, hippocampus, amygdala, olfactory bulb, septum +
parts of the thalamus and cerebral cortex.
Therefore, subcortical areas appear to generate emotional behaviours – the
cerebral cortex directs those behaviours towards appropriate targets as well as
trying to suppress them.
MacLean (1970) – evidence that the limbic system is important for emotion
based on observations of individuals suffering from temporal lobe epilepsy most with T.L.E. have no particular emotional experiences with their episodic
seizures. However, a substantial majority experience the following:
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Aggressive impulses
Dissociation of experience similar to multiple personality (Schenk,
1981)
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Uncontrollable laughter (Swash, 1972)
Sexual arousal (Rémillard et al, 1983)
Feeling of extreme bliss (sense of oneness with the universe)
Major pathways and connections of the Limbic System
Within the limbic system, MacLean distinguished three circuits:
a) amygdala & hippocampus – involved with self-preservation
b) cingulate gyrus (cerebral cortex) & septum – associated to sexual
enjoyment.
c) hypothalamus and ant. thalamus – believed to be important for
co-operative social behaviour (larger in primates than in other
animals).
Fear and Anxiety
With few exceptions, fear is either learned or modified by experience –
people with post-traumatic stress disorder (P.T.S.D.) show an enhanced
startle response compared to others (Shalev et al, 1992).
The startle response can be used to gauge levels of fear or anxiety – studies
indicate that if an animal (incl. humans) have learned an association between
some stimulus and shock, that stimulus becomes a “fear signal”.
e.g. presenting that stimulus just before a loud noise enhances the animal’s
response to the noise – links to phobias.
The Amygdala and its connections relevant to the leaning of fears and
anxieties
Key area associated with fear is the Amygdala. – a rat with damage to the
amygdala shows a normal startle response after a loud noise, but no
enhanced startle response to the combination of the “fear signal” plus loud
noise (Phillips & LeDoux, 1992).
In general, such animals neither learn new fears nor retain learned fears
acquired before brain damage (Kim & Davies, 1993).
The amygdala sends its output to various parts of the brain:
Its connections to:Hypothalamus – controls autonomic fear response e.g.
increased blood pressure.
Hindbrain – controls flinching, freezing and similar skeletal
responses.
Suggestion that there might also be links to the Pons and the Medulla.
Most incoming information to the amygdala comes directly from the thalamus,
not as suspected the cerebral cortex – i.e. amygdala does not rely on highly
detailed information, but gets the information very rapidly.
Likely that this might link some fears with pain – fear conditioning.
Anxiety-Reducing Drugs
Anti-anxiety drugs – barbiturates (habit forming) or, more recently, the
benzodiazepines (Valium, Librium, Xanax) –
Have the effect of relaxing muscles, induce sleep and decrease liklihood of
convulsions (anti-epileptic effect).
During 1980’s, benzodiazepine receptors discovered (GABA receptors).
The brain possess at least two major categories of such receptors:
GABAA - (tend to decrease anxiety) and GABAB receptors.
GABA receptor has a chloride channel – when open chloride ions cross the
membrane into the neurone – inhibits the synapse.
Surrounding the chloride channel are four units, three of which (α) also
contain the benzodiazepine receptor. When attached, benzodiazepine
molecule alters the shape of the receptor so that GABA attaches more
readily:
- results in greater flow of chloride ions across the membrane, ultimately
adjusting the anxiety levels of the person.
The GABAA receptor complex
Serotonin Synapses and Aggressive Behaviour
In terms of emotional behaviour, aggression is recognised as a
characteristic found in all species
Although it is unlikely that a single neurotransmitter system controls
aggression, evidence indicates that serotonin is implicated:
i.e. a low serotonin release =
with rise in aggression
Research evidence – people with history of violent behaviour (incl.
violence towards self) have lower-than-average serotonin turnover.
Those who have attempted or succeeded in committing suicide are found to
have low levels of 5-HIAA (5-hydroxy indole acetic acid, a serotonin
metabolite) in the blood, C.S.F. and urine – suggests lower than normal release
of serotonin.
Synthesis of Serotonin
Tryptophane
(from diet)
hydroxytryptamine)
5-hydroxytryptophan
Serotonin
(5-
Levels of 5-HIAA in the CSF of depressed people
according to suicide attempts (Roy, DeLong & Linnoila,
1989).
Such individuals also have more %-HT2 (serotonin2) receptors than
usual in the cerebral cortex – possibly brains means of compensating
for decreased serotonin release.
Serotonin turnover also depressed in those convicted of arson and
violent crimes.
Hypothesis suggests that serotonin synapses INHIBIT behavioural
impulses that might lead to punishment or other unfavourable
outcomes.
i.e.
when serotonin turnover is HIGH, have restrained behaviour;
when serotonin turnover is LOW, have impulsive behaviour, incl.
violent outbursts.
Now recognised that the measurement of serotonin turnover could be
used to predict the probability of a person with low serotonin levels
repeating specific actions – 84% probability.
e.g. person attempting suicide likely to repeat the act, possibly with fatal
consequences within 5 years (Roy, DeLong & Linnoila, 1989).
Also, person convicted of manslaughter or arson, more likely to repeat
crime after release from prison (Virkunnen et al, 1989).
Serotonin is particular neurotransmitter whose fluctuating presence in the
brain goes beyond the conditions considered so far.
As already demonstrated, it is linked to depressive episodes, when
serotonin levels decrease as well as aggressive or violent behaviours
Evidence also points to the involvement of the chemical in obsessive –
compulsive disorders.
Among such sufferers, many respond well to the drugs clomipramine and
fluvoxamine – both inhibit the reuptake of serotonin by the pre-synaptic
neurone i.e. the drugs prolong the effects of serotonin at the synapse.
Conversely, drugs that inhibit serotonin synthesis tend to aggravate the
symptoms of obsessive-compulsive disorder.
In view of these findings, there exists a strong case for suggesting that
measurement of serotonin levels could be used as a predictor of some
forms of human behaviour or mental disorder.
Whether or not anyone should use serotonin measures in such a way
raises ethical and legal issues – after all, predictions based on
serotonin turnover remain far from perfect.