Measurement of Stress Reaction: The Physiological Response, The Cognitive Response, The Behavioural Response

The Physiological Response

The process of physiological stress response starts from the moment the body realizes the presence of the stressor, followed by the sending of signals to the brain, and to the specific sympathetic and hormonal responses to eliminate, reduce or cope with the stress.

The Nervous System

When your body senses that a particular stressor is present, signals about that stimulus are sent to your brain. The master gland called the hypothalamus is then alerted to arouse the Autonomic Nervous System (ANS). The ANS is the system which controls most of the major organs of your body: the heart, lungs, stomach, glands and even the blood vessels. With these organs, you’ll readily notice that the ANS is responsible for the unconscious regulation of the heart beat and breathing.

Sympathetic Response

The SNS surely likes things to go very quickly when you are faced with stress. The neurotransmitter noradrenaline is released by the nerve endings and is sent to the SNS so that the latter can:

• Enhance the strength of your skeletal muscles. Have you heard news about people who were able to carry heavy furniture or equipments outside their house during fire? Well, fire is a very stressful situation, and thankfully we have our sympathetic response to aid us during these circumstances.
• Increase heart rate. During stressful moments, your heart beats faster than it usually does so that the parts of your body which are needed to cope up with the stress would be supplied by enough oxygenated blood to remain functional until the stressful situation subsides.
• Shoot up sugar and fat levels. We all know that sugar and fat provides our body with energy. During stressful situations, we need more energy to cope up, and so the SNS assists us to have more energy.

Hormonal Response

Other than the nervous system, the body’s stress response also includes the help of the adrenal glands. Situated on top of each kidney, the adrenal glands are also included in the physiologic stress response because the adrenal medulla (the centre part of the glands) has nerves that connect the gland to the SNS. The SNS stimulates the adrenal medulla to start releasing adrenaline and noradrenaline into the blood circulation. This action results in the “fight or flight” response, which is manifested by the increase in heart rate, dilation of bronchial airways and enhancement of the metabolic rate so more of the stored energy can be used.

The Cognitive Response

Despite evidence that psychological stress is an important risk factor for age-related cognitive loss, little research has directly evaluated psychological and physiological mediators of the relationship between stressful experiences and cognitive function. A key objective of the ESCAPE (Effects of Stress on Cognitive Aging, Physiology, and Emotion) project is to evaluate whether engaging in stress-related unconstructive repetitive thought (URT) is a pathway through which stressful experiences negatively affect cognitive health over the short- and long-term. Over the short-term, we hypothesize that engaging in URT will deplete attentional resources and result in worse cognitive performance in daily life. Over the long-term, we expect that the effects of chronic stress, from repeated exposure to stressors and regular engagement in URT, will be apparent in dysregulated hypothalamic-pituitary-adrenal (HPA) axis function and inflammation. Over time, stress-related physiological dysregulation will result in accelerated cognitive decline.

Cognitive symptoms of stress include:

• Constant worrying
• Racing thoughts
• Forgetfulness and disorganization
• Inability to focus
• Poor judgment
• Being pessimistic or seeing only the negative side

The Behavioural Response

Behavioural responses to stress are evoked from underlying complex physiological changes that arise consequently from stress.

Real or perceived threat in the environment elicits stress response in animals, which disrupts internal homeostasis. Physiological changes cause behavioural responses in animals, including: impairment of response inhibition and lack of motivation, as well as changes in social, sexual, aggression and nurture behaviour in animals. The extent of the impact is dependent upon the type and duration of the stress, as well as the animal’s past experiences. Behavioural responses to prolonged stress can also be transferred across generations.

Behavioural responses

Behavioural responses to stress are evoked from some underlying complex physiological changes that arise consequently from stress.

Impairment of response inhibition and lack of motivation

According to a study conducted by Mika and his colleagues, prolonged stress in rats causes response inhibition. It was evident through their experiment that stressed rats had inhibited premature responses (decreased timing of intervals to food), along with decreased intrinsic motivation to initiate a response. They link the decreased motivation to the stress-associated reduction in incentive motivation, as presented by another study conducted by Kleen and his colleagues. Decreased motivation was also seen in a study conducted by Beery and Kaufer, where they explained that stressed rodents are less likely to be motivated to interact with one another.

Change in social behaviours

Beery and Kaufer noted that social withdrawal and general reduction in social interaction after an exposure to a stressor are evident in rodents. They argue that this is due to the underlying physiological changes that the rodent goes through in response to stress. For instance, the changes that occur to the hypothalamic-pituitary-adrenal (HPA) hormonal axis is directly related to the changes in social behaviour. Social avoidance is another consequence of stress that can be seen in rodents. Rodents are more likely to avoid dominant rats and avoid social interactions amongst each other after the exposure to a stressor.

Sexual behaviours

Sexual interests change in many species when exposed to stressors. For instance, stressed male and female rats express inhibited mating behaviour, which is evident through the clear increase in the inhibitory hormone RF-amide. Another study suggests that masculine sexual behaviour in male rats is subject to changes in accordance to the type of stressors that the rats were subjected to. The female zebra finch’s mating choice is determined by the stressors that they are exposed to early in life, which remain consistent throughout adulthood. A study about stress effects on female songbird’s response to sexual signal for mating indicated that the response to this specific signal can be impaired if the female is exposed to developmental stress. Behavioural changes as a result from developmental stress impairs neural responses to sexual signals, which reduces mating.

Aggressive behaviours and anti-predator responses

Stressed animals would choose to avoid a novel situation rather than confront it. Aggressive behaviour is associated with sex hormones, such as testosterone, and specific brain regions and systems, such as the medial preoptic nucleus, prefrontal cortex-dependent response inhibition, and anterior hypothalamus. Stress negatively impacts sex hormones, which results in an imbalance and reduction in aggression related hormones and function. Also, chronic stress results in prefrontal cortex-dependent response inhibition. This results in reduction in aggression, thus promoting anti-predator responses.

Prolonged stress reduces parental behaviour toward offspring

Prolonged stress alters parental behaviour toward offspring and promotes parental neglect. According to a study performed by Tilgar and associates, predation stress alters parents’ behaviours, such as the reduction in provisioning rates, which negatively impacts the offspring’s performance. The hormones oxytocin and vasopressin are generally responsible for affiliative and pair-bonding behaviours in many species. Stress alters the level of both hormones, resulting in an abnormal behaviour from parents towards offspring. For instance, levels of oxytocin decrease as a result of prolonged stress, which has been shown to reduce pair-bonding behaviour and increase withdrawal behaviour. Prolactin is another important hormone that is associated with nurture by parents to offspring, and levels of this hormone can be altered as a result of stress. Reduced levels of prolactin as a result of stress decreases behaviours such as suckling, licking, and brooding.