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Monitoring the body’s reactions to stress

Class practical

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This investigation shows how the autonomic nervous system responds to stress, as measured in the students themselves. It could lead to a more detailed discussion of the ‘fight or flight’ response, and the wider context of how the autonomic nervous system controls the body. The Leiden Public Speaking Task has been developed as a robust technique for inducing stress to evaluate the effects of different hormones and environments on the stress response in humans. The procedure also permits students to assess the subjective experience of stress and identifies other physiological symptoms of stress.

The information and data provided in this practical have been supplied by members of the British Society for Neuroendocrinology.

Lesson organisation


This will depend on how many sets of apparatus you have for logging heart rate and/or blood pressure, and the extent to which you manage the session or ask the students to develop the protocol. You could prepare your students by explaining the task in an earlier lesson, but this may change the levels of nervousness experienced.

Apparatus and Chemicals


For the class – set up by technician/ teacher:

Datalogging equipment or electronic sensors for heart rate (Note 1)

For each group of students:

Handouts to record personal heart rate data

Stopwatches/ timers (Note 1)

Copies of the presentation/s (Note 2)

Health & Safety and Technical notes


Read our standard health & safety guidance

1 You could use a free-standing electronic heart rate monitor, or a datalogging device, or manual measurements of heart rate to collect data for this activity. The advantage of a data-logger are that it will record variations throughout the procedure without interrupting the student. Simple, free-standing monitors with a digital display are quicker and easier to read than counting the heart beats in 15 seconds. If you choose the simplest method, it will give more meaningful results if students assess one another’s heart rate rather than their own.

2 The topics of the speech should allow for expression of opinions, but avoid descriptions which might encourage animated arm movements, or ego-involving and arousing topics which could also contaminate the data. Suitable topics for the presentation would be themes such as the pros and cons of downloading pirated music compared with purchasing the real thing. The stress of the test should relate to that caused by standing up and speaking, rather than trying to make a particularly powerful argument. Sample speeches on two sides of the downloading music argument are attached below.

Sample speeches (54 KB)

Procedure


Preparation

a Consider issuing the  Student briefing and consent form and accepting ‘sign off’ agreement from any participating students.

b Prepare a speech for the students to deliver on a suitable subject (Note 1).

Investigation

c Measure students’ pulse rates at rest (not immediately after exercise) and before the task is announced.

d Ask the students to rate their nervousness on a scale of 1-10 (where 1 is completely relaxed, and 10 is extremely nervous).

e Ask the students to describe how their body feels at that level of nervousness. Can they describe their physiological state associated with any nervousness (or relaxedness!).

f Then tell the students that they will be giving a short presentation later in the lesson, and observing the effects of stress on the body.

g Provide students with notes for the presentation and give them some time to prepare for it.

h Just before they start speaking, measure pulse rate again.

i Ask the students to rate their nervousness on a scale of 1-10 (where 1 is completely relaxed, and 10 is extremely nervous).

j Ask the students to assess whether or not they have any physiological symptoms such as butterflies in the stomach or sweaty palms.

k Ask each student to deliver a prepared speech.

l After delivering the speech, measure the students' pulse rate again, and report on nervousness rating and physiological symptoms.

m Measure pulse rate every three minutes for the next 10 minutes.

n Collect the data and determine each data point as a percentage of base rate.

o Plot the data to see how stress affects heart rate, and how quickly heart rate returns to the baseline level of before the speaking task.

p Compare data from the nervousness questionnaire.

q Compare the descriptions of nervousness.

r See how stress affects the mind and emotional state.

Teaching notes


Previous scientific studies of this type have shown that the statistical analyses were not influenced by excluding participants who used oral contraceptives, or had eaten, smoked or consumed caffeinated soft drinks.

This practical provides an opportunity to collect data using a datalogger. Heart rate and blood pressure monitors are available. Your students may be interested to research other methods of measuring stress, such as changing skin conductance and levels of hormones such as cortisol in saliva.  However, skin conductance meters available for purchase may be too expensive, and salivary assays are quite complex and do not provide an instant result.

Background information: the autonomic nervous system


The following information about stress responses and the autonomic nervous system has been provided by the British Society for Neuroendocrinology.

The autonomic nervous system is the system responsible for conveying all of the outputs of the brain to the rest of the body, except for the motor innervation of skeletal muscle, and is largely outside voluntary control. It is formed from three main divisions – the sympathetic, parasympathetic and enteric nervous systems (which itself is closely linked with both the sympathetic and parasympathetic divisions) or SNS, PNS and ENS. Of these, the ENS, the series of nerves and ganglia that innervate the gastro-intestinal tract, is the only part capable of functioning without any CNS input. The SNS and PNS are absolutely controlled by the CNS, and are in many ways the more important parts of the autonomic nervous system. Between them, the main functions that the SNS and PNS control are the contraction and relaxation of smooth muscle, for example, in vasculature and bronchi, heart rate, sweating, and gastro-intestinal tract secretions and motility (with the aid of the ENS).

Both the SNS and PNS pathways from the spinal cord to the target organs consist of two neurone series. That is, a first neurone (pre-ganglionic) receives information from the spinal cord and transmits it to an autonomic ganglion lying outside the CNS, where the second neurone (post-ganglionic) picks up this information and transmits it to the target organ. This arrangement reveals the main anatomical difference between the SNS and the PNS: SNS ganglia lie in two chains running along either side of the spinal cord, so the SNS pre-ganglionic neurones are short, whilst post-ganglionic neurones are long. In contrast, PNS ganglia lie within or very close to the target organs themselves, so the PNS is characterised by long pre-ganglionic neurones and short post-ganglionic cells. The only exception to this rule is the innervation of the adrenal medulla, which secretes adrenaline into the blood in response to SNS activation: this has a single, long pre-ganglionic neurone – the adrenal gland itself effectively replacing the sympathetic ganglion and post-ganglionic cell in this case.

The second major difference between the SNS and the PNS lies in the chemical neurotransmitters used. In both systems the main neurotransmitter in the ganglion is acetylcholine; this is released by the pre-ganglionic neurone and targets nicotinic acetylcholine receptors (nAChR) on the post-ganglionic cell. Release of acetylcholine and activation of the nAChR is a very rapid process, taking a few milliseconds, and allowing the very quick transmission of information from one neurone to another. The main difference between the SNS and PNS in terms of transmitters lies in the post-ganglionic cells. All PNS post-ganglionic neurones again use acetylcholine as a neurotransmitter, but in this case it targets a different type of receptor, the muscarinic acetylcholine receptor (mAChR) which acts more slowly (seconds to a few minutes depending on the organ) and allows a more gradual activation. The SNS, in contrast, uses a different neurotransmitter, noradrenaline, which acts on a slow activating (seconds to minutes) family of receptors known as the adrenergic receptors, of which there are four different types, α1, α2, β1 and β2. α and β adrenergic receptors tend to have opposing actions (as a general rule, α are inhibitory and β are excitatory) which allows some organs to have only SNS input; this is most important in the control of blood vessel diameter, and through this the control of blood pressure.

The Autonomic Stress Response

The autonomic nervous system is the key player in the immediate response to stressful stimuli, whether physical or psychological; this is largely due to the actions of the SNS. Central to the rapidity of the SNS response to stress are the paravertebral chains of sympathetic ganglia. Due to their presence close to the spinal cord and the short pre-ganglionic neurones that innervate them, it is possible to activate the entire chain in a single co-ordinated response, allowing a rapid activation of the SNS as a whole. This brings about a series of interlinked events, best known as the 'fight or flight' response.

One of the most important effects of SNS stimulation following stress is to adjust blood flow, increasing delivery of both oxygen and glucose to tissues that may be key to survival – the brain, heart and skeletal muscle. This is achieved through several interacting mechanisms, principally an increase in heart rate and contractility, and an alteration in the pattern of blood flow.

Heart: The heart is innervated by both PNS and SNS fibres; the PNS acting through mAChR to both slow down heart rate and decrease its contractile force, and the SNS acting through β1 adrenergic receptors to speed it up, with the net heart rate and force being the sum of both. Stimulation of the SNS in response to stress quickly increases heart rate, and is responsible for the feeling of your heart racing – which in essence it is. This is the mechanism by which β-blockers work – they limit the ability of the SNS to speed up heart rate, and in doing so allow the PNS to dominate.

Vasculature: Alongside the heart, the other major contributor to blood pressure is the resistance of the vasculature. The walls of arteries and arterioles, the vessels supplying organs with blood, contain smooth muscle, and can contract and relax to change vessel diameter. The resistance of the vessel to blood flow significantly increases with decreasing diameter, so constriction of blood vessels can have a major impact on blood pressure. The blood vessels are only innervated by nerves from the SNS, with the effect of stimulation being dependent on the receptor types that any blood vessel has. In particular, stimulation of α1 adrenergic receptors causes a constriction in vessel diameter, whilst stimulation of β2 receptors causes a dilation. Following a stressful stimulus, activation of the SNS causes the constriction of the majority of blood vessels through stimulation of α1 receptors by noradrenaline released from SNS neurones, increasing blood pressure. At the same time, stimulation of adrenaline release (from the adrenal gland by SNS activation) targets β2 receptors on the blood vessels in the major skeletal muscles in the limbs, causing a local dilation. This dilation is not enough to significantly counteract the increase in blood pressure caused by the effects of SNS activation on the other blood vessels and on the heart; what it does do is to effectively shunt blood flow away from the gastrointestinal tract and towards the muscles – ensuring they have an enhanced supply.

Liver: As the most important energy store in the body, the liver has a key role to play in the autonomic nervous system response to stress. In response to SNS stimulation, either directly through noradrenaline acting on α1 receptors, or indirectly via adrenaline which then acts on β2 receptors, the liver begins to break down its stored glycogen into glucose; this is then released into the bloodstream. Glucose is then available at a relatively high level for metabolically-active tissues, in particular the muscles and the brain. In part, this increase in blood sugar is one of the factors underlying the so-called adrenaline rush, the rewarding feeling you get from some physiological stressful situations, such as extreme sports, and is analogous to some degree to the reward achieved when eating chocolate.

Gastro-intestinal tract: In addition to the effects on the circulation, another key effect of SNS stimulation is to slow gastro-intestinal transit. In essence this is due to a prioritisation of energy expenditure – it is more important to ensure the organism avoids the stressful situation than it is to make sure it has digested lunch properly. Moreover, the adaptation of the blood supply to favour muscles and the brain means that less is available to the GI tract, making nutrient transfer relatively inefficient – another reason to slow transit of food whilst still in a stressful situation. This is the main explanation for the feelings of 'butterflies in the stomach' occurring when under stress.

Sweating: The last major effect of stress is on sweating which, again, is controlled by the autonomic nervous system. Sweat glands in the skin are targeted by SNS nerve fibres, although these are slightly unusual in that both pre-ganglionic and post-ganglionic nerves use the neurotransmitter acetylcholine, which is more normally a feature of the PNS. However, the position of the ganglion in this pathway, close to the spinal cord, determines its classification as part of the SNS. The principal aim of sweating is thermoregulation (the control of body temperature); this can be highly important when undertaking physically exerting actions, such as running away or combating an attack. Where sweating occurs depends to some extent on the nature of the stressor – psychological or emotional stresses induce sweating which tends to be restricted to the palms, soles, and sometimes the forehead, whilst physical heat-induced sweating occurs throughout the body.

The Hypothalamo-Pituitary-Adrenal Axis

In addition to the autonomic nervous system, which enables the body to cope rapidly with a stressful stimulation and underlies most of the effects seen in this practical, there is a second system which co-ordinates the stress response, but over a longer time-frame, the hypothalamo-pituitary-adrenal or HPA axis. The hypothalamus, a small area in the centre of the base of the brain, in line with the top of the nose and between the ears, is the central controller of this axis. In response to stressful stimuli the neurones of this area release the hormone corticotrophin releasing hormone (CRH) into a collection of small blood vessels that flow to the pituitary gland, almost directly underneath the hypothalamus. The pituitary gland is the 'master' gland for virtually all endocrine responses, secreting a variety of hormones which then co-ordinate a wide range of physiological functions including stress, sex, feeding, sleep and growth. In the case of the stress response, CRH stimulates cells of the pituitary to release adrenocorticotrophic hormone (ACTH) which is secreted into the general circulation. ACTH then targets the adrenal glands to cause the release of the major hormone mediator of the HPA axis, cortisol.

Cortisol has an enormous range of actions, indeed it has been estimated that approximately 1% of all the genes in the human genome can be regulated directly by the hormone, and even more are indirectly controlled. The following are the main functions of cortisol in terms of the stress response:

  • increased secretion of glucose into the blood, partly through a block of insulin action, and partly through stimulation of glycogen breakdown in the liver
  • increasing blood pressure, by enhancing the sensitivity of the vasculature to noradrenaline and adrenaline
  • suppression of the immune response and inflammation which, although in the long term may delay healing, in the short term will serve to conserve energy and limit tissue damage from an overactive immune response.

The HPA axis acts over a longer time-frame than the autonomic nervous system response, with increases in circulating cortisol not seen until approximately an hour or so after the initial stimulus. This may reflect the different functions of the two systems – the autonomic nervous system allows a response immediately to a stressful stimulus, adapting the body’s physiology to maximise the chances of evading or tackling the stress, whilst the HPA axis is more concerned with adaptation to longer-lasting stressful situations.

Health & Safety checked, December 2010

Downloads


Download the student sheet Monitoring the body's reactions to stress (60KB) with questions and answers.
Download Sample speeches (54 KB) for the copyrght debate.
Download the Student briefing and consent form (63KB)

Related experiment


Observing the effects of exercise on the human body

Web links


www.salimetrics.com
This commercial company produces salivary assay kits for research purposes. Their website text is readable and approachable. Details of the methods can be downloaded.

(Website accessed October 2011)

 

Page last updated on 15 November 2011