coordinated responses

Question 1

fight or flight response

Answer 1

The fight or flight response is an instinct that all mammals possess. When a potentially dangerous situation is detected, the body automatically triggers a series of physical responses. These are intended to help mammals survive by preparing the body to either run or fight for life, hence the name of the response.
Once a threat is detected by the autonomic nervous system, the hypothalamus communicates with the sympathetic nervous system and the adrenal-cortical system. The sympathetic nervous system uses neuronal pathways to initiate body reactions whereas the adrenal-cortical system uses hormones in the bloodstream. The combined effects of these two systems results in the fight or flight response. The overall process is summarised in Figure 1.
The sympathetic nervous system sends out impulses to glands and smooth muscles and tells the adrenal medulla to release adrenaline and noradrenaline into the bloodstream.
These ‘stress hormones’ cause several changes in the body, including an increased heart rate.
The release of other stress hormones which have a longer-term action from the adrenal cortex is controlled by hormones produced by the pituitary gland in the brain. The hypothalamus stimulates the pituitary gland to secrete adrenocorticotropic hormone (ACTH). This travels in the bloodstream to the adrenal cortex, where it activates the release of many hormones that prepare the body to deal with a threat. Look back at Topic 14.1, Hormonal communication to remind yourself of the structure and function of the adrenal glands.
The physiological responses which occur as part of the fight or flight response are summarised in Table 1.

Question 2

physical response and purpose

Answer 2

heart rate increase-to pump more oxygenated blood around the body

pupils dilate-to take in as much light as possible for better vision

arteries in skin constrict-more blood to major muscle groups, brain, heart and muscles of ventilation

blood glucose level increases-increase respiration to provide energy for muscle contraction

smooth muscle of airways relaxes-to allow more oxygen into lungs

non-essential systems (like digestion) to shut down-to focus resources on emergency functions

difficulty focusing on small tasks-brain solely focused only on where threat is coming from

Question 3

action of adrenaline

Answer 3

One of adrenaline’s main functions during the fight and flight response is to trigger the liver cells to undergo glycogenolysis so that glucose is released into the bloodstream. This allows respiration to increase so more energy is available for muscle contraction.
Adrenaline is a hormone. It is hydrophilic therefore cannot pass through cell membranes. Adrenaline binds with receptors on the surface of a liver cell membrane and triggers a chain reaction inside the cell:
• When adrenaline binds to its receptor, the enzyme adenylyl cyclase (which is also present in the cell membrane) is activated.
-Adenylyl cyclase triggers the conversion of ATP into cyclic adenosine mono-phosphate (CAMP) on the inner surface of the cell membrane in the cytoplasm.
• The increase in CAMP levels activates specific enzymes called protein kinases which phosphorylate, and hence activate, other enzymes. In this example, enzymes are activated which trigger the conversion of glycogen into glucose.

This model of hormone action is known as the second messenger model.
The hormone is known as the first messenger (in this example, adrenaline) and CAMP is the second messenger. One hormone molecule can cause many CAMP molecules to be formed. At each stage, the number of molecules involved increases so the process is said to have a cascade effect (Figure 3).

1) The hormone adrenaline
approaches receptor site.
2) Adrenaline fuses to receptor site, and in doing so activates an enzyme inside the membrane.
3) The activated enzyme converts
ATP to cyclic AMP, which acts as a second messenger that activates other enzymes that, in turn, convert glycogen to glucose.