5.5.11 Coordination of Responses

Responding to change and stimuli

• Organisms must respond to changes in their environment in order to survive
• They can only survive if they are successful at:
  • Finding favourable conditions for living
  • Finding food
  • Avoiding being eaten

• If these vital requirements are not met then a species will die out or go extinct
  • For example, a red robin must find worms and insects to feed on and at the same time, they must also be watching out for predators such as crows

• Responses to change can vary in complexity depending on the type of organism involved and the specific circumstances they are responding to
• Responding to change requires detection
  • Detection involves a stimulus being detected by a receptor cell

• There are different types of receptors
  • Some receptor cells produce electrical activity in nerve cells in response to stimuli
  • Other receptor cells secrete messenger chemicals such as hormones in response to stimuli

• The impulses or hormones sent by receptor cells travel to a coordinator
• From the coordinators, the impulse is transported to the specific effector that will produce the appropriate response

The 'fight-or-flight' response

• An animal may produce a 'fight-or-flight' response in situations where there is a high level of stress, fear or aggression induced by environmental stimuli
• 'Fight-or-flight' responses are rapid and can be crucial for preserving life
• Using the earlier example of the red robin staying alert to predators:
  • A sudden movement by a crow (the stimulus) is detected by the receptors in the robin’s eye
  • The receptor cells send an impulse along the nerves and to the brain (coordinator)
  • The brain sends an impulse to the wing muscles (effectors) of the red robin so it can fly away (response)

The sequence of detection, coordination and action results in a 'fight-or-flight' response that saves the robin’s life

Coordination of the nervous system and endocrine system in the 'fight-or-flight' response

• There are two important coordination systems in the body, the nervous system and the endocrine system
• Both of these systems are involved in the 'fight-or-flight' response
• The nervous and endocrine systems work together in a complementary manner to coordinate this fast response
• The sympathetic nervous system is responsible for coordinating many of the responses to danger
• Its actions are supported by the effect of two hormones: adrenaline and cortisol (both secreted from the adrenal glands
• The initial part of the response is controlled by the nervous system, the response is continued by the endocrine system

Mechanism of the 'fight-or-flight' response

• Sensory neurones detect environmental stimuli associated with danger and send impulses to the brain
• The amygdala (a small region of the brain located in the cerebrum) sends impulses to various other parts of the brain, including the hypothalamus
• The hypothalamus is stimulated to send impulses via the sympathetic nerves to the adrenal glands
  • This causes the adrenal medulla to secrete the hormone adrenaline
  • Adrenaline stimulates target organs and tissues to increase sensory awareness, making the organism more alert and so improving its ability to respond to danger

• At the same time, the hypothalamus also releases a peptide hormone that stimulates the anterior pituitary gland to release ACTH (adrenocorticotropic hormone)
• ATCH is transported to the adrenal glands via the bloodstream
  • This causes the adrenal cortex to secrete the hormone cortisol
  • Cortisol stimulates target organs and tissues to increase blood pressure, blood glucose ensuring the tissues have sufficient glucose and oxygen needed for rapid response
  • Cortisol also suppresses the immune system

The roles of the nervous system and endocrine system in the fight-or-flight response. The release of the hormones adrenaline and cortisol is controlled by the nervous system.

The effects of adrenaline

• The hormone adrenaline is secreted from the adrenal glands (and sometimes the medulla oblongata)
  • It is often during times of stress or aggression that this hormone is secreted, which is why it is sometimes referred to as the 'fight-or-flight' hormone

• Adrenaline is transported via the bloodstream and it has a rapid effect on cells
• It can have a range of effects on a number of different cell types:
  • In the eyes it stimulates the muscles in the irises to contract, causing the pupils to dilate
  • It increases the diameter of the bronchioles by relaxing smooth muscle. This helps to increase the airflow to the alveoli
  • Adrenaline decreases the amount of blood flowing to the gut and skin via vasoconstriction
    • A higher blood pressure occurs due to increased resistance from vasoconstriction

  • It increases the amount of blood flowing to the brain and muscles via vasodilation
  • Heart rate and stroke volume (volume of blood pumped per beat) increase as a result of adrenaline
  • Adrenaline stimulates the breakdown of glycogen into glucose in the liver cells via enzymes, causing the blood glucose concentration to increase

The second messenger model and adrenaline

• When adrenaline is secreted it increases the concentration of blood glucose
• It does this by binding to different receptors on the surface of liver cells that activate the same enzyme cascade that occurs when glucagon binds to its specific receptors
• Adrenaline binds to specific receptors on the membrane of liver cells
• This causes the enzyme adenylyl cyclase to change shape and become activated
• Active adenylyl cyclase catalyses the conversion of ATP to the second messenger, cyclic AMP (cAMP)
• cAMP binds to protein kinase A enzymes, activating them
• Active protein kinase A enzymes activate phosphorylase kinase enzymes by adding phosphate groups to them
• Active phosphorylase kinase enzymes activate glycogen phosphorylase enzymes
• Active glycogen phosphorylase enzymes catalyse the breakdown of glycogen to glucose
  • This process is known as glycogenolysis

• The enzyme cascade described above amplifies the original signal from adrenaline and results in the releasing of extra glucose by the liver to increase the blood glucose concentration back to a normal level

The effect of adrenaline is amplified so that each molecule can stimulate many molecules of cAMP, which in turn activate many enzymes molecules

• Adrenaline also stimulates the breakdown of glycogen stores in muscle during exercise
• The glucose produced remains in the muscle cells where it is needed for respiration