Coordination of Responses (OCR A Level Biology): Revision Note

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