5.5.11 Coordination of Responses Flashcards
How do we respond to changes 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
What is 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)
What is the coordination of the nervous 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
What is the 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
What are 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
What is the second messenger model with reference to 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
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
