Coordinated Responses
This lesson covers:
- How the nervous and endocrine systems coordinate the fight or flight response
- The physiological responses in the fight or flight response
- The second messenger model
Nervous and endocrine coordination of the fight or flight response
The fight or flight response is an instinctive reaction to perceived threats. It involves a complex interaction between the nervous and endocrine systems, preparing the body to either flee or confront the danger.
Mechanism of the fight or flight response:
- The autonomic nervous system detects a threat, and the signal is passed on to the hypothalamus.
- The hypothalamus activates the sympathetic nervous system, sending nerve signals to glands and muscles.
- The sympathetic nervous system triggers the secretion of adrenaline and noradrenaline from the adrenal medulla, causing various physiological changes.
- The hypothalamus also communicates with the adrenal-cortical system, stimulating the pituitary gland to release adrenocorticotropic hormone (ACTH).
- ACTH travels via the bloodstream to the adrenal cortex, stimulating it to release several other hormones to help the body deal with the threat.
The physiological responses in the fight or flight response
There are several responses that are typically triggered during the fight or flight response to help escape from perceived danger.
| Physiological reaction triggered in the fight or flight response | Purpose |
|---|---|
| Increased heart rate | To circulate more oxygenated blood |
| Pupil dilation | To improve vision |
| Constriction of arterioles in skin | To divert more blood to major muscles, brain, and heart |
| Rise in blood glucose levels | To increase cellular respiration |
| Relaxation of airway muscles | To allow more oxygen into the lungs |
| Digestion stops | To prioritise emergency bodily functions |
The second messenger model
The second messenger model of hormone action involves a hormone (the first messenger) triggering the formation of a second messenger (cAMP) inside the cell, which activates enzymes to carry out a function.
The action of adrenaline as an example of the second messenger model
An example of the second messenger model in cell signalling is shown through the action of adrenaline, which is vital during the fight or flight response. Among its many functions, adrenaline plays a crucial role in rapidly making glucose available using glycogenolysis, which is when liver cells convert glycogen to glucose and release it into the bloodstream.
Cell signalling pathway for adrenaline:
- Adrenaline binds to complementary receptor on the cell-surface membrane of a liver cell.
- The binding of adrenaline causes the protein to change shape, activating a G protein.
- This activates the enzyme adenylyl cyclase.
- The activated adenyl cyclase converts ATP into cAMP.
- cAMP acts as a second messenger, binding to and activating many protein kinases via phosphorylation, amplifying the signal from adrenaline.
- Protein kinases activate enzymes that catalyse the breakdown of glycogen into glucose.
- Glucose moves out of liver cells by facilitated diffusion and into the blood through channel proteins.
- This increases the blood glucose concentration so that more glucose can be delivered to body cells for respiration.
The cascade effect in this process means that one hormone molecule can generate multiple cAMP molecules. This effect amplifies at each stage, increasing the number of molecules involved.