Biology Notes: The Fight or Flight Response
Endocrine and Nervous System Cooperation
The endocrine system and nervous system work together to control and coordinate the activities of the body
and maintain homeostasis by responding to stimuli. Both systems are used extensively every second of our
lives, and sometimes they work very intimately together.
Qualities of nervous system that make it ideal controller/coordinator/stimulus responder:
Connected to all sense organs, so gets loads of informat ion
Has an amazingly co mplex brain to make sense out of all that info
Uses high-speed electrical signals to send messages across the body very fast
Has motor neurons all over the body, so can send instructions just about everywhere
Can respond to a stimu lus very fas t
The nervous system is very expensive to operate. It takes a lot of energy to send all those electrical signals ,
making the nervous system poorly suited to long-term responses.
Qualities of endocrine system that make it ideal controller/coordinator/stimulus responder:
Indirectly connected to sense organs via nerve stimulat ion and the hypothalamus-pituitary connection
Some endocrine glands can detect certain stimu li directly (pancreas can detect changes in blood glucose
concentrations, thyroid can detect changes in Ca 2+ ion concentration)
It is very cheap to operate! The tiny tiny amounts of hormones made and released by the endocrine system can
have dramatic and long-lasting effects on the body
The endocrine system isn’t so speedy. It’s pretty fast, but response times start seconds rather than milliseconds
after the stimu lus.
The Fight or Flight Response
The body’s response to stress or danger
Similar response in most vertebrate animals
Prepares body for immediate physical activity: helping it to fight for its life or flee for its life
1. Brain interprets incoming stimu li as stress or danger
2. Brain activates sympathetic branch of the ANS whose nerves stimulate the effectors in the table below
Effector Sympathetic action Parasympathetic action
(fight or flight response) (return to normal)
Heart Increases rat e and strength of cont raction Decreases rate
Bronchial tubes Breathing tubes of lungs (bronchioles) dilate Constricts breathing tubes
Intestines Inhibits contraction of smooth muscles and Stimulates contraction, secretion of
reduces blood flow enzymes, increased blood flow
Sweat glands Stimulates sweat output
Liver Stimulates release of stored glucose
Salivary glands Stimulates secretion of thick, sticky saliva Stimulates high volume, watery
*Adrenal glands Stimulates release of epinephrine (adrenaline)
The net effect of all the changes brought on by the sympathetic nerve stimulation is to prepare the
body for vigorous physical acti vity.
3. If the danger or stress lasts a long time, the *epinephrine released by the adrenal glands maintains the response. The
hormone stimulates most of the same organs that the sympathetic nerves do, and the organs respond in almost exactly
the same way. In addition, epinephrine stimu lates blood vessels leading to skeletal muscles to open up and increases
ATP production in cells. Using epinephrine to maintain the response is much “cheaper” for the body than having to
send a continuous barrage of electrical signals through all those sympathetic nerves!
4. Once the danger has passed, the body is returned to its normal resting state by the action of the parasympathetic
branch of the ANS. These nerves stimulate many of the organs listed above, but their effects are opposite that of the
sympathetic nerves (see the table).