6.2.3 The Fight or Flight Response

• During situations that produce stress, fear or excitement, the neurons of the sympathetic nervous system stimulate the adrenal medulla (of the adrenal gland) to secrete adrenaline
  • Adrenaline is a hormone that prepares the body to deal with a fear/stress/excitement-inducing stimulus
  • This reaction is often called the "fight or flight" response
    • It is the effects of adrenaline that lead to the typical symptoms experienced during stressful situations such as increased heart rate, dry mouth, increased sweating etc.

The adrenal medulla is responsible for releasing the hormone adrenaline into the bloodstream to prepare the body for the "fight or flight" response

• Since adrenaline is a hormone, it is transported around the body in the bloodstream
• It will bind to receptors on its target organs
• One of the targets of adrenaline is the SAN, leading to an increase in the frequency of excitations
  • This, in turn, will increase the heart rate to supply blood to the muscle cells at a faster rate
  • More blood means more oxygen and glucose that reaches the muscle cells, which in turn, increases the rate of aerobic respiration
  • This releases more energy that will be used during the response to the stressful or dangerous situation
• Adrenaline will also stimulate the cardiovascular control centre in the medulla oblongata
  • This increases the impulses travelling along the sympathetic neurones affecting the heart, further speeding up the heart rate
• Blood vessels to less important organs (such as the digestive system and skin) constrict so that more blood can be diverted to organs that will be involved in the "fight or flight" response
  • Note that blood flow to the brain remains constant, regardless of whether the body is in a state of stress or relaxation
    • The brain is one of the most important organs in the body and needs a constant blood supply in order to function properly
• The changes experienced by the body during the "fight or flight" response are controlled by a combination of nervous and hormonal responses