Epinephrine & Melatonin (HL IB Biology)

Circadian rhythms

• Many physiological processes and behavioural patterns occur in regular, daily rhythms in organisms throughout the plant and animal kingdoms
  • Many animal species are only active for a specific part of the 24-hour cycle e.g. nocturnal animals are only active at night
  • Humans are diurnal meaning that we are more awake during daylight hours
• Humans are adapted to live in a 24-hour cycle and many aspects of our physiology and behaviour, including physical activity, sleep, body temperature, and secretion of hormones, follow specific and regular cycles throughout the 24-hour period
  • These daily cycles are known as circadian rhythms
• In humans, many circadian rhythms are influenced by the hormone melatonin
  • Melatonin is secreted by the pineal gland, which is located in the brain
  • Melatonin secretion increases in the evening in response to darkness and decreases at dawn in response to light, leading to our diurnal behaviour patterns

Melatonin and sleep patterns

• Although melatonin affects many aspects of human physiology and behaviour, one of the main circadian rhythms it controls is our sleep-wake cycle
  • The pineal gland secretes melatonin into the blood 
    
    • Production is influenced by the detection of light and dark by the retina of the eye
    • Signals are then transmitted to the pineal gland according to the amount of daylight a person is exposed to and varies with changes in day length (this is why the pineal gland is sometimes referred to as both an endocrine clock and an endocrine calendar)
  • Melatonin's target sites are found in many areas of the brain including the hypothalamus and pituitary glands, and also in the cells of the immune system, gonads, kidney, and the cardiovascular system, blood vessels, and intestinal tract
  • Increasing melatonin levels lead to feelings of tiredness and promote sleep
  • Decreasing melatonin levels lead to the body's preparation for waking up and staying awake during the day

• Experiments have also suggested that
  • Increased melatonin at night contributes to the night-time drop in core body temperature in humans
  • Melatonin receptors in the kidney enable melatonin produced at night to cause the night-time decrease in urine production in humans
  • Melatonin is still released in the absence of light and dark signals, but on a slightly longer cycle than the usual 24 hours
    • Subjects living in the dark with no access to natural daylight still release melatonin on a roughly 24 hour cycle
    • This suggests that the role of light is to reset the melatonin system every day to keep the circadian rhythm in line with daylight hours

Secretion of melatonin graph

The production of melatonin is influenced by the amount of daylight a person is exposed to: melatonin levels peak during 

• During situations that creates stress, fear or excitement, the neurones of the sympathetic nervous system will stimulate the adrenal medulla (of the adrenal gland) to secrete epinephrine (also called adrenaline)
  • Epinephrine is a hormone that will prepare your body for reacting to a stressful situation
  • This reaction is often called the "fight or flight" response
    • It is the effects of epinephrine that lead to the typical symptoms we experience during stressful situations such as increased heart rate, dry mouth, increased sweating etc

The adrenal gland diagram

The adrenal glands secrete the hormone epinephrine and prepare the body for vigorous activity

• Since epinephrine 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 epinephrine 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, vigorous or dangerous situation
• Epinephrine 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
• Epinephrine is also covered in the course with reference to the second messenger model, this can be found here