Epinephrine & Melatonin

Two hormones, two completely different timescales. One floods your blood in seconds and has your heart pounding before you have worked out what frightened you. The other rises quietly over hours and decides when you fall asleep tonight. Between them they show the full range of what the endocrine system can do.

Circadian rhythms

Many physiological processes and behavioural patterns happen in regular daily rhythms, in plants as well as animals. Many species are only active during a particular part of the 24-hour cycle — nocturnal animals at night, and humans, who are diurnal, during daylight.

Humans are adapted to a 24-hour cycle. Physical activity, sleep, body temperature and hormone secretion all follow regular cycles across the day. These are the circadian rhythms, and in humans many of them are influenced by melatonin.

How melatonin is controlled

• The pineal gland, in the brain, secretes melatonin into the blood.
• Production is influenced by light and dark detected by the retina of the eye.
• Signals travel from the retina to the pineal gland, and the amount released varies with how much daylight the person is exposed to. Because it tracks changing day length across the year as well as across the day, the pineal gland is sometimes called both an endocrine clock and an endocrine calendar.
• Melatonin’s target sites are widespread: the hypothalamus and pituitary, and also the immune system, gonads, kidney, cardiovascular system, blood vessels and intestinal tract.

What melatonin does

• Rising melatonin causes tiredness and promotes sleep.
• Falling melatonin prepares the body for waking up and staying awake during the day.
• Increased melatonin at night contributes to the night-time drop in core body temperature.
• Melatonin receptors in the kidney cause the night-time decrease in urine production — which is why you can sleep eight hours without needing the toilet.

The experiment that matters

Subjects living in constant darkness, with no access to natural daylight, still release melatonin on a roughly 24-hour cycle — but on a cycle slightly longer than 24 hours, so it slowly drifts.

That result is the whole argument. The rhythm is generated internally; it does not need light to exist. What light does is reset the system each day so it stays in step with real daylight hours.

Epinephrine

During stress, fear or excitement, neurones of the sympathetic nervous system stimulate the adrenal medulla to secrete epinephrine (also called adrenaline). This is the fight or flight response, and the familiar symptoms — racing heart, dry mouth, sweating — are the effects of the hormone.

Following the chain through

Epinephrine is a hormone, so it is carried in the bloodstream and binds to receptors on its target organs. Two of those targets speed the heart:

• At the SAN, epinephrine increases the frequency of excitations, so heart rate increases. More blood reaches the muscle cells per second.
• At the cardiovascular control centre in the medulla oblongata, it increases the impulses travelling along sympathetic neurones to the heart, speeding it up further.

Then follow the consequence: more blood means more oxygen and glucose reaching the muscle cells, which increases the rate of aerobic respiration, which releases more energy for the response.

At the same time, blood vessels supplying less urgent organs — the digestive system and the skin — constrict, so more blood is diverted to organs involved in the response. Blood flow to the brain remains constant whether the body is stressed or relaxed, because the brain needs a constant supply to function.

Worked examples