The hormonal message

The nervous system carries fast, targeted messages. The hormonal system does the opposite: it broadcasts slow messages throughout the body, to cells able to recognise them.

The principle

   ENDOCRINE GLAND
        |  secretes a HORMONE
        v
   BLOOD  ~~~~~~~~~~~~~~~~~~~~~~~~~>  circulates through the WHOLE body
                                          |
                                          v
                              TARGET CELLS
                              (those bearing the RECEPTOR)

The hormone reaches every cell in the body, but only those equipped with the matching receptor respond. It is the receptor, not the distribution, that defines the target.

The image is an announcement in an airport: everyone hears it, only the relevant passengers react.

The main glands

PITUITARY     the conductor: commands the other glands
THYROID       thyroxine: sets the general metabolic rate
PANCREAS      insulin and glucagon: set blood glucose
ADRENALS      adrenaline (emergency), cortisol (prolonged stress)
GONADS        testosterone, oestrogens: reproduction, sexual characters

Nervous or hormonal?

The two communication systems are complementary, and their properties are almost opposite:

                    NERVOUS                  HORMONAL
------------------  ----------------------   ----------------------
medium              electrical signal        molecule in the BLOOD
speed               milliseconds             seconds to minutes
precision           one precise target       all cells with the receptor
duration of effect  brief                    prolonged
example             pulling a hand away      growth, puberty

Nervous for emergencies, hormonal for the long run. Pulling a hand from a flame takes 50 milliseconds; growing twenty centimetres takes years. Neither system could do the other's job.

They also cooperate closely: facing danger, the nervous system triggers within a fraction of a second the release of adrenaline, which extends the reaction over several minutes — faster heartbeat, dilated airways, glucose release.

The effects of a hormone

The same hormone can have different effects depending on the target cell, because it is the receptor that determines the response:

adrenaline on the heart    ->  acceleration
adrenaline on the airways  ->  dilation
adrenaline on the liver    ->  glucose release

This economy of means is remarkable: a single messenger, coordinated yet distinct responses in several organs at once.

Summary

• A hormone is secreted by an endocrine gland and travels in the blood.
• It reaches the whole body but acts only on cells bearing the receptor.
• The nervous system is fast, targeted and brief; the hormonal one slow, diffuse and lasting.
• The two cooperate: the nervous triggers, the hormonal sustains.
• One hormone can produce different effects depending on the target cell.