The Endocrine System

2-6 How does the endocrine system transmit information and interact with the nervous system?

So far, we have focused on the body’s speedy electrochemical information system. But your body has a second communication system, the endocrine system (Figure 2.5). Glands in this system secrete hormones, another form of chemical messenger, which travel through our bloodstream and influence our behaviors and emotions.

Some hormones are chemically identical to neurotransmitters. The endocrine system and nervous system are therefore close relatives. Both produce molecules that act on receptors elsewhere. Like many relatives, they also differ. The speedy nervous system zips messages from eyes to brain to hand in a fraction of a second. Endocrine messages trudge along in the bloodstream, taking several seconds or more to travel from the gland to the target tissue. The nervous system delivers information to specific receptor sites with text-message speed. The endocrine system is more like sending a letter through the mail.

But slow and steady sometimes wins the race. The effects of endocrine messages tend to outlast those of neural messages. Have you ever felt angry, for no apparent reason? You may have experienced an “endocrine hangover” from lingering emotion-related hormones. Angry feelings can hang on, beyond our awareness of what set them off. When this happens, we need a little time to simmer down.

The endocrine system’s hormones influence many aspects of our lives: growth, reproduction, metabolism, mood. They work with our nervous system to keep everything in balance as we respond to stress, hard work, and our own thoughts. Consider what happens behind the scenes when you hear burglar-like noises outside your window. Your ANS may order your adrenal glands to release epinephrine and norepinephrine (also called adrenaline and noradrenaline). In response, your heart rate, blood pressure, and blood sugar will rise, giving you a surge of energy known as the fight-or-flight response. When the “burglar” turns out to be a playful friend, the hormones—and your alert, aroused feelings—will linger a while.

The endocrine glands’ control center is the pituitary gland. This pea-sized structure is located in the brain’s core, where it is controlled by a nearby brain area, the hypothalamus (more on that shortly). Among the hormones released by the pituitary is a growth hormone that stimulates physical development. Another is oxytocin, which enables contractions during birthing, milk flow in nursing, and orgasm. Oxytocin also promotes social interactions. Its pleasant release accompanies couples bonding, feelings of group togetherness, and experiences of social trust (De Dreu et al., 2010).

Pituitary secretions also direct other endocrine glands to release their hormones. The pituitary, then, is a master gland (whose own master is the hypothalamus). For example, under the brain’s influence, the pituitary triggers your sex glands to release sex hormones. These in turn influence your brain and behavior.

This feedback system (brain → pituitary → other glands → hormones → body and brain) reveals the interplay between the nervous and endocrine systems. The nervous system directs endocrine secretions, which then affect the nervous system. In charge of this whole electrochemical orchestra is that master conductor we call the brain.