Study topic · about 9 minutes

Hormones and the brain: the hypothalamus and pituitary

Hormones are chemical messengers carried in the blood, slower and broader than nerve signals. The brain runs much of this endocrine system from its base. The hypothalamus tells the pea-sized pituitary gland which hormones to release, and the pituitary's hormones steer the thyroid, adrenal glands, ovaries and testes. Negative feedback keeps each hormone in range, like a thermostat.

See the pituitary gland in 3D

Two messaging systems, linked by the hypothalamus

The body has two ways to send messages. The nervous system sends electrical signals along neurons, which pass chemicals across synapses to particular cells. The endocrine system uses glands that release hormones into the blood, which carries them everywhere. Only cells with the right receptor, the target cells, respond.

The main endocrine glands are the pituitary, pineal, thyroid, parathyroid and adrenal glands, the pancreas, and the ovaries or testes. Other organs, including the stomach and fat tissue, release hormones too.

The hypothalamus is where the two systems meet. This almond-sized area at the base of the brain keeps track of body temperature, water balance and energy stores. It can respond through nerves, by steering the autonomic nervous system, or through hormones, by commanding the pituitary gland below it. Some of its neurons, called neurosecretory cells, fire like neurons but release hormones into the blood.

The pituitary gland: two glands in one

The pituitary gland is about the size of a pea. It sits in a small bony cup in the base of the skull, the sella turcica ("Turkish saddle"). A stalk, the infundibulum, connects it to the hypothalamus above. It is often called the master gland because its hormones control other glands, but it takes its own orders from the hypothalamus.

The posterior pituitary: hormones made in the brain

The posterior pituitary is really an outgrowth of the brain. Neurosecretory cells in two groups of the hypothalamus, the supraoptic and paraventricular nuclei, make oxytocin and antidiuretic hormone (ADH), also called vasopressin. The hormones travel down the cells' axons through the stalk and wait in the axon endings. When the neurons fire, the endings release the hormones into the blood.

ADH tells the kidneys to hold on to water, which makes urine more concentrated. Sensors in the hypothalamus trigger its release when the blood gets too concentrated, for example when you are dehydrated or after a salty meal. At high levels it also narrows blood vessels, hence the name vasopressin. Alcohol blocks its release, which is one reason drinking makes people urinate more.

Oxytocin makes the uterus contract during childbirth and lets milk flow during breastfeeding, the let-down reflex. In the brain it is thought to help parents bond with their babies and to add to feelings of love and closeness. Its nickname, the love hormone, oversimplifies what it does.

The anterior pituitary: orders through the portal vessels

The anterior pituitary is gland tissue that grows up from the roof of the mouth in the embryo. It makes its own hormones, but on orders from the hypothalamus. Hypothalamic neurons release releasing hormones and inhibiting hormones into a small network of blood vessels, the hypophyseal portal system. It carries them down the stalk straight to the anterior pituitary, so they arrive undiluted.

Each releasing hormone calls up its own pituitary hormone: TRH releases TSH, CRH releases ACTH, GnRH releases FSH and LH, and GHRH releases growth hormone. Prolactin is the odd one out, because the hypothalamus mostly holds it back with an inhibiting hormone, dopamine. TSH, ACTH, FSH and LH are tropic hormones: their job is to make another gland release its hormones. The anterior pituitary also releases beta-endorphin, one of the body's natural painkillers.

Explore the hypothalamus in 3D

Negative feedback: a worked example

Most hormone systems regulate themselves by negative feedback: when a hormone rises, it switches off the signals that called for it. A thermostat works the same way, turning the heating off once the room is warm. The thyroid is the classic example.

1. Thyroid hormone in the blood runs low.
2. The hypothalamus releases TRH (thyrotropin-releasing hormone) into the portal vessels.
3. TRH makes the anterior pituitary release TSH (thyroid-stimulating hormone) into the blood.
4. TSH makes the thyroid gland in the neck release its hormones, thyroxine (T4) and T3, which speed up metabolism in cells all over the body.
5. As T4 and T3 rise, they turn down the release of TRH and TSH. The thyroid eases off, and its hormone level settles within a normal range.

When the loop breaks, the result makes sense. Without enough iodine in the diet, the thyroid cannot make T4 and T3, so nothing switches TSH off. The pituitary keeps pushing, and the thyroid swells into a goiter. Cortisol is controlled by the same kind of loop, as the next section shows.

A few hormones use positive feedback instead, where the effect boosts its own cause. In childbirth, the stretching cervix triggers more oxytocin, which makes contractions stronger. That stretches the cervix further, and the loop builds until the baby is born.

The adrenal glands: adrenaline and cortisol

An adrenal gland sits on top of each kidney, and each is really two glands in one. The inner adrenal medulla is part of the sympathetic nervous system. Within seconds of a scare, nerve signals make it release adrenaline (epinephrine), with some noradrenaline, into the blood. Heart rate, blood pressure and blood sugar rise, and the airways open.

The outer adrenal cortex makes steroid hormones, including aldosterone, which helps the kidneys hold on to salt, and cortisol, the main long-term stress hormone. Cortisol is run by the HPA axis, short for hypothalamic-pituitary-adrenal. CRH from the hypothalamus releases ACTH from the pituitary, and ACTH releases cortisol from the adrenal cortex. Cortisol raises blood sugar, calms inflammation and holds back the immune system. It also shuts off CRH and ACTH by negative feedback.

Cortisol follows a daily rhythm, peaking in the early morning. When stress drags on, cortisol stays high, which weakens the immune response. The endocrinologist Hans Selye described how the body copes with lasting stress in three stages, alarm, resistance and exhaustion: the general adaptation syndrome. Read more in emotion and stress.

Hormones that shape behavior: hunger, sleep and bonding

AP Psychology singles out five hormones that shape behavior:

Hunger: ghrelin and leptin

Hunger is a conversation between the gut, fat tissue and the hypothalamus. Before a meal, the stomach releases ghrelin, which makes you hungry. As you eat, the stomach stretches, sensory fibers of the vagus nerve report it, and ghrelin falls. Over the longer term, fat cells release leptin in proportion to their size. More leptin tells the brain that energy stores are full, which turns hunger down.

Both hormones act on a small area at the base of the hypothalamus, the arcuate nucleus. In many people with obesity, leptin levels are high but the hypothalamus responds less, a state called leptin resistance. The rare people born unable to make leptin feel constantly hungry and gain a great deal of weight.

Sleep: melatonin

The pineal gland, about the size of a grain of rice, sits deep in the middle of the brain. It releases melatonin as darkness falls, making you drowsy, and light shuts it off. Light works through a detour. The eyes signal the brain's master clock, the suprachiasmatic nucleus in the hypothalamus. The clock's message then runs down the spinal cord and back up through sympathetic nerves in the neck to the gland.

After a long flight, the melatonin rhythm can take several days to catch up with the new time zone, which is part of jet lag. Read more in sleep and the brain.

When the pituitary makes too much or too little

Most pituitary problems come from adenomas, noncancerous growths of the gland. Some make too much of one hormone. Others press on the healthy part of the gland so it makes too little, which is called hypopituitarism.

Diabetes insipidus has nothing to do with blood sugar. Because its old name was so often confused with diabetes mellitus, experts now call the form caused by too little ADH arginine vasopressin deficiency.

A large pituitary tumor can also press on the optic chiasm, which sits just above the gland. There, the nerve fibers from the inner half of each retina cross over. These fibers carry the outer half of what each eye sees. So the person loses the outer half of the visual field on both sides, called bitemporal hemianopia, as if wearing a horse's blinders.

Memory tricks

In these study guides

• MCATNeurons, the nervous system and hormones
• AP Psychology1.3 The neuron and neural firing
• Intro psychologyThe endocrine system (section 3.5)
• Medical schoolLimbic system and hypothalamus
• A-level PsychologyThe endocrine system and fight or flight
• GCSE BiologyThe endocrine system (4.5.3.1)

Take it further in 3D

• Emotion and rewardFear, drive and the value of things. A short guided lesson in the 3D app.

Sources

1. Anatomy and Physiology 2e, 17.3 The Pituitary Gland and Hypothalamus, OpenStax
2. Introduction to Behavioral Neuroscience, 16.4 Neural Control of Feeding Behavior, OpenStax
3. Pituitary tumor, MedlinePlus Medical Encyclopedia, National Library of Medicine (NIH)

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