Hormones and the hypothalamus

Two ways to send a message

Your heart pounds before a race. Nobody pulled a rope, and nobody told your heart to hurry. A chemical told it. The body has two control systems. The nervous system sends fast electrical signals along nerves, and each signal reaches only the cells that a nerve touches. It is over in a moment.

Many jobs need slow and lasting control: growth, the water and the salts of the body, the sugar in the blood, the cycles of reproduction. For these, the body uses chemical messengers called hormones.

A hormone is a non-nutrient chemical that acts as an intercellular messenger. It is made in trace amounts, which means in very small quantities. The blood carries it to the target cells, which have the matching receptors.

An endocrine gland has no duct. It pours its hormone straight into the blood or the tissue fluid. This is why endocrine glands are called ductless glands. An exocrine gland, such as a salivary gland or a sweat gland, sends its product along a duct. The pancreas is both: it makes digestive juice for a duct and hormones for the blood.

Simple animals such as the invertebrates have only a few hormones. In the vertebrates, a large number of chemicals act as hormones.

The human endocrine system

The main endocrine glands of the human body are these.

• The hypothalamus, the pituitary and the pineal gland in the head.
• The thyroid and the parathyroid glands in the neck, and the thymus in the chest.
• The adrenal glands and the pancreas in the abdomen.
• The gonads, which are the testis in the male and the ovary in the female.

The heart, the kidney and the gastrointestinal tract also make hormones. You will meet them in the third course.

The hypothalamus

The hypothalamus is the basal part of the diencephalon, the part of the forebrain that lies between the two halves of the cerebrum. Its cells are special. They are neurosecretory cells, which means nerve cells that release a hormone into the blood instead of passing a signal to the next nerve cell. Groups of these cells are called nuclei.

The hormones of the hypothalamus control the pituitary gland. There are two kinds.

• Releasing hormones stimulate the pituitary to make and release its hormones. Example: gonadotropin releasing hormone (GnRH) stimulates the pituitary to release LH and FSH.
• Inhibiting hormones stop the pituitary from releasing its hormones. Example: somatostatin inhibits the release of growth hormone.

The hormones reach the pituitary in two different ways.

• The anterior pituitary gets the releasing and inhibiting hormones through a portal circulatory system. A portal system is a set of blood vessels that carries blood from one capillary bed straight to a second capillary bed. Here it carries the hormones of the hypothalamus to the cells of the anterior pituitary.
• The posterior pituitary is under the direct control of nerves from the hypothalamus. The hormones oxytocin and vasopressin are made by the neurosecretory cells of the hypothalamus. They travel down the nerve fibres and are stored in the posterior pituitary, which releases them.

A-Level and IB books name more of the releasing hormones: thyrotropin releasing hormone (TRH), corticotropin releasing hormone (CRH) and growth hormone releasing hormone (GHRH). They also name dopamine as the factor that inhibits prolactin. NCERT names GnRH and somatostatin.

Feedback in brief

Many hormones are controlled by negative feedback. The hormone, or its effect, comes back to the gland that sent the first signal and switches it down. A heater with a thermostat works in the same way. When the room is warm, the thermostat switches the heater off. You will follow a full loop for the thyroid in lesson 3 and study feedback again in the third course.

You should find that the amount halves again and again, but it never reaches zero in a finite time. A hormone with a short half-life gives a short signal, and the gland must keep releasing it. Adrenaline is cleared within minutes. Thyroxine lasts about a week.

Try it in Python

The program halves the amount of hormone every half-life.

Practise with new numbers

Common mistakes

• Saying that a hormone travels along a duct. Endocrine glands are ductless. The blood carries the hormone.
• Thinking that oxytocin and vasopressin are made in the posterior pituitary. They are made in the hypothalamus and only stored and released there.
• Mixing up releasing and inhibiting hormones. GnRH releases LH and FSH. Somatostatin inhibits growth hormone.
• Saying that every hormone reaches every cell. The blood reaches every cell, but only the target cells have the receptor.
• Calling a hormone a nutrient or an enzyme. A hormone is a non-nutrient messenger made in trace amounts.

Remember these

• Hormones are non-nutrient chemical messengers made in trace amounts. Endocrine glands are ductless.
• The hypothalamus has neurosecretory cells (nuclei). It makes releasing hormones, inhibiting hormones, oxytocin and vasopressin.
• Releasing and inhibiting hormones reach the anterior pituitary by the portal system. The posterior pituitary is under direct nerve control.
• GnRH releases LH and FSH. Somatostatin inhibits growth hormone.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is a hormone? Why is an endocrine gland called a ductless gland?

2. Name two organs that are not main glands but also make hormones.

3. What are neurosecretory cells, and where are they found in the brain?

4. Give one example of a releasing hormone and one of an inhibiting hormone of the hypothalamus.

5. How do the hormones of the hypothalamus reach the anterior pituitary? How do oxytocin and vasopressin reach the posterior pituitary?

References

• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 17, The Endocrine System.
• Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, the chapters on introduction to endocrinology and on the pituitary hormones and their control by the hypothalamus.
• NCERT (2023). Biology, Textbook for Class XI, chapter 19.