SCN3230-ABED_1: Module 2 Lesson 1 - 6

Lesson 1 — The Endocrine System

Hypothalamus – Pituitary Complex

Read page 444 and study Figure 13.12 on page 445

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The hypothalamus lies just above the tiny, pea-sized pituitary gland in the middle of the head. The two structures are connected by a stalk. The hypothalamus has neural and endocrine functions. It also produces and secretes hormones, many of which can stimulate or inhibit pituitary hormones. The pituitary gland consists of anterior and posterior pituitary lobes. Each lobe secretes different hormones in response to the signal from the hypothalamus. Lesson 6 considers this topic in detail.

The Posterior Pituitary

The posterior pituitary stores and releases hormones produced in the hypothalamus. Anatomically, the posterior pituitary gland has two very different kinds of cells. The posterior pituitary has regular nerve cells as well as modified nerve cells that release hormones. The posterior pituitary does not produce any hormones. It is connected to the hypothalamus by neurosecretory cells that secrete two hormones - antidiuretic hormone (ADH, also called vasopressin) and oxytocin. Releasing hormones (releasing factors) from the hypothalamus signal the release into the blood stream of the stored ADH and oxytocin from the posterior pituitary.

Anterior Pituitary

The anterior pituitary synthesizes and releases six major hormones. It is connected to the hypothalamus by blood vessels.

Three of the hormones (follicle stimulating hormone, luteinizing hormone, and prolactin) are involved in the reproductive process and are studied in detail in the next unit. In this unit, you will learn their general functions. The other three hormones - adrenocorticotropic hormone (ACTH), thyroid stimulating hormone (TSH), and human growth hormone (hGH) are introduced in this module.

The hypothalamus and pituitary control their production and release through negative feedback loops. The hypothalamus is stimulated to secrete releasing hormones (RH) by conditions in the internal environment of the body. Moving through the blood vessels, the releasing hormones (RH) stimulate target cells that produce adrenocorticotropic hormone (ACTH), thyroid stimulating hormone (TSH), and human growth hormone (hGH). For example, the hypothalamus secretes thyroid releasing hormone (TRH), which stimulates the anterior pituitary to release thyroid stimulating hormone (TSH).

As indicated by its name, ACTH stimulates the outside layer of the adrenal gland called the adrenal cortex, which releases cortisol, a major stress hormone, and aldosterone. TSH stimulates the thyroid gland and its production of thyroxine.

Watch and Listen

You can review some of the concepts on the anterior pituitary by watching the following segment of Biologix-09 on Anterior Pituitary (1 minute).

©Alberta Education. The Hypothalamus and Pituitary: The Master Complex (14:01 - 15:14); Series 09. LearnAlberta.ca

Try This

Practice identifying the names of the structures of the hypothalamus/pituitary complex by completing this drag and drop activity.

Self-Check

To practise applying your knowledge of specific tropic hormones and their regulation by the hypothalamus and anterior pituitary gland, complete the questions below.

What is a tropic hormone? Give three examples.

Use the following flowchart that illustrates regulation by tropic hormones to respond to the next six items.

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How is the secretion of tropic hormones from the pituitary gland regulated?

If Hormone 2 is TSH, what is the target gland?

If Hormone 2 is TSH, what is Hormone 3?

Explain how Hormone 3 can inhibit or stimulate the release of Hormone 1.

Explain how Hormone 3 can inhibit the release of Hormone 2.

If the target gland is the adrenal cortex, identify the specific tropic hormone at work.

Identify Hormone 3 produced by the adrenal cortex.

Explain how the anterior and posterior pituitary differ with respect to their relationship to the hypothalamus.

Explain how the differences between the two regions of the pituitary relate to the nature of their hormonal secretions.

Check your work.

Self-Check Answers

A tropic hormone is released by a gland and travels through the blood to another endocrine gland, which it stimulates to release another hormones.

Other tropic hormones are releasing hormones from the hypothalamus, thyroid stimulating hormone (TSH), human growth hormone (hGH), LH, and FSH.
The secretion of tropic hormones from the pituitary gland is regulated by releasing hormones from the hypothalamus, which either stimulate or inhibit the anterior pituitary gland.
If Hormone 2 in the tropic hormone pathway is TSH, then the target gland is the thyroid.
If Hormone 2 is TSH, Hormone 3 is thyroxine.
Excessive amounts of thyroxine (Hormone 3) inhibit the anterior pituitary gland from releasing TSH and the hypothalamus from secreting releasing hormones. The thyroid gland is inhibited and does not release thyroxine, thus lowering the amount circulating in the blood.

Inadequate amounts of thyroxine stimulate the hypothalamus to secrete releasing hormones, which stimulates the anterior pituitary to release TSH, which stimulates the thyroid gland to release more thyroxine.
If the target gland is the adrenal cortex, then the tropic hormone is ACTH.
The adrenal cortex is stimulated to produce hormones such as cortisol.
The posterior pituitary is an extension of the hypothalamus and is composed of specialized neurons, but the anterior pituitary is a true endocrine gland and is composed of several types of secretory cells that produce and release hormones. Both lobes of the pituitary gland are connected to the hypothalamus by a stalk, but the posterior pituitary is connected to the hypothalamus by neurons, and the anterior pituitary is connected to the hypothalamus by blood vessels.
The neurosecretory cells of the hypothalamus secrete the hormones and move them along the axons to the posterior pituitary where they are stored and released into the blood stream as needed. Neurons from the hypothalamus stimulate the secretory cells of the anterior pituitary to synthesize the hormones, and releasing hormones circulating in the bloodstream stimulate the release of the anterior pituitary hormones.

Watch This

Who is in charge of the endocrine system? Is it the hypothalamus, the pituitary gland, or both? To review the connections between the hypothalamus and the pituitary gland, watch the following segment of Biologix-09 on Relationship between the Hypothalamus and the Pituitary Gland (1 minute).