1. Module 2

1.43. Page 3

Lesson 6

Module 2—The Endocrine System

The diagram of the brain illustrates how the nervous system and the endocrine system are connected through the hypothalamus of the brain and the pituitary gland of the endocrine system.

© Cristian Alexandru Ciobanu/shutterstock

The Brain and the Pituitary Gland

 

The hypothalamus, which is located just above the pituitary gland, is an important part of the brain. The pituitary gland, however, is an important part of the endocrine system. Together, the hypothalamus and the pituitary gland form an important communication link between the nervous system and the endocrine system.

 

The hypothalamus contributes to the homeostasis of the nervous system. Information from the sense organs is transmitted through sensory pathways to the hypothalamus. Based on this information, the hypothalamus co-ordinates many basic physiological activities, including the reflex activity of the autonomic nervous system.

 

The hypothalamus also contributes to the homeostasis of the endocrine system. There are essentially three groups of nerve centres in the hypothalamus. These centres allow communication between the nervous and endocrine systems. They send instructions to either the anterior or the posterior lobe of the pituitary. The pituitary, the master gland of the endocrine system, is then stimulated to increase or decrease secretion of its hormones into the bloodstream. The first nerve centre controls the secretion of releasing hormones from the hypothalamus. These releasing hormones are carried through the blood vessels to the anterior pituitary, where they regulate the secretion of hormones, such as thyroid stimulating hormone (TSH), from the anterior pituitary. The second nerve centre performs two functions in its regulation of the pituitary gland:

  • It secretes hormones that are carried by the nerve cells to the posterior pituitary, where they are stored in the axon terminals.

  • It sends nerve impulses to the posterior pituitary, stimulating the release of the stored hormones, such as antidiuretic hormone (ADH) and oxytocin.

The third centre in the hypothalamus is connected by sympathetic neurons to the adrenal gland, regulating the release of epinephrine.

 

These examples illustrate the importance of the hypothalamus as an important command centre for co-ordinating the activity of the nervous and endocrine systems.

 

Depending on your learning style, you may choose to compete the Read activity or the Watch and Listen activity that follows it.

 

Read

 

Review page 437 and page 463 of your textbook, and finalize your notes about how the nervous and endocrine systems maintain homeostasis, how they are co-ordinated, and how they act together. Developing a flow chart may help you visualize these relationships. You may want to review your notes from Lessons 1, 2, 3 and 4 of Module 2 before you begin. Post your notes for discussion with your peers and your teacher. File your notes in your course folder.

 

Watch and Listen

 

Watch the following segments of “The Neuroendocrine System: Working Together to Maintain Homeostasis.” Make notes as you watch. File your notes in your course folder. You may be required to enter a username and password in order to access these videos. Contact your teacher for this information.

  • “Nervous and Endocrine System”
  • “Bio Bit: Nervous vs. Endocrine Action”
  • “Bio Review: Basic Nervous/Endocrine Differences”
  • “Blood Glucose Regulation”
Self-Check

 

Complete the following questions to support your understanding. Check your answers and then file your work in your course folder. Consult with your teacher if you have questions.

 

SC 2. Using an example, explain how the nervous and endocrine systems work together to regulate a response in the body.

 

SC 3.

  1. Why has the pituitary gland often been called the “master gland”?
  2. Explain how the hypothalamus controls the pituitary gland.
  3. Is “master gland” a suitable name for the pituitary gland?

For the child in the photo, growth and development is dependent on the nervous and endocrine systems. His hypothalamus secretes releasing hormones to stimulate the production of hGH required for growth and development of children, and his endocrine system releases thyroxine that influences the development and function of the nervous system in children and adults.

© Enge/iStockphoto

SC 4. Explain how the anterior pituitary and posterior pituitary differ with respect to their relationship to the hypothalamus.

 

SC 5. How are neurotransmitters and hormones similar? How are they different?

 

SC 6. Using an example, explain how the hormones of the adrenal medulla complement the actions of the sympathetic branch of the autonomic nervous system.

 

SC 7. Compare and contrast the role of the norepinephrine in the nervous system with its role in the endocrine system.

 

SC 8. How is the hypothalamus involved in the release of epinephrine and norepinephrine by the adrenal gland?

 

SC 9. How is the secretion of tropic hormones from the pituitary gland regulated?

 

SC 10. Compare the anatomy of the pituitary gland and adrenal medulla.

 

SC 11. Why is control of the adrenal medulla by the nervous system important?

 

Check your work.
Self-Check Answers

 

SC 2. Your answers may contain one of the following:

  • Some nervous system tissues secrete hormones. For example, neurosecretory cells in the hypothalamus produce antidiuretic hormone and oxytocin, which are stored and released by the posterior pituitary.

  • Several chemicals function as body neurotransmitters and hormones, depending on their location in the body. For example, both epinephrine and norepinephrine act as a neurotransmitters in synaptic transmission in certain neural pathways, such as the sympathetic nervous system, as well as hormones released by the adrenal medulla in the fight-or-flight response (short-term stress).

  • The regulation of several physiological processes involves both the nervous and endocrine systems acting in conjunction with each other. When a mother breast feeds her baby, the baby’s suckling initiates sensory messages in the mother’s neurons that travel to the hypothalamus, which in turn triggers the pituitary to release a hormone called oxytocin. Oxytocin travels in the bloodstream to the mammary glands of the breast, causing the secretion of milk. Another example that might be outlined is that of sensory messages of something that causes excessive fear or rage. In such a case, a message is relayed to the hypothalamus which initiates nerve impulses in the sympathetic nervous system, which in turn stimulates secretory cells in the adrenal medulla to secrete epinephrine, which initiates a response to the stress.

SC 3.

  1. The pituitary gland is often referred to as the “master gland” because it synthesizes many hormones and releases many tropic hormones that regulate the action of other endocrine glands.

  2. Neurosecretory cells in the hypothalamus produce releasing and release-inhibiting hormones. These hormones are secreted into the blood vessels between the hypothalamus and anterior pituitary. Each type of hormone from the hypothalamus either stimulates or inhibits production and secretion of an anterior pituitary hormone into the bloodstream.

  3. Your answers could either support or reject the “master gland” label assigned to the pituitary gland. Some may argue that the hypothalamus is part of the brain and not a separate endocrine gland. In this case, the name “master gland” given to the pituitary gland is suitable. Others may argue that the hypothalamus should be called the “master gland” because it is actually controlling the pituitary gland through releasing hormones.

SC 4. The posterior pituitary is an extension of the hypothalamus, and both are composed of specialized nerve cells. Hormones produced in the hypothalamus are transported by nerve cells to the posterior pituitary, where they are stored and released when stimulated by neurons from the hypothalamus. On the other hand, the anterior pituitary is composed of secretory cells that produce and release hormones when stimulated by hormones from the hypothalamus, which travel to the pituitary by way of the bloodstream.

 

SC 5. The similarities between neurotransmitters and hormones include the following:

  • Both are stored in cells for later release.
  • Stimulation of cells prompts release.
  • Some molecules function as both a hormone and a neurotransmitter.
  • Both require a specific receptor for action.
  • Both include responses that are regulated by negative feedback loops.

The differences between neurotransmitters and hormones include the following:

  • Neurotransmitters act more rapidly than hormones.

  • Neurotransmitters are released in synapses between neurons, whereas hormones are secreted into the bloodstream, where they are transported to a broader range of cell types.

SC 6. The sympathetic nervous system and the adrenal medulla produce hormones that regulate a short-term response to stress, commonly referred to as the fight-or-flight response. In response to a stress stimulus, such as fear, neurons of the sympathetic nervous system release a neurotransmitter called norepinephrine. Other neurons from the sympathetic nervous system carry a signal from the hypothalamus directly to the adrenal medulla. These neurons, rather than hormones, stimulate the adrenal medulla to secrete epinephrine or norepinephrine. The combined action of the neurotransmitter and the hormones triggers the fight-or-flight response, which is characterized by an increase in breathing rate, heart rate, blood pressure, blood flow to the heart and muscles, and the conversion of glycogen to glucose in the liver.

 

SC 7. Norepinephrine is released by neurons of the sympathetic nervous system and by the cells of the adrenal medulla. The sympathetic neurons release norepinephrine as a neurotransmitter for synaptic transmission. In response to a stress stimulus, neurons of the sympathetic nervous system carry a signal from the hypothalamus directly to the adrenal medulla. These neurons, rather than hormones, stimulate the adrenal medulla to secrete norepinephrine or epinephrine. These hormones also trigger the fight-or-flight response. One difference is that the response to danger by the nervous system is much faster than that of the endocrine system. Another difference is that the effects of norepinephrine released by the adrenal medulla last longer.

 

SC 8. Centres in the hypothalamus initiate an impulse in a sympathetic neural pathway which connects to the secretory cells in the adrenal medulla and stimulates them to release epinephrine or norepinephrine, resulting in the fight-or-flight response.

 

SC 9. Typically, the hypothalamus secretes a releasing hormone that is transported by the blood to the anterior pituitary gland. This causes the anterior pituitary to release a tropic hormone into the bloodstream. The pituitary tropic hormone then stimulates the target gland to release its hormone into the blood. This third hormone travels to another target tissue and produces an effect. Levels of the third hormone prevent further release of the first two hormones in the pathway by way of negative feedback.

 

SC 10. The anatomy of the posterior pituitary is similar to that of the adrenal medulla as both are composed of modified nerve cells and release hormones. The anatomy of the anterior pituitary and the adrenal cortex are similar in that both are composed of secretory cells that synthesize hormones.

 

SC 11. It is important for the adrenal medulla to be stimulated by the nervous system so that responses are very rapid rather than slower, as would occur if it was stimulated by the endocrine system. Fight-or-flight responses are often life-preserving and require quick action.