Module 2 Lesson 7 - 3
Lesson 7 — Endocrine System vs Nervous System
The Brain and the Pituitary Gland
Read page 437 and 463
The hypothalamus, an important part of the brain, is located just above the pituitary gland. Information is transmitted to the hypothalamus through sensory pathways from the sense organs. Based on this information, the hypothalamus controls and integrates many basic physiological activities including the reflex activity of the autonomic nervous system, of which the sympathetic nervous system is a part.
In the hypothalamus are two groups of nerve centres. One is connected to the anterior lobe of the pituitary by blood vessels, and the other is connected to the posterior lobe by nerve cells. These centres send instructions to the two parts of the pituitary,
telling them whether to release their hormones into the bloodstream.
Anterior Pituitary
However, the nerve centres communicate with the two parts of the pituitary gland in different ways. 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. For example, adrenocorticotropic
hormone (ACTH) stimulates the adrenal cortex to secrete cortisol. Levels of the third hormone prevent further release of the first two hormones in the pathway by way of negative feedback.
Posterior Pituitary
A second nerve centre, connected to the posterior lobe by nerve cells, performs two functions: (1) it secretes hormones, antidiuretic hormone and oxytocin, which are carried by the nerve cells to the posterior pituitary where they are stored in the axon terminals and (2) it sends nerve impulses to the posterior pituitary, stimulating the release of the stored hormones.Adrenal Glands
A third centre in the hypothalamus is connected by sympathetic neurons to the adrenal gland. 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, 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/epinephrine.
In response to a stress stimulus such as fear, neurons of the sympathetic nervous system also release neurotransmitters called norepinephrine for synaptic transmission. One difference between these two types of responses is that the norepinephrine released through the nervous system acts much faster than that of the endocrine system. However, the effect of norepinephrine released by the adrenal medulla lasts much longer.
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. 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.
These examples illustrate the importance of the hypothalamus as an important command centre for coordinating the activity of the nervous and endocrine systems.
Watch and Listen
Read page 437 and page 463 of your textbook and make notes on how the nervous and endocrine systems maintain homeostasis, how they are coordinated, and how they act together. Developing a flow chart may help you visualize these relationships.
You may want to review your notes, particularly from Module 2 Lessons 1 to 4.
You can investigate the same concepts by watching and making notes on the following segment of Biologix -13.
©Alberta Education. The Neuroendocrine System: Working Together to Maintain Homeostasis (2:07-6:13, 9:29-10:20); Series 13. LearnAlberta.ca