Module 2 Lesson 1 - 2

In the previous module, you learned about the basic unit of the nervous system, the neuron, and how communication occurs through neural pathways. Now, you will learn about the major glands of the endocrine system and how communication occurs through special chemical messengers called hormones.

Endocrine glands are ductless glands. They do not release their secretions into ducts as exocrine glands do. Instead, endocrine glands secrete their hormones directly into the blood, which acts as their transport medium. As the hormones travel in the blood and encounter cells, only those cells with special receptors react to their presence. These cells are called target cells.

As a result, hormones usually are slower to prompt responses and, typically, are less specific than the nervous system is.

Lipid-soluble hormones and water-soluble hormones activate their target cells very differently. Lipid-soluble hormones diffuse through the lipid bilayer and attach to receptor proteins inside the cell, resulting in changes to the cell. Water-soluble hormones attach to the receptor proteins located on the surface of the cells, resulting in a cascade of reactions.

Hormones interact, and in many instances, one hormone counteracts the action of a second hormone. Hormones such as these are called antagonistic hormones.