Study hint: Your procedure for mastering the endocrine system deviates from our usual technique. Use chapter 23 in your textbook as a reference to help you understand the information in the Endocrine Chart in your syllabus. I have gathered together the important facts about the endocrine glands of the body, and compiled that information into the chart in your syllabus.
Key Features of the Endocrine System
Major Tissue Type: Epithelial tissue
Characteristic Cell Type: Variable; depends on gland
Functional Unit: Endocrine cells
Confusing Terms: Endocrine glands versus endocrine tissue which will be studied in detail in Human Physiology.
It is appropriate that we follow our study of the nervous system, the first master of the body, with a second, very powerful control system, the endocrine system. These two systems meet at the diencephalon in the hypothalamus. Whereas the nervous system secretes chemicals called neurotransmitters at synapses throughout the body, the endocrine system secretes hormones into the bloodstream. Both systems use the ligand-receptor paradigm we discussed at the beginning of the course. Even though all of the hormones move around the entire body, each hormone only influences the functions of cells with receptors for it. More specific information about the interactions between hormones and their target cells is an important subject for Physiology.
The hypothalamus controls the pituitary gland, the so-called master gland, one of two ways:
Cells in the hypothalamus secrete hormones into a plexus of veins that extends to the part of the pituitary gland called the anterior pituitary aka adenohypophysis. The adenohypophysis contains endocrine cells that synthesize and release one of seven different hormones: human growth hormone, thyroid-stimulating hormone, follicle-stimulating hormone, luteinizing hormone, prolactin, adrenocorticotropic hormone, and melanocyte-stimulating hormone. Refer to your endocrine chart for additional information on this topic. We will consider the nature of this portal system of veins when we study the systemic circulation.
Two hypothalamic nuclei, the paraventricular nucleus and the supraoptic nuclei, synthesize hormones. Axons from their nuclei enter the infunibulum and reach the posterior pituitary aka neurohypophysis. When neuron cell bodies in these nuclei are stimulated, action potentials travel down their axons and cause release of oxytocin or antidiuretic hormone (aka vasopressin) from their axon terminals. Isn't it amazing that these hormones are released like any neurotransmitter--except for one fact. These hormones are released into the bloodstream, and influence target cells at distant locations. Notice that the posterior pituitary does not contain endocrine cells.