This sheet is for the Introductory part of the chapter on generalized hormone classes,
function, etc. For the specifics of individual gland/hormone function, you will need to know the 
information on the "HORMONES AND NEUROHORMONES SECRETED BY THE BODY"  
handout. For each of the hormones on that sheet, you will need to know several things:

1. Where the hormones are produced/released from (for all hormones except those of the
   posterior pituitary [neurohypophysis] the site of production and release is the same.)
2. What stimulates the release of the hormone.
3. Targets of the hormone.
4. Effects of the hormone on the targets.

___________

    The Endocrine System is the second of two control systems (nervous is the other); controls 
slowly developing aspects of the body (growth, sexual maturation), balance of substances in the 
bloodstream, stress, etc. Works together with NS for some jobs; remember, hypothalamus 
partly responsible for endocrine system control (hypothalamus called a neuroendocrine organ).

Hormones -- substances that are released into the bloodstream and have effects on specific targets.
        Can travel anywhere; targets determined by cells having hormone-specific receptors.

Hormone Classes:
        1. Amino Acid (Protein) based B affect targets by binding to cell surface (membrane) 
            receptors, and working through secondary messenger systems; one of commonest is 
            cyclic AMP (cAMP), which activates protein kinase enzymes. These kinases in turn 
            activate/inhibit potentially a wide variety of other proteins in the cell by phosphorylation  
            (dramatically alters cell activity).
        2. Steroid based B all made from cholesterol (released from two glands only: the adrenal 
            cortex and the gonads). Steroids typically small and non-polar (can pass through cell 
            membranes), so receptors are in the cyto- or nucleoplasm. The hormone binds to the 
            receptor, which in turn binds to DNA to activate (or inhibit) transcription (make RNA). 
            The RNA is translated into a "new" protein, which alter cell action.

    Though the mechanisms are different, the end result is similar for the two hormone classes --  
newly active proteins inside the cell which can dramatically alter cell activity.

Target Cell Response
        Up-regulation (similar to the nervous system concept of potentiation) -- a cell exposed to
            a rising level of a hormone develops more receptors for that hormone.
        Down-regulation (similar to adaptation) -- prolonged exposure to a constant high level of 
            hormone desensitizes the cell (the cell reduces its # of receptors).

Duration of Hormone Activity
    As with neurotransmitters, do not want hormone to have significantly long effects -- hormone 
effects last from 10 seconds to several hours (different for dif. hormones). Hormones must be 
removed from bloodstream (in #30 minutes by targets, or by kidney or liver).

Interactions of Hormones at Targets
        Synergistic -- hormones work together; additive effects
        Antagonistic -- hormones have opposite effects; frequently seen with pairs of hormones 
            responsible for balance of substances in the blood stream
        Permissiveness -- one hormone affects target by making it responsive to another; in 
            other words, one hormone permits another hormone its affects

Control of Hormone Release
    Three different stimuli can cause glands to release their hormones:
        1. Humoral stimuli -- levels of substances in the bloodstream
        2. Neural stimuli -- direct NS stimulation (as is the case with the adrenal medulla)
        3. Hormonal stimuli -- many hormones are released from glands in response to other 
            hormones targeting those glands; hormones whose function it is to cause other glands 
            to release their hormones are called tropic hormones.

    Negative feedback is also a common mechanism for balancing release of hormones, 
particularly of the tropic hormones; typically the target gland hormones will rise to a certain 
level and ultimately turn off their own production by inhibiting release of their own tropic 
hormones (will talk about hypothalamic/pituitary examples in class).