NOTICE: Posting schedule is irregular. I hope to get back to a regular schedule as the day-job allows.

Sunday, May 15, 2011

*I* am in control! (Controlling the body, part 2). [Full link to blog for email clients.][FT:C44]

The previous blog concentrated on direct nervous control of the body.  This blog will concentration on *indirect* control via chemicals released by the brain to affect targets in the rest of the body. 

Most people should be familiar with the common hormones testosterone, estrogen, progesterone produced by testes and ovaries; adrenalin and cortisol produced by adrenal glands, thyroxin, produced by the thyroid gland, and growth hormone, secreted by the pituitary gland.  With the exception of growth hormone, each of these hormones is produced outside the brain by specialized tissues that receive extensive blood and neural inputs.  In adittion to the obvious effects (e.g. reproductive effects and secondary sex characteristics), these hormones have profound effects on muscle development, blood pressure, metabolism, fat deposition, and bone growth. 

What is less obvious is that the production of each of these hormones is under strict control of the brain - both through direct neuron connections (i.e. to the adrenal gland), or indirectly through specialized "tropins" or regulatory hormones produced by tissues in the brain.  The hypothalamus is as much a "gland" be definition, as it is a nucleus of the brain.  Neurons in hypothalamus controls the autonomic (i.e. automatic or involuntary) functions of the nervous system and body, regulate homeostasis - body temperature, blood pressure, digestion, heart rate, respiration rate, etc. - and gives instructions to the pituitary gland (distinctly a *gland* and not a nucleus) through specific releasing hormones and inhibiting hormones.  In this manner, it also controls the functions of all the other glands, directly or indirectly, of the body, forming what is officially termed the "endocrine" (hormonal) system.

The hypothalamus produces antidiuretic hormone (ADH) or vasopressin which regulates smooth muscle contraction around the blood vessels, and hence the blood pressure throughout the body.  This is a very important function, since blood must be under pressure to penetrate the small capillaries that serve all tissues of the body.  Another important, similar, hormine produced in hypothalamus is oxytocin, which produces contraction in smooth muscle of the mammaries and uterus, promoting lactation and inducing labor during pregnancy.  Both hormones are produced by the hypothalamus, but stored and released by the pituitary.  This is accomplished by neurons with cell bodies in the hypothalamus, and axon terminals which store and release the hormones onto blood vessels in the posterior pituitary in much the same manner as neurotransmitter release onto other neurons.

A series of "stimulating" or "releasing" hormones are produced and released in the anterior pituitary by neurons that receive inputs from other areas of hypothalamus. Prolactin (stimulates mammary glands), human growth hormone (affects bone and muscle growth), melanocyte stimulating hormone (affects skin color, and has some interesting interactions with the enkephalin/pain system), thyroid stimulating hormone (causes thyroid to produce thyroxin and thyrotropin), adrenocorticotropic hormone (causes the adrenal cortex to produce cortisol, one of the bodies primary stress-response hormones), follicle stimulating hormone (causes the development of follicles and ova in the ovaries and also promotes estrogen synthesis), and lutenizing hormone (causes ovulation, the release of ova from follicles, and transforms the follicle into a "corpus luteum" which produces progesterone).

The pineal gland is the other major gland of the brain which produces melatonin and releases it at night to assist in the sleep/wake cycle.

There are also lesser known neurotransmitters and neuromodulators produced in various areas of the brain which also have hormone-like functions both within the brain and throughout the body.  For example: orexin, is a neurotransmitter produced in hypothalamus and a few other brain areas that regulates neural activity, promotes wakefulness, and is involved in feeding behavior.  Ghrelin is a neuromodulator and hormone (growth hormone releasing) in the hypothalamus and is also produced by the stomach, associated with satiety (the full, satisfied feeling after a big meal).  Cholecystokinin (CCK) is a minor neurotransmitter, that is also produced in the intestines and regulates smooth muscle contraction, fat uptake and gall bladder function. Norepinephrine is a major neurotransmitter that is structurally similar to adrenaline (epinephrine) and is involved in stress response. 

Together, the thalamus and hypothalamus are intricately involved in control of the body - the thalamus (and midbrain and spinal cord) in the direct neural control, and the hypothalamus for endocrine or hormonal indirect control.  Many systems of the body will certainly continue to function with inputs from the rest of the brain, but each of these systems has an override or regulatory function to control every aspect of the body.

Thus the brain really *is* in control of the whole body.  So, don't let the *body* tell you hat to do.  *You* are in control!

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