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Physiology 3120 Lecture Notes - Circadian Rhythm, Neuroendocrinology, Pineal Gland

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Tom Stavraky

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Human Physiology
Monday, February 22, 2010
“Endocrine II”
Portal bloodstream travels to anterior pituitary and stimulates release of tropic hormones
Also have long axons that terminate in the posterior pituitary (PP)
Hormones produced in these cells sit there waiting to be released
Vasopressin and oxytocin are stored in PP
3 types of neurons
Magnocellular (typical)
Hypophysial tropic (majority of neurons)
Axon terminates in portal capillary bed
Hypothalamic projection neuron
Communicates with other neurons; have connective functions
Episodic endocrine secretion
Circadian rhythm (~24 hrs.)
Diurnal rhythm (exactly 24 hrs.); GHRH, CRH, and cortisol follow this rhythm; often used
interchangeably with circadian rhythm
Ultradian (not circadian or diurnal; minutes/hours); GnRH, and LH follow this rhythm)
Some hormones have multiple types of patterns depending on context
GH secretion is both diurnal and ultradian (large peak during sleep, but smaller peaks
during the day)
Generated in suprachiasmatic nucleus (SCN) of the hypothalamus
Intrinsic circadian pattern of secretion
Regulated expression of “clock” genes (i.e. cryptochromes, and period)
Direct input from retina (non-visual); light resets the pattern (using melatonin from pineal
Light causes signal from SCN to be sent down the spinal cord and then back up to the pineal
gland, which then releases melatonin
Major axes regulated by the hypothalamus
HP-adrenal axis (“stress axis”)
Hypothalamus releases CRH (corticotropic releasing hormones); stimulates release of
adrenocorticotropic hormone (ACTH) from AP; stimulates adrenal cortex to release
Cortisol increases blood glucose (via gluconeogenesis, muscle catabolism, inhibit
GLUT4 activity); also suppresses immune response
Made from POMC (proopiomelanacortin); also a precursor for many other
hormones (i.e. MSH, and endorphins)
HP-thyroid axis
Hypothalamus releases TRH and SRIF; stimulates release of thyroid-stimulating
hormone (TSH); stimulates release of thyroxine (T3) and triiodothyronine (T4)
Increase metabolism and heart rate
HP-growth axis
Hypothalamus releases GHRH and SRIF (somatotropin release inhibiting factor);
stimulates release of GH; travels to liver, which causes release of insulin growth factor 1
IGF-1 resembles insulin and is responsible for most of the actions of GH
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