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University of California - Irvine
Biological Sciences
Peter A.Bowler

E109 Oct 10 Wednesday Lecture Feedback loops and control systems Sensor: detects whether regulated variable is within desired range (thermometer is a sensor that detects temperature) Input signal sent Controller (or integrating center): compares regulated variable level (input from sensor) to set point or desired range, sends output to effector Output signal sent Effector (or target): causes a change in a regulated variable Response results -Sensor is constantly monitoring the changing stimulus (continuous loop) -Sometimes, different types of effectors are needed for different responses Fig 1.9 Negative vs. positive feedback loop (thermoregulation) Negative feedback loop--response by effector returns regulated variable BACK to set point or range; homeostatic control Positive feedback loop—response by effector sends regulated variable to extreme (runaway process); not used in homeostatic control Fig 1.10 -Negative feedback controls the temperature to return to normal if it gets too high or too low Fig 1.12 -Positive feedback example: delivery of baby—drives process to completion (delivery of baby) Regulated variable: baby moving down in uterus Sensor: cervical stretching felt by the mother Controller: brain (when it gets oxytocin chemical signals, it sends output signal of uterine contractions) Effector: pushes baby against cervix Negative control -Set points for regulated variables can change with time of day/night (circadian rhythm) or season 1.13 Example: body temperature changes from day to night Example: hormone levels can change during the day i.e. cortisol (stress hormone) +hormone peak at 9AM Cannon’s Postulates 1.The nervous system has a role in preserving the “fitness” of the internal environment. 2. Some systems of the body are under tonic control (up/down control) +Neural regulation of diameter in blood vessels (i.e. decreased signal rate of neurons = vessel dilation; increased signal rate of neurons = vessel constriction) 3. Some systems of the body are under antagonistic control. +systems not under tonic control are usually under antagonistic control +uses different signals to send a parameter in opposite directions +sympathetic neuron increases/speeds up heart rate, while parasympathetic neuron decreases/relaxes heart rate 4. One chemical signal can have different effects in different tissues +epinephrine constricts or dilates blood vessels, depending on whether the vessel has an alpha or beta adrenergic receptor Hormones Ch. 7 -Hormones differ in location (where they were synthesized), primary targets, action, and class -Hormones are +secreted by cell or group of cells (endocrine glands) +usually transported to distant target (s) +act and are present in low concentrations +bind to receptors to act on target cells +degraded (half-life) Hormone classification by chemical type -Peptide or protein hormones: -Steroid hormones -Amino acid derived hormones Table 7-1 Peptide hormones -Made in advance and stored in secretory vesicles; vesicles stored until cell receives signal to release! Peptides cannot easily cross wall of vesicle. +different parts of hormone chopped off to get active form -Released from parent cell by exocytosis into the ECF (extracellular fluid) -transported in blood by being dissolved in plasma (lipophobic or hydrophilic) to target and nontarget cells -peptide hormone zips across the capillary endothelium by vesicular transport +combination of endocytosis, vesicular transport across cell, and exocytosis (interstitual fluid to plasma What happens when peptide hormones reach target cells? -peptide hormones cannot enter their target cells and must combine with membrane receptors that initiate signal transduction processes +can open ion channel +can cause changes to protein (phosphorylation) to generate a cellular response +Hormone is considered first messenger and the other signal molecules generated inside the cell are called second messengers +nontarget tissues lack receptors Hormone classification -Peptide or protein hormones +most hormones are this type +water soluble (transported in blood) +don’t enter cells but bind to receptors located in cell membrane +half-life usually in minutes -Steroid hormones +derived from cholesterol (lipid) #sex hormones are steroids (in ovary/testes or gonads)—estrogen #cortisol and aldosterone are steroids made in the adrenal cortex +made in adrenal glands (outer portion called adrenal cortex) and gonads +not stored (lipophilic) or hydrophobic +transported in blood by carrier molecules +half-life of an hour to hours Steroid hormone action -Steroid hormone arrives the target cell and crosses the cell membrane barrier because its lipophilic -binds to receptors in the cytoplasm or in the nucleus => >> binds to DNAto activate or repress transcription (production of mRNA) and translation -rarely will steroid hormones bind to cell surface receptors—which would generate a rapid response -Amino-acid derived hormones +Tyrosine = catecholamines (made in adrenal glands (inner portion called adrenal medulla)) and thyroid hormones (made in thyroid gland) -thyroid hormones incorporate iodine ions; iodine is needed to synthesize these thyroid hormones -chemical similarity with parent amino acid The hypothalamus and the pituitary -Two very important endocrine glands -Both parts of the brain -Release a large number of hormones that coordinate growth, metabolism, and reproduction Hypothalamas: Pituatary: two distinct parts of this gland—Anterior pituitary and posterior pituitary; release different hormones -Posterior pituitary is from neural tissue -Anterior pituitary is derived from the epithelium Posterior pituitary -2 hormones release: oxtyocin (release in response to stimulation of cervical stretch to cause muscular contractions in females) and vasopressin (ADH—anti-diuretic hormone) -An extension of the brain that secretes neurohormes made and packaged in the hypothalamus 1. Neurohormone is made and packaged in cell body of neuron 2. Vesicles are transported down the cell 3. Vesicles containing neurohormone are stored in posterior pituitary 4. Neurohormones (either oxytocin or vasopressin) are released into blood. Hormones of the anterior pituitary -Prolactin, TSH, ACTH, GH, FSH, LH hormones -makes its own hormones also but it is released in response to other hormones released from the hypothalamus cells (trophic hormones) -trophic hormones control secretion of other hormones -RH: releasing hormone -IH: inhibiting hormone +GHIH causes the inhibition of release of the growth hormone from the anterior pituitary Negative feedback in hypothalamus/anterior pituitary hormones -Hypothalams releases a trophic hormone, causing the anterior pituitary to release a trophic hormone, which causes the endocrine gland to release a hormone, which acts on the target tissue to elicit a response -“trophic hormone: controls secretion of another horomone -Short-loop negative feedback: trophic hormone secreted from anterior pituitary inhibits hypothalamus from releasing more trophic hormones -Long-loop negative feedback: hormone secreted by the endocrine gland inhibits hypothalamus from releasing more trophic hormones or inhibits anterior pituitary from releasing more trophic hormones Negative feedback loops in the cortisol secretion pathway CRH=corticotrophin-releasing hormone ACTH=adrenocorticotropic hormone (corticotrophin) HypothalamusCRHAnterior PituitaryACTHAdrenal cortex CortisolTarget tissueResponse!! -Long-loop negative feedback: Cortisol inhibits the anterior pituitary from releasingACTH or it inhibits the hypothalamus from releasing more CRH Next Lecture---- Portal system in hypothalamus/anterior pituitary -Portal system is 2 capillary beds in succession connected by portal vessels---hormones from hypothalamus go directly to anterior pituary and don’t mix with the blood -only 3 portal systems in the entire body: kidney, digestive system, hypothalamus/anterior pitu -capillary bed is collection of the smallest blood vessels: “exchange between the blood and surro
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