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12 Cards in this Set
- Front
- Back
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Where is the hypothalamus? What does it do?
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-located within the diencephalon region of the brain
-it is located below the thalamus and above the pituitary gland -it is composed of anatomically distinct nucleui that possess extensive synaptic connections with other regions of the nervous system -it controls a number of metabolic functions s well as being one of the main regulators of the autonomic nervous system through direct connection with autonomic nuclei in the brainstem and spinal cord |
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Describe the neural and vascular connections between the hypothalamus and the anterior and posterior lobes of the pituitary gland
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-the hypothalamus has specific nuclei that produce ADH and oxytocin (OT) and transport them down their axons to the posterior pituitary for secretion into blood
-the hypothalamus also contains other specialized neurons that synthesize releasing or inhibiting hormones into a plexus of veins called the portal system that carriers the hormones to the anterior pituitary where they stimulate anterior pituitary cells to secrete additional hormones |
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Where is the pituitary gland?
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-composed of 2 separate glands: anterior pituitary (adenohypoposis) and posterior pituitary (neurophypophysis)
-it is located in the sella turcica at the base of the brain inferior to the hypothalamus and is connected to the hypothalams via the pituitary stalk (infundibulum) that contains vascular and neural connections -the hypothalamus controls secretions of hormones in both the anterior and posterior pituitary but in very different mechanisms |
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What is the structure of the posterior pituitary?
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-an extension of the hypothalamus and is composed of neural tissue
-large bodied neurons in the paraventriclar and hypothalamus synthesize oxytocin and ADH -these hormones together with a carrier protein neurophysin travel down the axons of the neurons (within the hypothalamo-hypophyseal tract) to the posterior pituitary where the hormones are stored in the nerve terminals in vesicles until released into a plexus of blood capillaries following the appropriate stimuli -release is by exocytosis when the nerve terminals are depolarized |
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What is the structure of the anterior pituitary?
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-contains 5 trophic cells that secrete 6 peptide hormones into the systemic circulation
♦thyroid stimulating hormone (TSH) produced by thyrotrophs ♦Adrenocorticotropic hormone (ACTH) produced by corticotrophs ♦Prolactin (PRL) produced by lactrotrophs ♦Follicle stimulating hormone (FSH) produced by gonadotrophs ♦Growth hormone (GH) produced by somatotrophs |
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How are hormones secreted in the anterior pituitary?
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-cells that produce anterior pituitary hormones are not directly innervated but regulated by hypothalamic hormones that are transported to the anterior pituitary by the blood
-neurons in the hypothalamus secrete releasing or inhibitory hormones into a specialized circulatory system: the hypothalamo-hypophyseal portal system -small bodied neurons in several nuclei of the hypothalamus secrete releasing and inhibitory factors in response to neural inputs from other areas of the CNS into a rich runnel shaped plexus of capillaries that penetrates the median eminence and surrounds the infundibular recess -these include: the arcuate, the paraventricular and ventromedial nuclei, and the medial preoptic and periventricular regions -the capillaries (primary plexus), which are outside of the blood brain barrier, coalesce into long portal veins that carry the releasing and inhibitory factors down the pituitary stalk to the anterior pituitary -other neurons secrete their releasing factors into a capillary plexus that is much further down the pituitary stalk; short portal veins carry these releasing factors to the anterior pituitary -there, the portal veins break up into the secondary capillary plexus of the anterior pituitary and deliver the releasing and inhibitory factors to the anterior cells that actually secrete the anterior pituitary hormones |
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Describe the growth hormone/insulin like growth factor I axis and the implications for postnatal growth
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-growth hormone is a polypeptide of 191 amino acids secreted from cells called somatotrophs in the anterior pituitary
-post-partum growth is not sustained in the absence of growth hormone, whereas GH administration to GH-deficient animals results in accelerated somatic growth, including the stimulation of longitudinal bone growth, skeletal muscle development and weight increased -long bones that are subject to longitudinal growth have a cartilagenous growth plate, the epiphyseal cartilage, at the transition between the epiphysis and the shaft -this cartilage comprises the growth zone -cartilage cells in the epiphyseal cartilage divide when the cartilage adjacent to bone tissue ossifies -growth hormone stimulates differentiation of immature cartilage cells in the epiphyseal growth plate -as these cells mature, they are stimulated by GH to produce IGF-1, which increases the rate of mitosis in the epiphyseal cartilage cells -longitudinal growth ceases when the epiphyseal cartilage becomes ossified |
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What are the direct effects of growth hormone?
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-decreases glucose uptake into cells (diabetogenic)
-increases lipolysis -increases protein synthesis in muscle and increases lean body mass -increases production of IGF |
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What are the indirect effects of growth hormone
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-GH casues release from target tissues (eg. liver) of IGF-1 and IGF II that promote cellular proliferation/tissue growth and development by increasing the number of cells (hyperplasia) and increasing the size of cells (hypertrophy)
-IGF-I are also responsible for the growth promoting action on epiphyseal cartilage -bones grow in length as a result of proliferation and enlargement of chondrocytes (cartilage cells) -this causes the epiphyseal growth plate to increase in width, calcification of the exracellular matrix on the diphyseal border and old nutrient deprived chondrocytes die and are cleared by osteoclasts and replaced by osteoblasts that deposit bone |
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Describe the regulation of growth hormone secretion
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-primarily regulated by the secretion of 2 hypothalamic neurohormones, a stimulatory hormone (GHRH( and an inhibitory hormone (GHIH)
-GH-producing cells in the anterior pituitary are continuously influenced by both GHRH and GHIH -such a dual control facilitates precise regulation of hormone secretion -variations in GH secretion are primarily due to variations in GHRH secretion, whereas GHIH plays a relatively minor role -GHRH stimulates somatotrophs to release GH via Gs coupled receptors modulating cAMP levels to regulate exocytosis of preformed GH -GHIH inhibits cAMP producing via Gi receptors -growth hormone stimulates production of a peptide that mediates the growth stimulating action of GH -this peptide hormone is structurally similar to proinsulin and is called insulin-like growth factor 1 -IGF-1 exerts a potent negative feedback on the secretion of GH by acting both on the pituitary gland and the hypothalamus -changes in the plasma concentration of several nutrient are affected by variation in GH also influence the secretion of GH -in addition, the hypothalamic regulation of growth hormone secretion is influenced by input from other parts of the central nervous system -streneous physical activity, starvation, stress and low plasma glucose are among the physiological factors that result in increased growth hormone secretion -other hormones, such as thyroid hormones, also participate in the regulation of growth hormone production -if the production of thyroid hormones is reduced, GH secretion is also reduced -sex hormones also stimulate growth hormone production and contribute to increased GH secretion during puberty |
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What is pituitary dwarfism?
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-caused by hyposecretion of GH during the growth years
-this causes slow bone growth and the epiphyseal plates to close before normal height is reached -other organs of the body also fail to grow -may be due to genetics with lack of normal development of the pituitary gland |
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What is gigantism and acromegaly?
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✩GIGANTISM
-hypersecretion of GH (early in life) results in gigantism whic is an abnormal increase in the length of long bones ✩ACROMEGALY -hypersecretion of GH during adulthood (cats 8-14 years of age) -further lengthening of the long bones cannot occur because the epiphyseal plates are already closed -instead the bones of the limbs, paws, cheeks, and jaw thicken -growth in thickness of bone is achieved by the addition of new bone on top of existing bone through the activity of osteoblasts within the periosteum (connective tissue sheath that covers the outer bone surface) -other tissues also grow = cardiomegaly & liveromegaly -the eyelids, lips, tongue, and nose enlarge, and the skin thickens -azotemia develops in 50% of cats |
