17.1 Overview of Endocrine 17.2 Hypothalamus and Pituitary
flow of events for secretion of hormone?
1. The hypothalamus secretes a releasing hormone 2. pituitary cell to secretes its hormone 3. that hormone is usually targeted to another endocrine gland elsewhere in the body 4. finally that gland secretes a hormone with an effect of its own
the two structures of the pituitary
1. anterior pituitary: arises from a pouch that grows upward from the embryonic pharynx 2. posterior pituitary: arises as a bud growing downward from the brain They come to lie side by side and are so closely joined that they look like a single gland. fig 17.3
neuroendocrine reflexes
a way of controlling the pituitary; the release of hormones in response to nerve signals. ex 1: 1. dehydration raises osmolarity of the blood 2. this is detected by hypothalamic neurons called osmoreceptors 3. osmoreceptors trigger ADH release 4. ADH promotes water conservation ex 2: 1. suckling of infant stimulates nipple 2. stimulation sends nerve signal to hypothalamus and then to posterior pituitary 3. due to stimulation oxytocin is released 4. oxytocin stimulates release of milk *neuroendocrine reflexes can involve higher brain centers such that hearing a baby cry can lead to release of milk or stress can affect gonadotropin secretion which disrupts menstrual cycle
hypothalamic control
brain monitors conditions outside of the body and tells pituitary to release hormones depending on what is happening
target cells
cell that can respond to specific hormone because it has the relevant receptor on it. target cells can also due to the fact that the circulating hormone is inactive and only the target cells have the enzyme needed to convert it to active form.
functions of antidiuretic hormone (ADH)
functions include: -increased water retention in kidneys -reduce urine volume -help prevent dehydration -brain neurotransmitter (also called arginine vasopressin AVP) -causes vasoconstriction
lactotropes (mammotropes)
pituitary cell that secretes prolactin (PRL) and increase greatly in size and number during pregnancy. PRL secretion rises proportionately (growth of lactropes), but it has no effect on the mammary glands until after a woman gives birth. Then, it stimulates them to synthesize milk. Male: Increased LH sensitivity
hypothalamus
regulates primitive functions of the body like water balance, thermoregulation, sex drive, and childbirth. Many of its functions are carried out by the pituitary gland
axis
relationship between the hypothalamus, pituitary, and another downstream endocrine gland. ex: hypothalamo-pituitary-thyroid axis 1. hypothalamus secretes thyrotropin-releasing hormone (TRH) 2. above causes anterior pituitary to secrete thyroid-stimulating hormone (TSH) 3. TSH stimulates the thyroid gland to release thyroid hormone (TH) 4. thyroid hormone (TH) exerts its metabolic effects throughout the body.
how is pituitary secretion controlled?
timing and amount of pituitary secretion are regulated by the: 1. hypothalamus 2. other brain centers 3. feedback from target organs
negative feedback inhibition
*regulation of endocrine glands is not simply from pituitary because the glands can control it via feedback
6 main hormones of the anterior pituitary
-Follicle-stimulating hormone (FSH) -Luteinizing hormone (LH) -Thyroid-stimulating hormone (TSH) -Adrenocorticotropic hormone (ACTH) -Prolactin (PRL) -Growth hormone (GH)
hypophyseal portal system
-consists of a network of primary capillaries in the hypothalamus, a group of small veins called portal venules that travel down the stalk, and a complex of secondary capillaries in the anterior pituitary. The hypothalamus controls the anterior pituitary by secreting hormones that enter the primary capillaries, travel down the venules, and diffuse out of the secondary capillaries into the pituitary tissue.
anterior pituitary (anterior lobe or adenohypophysis)
-constitutes about three-quarters of the pituitary as a whole. -has no nervous connection to the hypothalamus, but is linked to it by a complex of blood vessels called the *hypophyseal portal system* -The anterior lobe contains Basophils (gonadotropes, thyrotropes, and corticotropes), Acidophils (somatotropes and lactotropes), and chromophobes
posterior pituitary (posterior lobe or neurohypophysis)
-constitutes the posterior one-quarter of the pituitary. -It is actually nervous tissue (unmyelinated nerve fibers and neuroglia (called pituicytes), thus NOT A TRUE GLAND. -Nerve fibers arise from certain cell bodies in the hypothalamus, pass down the stalk as a bundle called the hypothalamo-hypophyseal tract, and end in the posterior lobe. -Hormones are made in the hypothalamic neurons and move down the nerve fibers by axoplasmic flow to the posterior pituitary. Here they are stored in the nerve endings until a nerve signal coming down the same axons triggers their release.
what are the 8 hormones produced in the hypothalamus?
-oxytocin (OT) -antidiuretic hormone (ADH) -Thyrotropin-releasing hormone (TRH) -Corticotropin-releasing hormone (CRH) -Gonadotropin-releasing hormone (GnRH) -Growth hormone−releasing hormone (GHRH) -Prolactin-inhibiting hormone (PIH) -Somatostatin first two are stored in the posterior pituitary and released on demand. last six are used to regulate the anterior pituitary.
four principal ways of communication from cell to cell
1. Gap junctions join single-unit cells to each other. They enable cells to pass nutrients, electrolytes, and signaling molecules directly *from the cytoplasm of one cell to the cytoplasm* of the next through pores in their plasma membranes 2. Neurotransmitters are released by neurons, diffuse across a narrow synaptic cleft, and bind to receptors on the surface of the next cell. 3. Paracrines are secreted by one cell, diffuse to nearby cells in the same tissue, and stimulate their physiology. Some call them local hormones. 4. Hormones are chemical messengers that are transported by the bloodstream and stimulate physiological responses in cells of another tissue or organ, often a considerable distance away.
Hypothalamo-Pituitary-Target Organ Relationships
1. TRH 2. GnRH 3. CRH 4. GHRH 5. PRL 6. TSH 7. LH/FSH 8. ACTH 9. IGH 10. GH TABLE 7.4*******
communication by endocrine system
Endocrine cells secrete a hormone into the bloodstream. At a point often remote from its origin, the hormone leaves the bloodstream and enters or binds to its target cells fig 17.2?
endocrine vs exocrine glands
Endocrine glands are ductless and release their secretions into the bloodstream while exocrine glands secrete their products by way of a duct onto an epithelial surface such as the skin or the mucosa of the digestive tract. Exocrine secretions have extracellular effects such as the digestion of food. endocrine secretions have intracellular effects—they alter cell metabolism.
hormones of posterior pituitary
OT comes from neurons in the right and left paraventricular nuclei of the hypothalamus ADH comes mainly from the supraoptic nuclei
compared speed with which nervous and endocrine systems start and stop responding to a stimulus
The nervous system responds within a few milliseconds while endocrine system takes from several seconds to days for a hormone to act. When a stimulus ceases, the nervous system stops responding almost immediately, whereas hormonal effects can last for days or even longer. *under long-term stimulation, most neurons quickly adapt and their response declines. The endocrine system shows more persistent responses.
neuroendocrine cells
act like neurons in many respects, but like endocrine cells, they release their secretions (such as oxytocin) into the bloodstream
effect of endocrine vs nervous
efferent nerve fiber innervates only one organ and a limited number of cells within that organ, *so its effects are usually precisely targeted and relatively specific*. Hormones circulate throughout the body and have more widespread effects than a neuron
endocrine system and endocrine glads
glands, tissues, and cells that secrete hormones the pituitary, thyroid, and adrenal glands are endocrine glands, but organs such as brain, heart, small intestine, bones, and adipose tissue also release hormones
gonadotropins
hormones that target the ovaries and testes (gonads): -Follicle-stimulating hormone (FSH): Female: Growth of ovarian follicles and secretion of estrogen Male: Sperm production -Luteinizing hormone (LH): Female: Ovulation, maintenance of corpus luteum (follicle after ovulation) Male: Testosterone secretion
17.2 regulator of endocrine system
no master control center that regulates the entire endocrine system, but the pituitary gland and hypothalamus have the most wide-ranging influence
fenestrated capillaries
permeable capillaries which have patches of large pores in their walls allowing for the easy uptake of matter from the gland tissue
corticotropes
pituitary cell that secretes Adrenocorticotropic hormone (ACTH), or corticotropin. ACTH stimulates the adrenal cortex to secrete hormones called glucocorticoids (especially cortisol), which regulate glucose, protein, and fat metabolism and are important in the body's response to stress.
thyrotropes
pituitary cell that secretes Thyroid-stimulating hormone (TSH), or thyrotropin. TSH stimulates growth of the thyroid gland and the secretion of thyroid hormone, which has widespread effects on metabolic rate, body temperature, etc.
releasing / inhibiting hormones of anterior pituitary
releasing hormones: Among the last six hormones produced in the hypothalamus, those that stimulate the pituitary to release hormones of its own inhibiting hormones:suppress pituitary secretion
functions of oxytocin (OT)
reproductive functions in situations like: - sex -breastfeeding -sexual satisfaction -*emotional bonding between partners* -stimulates labor contractions -stimulates flow of milk from mammory gland to nipple
comparison of nervous and endocrine system
similarities: - some hormones and neurotransmitters produce identical effects on the same organ -communicate chemically -some chemicals function as both neurotransmitters and hormones—for example, norepinephrine, dopamine The nervous and endocrine systems complement each other. Neurons often trigger hormone secretion, and hormones often stimulate or inhibit neurons.
somatotropes
somatotropes are the most numerous cells of the anterior pituitary and they secrete GH or somatotropin. The pituitary produces at least a thousand times as much GH as any other hormone. The general effect of GH is to stimulate mitosis and cellular differentiation and thus to promote tissue growth throughout the body.
hypophysis (pituitary gland) location
suspended from the floor of the hypothalamus by a stalk (infundibulum) and housed in a depression of the sphenoid bone, the sella turcica. 1.3cm wide. it grows about 50% larger in pregnancy