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D5 - Hormones and metabolism (Hormones (Steroid hormones (Enter the cell…
D5 - Hormones and metabolism
Lactation
The production and secretion of milk by maternal mammary glands following birth
Oxytocin
Responsible for the release of milk from the mammary glands (milk ejection reflex)
Produced in the hypothalamus and secreted by neurosecretory cells that extend into the posterior pituitary
Oxytocin release is triggered by stimulation of sensory receptors in the breast tissue by the suckling infant
Positive feedback loop - continuous oxytocin secretion until the infant stops suckling
Prolactin
Responsible for the development of the mammary glands and the production of milk
Secreted by the anterior pituitary in response to the release of PRH (prolactin releasing hormone) from the hypothalamus
Inhibited by progesterone, which prevents milk production from occurring prior to birth
Placenta gets expelled from the body, leading to a sudden drop in progesterone levels
Hypothalamus and the Pituitary Gland
Hypothalamus - section of the brain that links the nervous and endocrine systems in order to maintain homeostasis
Receives information from nerves throughout the body and other parts of the brain --> initiates endocrine responses
Secretes neurochemicals (releasing factors) into a portal system which target the anterior lobe of the pituitary gland
Portal system - any system of blood vessels which has a capillary network at each end.
Secretes hormones directly into the blood via neurosecretory cells that extend into the posterior pituitary lobe
Pituitary gland
Lies adjacent to the hypothalamus and is in direct contact due to a portal blood system. It receives instructions from the hypothalamus and consists of two lobes
Anterior lobe
PRODUCES hormone
The hypothalamus produces releasing factors, which are released into portal vessels by neurosecretory cells
Example of releasing factor - GnRH, which triggers the release of LH and FSH from the anterior pituitary
Releasing factors cause endocrine cells in the anterior pituitary to release specific hormones into the bloodstream
Posterior lobe
SECRETES/RELEASES hormone (produced by the hypothalamus, transported via neurosecretory cells)
Neurosecretory cells extend into the posterior lobe from the hypothalamus and release hormones into the blood
Controls the secretion of a number of other endocrine glands and many vital processes in the body
MARGE (metabolism, adult growth, reproduction, growth, equilibrium/homeostasis)
Neurosecretory cells
Similar to non secretory nerve cells, consists of fine projections (an axon and neurofibrils) and a cell body
Extends into posterior lobe
Extends into the tip of anterior lobe, then is connected to portal vessels
Endocrine Gland
Ductless glands that release hormones, chemical substances into the the circulatory system (blood) to regulate bodily functions
May affect one or several organs throughout the body
Slower to initiate but create a more prolonged reaction compared to the nervous system
Major endocrine glands include the pancreas, adrenal gland, thyroid gland, pineal gland and the gonads (ovaries and testes)
The hypothalamus and pituitary gland are neuroendocrine glands and function to link the nervous and endocrine systems
Hormones
A chemical messenger that is transported indiscriminately via the bloodstream to act on target cells/tissues with the appropriate target receptor
Some organs may also secrete hormones despite not being endocrine glands (e.g. adipose tissue secretes leptin)
Steroid hormones
Enter the cell by passing through plasma membrane -> bind to receptor cells in the cytoplasm to form a hormone receptor complex --> travels to nucleus of the target cell
The complex regulates the transcription process, transcription of some genes may be promoted, while some may be inihibited
Control synthesis of enzymes and proteins --> control activity and development of target cells
Lipophilic (fat loving), meaning they can pass through membranes
Example - hormones produced in the gonads (i.e. estrogen, progesterone and testosterone)
Peptide hormones
Activates or inhibits enzymes
Do not enter cells
Bind to receptors on the plasma membrane of target cells --> release of secondary messenger/relay molecules, triggering a cascade of reactions
Signal transduction - the external signal (hormone) is transduced via internal intermediaries (ie. secondary messengers)
Example of secondary messengers - calcium ions (Ca2+)
Example - insulin, glucagon, leptin, ADH and oxytocin
Hydrophylic and lipophobic (fat-hating), meaning they cannot freely cross the plasma membrane
Growth hormone
Anabolic peptide hormone that stimulates growth
Due to its role in promoting growth and regeneration, it is used by some athletes as a performance enhancer
Banned in sports
Traditional urine testing could not detect doping, which historically made bans difficult to enforce
Recent blood tests can now identify between natural and artificial variants of growth hormone
Reduces formation of adipose cells and increases muscle mass, bone size
Secreted by the pituitary gland
