Endocrine principles

• Homeostasis does not equal equilibrium, is just a dynamic steady state of the body
• Negative feedback for homeostasis

  ex. cortisol secretion regulation

  • cortisol is somehow stimulated to be released
  • once body senses there is enough it suppresses the upstream release of ATCH and the release of CRH
  • once you need to start the cycle again, negative feedback stops
• Positive feedback for change → moving away from homeostasis

  ex. oxytocin and control of uterine contractions

  • childbirth → when its time for baby to come out, labour beginning is the stimulus
  • cervical stretch stimulates oxytocin which causes uterus to contract more and more, which causes more stretch and then more oxytocin
  • only stops once baby is out
• • gap junctions
  • small ions and molecules move through gap junctions connecting cells
• • contact-dependent
  • membrane protein binds to membrane protein
• • autocrine
  • molecules move through interstitial fluid, short distances and through close by cells
• • nervous and endocrine system serve as long distance communicators
  • an endocrine cell is able to release a hormone that then travels through blood stream all the way down to the target cell
  • hormones finds the correct receptor to elicit response
• Simple and Complex Reflexes

  • simple reflexes are mediated either only by NS or ES
  • complex reflexes are more common and go through several integrating systems
• • Simple neural reflex
  • stimulus -> sensory neuron -> CNS integrates that so respond through the efferent neuron -> releases neurotransmitter to target cell -> response occurs
• • Simple endocrine reflex
  • change occurs -> sensed by ES -> outputs a hormone -> elicits response
• Exocrine

  • secretion outside
  • into a duct
  • substances secreted to environment external to self ex. sweat
• Endocrine

  • secretion inside
  • into the bloodstream
  • ex. hormones
• Primary endocrine organs (main function is hormone release)

  • hypothalamus
  • pineal gland
  • pituitary gland
  • thyroid gland
  • parathyroid gland
  • adrenal glands
  • ovaries
  • testes
• Secondary endocrine organs (release hormones + something else)

  • heart
  • liver
  • pancreas
  • kidneys
  • stomach and small intestine
  • adipose tissue
  • skin
• Features of hormones:

  • can be made in different places in body
• Features of hormones:

  • chemicals made by cells in specific endocrine glands or other tissues
• Features of hormones:

  • transported in the blood to distant targets
• Features of Hormones:

  • bind to specific receptors
• Features of Hormones:

  • may act on multiple tissues
• Features of Hormones

  • alter activity of target cells
• Features of Hormones:

  • action must be terminated
• Features of Hormones

  • maintain homeostasis or precipitate change in many physiological processes
• How were hormones identified?

  • removed glands and observed results
  • replacing it and seeing if results are restored
  • replace extract from gland
  • give excess gland (or extract)
  • purify extract and test in biological assay
• cell-cell communication often involves the nervous system or the endocrine system
• endocrine system communicates via messengers known as hormones
• hormones produced in endocrine glands have various effects on target cells
• hydrophilic hormones (lipophobic)

  • water soluble, can dissolve in plasma
  • water soluble, can dissolve in plasma
  • not lipid soluble, cannot cross plasma membranes
  • synthesized in advance and stored
  • released by exocytosis
  • dissolved in the blood
  • ex. peptide hormones, protein hormones and catecholamines
• hydrophobic hormones (lipophilic)

  • not water soluble, do not dissolve in plasma
  • lipid soluble, readily cross plasma membrane
  • synthesis is on an on-demand basis
  • released by diffusion
  • bound to carrier proteins in the blood
  • ex. steroid and thyroid hormones
• Three main types of hormones

  • peptide/protein (3 or more AAs in chain)
  • steroid (derived from cholesterol)
  • amine (derived from single AA)
• majority of hormones are peptide hormones
• peptide hormones are stored in vesicles and released by exocytosis upon a signal
• • In hormones the first protein that is created to encode for the hormone usually has a bunch of signals on it. Happens in the ER via ribosomes, long chain of proteins has a signal sequence which is cleaved off. Cleaved into a pro-hormone which is pinched off the ER, further processed in the Golgi. Reaches secretory vesicles, where it is cleaved and creates the active hormone. Released by exocytosis into the bloodstream.
  • OG peptide goes through several cleavages and processing until it gets to the active form of the hormone
• Single pre-prohormone can contain:

  • one single long protein→ many copies of the same hormone
  • PreproTRH → cleaved to get 6 molecules of TRH
  • Gene will code for a prohormone that encodes for more than one type of hormoneActive peptides released depends on specific proteolytic processing enzymes
• Proinsulin

  • c peptide gets cleaved and active insulin is released
• steroid hormones all synthesized only from cholesterol
• steroid hormones

  • made on demand
  • not stored in vesicles
• peptide hormones have a short half life
• steroid hormones have long half life
• steroid hormones

  • diffuse into target cells or taken up by endocytosis of steroid hormone carrier proteins
  • cytoplasm or nucleus receptors (but can act on plasma membrane receptors)
• type of steroid hormone depends on which enzymes are present in the cell

  • cells have different enzymes that can take out or add on to the cholesterol molecule