Endocrine system I Flashcards
Analysis of body composition
- 6%: minerals, hormones, vitamins, neurotransmitters
- 60%: liquid
- 18%: protein
- 16%: fat
Intercellular communication
- despite the vast extent of the communication network requires, there are only a few mechnisms for communication
Direct communication
via gap junctions or direct linkup of cell surface markers
Indirect communication
via an assortment of chemical messengers or signal molecules
e.g. paracrine, neurotransmitters, hormones, neurohormones
Direct communication via gap junctions
- gap junctions link adjacent cells together
- connexons form channels that link the cytosol of adjacent cells permitting transfer of ions between the two cells
- e.g. where gap junctions are crucial to survival is in the heart
Direct linkup of cell surface markers
- complementary surface markers = surface receptors
- e.g. to activate an immune cell
Indirect chemical messengers or signal molecules
- secretory cell released chemical messenger into EFC
- messenger binds to specfic receptor on the ‘target’ cell
- binding of messenger to receptor triggers a response in the target cell
Paracrines (local)
e.g. (3)
- local chemical messengers
- exert effect only on neighbouring cells in immediate environment of secreiton site - secreted by once cell and diffuse to nearby target cell
Histamine: released in damaged tissue and causes inflammatory response
Epidermal growth factor: stimulates cell division
Vascular endothelial growth factor: stimulates growth and branching of blood vessels
Autocrines (self)
e.g. (1)
- bind to receptors and exert their effects on the same cell that secreted them
- often autocrines also function as a paracrine or other messenger
e. g. growth factor
Cytokines
- secreted by cells of the immune system
- e.g. interleukins and interferons = groups of small proteins released from white blood cells
Neurotransmitters
E.g. (3)
- released from neurons to axon terminal
- short range chemical messengers
- diffuse across narrow space to act locally on adjoining target cells = synpatic signalling (another neuron, a muscle or a gland)
- released from axon terminals of nerves
- E.g. acetylcholine: involved in triggering contractions of skeletal muscle
Dopamine: precursor of norepinephrine
Serotonin: especially active in constricint smooth muscles, contributing to wellbeing and happiness
Hormones
- Long range chemical messengers
- secreted by endocrine glands into the blood
- exert effect on target cells some distance away from release sight
Endocrinology
study of hormones and endocrine organs
Hormones II
- represent a very small percentage of the body
- effects exerted at very low concentrations
- chemical messengers secreted directly into the blood
- circulatory syste, transports them to distant target organs
- hormones bind to cell receptors and trigger a response
- same hormone can cause different responses in different cell types
- acts with nervous system to coordinate and integrate activity of body cells
- responses slower but longer lasting than nervous system responses
Endorcrine system controls and integrates
- maintenance of electrolyte, water and nutrient balance of blood. e.g. in kidney
- regulation of cellular metabolism and energy balance e.g. insulin
- growith and development e.g. growth hormone
- mobilisation of body defences e.g. histamine
- reproduction e.g. testosterone
Exocrine glands
- produce non-hormonal substances
- have ducts to carry secretion to membrane surface
- e.g. sebaceous glands, salivary glands
Endocrine glands
- typically have a rich vascular and lymphatic drainage for rapid dispersal of hormones throughout body
- produce hormones
- lack ducts (secretion into blood)
- e.g. pituitary, thyroid gland
Example of endocrine and hormones
- Pituitary gland: FSH, LH, prolactin, GH
- Thyroid gland: thyroid hormones, calcitonin
- Parathyroid gland: parathyroid hormones
- adrenal glands: aldosterone, cortisol, epinephrine, norepinephrine
- pineal glands: melatonin
- gonads: ovaries and testes: progesterone, oestrogen, testosterone
Combines exocrine and endocrine
- hypothalamus is neuroendocrine organ
- some have exocrine and endocrine functions (pancreas, gonads, placenta)
- other tissues and organs that produce hormones
- adipose cells, thymus and cell inwalls of small intestine, stomach, kindneys and heart
Mechanism of hormone action
- for hormones to exert their effects, they must bind to a specifc receptor
- target cell receptors are located outside the cell in/ on the cell membrane, in the cytosol or in the nucleus
- receptors bind ligands (hormoens) -> then translate ‘message’ of signal ligand into a cellular response = cell signalling
Cell signalling usually involves
- activation of transcriptio factors in the nucleus of the cell -> increased production of proteins and other substances
- activate or deactivate enzymes
- induce secretory activity
- stimulate mitosis
Receptors and their regulation
- the amount of receptors on target tissues is not static
- amount of hormones can influence number of receptors for that hormone
Upregulation of receptors
Number of receptors increases on the target cell when hormone levels are low
Down regulation of receptors
Number of receptors decreases on the target cell when hormone levels are high
de-sensitised the target cells to prevent them from overreacting to persisttently high levels of hormones
Hormone release
- blood levels of hormones
- controlled by negative feedback systems
- increased hormone effects on target organs can inhibit further release
- levels vary only within narrow, desirable range
Hormone release is triggered by
Endocrine gland stimuli
Nervous system modulation
Control of endocrine activity
When hormone concentrations are either too high or too low -> disease -> precise control circulating concentraton of hormones is crucial
the concentration of hormone as seen by target cells is determined by 3 factors
Rate of production
- synthesis and secretion of hormones are the most highly regulated aspect of endocrine control
- control mediated by positive and negative feedback circuits
Rate of delivery
an example of this effects if blood flow to a target organ or group of target cells - high flow delivers more hormone than low blood flow
Rate of degradation and elimination
hormones, like all biomolecules, have characteristics rates of decay -> metabolised and excreted from the body through several routes
Shutting off secretion of a hormone that has a very short half-life causes circulating hormone concentration to plummet
- if a hormone’s biological half-life is long, effective concentrations persist for some time after secretion ceases
Mechanisem of hormone action
- in most cases, cells release chemical messnegers to casue an effect on a nearby target cell
- hormones have the advantage of being transported in the blood
- secretion of hormones can occur in two ways bases on their solubility properties
Dissolved in the blood (hydrophilic messenger, highly water soluble, low lipid solubility)
Attached to a carrer protein (hydrophobic messenger, high lipid solubility, low water solubility)
Mechanisms of hormone action
- lipophilic hormones diffuse across cell membrane and bind to cytosolic or nuclear receptors
- lipophobic hormones bind to cell membrane receptor
- binding of hormone to intracellular receptor leads to hormone response element int the mucleus, altering gene expression
Surface receptors
- binding of extracellular messenger to matching receptor leads to response by:
1. opening or closing of channels
2. activating receptor enzymes
3. activating second messenger systems
the 3 types of cell surface receptors
- different types of cell surface receptors can result in different outcomes:
1. channel linked receptors
2. enzyme linked receptors
3. G-protein linked receptors
Opening or closing of channels
- channel linked receptors
- involved in rapid signalling/electrical transmission
- neurons have a cation linked receptor
Activating receptor enzymes
- enzyme linked receptors
- most common are tyrosine kinases
- hormones and growth factors bind to receptor, causing intracellular signalling to occur (e.g. insulin)
Activating second messenger systems
- activation of second messenger pathway via G protein coupled receptors
- G protein linked receptors
- odorant molecules, hormones and neurotransmitters
- cause cAMP signalling or PI singalling (e.g. epinephrine, ADH)
Amplication of signal
very little messenger necessary to bind target ce;; and cause large effect
The downsife of G proteins - Cholera
- affects the small intestine and causes violent diarrhoea
- transmitted by contaminates water/food
- caused by bacterium Vibrio cholera
- secretion of chlorea toxin - choleragen (complex)
- results in the activation of chloride channel proteins, which leads to ATP- mediated efflux of chloride ions and leads to secretion of H2O, Na+, K+, HCO3- into the intestinal lumen
- entry of Na+ and consequently the entry of water into enterocytes are diminished
