3. Principles of Endocrinology Flashcards
Overall Function Of The Endocrine System
regulates internal environment in response to external
controls activities that require duration (instead of speed) - e.g. nervous system
regulates, coordinate and integrates cellular and organ function at a distance
Examples of the Endocrine System
- regulates metabolism
- regulates water and electrolyte balance
- induces adaptive changes to help the body cope with stressful situations
- promotes smooth, sequential growth and development
- controls reproduction
- Regulates red blood cell production
- Together with the autonomic nervous system, it controls and integrates both circulation and the digestion and absorption of food
Chemical Messengers - 3 types
- Paracrines
- Neurotransmitters
- Hormones
[when released into the interstitial fluid, each of these transmits a signal by binding receptors to a target cell]
Paracrines
Chemicals that communicate with neighbouring cells
Target cell must be close enough that once paracrine is secreted into the extracellular fluid, it can reach target cell by SIMPLE DIFFUSION
- includes growth factors, clotting factors, cytokines
Neurotransmitters - The electrical signal
Neurons transmit both electrical signals within the cell and chemical signals between cells
Neurons communicate across long distances
The initial signal is electrical and is transmitted along the axon to the axon terminal
Here the electrical signal initiates a chemical signal
Neurotransmitters - The Chemical Signal
Released from a specialised portion of the (presynaptic) neuron called the axon terminal
axon terminal is v close to the target cell
juncture is called a synapse
neurotransmitter (e.g. acetylcholine) is released from the axon terminal and quickly diffuses across the synaptic cleft (chemical signal)
the neurotransmitter binds to its receptor on the postsynaptic cell, triggering a response
how do hormones carry out their function
Secreted by endocrine cells into the interstitial fluid
Hormones diffuse into the bloodstream for transport to target cells in the body
Target cells are identified by receptors for the specific hormone
(Cells without receptors cannot response to a hormone’s signal)
Hormones are chemical substances that are…
- synthesised by a specific cell type
- secreted by endocrine glands
- transported in the circulation (of blood)
- present in v low concentrations (10^-6 to 10^-12)
- elicit a specific response in target tissues
- are responsible for homeostatic chemical adjustments
Hormone Classification
- classified based on solubility propterities - hydrophilic or lipopilic.
- hormones within each group are further classified to biochemical structure and/or source
Hydrophilic Hormones
- highly water soluble
- low lipid solubility
- are stored in large amounts in secretary granules
- transported free in the blood
- target receptors are on the cell surface
- produce effect by either:
- changing the configuration of a protein channel
- activating a 2nd messaging system to alter activity of a pre existing protein that produces the effect - signal transduction
Signal transduction
- transfer of the signal carried by the extracellular messenger into the cell for execution
- the binding of the extracellular chemical messenger that cannot gain entry into cell, (e.g. protein hormone - 1st messenger) to a membrane receptor
- that triggers a cascade of cellular responses by two major methods
- opening or closing specific channels
- activating an intracellular messenger (2nd messenger)
- acts to amplify 1st message by cascade
- activation of 2nd messenger system alters the activity of pre-existing intracellular proteins (enzymes) to produce desired effect
Signal transduction - example
Lipophilic Hormones
- highly lipid soluble
- low water solubility
- are transported in the blood bound to plasma proteins
- target receptors are inside the target
- effects are produced through activation of specific genes to synthesise new proteins (that produce the effect)
Lipophilic Hormones extended example
- a lipophilic hormone diffuses through the plasma and nuclear membranes of its target cells and binds w a specific nuclear receptor
- the hormone receptor complex in turn binds with the hormone response element, a segment of DNA specific for the hormone receptor complex
- DNA binding activates specific genets produce complimentary RNA
- mRNA leave the nucleus
- in cytoplasm, mRNA directs synthesis of new proteins
- new proteins, either enzymatic or structural, accomplish the target cell’s ultimate physiological response to the hormone
Chemical Classification of Hormones
- a lipophilic hormone diffuses through the plasma and nuclear membranes of its target cells and binds w a nuclear receptor specific for it
- the hormone receptor complex in turn binds with the hormone response element, a segment of DNA specific for the hormone receptor complex
- DNA binding activates specific genets produce complimentary RNA
- mRNA leave the nucleus
- in cytoplasm, mRNA directs synthesis of new proteins
- new proteins, either enzymatic or structural, accomplish the target cell’s ultimate physiological response to the hormone
Chemical classification of hormones
Plasma Hormone Concentration
- primary function of most hormones is regulation of homeostatic activities
- hormones’ effects are proportional to their concentrations in plasma
- concentrations in plasma are subject to control according to homeostatic need
Factors affecting the plasma concentration of hormones (4 steps)
plasma conc. of a free, biologically active hormone to its target cells to produce a physiological response, depends on
- the hormone’s rate of secretion by the endocrine gland - major factor
- rate of metabolic activation (not all hormones)
- extent of binding to plasma proteins - lipophilic hormones
- rate of metabolic inactivation and excretion
Controlling Hormone Secretion [3]
3 mechanisms to help regulate the plasma concentration of hormones
- Negative feedback control
- neuroendocrine reflexes
- diurnal and circadian rhythm’s
Negative feedback control
- is a prominent feature of hormonal control systems
- exists when the output of a system counteracts a change in input, maintaining a control variable within a narrow range around a set level
- maintains the plasma concentration of a hormone at a given level/set point
Neuroendocrine Reflexes
- includes neural as well as hormonal components
- purpose of such reflexes is to produce a sudden increase in hormone secretion in response to a specific stimulus
- an example is the increased secretion of cortisol during a stress response
The suckling reflex
- a neuroendocrine reflex
- suckling stimulates the release of oxytocin and prolactin
- oxytocin causes the smooth muscle in the breast to contract and milk is ejected
- prolactin causes milk to be secreted
comparison of neural, neuroendocrine and endocrine reflexes
Diurnal Rhythms
Set points
- Negative feedback control mechanisms operate to maintain whatever set point is established for a given physiological parameter for that time of day and environment - Internal and external
- varies from person to person and over time (biorhythms)
- factors that impact are genetics and acclimatisation to the environmental (inherent vs acquired)
Endocrine Dysfunction
2 common causes for abnormalities in hormone plasma concentration
Hyposecretion
- primary hyposecretion - too little hormone is secreted due to abnormality in gland ——causes: genetic, dietary, chemical or toxic, immunologic, and other diseases such as cancer etc
- secondary hyposecretion - gland is normal but too little hormone is secreted due to deficiency of its tropic hormone
Hypersecretion
- primary hypersecretion - too much hormone is secreted due to abnormality in gland
- secondary hypersecretion - excessive stimulation from outside the gland causes over secretion
causes: tumours ignore normal regulatory input and continuously secrete excess hormone, immunologic factors
General principles of endocrinology [3]
- Hormones can influence activity of other hormones at given target cell in one of 3 ways
- permissiveness - one hormone must be present in adequate amounts for full exertion of another hormone’s effect
- synergism - when several hormones are complimentary and combined effect is greater than the sum of their separate effects
- antagonism - when 1 hormone causes loss of another hormone’s receptors and it reduces effectiveness of second hormone
Permissiveness example
Synergism Example
Antagonism Example
Tropic Hormones
- Regulate hormone secretion by another endocrine gland, rather than having direct effect on the target cell
- stimulate and maintain their endocrine target tissues
- example: thyroid-stimulating hormone (TSH) secreted from anterior pituitary stimulates thyroid hormone secretion by thyroid gland
- also maintains structural integrity of thyroid gland
- example: thyroid-stimulating hormone (TSH) secreted from anterior pituitary stimulates thyroid hormone secretion by thyroid gland
Complexity of Endocrine Function
- a single endocrine gland may produce multiple hormones
- a single hormone may be secreted by more than one endocrine gland and may have more than 1 type of target cell and induce more than one type of effect
- the same chemical messenger may be a hormone or neurotransmitter e.g. norepinephrine
- the rate of secretion of some hormones varies considerably over time, in a cyclical pattern - to provide temporal coordination of function e.g. menstrual cycle
- some organs are exclusively endocrine in function (they specialise in hormone secretion alone) whereas other organs of the endocrine system perform non-endocrine function and secrete e.g. testes
