Week 3 Flashcards
Homeostasis
Process of maintaining a constant internal environment despite changing conditions
Dynamic steady state
Ions can have movement between ECF and ICF
Feedback control
Stimulus –> sensor/receptor/afferent pathway –> integrating centre –> efferent pathway –> target/effector organ –> response
Oscillation around a set point
In our bodies we have ranges. When the top limit of the range is reached, native feedback turns the response loop off. when the lower limit of the range is reached the response loop turns on.
Negative feedback
In a negative feedback loop, feedback reduces an excessive response and keeps a variable within the normal range. Examples of processes controlled by negative feedback include body temperature regulation and blood glucose control.
STABILIZING
Positive feedback
A positive feedback mechanism is a process that amplifies or increases a change or output.
Starting Point: Something happens in a system that creates a change.
Amplification: This change triggers a response that makes the change happen even more.
Cycle Continues: As the response increases the change, it can create a cycle that keeps going until something stops it.
REINFORCING
Example of negative feedback
Regulation of cortisol secretion
Negative feedback action of cortisol suppresses CRH and ACTH release, so we ultimately stop releasing cortisol.
Example of positive feedback
Oxytocin and the control of uterine contractions.
the baby drops lower in the uterus to initiate labor causing cervical stretch, this stimulates oxytocin release which causes uterine contractions further pushing the baby against the cervix, and the cycle continues as the cervix stretches more.
This cycle stops once the baby is delivered.
Gap junctions
They form direct cytoplasmic connections between adjacent cells, allowing small ions and molecules to move through
Local control - intercellular communication
Gap junctions, contact-dependent, autocrine
Contact dependent signals
they require interaction between membrane molecules on the two cells
Autocrine signals
They act on the same cell that secreted them
Paracrine signals
They are secreted by one cell and diffuse to adjacent cells
Long distance communication
1st major system - nervous system
2nd major system - endocrine system
neurotransmitters
Chemicals secreted by nerons that diffuse across a small gap to the target cell
neurohormones
chemicals released by neurons into the blood for action at distant targets
Simple reflexes
They are mediated either by the nervous or the endocrine system
hormones
they are secreted by endocrine glands or cells into the blood. only target cells with receptors for the hormone respond to the signal.
Complex reflexes
They are mediated by both the nervous and the endocrine system
exocrine system
secreted into a duct - outside
Sensor
Sensors can be specialized cells or structures that convert various stimuli into electrical signals eg. eye, nose, chemoreceptor, thermoreceptor OR cell membrane or intracellular receptor proteins
endocrine system
hormones secreted into the bloodstream
features of hormones
- can be made in different places in the body.
- chemicals made by cells in specific endocrine glands or other tissues
- transports in the blood to distant targets
- bind to specific receptors
- may act on multiple tissues
- alter activity of target cells
- action must be terminated
- maintain homeostasis or precipitate change in many physiological processes.
How were hormones identified?
- removing the gland and observing the results
- replacing the gland
- replacing extract from gland
give excess gland/ extract - purify extract and test in biological assay
Hydrophilic hormones
- Water soluble, can dissolve in plasma
- not lipid soluble (lipophobic), cannot cross plasma membranes
eg. peptide hormones, protein hormones and catecholamines
Hydrophobic hormones
- Not water soluble, do not dissolve in plasma
- Lipid soluble (lipophilic), readily cross plasma membrane
eg. steroid and thyroid hormones
Synthesis of hydrophilic and hydrophobic hormones
Hydrophilic: in advance, stored
Hydrophobic: on demand
Release of hydrophilic and hydrophobic hormones
Hydrophilic: exocytosis
Hydrophobic: Diffusion
Transport in blood of hydrophilic and hydrophobic hormones
Hydrophilic: dissolved
Hydrophobic: bound to carrier proteins
3 main types of hormones
- peptide/protein (hydrophilic)
- steroid (hydrophobic)
- amine - single amino acid (hydrophilic)
example pf steroid hormones
sex steroids (estrogen), cortisol
example of amine hormones
catecholamines (epinephrine), thyroxine
example of peptide hormones
insulin
Peptide hormones - most hormones
- they are linked amino acids.
- they are made in advance
and synthesized like secreted proteins. - stored in vesicles.
- release by exocytosis upon a signal
- water-soluble dissolved in plasma
- short half life in plasma
- bind to membrane receptors
Explain the synthesis, packaging and releasing of peptide hormones
Endoplasmic reticulum:
mRNA+ribosome+preprohormone+signal sequence
the signal sequence is cleaved off.
Transport vesicles:
prohormone
golgi complex:
prehormone moves into the Golgi
secretory vesicle:
the active hormone and peptide fragments split
there is a release signal and the hormone goes to the target
WHat can the preprohormone process to?
hormone, other peptides and signal sequence
What can prohormone process to?
active hormones and other peptide fragments
Proinsulin
insulin + C peptide
Steroid hormones
- Synthesized only from cholesterol
- Made on demand
- Not stored in vesicles
- Released from cell by simple diffusion
- Water insoluble (bound to carriers in blood)
- Long half life
- Diffuse into target cells or taken up by endocytosis of steroid hormone carrier proteins
- Cytoplasm or nucleus receptors (but can also
act on plasma membrane receptors)
Organs that can modify cholestrol
Adrenal cortex and ovries
Amine hormones
Synthesized from Tryptophan or tyrosine
Tryptophan derivative:
Melatonin (behaves like peptides or steroids)
Tyrosine derivatives:
Catecholamines (behave like peptides) –> epinephrine, norepinephrine, dopamine
Thyroid hormones (behave like steroids) –> thyroxine, triiodothyronine
Melatonin
Darkness hormone
Secreted at night (Sleep)
Made in pineal gland (also gi tract, leukocytes, other brain regions)
Diverse effects:
1. Transmits information (light-dark cycles to govern the biological clock)
2. Immune modulation
3. Anti-oxidant
Synthesis of catecholamines
- synthesized in adrenal medulla
(mainly in cytosol) - Stored in vesicles prior to release
- Released via exocytosis
- Lipophobic, water soluble
- Bind to membrane receptors
How do the stimuli trigger hormone release?
Act through intracellular pathways to:
* change the membrane potential
* increase free cytosolic Ca2+
* change enzymatic activity
* increase the transport of hormone substrates into the cell
* alter transcription of genes coding for hormones or enzymes needed for hormone synthesis
* promote survival and in some cases growth of the endocrine cell
Permissive effects
One hormone enhances the target organ’s response to a
second later hormone
Estrogen prepares uterus for action of progesterone
Synergistic effects
Multiple hormones act together for greater effect
Synergism between FSH and testosterone on sperm production
Ngetaive feedback loop
hyothalamus- anterior pituatary - peripheral endocrine gland
Antagonistic effects
One hormone opposes the action of another
Insulin lowers blood glucose and glucagon raises it
How do hormones signal?
- Hormone binds to receptor
- Changes the conformation and activity of the receptor
- Alters the activity of intracellular signaling pathways
- Leads to changes in synthesis of target proteins and
modification of existing target proteins
Properties of receptors
- High affinity
- Saturable
- Specific
- Reversible
What characteristics do receptors share?
- Large proteins
- Families
- Can be multiple receptors for one ligand or more than one ligand for a receptor
- Variable number in target cell (~500-100,000)
- Can be activated and inhibited
- Located in the cell membrane, cytoplasm, nucleus
Hormones and signaling
- Hormone binds to receptor
- Changes the conformation and activity of the receptor
- Alters the activity of intracellular signaling pathways
- Leads to change in the synthesis of target proteins (slow)
and modification of existing target proteins(fast).
Intracellular receptors (bind lipid soluble hormones)
Cytosolic and nuclear
Directly alter gene transcription = genomic effects
What are the two main types
of receptors?
Intracellular and plasma membrane receptors
Plasma membrane receptors
- G protein coupled receptors
- receptro enzyme receptors
- receptor channel
- integrin receptors
Mode of action of peptide hormones
- they cnanot pentrate target cell
- they bind to surface receptors and activate intracellular processes through second messengers.
Mode of action of steroid hormones
- pentrate plasma membrane and bind to internal receptors in nucleus
- influence expression of genes of target cell
