Hormone Signalling steps:
1- Biosynthesis: by a particular tissue.
2- Storage and secretion: by exocytosis.
3- Transport: usually blood.
4- Recognition by target cell: by the cell membrane or intracellular receptor
5- Relay and Amplification of signal (by signal transduction): cellular response
6- Breakdown
Hormone classification:
Hormones are carried by blood stream, only certain cells (target cells) are affected by
hormones. Target cells usually have thousands of receptors for one particular hormone,
and some target cells have more than one type of receptor so they can be affected by
different hormones.
The response to the hormone is determined by the target cell: so different cells may
respond in different ways to the same hormone.
Protein and polypeptide hormones:
- Secreted from anterior and posterior pituitary, pancreas, and parathyroid glands.
- Stored in secretory vesicles.
- Released by exocytosis.
- Consist of chains of amino acids
- Mechanism: They bind to cell surface receptors (the receptor is usually
transmembrane protein with a part outside cell to bind with hormone and a part
inside cell that signals intracellularly). Binding of hormone activates intracellular
signaling (like cAMP and phosphoinositide) which cause cellular response.
- Insulin, growth hormone, Thyroid-stimulating hormone (TSH), Follicle-
stimulating hormone (FSH), luteinizing hormone (LH)
Steroid Hormones:
- Secreted from adrenal cortex, ovaries and placenta.
- From cholesterol and they are not stored.
- Mechanism: They diffuse into the cell and bind with intracellular receptors, forming
hormone-receptor complex. This complex enters the nucleus and regulate gene
expression by binding to specific DNA sequences. Causes synthesis of new proteins
and cell function changes.
- Estrogen, progesterone, testosterone, cortisol and aldosterone
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Amine Hormones:
- Secreted by thyroid and adrenal medulla.
- Derived from tyrosine (or tryptophan)
- Mechanism: varies depending on the hormone, catecholamines (ex. Adrenaline
and noradrenaline) act similar to peptide by binding to transmembrane receptors on
cell surface that activate signalling pathways. Although, Thyroid hormones (T3,T4)
act like steroids and enter the cell and bind to intracellular receptors.
- Catecholamines (epinephrine, norepinephrine), thyroid hormones
(triiodothyronine, thyroxine)
Gasotransmitters:
- Small, gaseous molecules that act as signaling molecules.
- Mechanism: diffuse freely into the cell and interact with intracellular enxymes or
ion channels. Can cause change to cell function, such as smooth muscle relaxation
or neurotransmitter release.
- Nitric oxide (NO), carbon monoxide (CO), Hydrogen sulfide (H 2S).
Eicosanoids:
- Lipid derivatives from arachidonic acid, can act as signaling molecules.
- Mechanism: Act locally within tissues and bind to receptors on cell surface. Results
in inflammation, immunity, and cell signaling.
- Prostaglandins, leukotrienes, thromboxanes.
- Act like hormones but do not travel.
Hormone transport:
Lipid-soluble hormones:
- Usually cause gene expression alteration (like steroids, thyroid hormones, hormonal
form of vit D)
- Detach from carrier in blood and enter target cell and bind to receptor inside the
cell.
- Hormone-receptor complex then acts as transcription factors and regulate
transcription of genes.
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Water-soluble hormones:
- Binding of hormone causes signal transduction and cause change in gene
expression.
- Epinephrine, binds to receptors on plasma membrane of liver cell, triggers release
of messenger molecule that activate enzymes and cause release of glucose in
blood.
Reactions of hormones:
- Hormones are effective in extremely small amounts (only a few molecules can
cause dramatic response in a target cell).
- Hormones help the target cell in three ways:
1- A hormone can increase the rate some substances enter or leave the
cell.
2- It can also stimulate the cell to produce enzymes, proteins or other
substances.
3- It can prompt the target cell to activate or suppress existing cellular
enzymes
- Hormones are like enzymes, they are not changed by the reaction they regulate.
Feedback control system:
A control system that monitors changes in the animal or external environment and sends
information to a control unit which makes adjustments.
Positive feedback:
- Reinforces the initial stimulus
- Rare in animals because they lead to instability or pathological states.
- Surges of hormones may occur.
Negative feedback:
- Conteracts the initial stimulus
- More common.
- Monitor amount of hormone secreted, maintain homeostasis.
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Lecture 2:
Hypothalamus:
- Controls release of hormones from anterior and posterior pituitary glands
- Regulates emotion, feeding/satiety, thirst, body temperature.
- Attached to roof of third ventricle near thalamus.
- Continuously receive information on status of body system via nerve impulses.
- Interpret messages and then sends messages via nerves or hormones.
- Role in feedback system.
Pituitary gland:
- Connected to the hypothalamus by infundibulum.
- Two parts:
Anterior pituitary (adenohypophysis):
- Larger part
- Portal system (two capillaries one vein) lies between the hypothalamus and anterior
pituitary.
- Hypothalamus controls the anterior pituitary by (hypothalmic-releasing and
hypothalmic-inhibiting hormones) which pass from hypothalamus to anterior
pituitary by portal system.
- Ex: The hypothalamus secretes Thyroid-releasing hormone (TRH) and Thyroid-
inhibiting hormone (TIH). The TRH stimulates anterior pituitary to secrete Thyroid-
stimulating hormone TSH, and TIH inhibits the pituitary from secreting TSH.
- Anterior pituitary release seven hormones:
1- Growth hormone GH
2- Prolactin hormone PH
3- Melanocytes-stimulating hormone MSH
4- Thyroid-stimulating hormone TSH
5- Adrenocorticotropic hormone ACTH
6- Follicle-stimulating hormone FSH
7- Luteinizing hormone LH
Posterior pituitary (Neurohypophysis)
- Oxytocin: stimulate smooth muscle contraction of uterus and mammary glands.
(controlled by positive feedback)
- Antidiuretic hormone: stimulates water reabsorption in collecting ducts, stimulate
vasoconstriction (vasopressin)
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Thyroid gland :
- Inferior to larynx, two lobes.
- Have two types of cells: follicular and parafollicular cells
- Follicular cells produce hormones and store them in follicles.
- Triiodothyronine T3 has three iodine atoms, while thyroxine has four.
- They increase metabolic rate, act on all cells of the body.
- Parafollicular cells (C-cells): produce Calcitonin (CT) which reduces blood calcium
by reducing absorption of calcium from bones.
Parathyroid gland:
- In the neck, 4 small glands
- Release Parathyroid hormone (PTH)
- Increase blood calcium by activating osteoclasts that release calcium from bones.
Adrenal gland:
- On top of kidney, two separate gland structures.
- Adrenal cortex: 3 zones make steroids
• Outer zone: mineralocorticoids (aldosterone)
• Middle zone: glucocorticoids (cortisol)
• Inner zone: androgens (testosterone)
- Adrenal medulla: produces epinephrine (adrenaline) and norepinephrine
(noradrenaline).
Pancreas:
- Fish shaped organ behind stomach near duodenum.
- Not under pituitary gland control.
- Two tissue: Exocrine: Produces and secrets digestive juices
Endocrine: pancreatic islets
- The islets, are the control center of blood glucose, insulin from Beta cells and
glucagon from Alpha cells
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Glucagon
- Low blood glucose stimulates glucagon release.
- Target tissues are the liver and adipose tissue.
- Glucagon stimulates liver to release glucose
- The adipose tissue cells breakdown fat to glycerol and fatty acid- used by liver for
glucose formation.
- Increases blood glucose.
Insulin
- High glucose levels stimulate insulin release.
- Insulin increases glucose transport into skeletal muscle and adipose cells.
- Decreased blood glucose.
- Insulin promotes amino acid uptake, protein synthesis, and lipid storage
- Autonomous nervous system also modulates hormone release.
Gonads:
- Testosterone in males
- Estrogen and progesterone in females, and Inhibin that inhibits FSH release.
Regulated by :
➢ GnRH from hypothalamus
➢ FSH + LH from anterior pituitary.
Pineal gland
- Small gland attached to roof of third ventricle of brain.
- Produces melatonin.
- Sets body’s biological clock.
- More released in darkness, less in sunlight.
-
Thymus gland
- The lobular thymus gland lies just beneath the sternum.
- Reaches its large size and is most active during childhood – with aging gets smaller
and become fatty
- Secretes thymosins- which stimulate T-lymphocyte production and maturation.
