hub2019f Endocrine system

neural siganlling | endocrine signalling

Short distance communication is done via [...], while long distance communication is done via [...]:

direct, autocrine (same cell), paracrine (local), endocrine and synaptic

The 5 types of intercellular communication include:

remain in the blood/cells they work on | remain in blood/cells they work on OR can enter the lumen of some organs

Endocrine hormones [...] while exocrine hormones [...]:

amine, peptide, protein, glycoprotein and steroid

The 5 types of hormones (water and lipid soluble) include:

tyrosine and tryptophan

Amine hormones are derived from 2 AAs:

thyroid, parathyroid and pituitary

Peptide hormones are made in three organs:

adrenal and gonads

Which 2 glands secrete lipid/steroid hormones:

T3 and T4 (3 and 4 iodine atoms)

Thyroid hormones are small and act as lipid-hormones, and include 2:

the cell membrane, in the cytoplasm and in the nucleus

The three places receptors are located include:

slower | faster

Cytosolic and nuclear hormones have a [...] response, while cell membrane hormones have a [...] response:

1. The hormone dissociates off of its carrier protein and enters the cell.
2. It binds to a receptor and forms the hormone-receptor protein complex.
3. It then translocates into the nucleus and binds to a segment of DNA called the hormone-response element at its half sites.
4. Two H-RPCs bind together on the H-RE sites and form a homodimer (the ligand domain to the hormone and the DNA domain to the DNA).
5. This then activates genes and leads to modified transcription.

The mechanism of steroid hormones has 5 steps:

1. Unbound thyroxine 4 (from TBG) and thyroxine 3 enter the cell, where T4 is converted to T3 (bound hormones act as a resevoir).
2. T3 binds to the TR receptor (ligand site) in the nucleus (after being transported via a seperate protein).
3. 9-cisRA binds to the RXR receptor (ligand site) in the nucleus.
4. The RXR and TR receptors form a heterodimer that at its DNA sites binds to the hormone-response element at the half-sites.
5. This then has an effect on gene expression and transcription.
Note: If there is no T3 present, co-repressors inhibit transcription which exists in a negative-loop with T3

The mechanism of thyroid hormones has 5 steps:

1. Hormones bind to cell membrane receptors that once bound activate a G protein.
2. The G protein activates adenylyl cyclase.
3. Adenylyl cyclase converts ATP to cAMP (a 2° messenger).
4. cAMP phosphorylates protein kinases (activation) which then phosphorylates (activates) other proteins.
5. These proteins go on to carry out cellular functions and pathways.

The mechanism for water-soluble hormones has 5 steps:

Downregulation (increase hormone levels ~ decrease receptor numbers; decrease in cell sensitivity)
Upregulation (decrease hormone levels ~ increase receptor numbers; increase in cell sensitivity)

Hormone receptor regulation is carried out in two ways:

1. The permissive effect (one hormone enhances/enables a second hormone; thyroid linkage to gonad hormones)
2. The synergistic effect (two hormones have similar effects/needed together and thus will create an amplified response; FSH and oestrogen)
3. The antagonistic effect (one hormone diminishes/disables a second hormone due to opposing effects; insulin vs glucagon)

The three types of interactions between hormones include:

Positive-feedback loops (an initial release of a hormone stimulates its further release; oxytocin during childbirth)
Negative-feedback loops (low levels stimulate an increase in its release | high levels disable its secretion; insulin or glucagon)

The two types of feed-back loops include:

1. Hormonal stimulus (most common and where one hormone is secreted because of aonother's presence)
2. Humoral stimulus (changes in fluids like blood and bile regulate hormone levels; insulin and glucagon, or ADH and DH)
3. Neural stimulus (when a nerve (sympathetic) stimulates a hormone's secretion)

The three types of endocrine reflexes include:

posterior (neurohypophysis) and anterior (adenohypophysis)

The pituitary gland is divided into two segments:

1. GH
2. FSH
3. LH
4. Prolactin
5. Thyroid-stimulating hormone
6. Adrenocorticotropic hormone

The 6 peptide hormones (trophic -> influence growth) secreted by the anterior pituitary are:

anabolic (protein synthesis and bone development) and catabolic (lipolysis of adipose tissue)

IGF-1 and 2 support the two types of activity that GH mediates:

1. Negative feedback loops (TRH -> TSH -> thyroxine ↯ TRH; GnRH -> LH and FSH -> sex steroids ↯ GnRH)
2. Positive feedback loops (LH -> estradiol (mentrual cycle) ↥ LH; later on in the cycle this becomes negative)
3. Short feedback loops (TSH can inhibit TRH or thyroxine inhibits TRH)

There are three feedback mechansisms of the hypothalamo-hypophyseal portal system (the hormone stimulants for the 6 anterior pituitary hormones):

1. pituitary-gonad axis (emotional responses)
2. pituitary-adrenal axis (stress)

There are two axes that involve the anterioir pituitary and target organs:

ADH (vasopressin) and oxytocin

There are two hormones fabricated in the hypothalamus and secreted in the posterior pituitary:

1. Production takes place in the paraventricular and supraoptic nuclei.
2. ADH and oxytocin travel down the axons to be stored in the neuronal terminals of the posterioir pituitary.
3. Neuroendocrine reflexes stimulate hormone secretion.

Posterior pituitary hormone producttion pathway: