Here is some facts on TSH, FSH, LH

I hope your studying for step 1 is going well.  Do study the First Aid.  The First Aid Q & A is an excellent source of questions to do while studying. 

First Aid for the USMLE Step 1 2011 (First Aid USMLE)

First Aid Q&A for the USMLE Step 1, Second Edition (First Aid USMLE)

I recommend making a realistic schedule and sticking to it.  You know yourself better so time yourself.  Have fun studying.  If you need a good Physiology book, I recommend:

Physiology: with STUDENT CONSULT Online Access (Costanzo Physiology).  Excellent book! 

Here is some facts on TSH, FSH, LH

• Glycoproteins ( with sugar moieties covalently linked to asparagines)
• Each is made up of an alpha and beta subunits (not covalently linked)
• Alpha subunits of TSH, FSH, LH are identical (synthesized from the same mRNA)
• Pairing of alpha and beta – in the ER and continues in the Golgi
• HCG – structurally similar to TSH, FSH and LH. 
• Glycoprotein with same alpha chain. 

Pituitary Gland

• 2 fused glands (anterior and posterior)
• Extends down from the brain – connected by thin stalk

Anterior Pituitary Gland (adenohypophysis)

• Epithelial origin – embryonic tissue that formed the roof of the mouth
• 6 hormones – all controlled by hypothalamus
• Prolactin, TSH, ACTH, GH, FSH, LH
• A.p. hormones control growth, metabolism, and reproduction
• Prolactin – milk production
• GH (somatotropin)
• Metabolism of many tissues
• Activates hormone production by liver
• FSH, LH – gonadotrophins (ovaries and testes)
• TSH (thyrotropin) – controls hormone synthesis + secretion in the thyroid gland
• ACTH (corticotrophin) – acts on adrenal cortex à cortisol

Amine Hormones

·         Derived either from tryptophan (melatonin), tyrosine (all other amine hormones – catecholamine + thyroid hormones)

o   Catecholamine à have 1 tyrosine molecule

o   Thyroid hormones – 2 tyrosine molecules + iodine atoms (thyroid hormones behave more like steroid hormones)

Cellular Mechanism of Action of Steroid Hormones

·         Destination – to the nucleus.  Activates transcriptional factor à binds to DNA and either activating or inhibiting one or more genes. 

·         Activation of genes à create new mRNA à new protein (genomic effect on the target cells)

·         Exception: Estrogen and aldosterone à have cell membrane receptors linked to signal transduction pathways (like peptide hormones).  These receptors enable those hormones to initiate rapid non-genomic responses in addition to their slower genomic effect. 

Neural Reflexes

Neural Reflexes:

1.       Efferent division that controls the effector

a.       Somatic motor neurons control sk. Muscle

b.      Autonomic neurons control smooth and cardiac muscle, glands, and adipose tissue

2.       Integrating region within the central nervous system

a.       Spinal reflexes don’t require input from the brain

b.      Cranial reflexes are integrated within the brain

3.       Time at which the reflex develops

a.       Innate (inborn) reflexes are genetically determined

b.      Learned reflexes are acquitted through experience

4.       The number of neurons in the reflex loop

a.       Monosynaptic reflexes have only two neurons: sensory – afferent and one efferent.  Only somatic motor reflexes can be monosynaptic

b.      Polysynaptic reflexes – one or more interneurons between the afferent and the efferent neurons.  All autonomic reflexes are polysynaptic because they have three neurons: one afferent and two efferent. 

Endocrine: Steroid Hormones - some facts for Step 1

·         Derived from Cholesterol

·         They all have similar structures

·         Only made in few organs for example

o   Adrenal cortex

o   Gonads produce sex hormones

Steroid Hormone Synthesis and Release

·         Cells that secrete steroid hormones have abundant SER

·         SER – location of steroid synthesis

·         Lipophilic and therefore easily diffuse across the cell membrane

·         Synthesized as needed EXCEPT for thyroid hormones

Transport in the blood & half life of steroid hormones

·         Lipophilic – not soluble in plasma

·         Most steroid hormones are bound to protein carrier molecules

o   Some have specific carriers – corticosteroid binding globulin

o   Others simply bind to general plasma protein – ie: albumin

o   Exception – adrenal androgens – no carrier proteins      

·         Half life of steroids are much longer than that of peptide hormones

·         There is always and equilibrium between bound hormones and unbound hormones.  So the ratio between the two in the plasma is constant.  Carriers release the hormone and they then simply diffuse across the cell.

·         Unbound (free) hormones are available to tissues

·         Free form creates the negative feedback à regulated!

3 hormones that increase blood glucose level are:

- Epinephrine
- Glucagon
- Cortisol

USMLE Step 1 - Endocrine: peptide hormones

Hello all fellow students,  

I will be posting some study material for USMLE Step 1 that I am preparing for myself.  I find them helpful.  I am starting with Endocrine: peptide hormones.  I hope you will find them helpful.  If you want any specific topics to be posted, please let me know.  I will see if I have notes for them.   

Hormones

Three types of classification

1)      Control – is it controlled by brain or not

2)      Receptors

a.       Tyrosine Kinase linked

b.      Intracellular receptors

c.       Others

3)      Chemical composition

a.       Peptide/protein

b.      Steroid – derived from cholesterol

c.       Amine hormones – derived from

                                                               i.      Tyrosine or

                                                             ii.      Tryptophan

Properties Peptide Hormones

1)      Made in advance

2)      Stored in secretory vesicles

3)      Released by exocytosis

4)      Transport in blood by dissolved in plasma

5)      Short half life

6)      Activation of second messenger systems

7)      Modification of existing proteins + induction of new protein sysnthesis

8)      Examples: insulin, parathyroid

9)      Water soluble

10)   No carrier protein is need for transport EXCEPT:  IGF (lipid soluble – need a protein carrier)

Peptide Hormone Synthesis, Storage, and Release

1)      mRNA binds to ribosome and links amino acids into a peptide chain èpreprohormone (which is directed to RER by a signalling sequence).

2)      Prohormone is produced in RER by è an enzyme cleaving the signalling sequence.

3)      Prohormone passes from RER to Golgi

4)      Secretory vesicle è enzymes + prohormone bud off Golgi

a.       In the secretory vesicle prohormone is cleaved è active peptide + peptide fragments

5)      Secretory vesicles releases the fragments into the ECF by excocytosis (Calcium dependent)

6)      Hormone is in circulation; heading for the target organ. 

·         Secretory vesicles containing peptides are stored in the cytoplasm of the endocrine cell until the cell receives a signal for secretion. 

Post Translational Modification of Prohormones

1)      Examples:

a.       Prohormone – TRH (thyrotropin releasing hormone) contains multiple copies of the hormone

b.      Pro-opiomelanocortin:  prohormone splits into 3 active peptides + 1 inactive fragment.

c.       Proinsulin = insulin + c peptide (clinical correlate:  measuring the levels of the C peptide in the blood of diabetics to monitor how much insulin the pancreas is producing). 

Cellular Mechanism of action of peptide hormones

Peptide hormones are:

1)      Lipophobic

2)      Unable to enter target cell

3)      Binds to surface receptor complex – initiates the cellular response by means of signal transduction system

4)      Response of cells t peptide hormones is rapid because of the second messenger system

5)      Changes triggered by peptide hormones include opening or closing membrane channels and modulating metabolic enzymes or transport proteins

6)      Some peptide hormones also have long lasting effects when the second messenger systems activate gene synthesis producing new proteins.