PHYSIOLOGY 1B Flashcards
The thyroid gland is located in front of and wrapped around the sides of what structure?
trachea
The thyroid hormones are transported around the body in the blood attached to …?
thyroxine-binding globulin
The thyroid gland function is regulated by a hierarchical control axis
hypothalamus and anterior pituatry gland
What is the name of the hormone secreted by the hypothalamus, which contributes to the hypothalamic-pituitary-thyroid axis?
thyrotropin-releasing hormone
The thyrotropin releasing hormone (TRH) then travels via the hypothalamic-pituitary portal vein to the anterior pituitary where it stimulates the release of what hormone?
thyroid stimulating hormone
The thyroid gland secretes thyroid hormones in response to stimulation from TSH. The secretion of thyroid hormones is then regulated by negative feedback. Which of the following are correct? (there may be more than one!)
Negative feedback onto the hypothalamus
Negative feedback onto the anterior pituitary
If for some reason there was an increase in the secretion of thyroid hormone, what would happen to the negative feedback by T3 and T4 on the hypothalamus and anterior pituitary?
there would be more
As a consequence, what would happen to the secretion of thyroid stimulating hormone (TSH)?
there would be less
negative feedback
The secretion of thyroid hormones is regulated by negative feedback loops onto the hypothalamus and anterior pituitary.
If thyroid hormone levels are too high, then there is more negative feedback. This will suppress the synthesis and secretion of thyrotropin releasing hormone (TRH) from the hypothalamus, suppress synthesis and secretion thyroid stimulating hormone (TSH)from the anterior pituitary, and consequently thyroid hormone secretion will also be suppressed.
Similarly, if there was a decrease in thyroid hormone levels, then the negative feedback would be less. This would allow more TRH secretion from the hypothalamus, more TSH from the anterior pituitary and so thyroid hormone secretion would also increase
To synthesize thyroid hormone, iodide must be accumulated by the thyroid follicular cells. This is achieved by which protein?
sodium iodide symporter
What is the approximate concentration of iodide inside the follicular cells of a normal thyroid gland compared to the circulating concentration of iodide?
20 times greater
Once inside the follicular cell, the higher concentration of iodide (20-times greater) will drive the movement of iodide across the apical membrane into the follicular lumen. This is facilitated by which TWO of the following membrane proteins?
Anoctamin
Pendrin
Upon entering the follicular lumen, the iodide takes part in a series of reactions that leads to the synthesis of thyroid hormone. Place these reactions in the correct order for the synthesis of thyroid hormone:
- oxidation of iodide to iodine
- iodination of tyrosine residue
- conjugation of adjacent tyrosine residue
What is the name of the enzyme that is responsible for the oxidation, iodination and conjugation reactions?
thyroperoxidase TPO
To secrete thyroid hormones, the thyroglobulin is first endocytosed from the follicular lumen. This process is stimulated by:
thyroid-stimulating hormone TSH
The endocytotic vesicle then fuses with a lysosome. The contents of the lysosome then enter the endocytotic vesicle and as a result, T3 and T4 are:
cleaved from the thyroglobulin
Thyroid hormone secretion
Secretion
The cleavage of T3 and T4 from the thyroglobulin by proteases is accompanied by degradation of thyroglobulin itself. The vesicle then breaks down, allowing T3 and T4 to be exported to the circulation via transport proteins in the basal membrane.
Thyroid stimulating hormone (TSH) binds to receptors on the basal membrane of follicular cells and initiates signalling pathways inside the follicular cell. In this way, TSH has three major effects on follicular cells. Which THREE from the list below are actions of TSH?
TSH increases:
Iodide uptake
Synthesis of thyroglobulin
Endocytosis of thyroglobulin
Major actions of thyroid stimulating hormone (TSH)
TSH increases:
iodide uptake
thyroglobulin synthesis
endocytosis of thyroglobulin for secretion of thyroid hormone
Which of the following characteristics are visible in the follicular cells from the thiouracil treated glands?
pseudopodia on apical surface;The follicular cells show signs of a ‘fuzzy’ apical surface, indicating the presence of pseudopodia.
evidence of endocytosis of colloid; There is evidence of vesicles in the cytoplasm that indicates there is endocytosis.
evidence of protein synthesis;The chromatin in the nucleus is dispersed, which indicates transcription and so protein synthesis is occurring in the cells.
. Follicular cells from normal thyroid glands show
less
of these features compared to thiouracil treated glands
less
Follicular cells from thyroxine treated thyroid glands show
less
of these features compared to thiouracil treated glands.
less
chemical messenger
chemical messenger can be classified according to their function or chemical structure
chemical messengers can be classified into 3 categories
1 paracrines
2neurotransmitters
3 hormones.
each type of messenger transmits a specific signal by binding to receptors on a target cell.
paracrines
are chemicals that communicate with neighbouring cells.
the target cells must be close enough that once the paracrine is secreted into the extracellular fluid, it can reach the target cell by simple diffusion. Paracrines generally include growth factor, clotting factor, and cytokines. Growth factor are proteins that stimulate proliferation and differentiation cells. For example, nerve growth factor is necessary for embryonic development of the nervou system and for maintenanace or regeneration of neurons; vacular endothelial growth factor stimulates proliferation of blood vessels.
clotting factors are proteins that stimulate formation of a blood clot.
cytokines are peptides; usually released from immune cells, their function is to coordinate the body’s defense against infections. although most cytokines function as paracrines, others travel in the bloodstream to distant target cells and function more like hormones.
histamine
is an exmaple of a paracrine chemical messenger that is important in allergic reactions and inflammation. It is secreted by mast cells scattered throughout the body. it is responsible for the runny nose, red, watery eyes associated with allergic reactions. occurring in response to bacterial infections and various forms of tissue damage, the release of histamine by mast cells is a part of a complex called inflammation. in inflammation, histamines increases blood flow to affected tissues and causes fluid to leak out of the blood vessels and into the tissue.
autocrine
are a subclass of paracrine that act on the same cell that secreted them. thus the seceretory cells is also the target cell. often an autocrine regulates its own secretion.
neurotransmitters
are chemicals released into interstitial fluid by neurons. these substances are released from a specialised portion of the neuron called the axon terminal, which is very close to the target cell. Because the juncture between the axon terminal and the target cell is called synapse, communications by neurotransmitters is often called synaptic signaling.
the cell that releases the neurotransmitter is called the presynaptic neuron, whereas the target cell is called the postsynaptic cell. upon release from the presynaptic neuron, the neurotransmitter quickly diffuses over the short-distance from the axon terminal and binds to receptors on the postsynaptic cell triggering a response.
communication between a neuron and its target cell is very specific because the neurotransmitter is directed only to cells with which the neuron has an anatomical association; the synapse. an example of neurotransmitter is acetylcholine, which is released by the neurons that triggers contraction of skeletal muscles.
hormones
are chemicals released from endocrine glands into the interstitial fluid, where they can diffuse into the blood. the hormone then travels in the blood to its target cells, which can be distant from the site of hormone release. only cells possessing receptors specific for the hormone are able to respond and thus serve as its target cell. an example of hormone is insulin, which is secreted by the pancreas and acts on target cells throughout the body to regulate energy metabolism.
neurohormones
are relased by a special class of neurons called neurosecretory cells through a mechanism similar to that of neurotransmitter release. an example of neurohormone is vasopressin, or antidiuretic hormone ADH, which is synthesised by neurosecretory cells originating in an area of the brain called the hypothalamus. Once vasopressin is released from the axon terminals of these neurosecretory cells, which are located in the posterior pituitary gland, it travels in the blood to its target cells. the primary target cells are located in the kidneys where vasopressin affects the volume of urine that is excreted.
the most important chemical characteristic
is whether the messenger can dissolve in plasma (primarly water) or cross lipid bilayer in plasma membrane of cells.
lipophilic (hydrophobic) molecules
are lipid soluble and therefore readily cross the plasma membrane but they do not dissolve in plasma.
lipophobic (hydrophilic)
molecules that are soluble in water and therefore dissolve in plasma but do not cross the plasma membrane.
the chemical classifications of chemical messengers are
1 amino acids
2 peptides/proteins
3 steroids
4 eicosanoids
5 amines
amino acid messengers ( neurotransmitter)
glutamate, aspartate, glycine, and gamma-aminobutyric acid (GABA) are amino acids that are classified as chemical messengers becuase they function as neurotransmitters in the brain and spinal cord. Glutamate, aspartate, and glycine are among the 20 alpha amino acids, that are used in protein synthesis. Whereas GABA belongs to gamma amino acids. amino acids are lipophobic.