on 04-Nov-2016 (Fri)

ACh is the only neurotransmitter that is utilized at the neuromuscular junction.

In the nucleus, the gene for the hormone is tran- scribed into an mRNA. Generally, a single gene is responsible for directing the primary structure of each peptide hormone. (Because the genes for almost all peptide hormones have been cloned, recombinant DNA technology makes it possible to synthesize human peptide hormones.) 2. The hormone mRNA is transferred to the cytoplasm and translated on the ribosomes to the first protein pro- duct, a preprohormone. Translation of the mRNA begins with a signal peptide at the N terminus. Translation ceases, and the signal peptide attaches to receptors on the endoplasmic reticulum via “docking proteins.” Translation then continues on the endoplasmic reticulum until the entire peptide sequence is produced (i.e., the preprohormone). 3. The signal peptide is removed in the endoplasmic reticulum, converting the preprohormone to a prohormone. The prohormone contains the complete hormone sequence plus other peptide sequences, which will be removed in a final step. Some of the “other” peptide sequences in the pro- hormone are necessary for proper folding of the hormone (e.g., formation of intramolecular linkages). 4. The prohormone is transferred to the Golgi appa- ratus, where it is packaged in secretory vesicles. In the secretory vesicles, proteolytic enzymes cleave peptide sequences from the prohormone to produce the final hormone. Other functions of the Golgi apparatus include glycosylation and phosphoryla- tion of the hormone. 5. The final hormone is stored in secretory vesicles until the endocrine cell is stimulated. For example, parathyroid hormone (PTH) is synthesized and stored in vesicles in the chief cells of the parathyroid gland. The stimulus for secretion of PTH is low extracellular calcium (Ca 2+ ) concentration. When sensors on the parathyroid gland detect a low extra- cellular Ca 2+ concentration, the secretory vesicles are translocated to the cell membrane, where they extrude PTH into the blood by exocytosis. The other constituents of the secretory vesicles, including

The amine hormones are catecholamines and thyroid hormones

Feedback mechanisms are more common than neural mechanisms

Temperatura odczuwalna

M₂ muscarinic receptors act via a G_i type receptor, which causes a decrease in cAMP in the cell, generally leading to inhibitory-type effects. They appear to serve as autoreceptors.^[10]

In addition, they modulate muscarinic potassium channels.^[11][12] In the heart, this contributes to a decreased heart rate. They do so by the G beta gamma subunit of the G protein coupled to M₂. This part of the G protein can open K⁺ channels in the parasympathetic notches in the heart, which causes an outward current of potassium, which slows down the heart rate.

The M₂ muscarinic receptors are located in the heart

ACh is released from all preganglionic and most postganglionic neurons in the parasympathetic nervous system.

In the nucleus, the gene for the hormone is tran- scribed into an mRNA

he hormone mRNA is transferred to the cytoplasm and translated on the ribosomes to the first protein pro- duct, a preprohormone

The hormone mRNA is transferred to the cytoplasm and translated on the ribosomes to the first protein pro- duct, a preprohormone

According to Bernard, animals have two environments: the milieu extérieur that physically surrounds the whole organ- ism; and the milieu intérieur, in which the tissues and cells of the organism live

milleu interiuer = the well-controlled liquid environment

At still higher concentrations, phospholipids spontaneously form bilayers

In phospholipid bilayers, the phospholipid molecules arrange themselves into two parallel sheets or leaflets that face each other tail to tail

An individual phospholipid molecule is free to diffuse within the entire layer of the membrane where it resides

The rate at which this two-dimensional diffusion occurs is extremely temperature dependent. At high temperatures, the thermal energy of any given lipid molecule is greater than the interaction energy that would tend to hold adjacent lipid molecules together. Under these con- ditions, lateral diffusion can proceed rapidly, and the lipid is said to be in the sol state. At lower temperatures, interac- tion energies exceed the thermal energies of most individual molecules. Thus, phospholipids diffuse slowly because they lack the energy to free themselves from the embraces of their neighbors. This behavior is characteristic of the gel state

The temperature at which the bilayer membrane converts from the gel to the sol phase (and vice versa) is referred to as the transition temperature.

The transition temperature is another characteristic that depends on the chemical makeup of the phospholipids in the bilayer. Phospholipids with long, saturated fatty acid chains can extensively interact with one another. Consequently, a fair amount of thermal energy is required to overcome these interactions and permit diffusion. Not surprisingly, such bilayers have relatively high transition temperatures. For example, the transition tem- perature for dioctadecanoic phosphatidylcholine (which has two 18-carbon fatty acid chains, fully saturated) is 55.5°C. In contrast, phospholipids that have shorter fatty acid chains or double bonds (which introduce kinks) cannot line up next to each other as well and hence do not interact as well. Con- siderably less energy is required to induce them to partici- pate in diffusion. For example, if we reduce the length of the carbon chain from 18 to 14, the transition temperature falls to 23°C. If we retain 18 carbons but introduce a single, double bond (making the fatty acid chains monounsaturated), the transition temperature also falls dramatically

The glycerol-based phospholipids, the most common membrane lipids, include the phosphatidyletha- nolamines described earlier (Fig. 2-1A) as well as the phos- phatidylinositols (Fig. 2-2A), phosphatidylserines (Fig. 2-2B), and phosphatidylcholines

sphingolipids (deriva- tives of sphingosine), are made up of three subgroups: sphin- gomyelins (Fig. 2-2D), glycosphingolipids such as the galactocerebrosides (Fig. 2-2E), and gangliosides

Cholesterol molecule rigid steroid ring binds to and partially immobilizes fatty acid side chains.

Therefore, at modest con- centrations, cholesterol decreases fl uidity. However, when it is present in high concentrations, cholesterol can substan- tially disrupt the ability of the phospholipids to interact among themselves, which increases fl uidity and lowers the gel-sol transition temperature.