Human Physiology Week 2 Notes
Human Physiology Week 2 Notes BISC 3122
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This 8 page Class Notes was uploaded by PhenomenalNotetaker on Sunday January 31, 2016. The Class Notes belongs to BISC 3122 at George Washington University taught by Dr. Packer in Fall 2016. Since its upload, it has received 56 views. For similar materials see Human Physiology in Biology at George Washington University.
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Date Created: 01/31/16
Human Physiology Lecture #2 : Cell-to-cell communication In order for homeostasis to go on smoothly, cells have to communicate with each other. The two main communication systems in the body are endocrine and nervous systems. Both signals rely on chemical signaling but differ in that while nerve cells communicate over shorter distances (synaptic cleft), endocrine glands are able to secrete signal substances that can travel longer distances to where they are needed. Thus, ‘distance of travel’ is the major difference between them. Some examples as shown above are: Gap junctions: these enable cells next door to each other to communicate. Contact-dependent signals: these are common in immune systems. Autocrine/Paracrine signals: these are chemicals released that can act on receptors on the same cell (autocrine) or on another cell (paracrine). They travel within the tissue and communicate through interstitial fluid. Long distance cell-signaling cellcell chemical and/or electrical as shown above. Chemical or electrical signals are secreted into the interstitial fluid then to the blood then to target cells. These signals have access to all cells but only those with the appropriate receptors respond. * Neurotransmitters and neurohormones can be seen as special types of paracrine signals. Signal Amplification Signals are very small but when they meet a receptor, their effect is greatly amplified. Strength of amplification depends on the answers to these questions; is the receptor present? How many receptors are present? Signal Transduction pathway Components of the pathway: - Receptors can bind the ligand, an agonist or an antagonist - Certain ligands can bind more than 1 type of receptor (Ex. Epinephrine binds Alpha and Beta Receptors) - The number of receptors can be up-regulated or down-regulated *A ligand is a molecule that binds to another molecule to serve a biological purpose so for example the neurotransmitters involved in communication in nervous systems. There are 5 categories of cell signaling pathways Some rely on cell membrane receptors for ligands and other for intracellular 1. Ligand-gated ion channels Ligands are always in solution. These ligands bind these said channels as shown in the picture and activate the channel. These protein channels are usually closed until a ligand binds to and activates them then they open to let in the ions e.g. Cl , Na etc. A typical example would be the neurons that have nicotinic acetylcholine (ACh) receptors. 2. Tyrosine kinase – a receptor enzyme Initially the alpha helices (with tyrosines) are inactive and exist as monomers. When the ligand binds, the monomers form a dimer and the tyrosines are phosphorylated (phosphate group is attached to them) with the help of ATP. The channel is then fully activated and can activate relay/effector proteins which in turn promote cellular responses. A typical example is the communication between cell membranes and insulin and other growth factors. 3. G- Protein coupled receptors – this is the most common. - This is a much slower process however the channel stays open longer than when the ligand has direct control of the channel. - The channel is a G-Protein Coupled Receptor (GPCR). The 3 subunits (alpha, beta and gamma) bound to it are called G proteins. - Inactive G proteins (usually the alpha subunit) are bound to GDP - Binding to GTP activates the G protein (activation can result in stimulatory or inhibitory pathway activation) - When GTP binds and the G protein is activated, it does 1 of 2 things: - (A) Open or close an ion channel - (B) Regulate the activity of an amplifier enzyme bound to the membrane - Phosphodiesterases dephosphorylate phosphorylated proteins to release an inorganic phosphate group (Pi) and in this case, reverts the subunit to its inactive form. 4. Contact dependent integrin receptors - This is the most short range of them all - They involve large proteins in the cell membrane attached to cellular cytoskeleton. As a result, they can cause a change in conformation. - It typically does not require the release of a secreted molecule - In contact-dependent signaling, transduction is completed when the signaling molecule anchored in the phospholipid membrane of the signaling cell binds to the receptor molecule embedded in the phospholipid membrane of the target cell. - Contact-dependent signaling can also occur in the form of a cell interacting with the extra cellular matrix (ECM). An example of a receptor involved in this is integrin. * Integrins are cell surface receptors that interact with the extracellular matrix. They mediate intracellular signals in response to the extracellular matrix including cellular shape, mobility, and progression through the cell cycle. (In class, this pathway was centered on this example with integrins so I would focus on this. I added the rest of the points above it just to give an idea of what the pathway was about). 5. Steroids - All steroids are derived from cholesterol - They are all built from the organic backbone below - They are all fairly hydrophobic - They are amphipathic; they have both non-polar and polar regions. - They have a lot of polar regions and thus can cross a good number of pathways easily. - Very often, steroids (e.g. estrogen) travel in blood while bound loosely to plasma proteins - Transport via this method is slower but it is quite powerful Below is a diagrammatic summary of the pathways discussed. Tonic and antagonistic control systems Tonic control - Regulate by changing frequency of signals - For instance in the above example, if there are only a few signals coming (i.e. the rate of firing is at an all time low), the smooth muscle won’t contract as much. - The reverse however leads to a more contracted smooth muscle. This in turn leads to an increase in blood pressure and thus vasoconstriction. Blood flow to skin is diminished leading to pale skin. - The above process is exemplified in epinephrine/norepinephrine action. That is why you hear the saying “pale with fear”. Antagonistic control - Recognize the electrocardiogram on the side? (C’mon it’s in every hospital scene when the patient’s heart stops/starts). - A sympathetic neuron, when active, releases norepinephrine which causes the heart rate to rise, (and also vasoconstriction). It’s obviously the purple neuron in the image. - A parasympathetic neuron, when active, releases acetylcholine which decreases the heart rate. It’s the blue neuron in the image. *In essence, they are acting antagonistically to control heart rate.
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