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Upon completion of this lecture, students should be able to:1. Compare the three major functional classes of chemical messengers with respect to the cell that releases them and the ways the messenger reaches the target cell.
* Paracrines: Released by a source cell in response to a stimulus; transmit signals to neighboring cells within the same tissue or local vicinity.
* Neurotransmitters: Produced and released by neurons into the synaptic cleft; transmit signals between nerve cells or from nerves to target cells (e.g., skeletal muscles).
* Hormones (Endocrines): Produced by specialized endocrine cells; secreted into the bloodstream via interstitial fluid, allowing transport to distant target cells throughout the body.2. Describe the basic function of each chemical class of messenger concerning release, transport, and signal transduction.
* Amino Acids: Function exclusively as neurotransmitters (e.g., glutamate, aspartate, glycine, GABA). They are lipophobic, bind to cell membrane receptors, and are released into the synaptic cleft for local communication.
* Amines: Function as paracrines, neurotransmitters, or hormones (e.g., catecholamines like dopamine, norepinephrine, epinephrine). Mostly lipophobic and bind to cell membrane receptors. Thyroid hormones are an exception, being lipophilic and binding intracellularly. Their transport varies based on function (local, synaptic, or bloodstream).
* Peptides/Proteins: The most common type of chemical messengers. Function as paracrines, neurotransmitters, or hormones. They are lipophobic and bind to cell membrane receptors. Their transport varies based on function.
* Steroids: Primarily function as hormones. They are lipophilic and easily diffuse across cell membranes to bind to intracellular receptors (in the cytoplasm or nucleus), typically influencing gene expression and protein synthesis. Transported via the bloodstream.
* Eicosanoids: Mostly function as paracrines. They are lipophilic and act inside the target cell cytosol, released locally and diffusing to neighboring cells.3. Compare and contrast the signal transduction mechanisms for lipophilic and lipophobic messengers.
* Lipophobic (Hydrophilic) Ligands: Cannot easily cross the cell membrane and require specific pumps, channels, or transporters. They bind to receptors located on the cell membrane surface. Their general actions often trigger enzyme activation or alter membrane ion permeability.
* Lipophilic (Hydrophobic) Ligands: Easily diffuse across cell membranes due to their lipid solubility. They bind to intracellular receptors, which are found in the cytoplasm or nucleus of the target cell. Their general actions typically influence gene expression and protein synthesis.4. Describe the following functional classes of receptors: channel-linked receptors, enzyme-linked receptors, and G protein-linked receptors.
* The provided notes outline "III. Signal Transduction Mechanisms" as a topic but do not describe the specific functional classes of receptors such as channel-linked, enzyme-linked, or G protein-linked receptors within the given text.