A little order can be brought to this confusing abundance of receptor types because many receptors can belong to the same family or superfamily (a family is a closely related group of receptors, a superfamily is a grouping of families of receptors that have something (either structural or functional or both) in common. Muscarinic ACh receptors are linked to G-proteins and second messengers. Nicotinic ACh receptors are found within the sympathetic ganglia and at the neuromuscular junction. There are two main classes of ACh receptor for example nicotinic and muscarinic (named after two chemicals that bind with high affinity to the receptors and mimic the actions of ACh, nicotine and muscarine respectively. Many first messengers have more than one type of receptor. By and large, there will be at least one receptor for each first messenger substance. ATP) and opiates (enkephalins and endorphins). glutamine, glycine), prostaglandins, nucleotide-phosphates (e.g. A few that spring immediately to mind include: histamine, g -aminobutyric acid (GABA), dopamine, peptides (e.g substance P and including cytokines and peptide hormones such as insulin), amino acids (e.g. There are a great many substance other than acetylcholine and noradrenalin that can act as first messengers. A receptor is an intrinsic, membrane spanning protein that can maintain contact with both the extracellular and intracellular environments.Receptors keep cells in touch with one another and allow them to respond to external control signals. A notable exception to this general scheme are the steroid hormones which are cell membrane permeable and so can enter the cell, bind to intracellular receptors and have a direct effect on the target biological process.Ī receptor in the plasma membrane is a communication device that connects the inside of the cell to the extracellular environment. In either event, most extracellular signals cannot cross the plasma membrane and so the extracellular signal carried by the first messenger must be passed on as an intracellular signal carried by a second messenger. The messenger may be a hormone that has been released in another part of the body and has travelled in the bloodstream to the target, or it may be a neurotransmitter released by a nerve cell very close to the surface of the target cell. The final extracellular step in most signalling pathways is release of a chemical messenger at the surface of the target cell. All of these signals or messages will have some action upon a target organ or cell. There are a lot of messages flying around the body to keep everything working smoothly and to keep the different parts of the body co-ordinated with one another. For example, the signal contained in the action potential of a motor neurone is transduced into contraction of a muscle. Signal transduction is the process of converting a control signal into a change in the biological activity of a cell or organ. Sigtrans Body Fluid compartments Signal Transduction Getting the Message Across
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