Chapter 5 Notes - Chapter 5 Chemical Messengers Mechanisms...

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Chapter 5: Chemical Messengers October, 2007 Mechanisms of Intercellular Communication - cells can be physically linked via gap junctions - cells may communicate via chemical messengers Direct Communication Through Gap Junctions - movement of small molecules through gap junctions metabolically couples the cells so that one cell provides necessary nutrients to other cells (e.g. gap junctions allow nutrients to reach certain bone cells distant from bloodstream) Indirect Communication Through Chemical Messengers - ligands: molecules that bind to proteins reversibly (cells mainly communicate via chemical messengers) General process - one cell releases chemical into interstitial fluid usually via secretion - another cell (target cell) responds to chemical messenger - target cell responds to chemical messenger because it has certain proteins (receptors) that specifically recognize and bind the messenger - binding of messenger to receptors produces response in the target cell through mechanisms known as signal transduction - target cell response ↑ as number of bound receptors ↑ Chemical Messengers Functional Classification of Chemical Messengers - four types: 1) paracrines 2) autocrines 3) neurotransmitters 4) hormones Paracrines - chemicals that communicate with neigboring cells - target cell must be close enough that once paracrine is secreted into ECF, it can reach target cell by simple diffusion - growth factors, clotting factors, cytokines Growth factors - proteins that stimulate proliferation and differentiation of cells Clotting factors - proteins that stimulate formation of blood clot Cytokines -peptides usually released from immune cell that act in coordination with the body’s defense against infections Autocrines - similar to paracrines- except autocrines act on the same cell that secreted them - secretory cell = target cell - may act as a paracrine - regulates its own secretion Neurotransmitters - chemicals released into interstitial fluid from nervous system known as neurons - neurotransmitters released from specialized portion of the neuron called axon terminal - this is very close to the target cell - axon terminal very close to target cell - juncture between two cells is the synapse - communication by neurotransmitters called synaptic signalling - presynaptic neuron: cell that releases neurotransmitter - postsynaptic cell: target cell (may be another neuron, gland or muscle cell) Hormones - chemicals released from endocrine glands into interstitial fluid - may diffuse into blood - then it travels to its target cells which may be distant from site of hormone release - hormone sent to basically all cells in the body, but only those with receptors able to respond and thus serve as target cells Neurohormones
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