Video explaining intracellular signaling

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Most signal transduction involves the binding of extracellular signaling molecules (and ligands) to cell-surface receptors. Such receptors typically face outward from the plasma membrane while triggering events inside the cell. Intracellular signaling cascades can also be triggered through cell-substratum interactions, as in the case of integrins, which bind ligands found within the extracellular matrix. Steroids represent another example of extracellular signaling molecules that may cross the plasma membrane due to their lipophilic or hydrophobic nature. Many, but not all, steroid hormones have receptors within the cytoplasm, and usually act by stimulating the binding of their receptors to the promoter region of steroid-responsive genes. Within multicellular organisms, numerous small molecules and polypeptides serve to coordinate a cell's individual biological activity within the context of the organism as a whole. Read more: http://en.wikipedia.org/wiki/Signal_transduction