Molecules found within neurons that serve to propagate or distribute an incoming signal to the neuron and that mediate the biological responses of the cell. In contrast to first messengers such as NEUROTRANSMITTERS or neuroendocrine signals, which are responsible for communication between cells, the role of second messengers is to mediate communication within the cell, thus controlling the cell’s physiological response to the incoming signal. The process by which an external signal is converted into an internal signal is known as SIGNAL TRANSDUCTION. There are two major second messenger systems.
One employs cyclic adenosine monophosphate (CYCLIC AMP or cAMP) and the other utilizes a combination of molecules that includes CALCIUM ions (Ca2+), INOSITOL triphosphate (IP3) and DIACYLGLYCEROL (DAG). Both these pathways utilize an initial coupling mechanism requiring membrane-bound G PROTEIN, which in turn stimulates the production of second messengers. The final action of second messengers is generally to induce cellular proteins to change their structure. The most common mechanism is PROTEIN PHOSPHORYLATION; that is, second messengers activate a type of ENZYME called PROTEIN KINASE, which in turn phosphorylate (add phosphate groups) the target protein. Second messengers may also bind to proteins directly. Phosphorylation is a final common pathway of fundamental importance in regulation of neuronal activity and gene expression in the nervous system.
ANN E.KELLEY
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