An adrenergic synapse employs the neurotransmitter norepinephrine (NE), also called noradrenaline. NE, other monoamines, and neuropeptides act through second-messenger systems such as cyclic AMP (cAMP). The receptor is not an ion gate but an integral protein associated with a G protein. The binding of NE activates the G protein, which activates adenylate cyclase, which converts ATP to cAMP (fig. 12.20). The cAMP can have multiple effects such as stimulating the synthesis of new enzymes, activating preexisting enzymes, or opening ligand-regulated gates and producing a postsynaptic potential.
Figure 12.20 Transmission at an Adrenergic Synapse. The norepinephrine receptor is not an ion channel. It activates a second-messenger system with a variety of possible effects in the postsynaptic cell.
As complex as synaptic events may seem, they typically require only 0.5 msec or so—an interval called synaptic delay. This is the time from the arrival of a signal at the axon terminal of a presynaptic cell to the beginning of an action potential in the postsynaptic cell.
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