Diagram of neuron with synapse. Individual nerve cells, or neurons, both send and receive cellular signals to and from neighboring neurons, but for the purposes of this diagram only one activity is indicated for each cell. Neurotransmitter molecules are released from the neuron terminal and move across the gap between the "sending" and "receiving" neurons. A signal is transmitted to the receiving neuron when the neurotransmitters have bound to the receptor on its surface. The effects of a transmitted signal include:
- Changing the cell's permeability to ions, such as calcium and potassium.
- Turning a particular gene on or off.
- Sending a signal to another neuron.
- Increasing or decreasing the responsiveness of the cell to other cellular signals.
Those effects can lead to cognitive, behavioral, or physiological changes, depending on which neuronal system is activated.
The expanded view of the synapse illustrates a variety of ligands, that is, molecules that bind to receptors. Anandamide is a substance produced by the body that binds to and activates cannabinoid receptors; it is an endogenous agonist. THC can also bind to and activate cannabinoid receptors but is not naturally found in the body; it is an exogenous agonist. SR 141716A binds to but does not activate cannabinoid receptors. In this way it prevents agonists, such as anandamide and THC, from activating cannabinoid receptors by binding to the receptors without activating them; SR 141716A is an antagonist, but it is not normally produced in the body. Endogenous antagonists, that is, those normally produced in the body, might also exist, but none has been identified.
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