General Principles Of Pharmacology/synapse
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Synapses are the target area for psychoactive drugs; they are the location within the brain where these substances exert their effect. The synapse is the gap between neurons; each neuron may form thousands of synapses upon other neurons providing for trillions of synapse within the brain. The term synapse was coined by Charles Sherrington in 1897; this work won him the Nobel Prize in 1932. For the most part, synapses are formed between the terminal button of the sending or presynaptic neuron and the dendrite of the receiving or postsynaptic neuron, however, the sending neuron may synapse on the cell body or even the axon of the receiving neuron.
When the action potential arrives at the terminal button, vesicles, containing neurotransmitter, merge with presynaptic membrane and burst, spewing neurotransmitter into the synaptic gap or cleft. The postsynaptic membrane of the dendrite of the receiving neuron contains receptors. Receptors consist of a complex of membrane-spanning proteins, which make up a binding site and a chemical-dependent ion channels. Receptors are classified based upon how they control their ion channel, directly (metabolic) or indirectly (metabotropic), through the use of a second messenger (G-protein). For example, when a neurotransmitter is in the synapse it may bind (form a covalent bond) with the receptor, if the receptor directly opens an ion channel, this binding causes a distortion in the proteins that make up the receptor causing a pore or channel to be formed in the center of the receptor. If the receptors control an ion channel indirectly, after the neurotransmitter binds, a second messenger are released within the postsynaptic dendrite that opens a nearby channel. These channels allow ions to flow into the postsynaptic neuron, depending upon their electrical charge; they exert either an excitatory or inhibitory effect upon the postsynaptic neuron. These signals are summed within the soma of this neuron and if the threshold of excitation is reached, the neuron fires and the process is repeated at all of the synapses of that neuron. Any neurotransmitter that is left in the synapse is deactivated by reuptake or by enzymes.
Next Page: Drug-Receptor Affinity: Agonists and
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