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Terminal buttons exist at the ends of the many branches that divide out from the axon. The terminal buttons receive the message transferred down the axon, store them in their synaptic vesicles and are responsible for then secreting these transmitter substances.
These transmitter substances are called neurotransmitters. The terminal buttons release these message carriers into the synapses the junctions between two neurons, usually between the terminal buttons of one neuron and the dendrites of another neuron. These messages are transferred in one direction. For this reason, the transferring neuron is the presynaptic membrane, while the receiving neuron is the postsynaptic membrane. The area between these membranes into which the neurotransmitters are released is called the synaptic cleft.
When the axon of a neuron fires ( i.e. propagates an electrical charge ), the synaptic vesicles fill up with the transmitter substance, adhere to the presynaptic membrane and then rupture. The transmitter substance is then released into the synaptic cleft, where it may or may not be picked up by the postsynaptic receptors of another neuron's membrane.
The postsynaptic neuron's receptors can find these chemicals, or messages, either excitatory ( depolarizations ) or inhibitory ( hyperpolariztions ). This will affect the permeability of the postsynaptic membrane to the neurotransmitter.
The presynaptic terminal buttons only release their neurotransmitter substances briefly. The process is stopped by a process of reuptake by the presynaptic terminal buttons and causes a very fast removal of the transmitter substance from the synaptic cleft. In effect, the terminal buttons release the chemicals, then quickly take them back, unless quickly absorbed into the postsynaptic membrane of other neurons.
The drug Phentermine is believed to affect the reuptake of norephinehprine and serotonin, resulting in increased levels of these neurotransmitters in the brain. In this way, Phentermine produces its appetite suppressing effects.
Types of synapses:
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