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9 Cards in this Set

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Step 1

Step 1

It begins when the nerve impulse reaches the presynaptic axon terminal. Depolarization of the presynaptic membrane initiates the sequence of events leading to transmitter release and activation of receptors on the postsynaptic membrane.

Step 2

Step 2

Some of the free synaptic vesicles make their way to the plasma membrane and dock, as a series of priming reactions prepares the vesicular and plsama membranes for fusion.

Step 3

Step 3

The membranes of the synaptic vesicles are drawn together via protein complexes that are expressed on the vesicle and presynaptic membranes.

Step 4

Step 4

When the axon terminal is depolarized, voltage-gated calcium channels open, and calcium ions rush into the axon terminal. Some of the calcium ions bind to a protein on the synaptic vesicile membrane called synaptotagmin.

Step 5

Step 5

When calcium binds to synaptotagmin on the synaptic vesciles nearest the active zone, the vesicles are drawn even closer to the presynaptic membrane. The vesicles fuse with the axon terminal membrane, and release their transmitter cargo into the synaptic cleft. Some of the transmitter molecules bind to special receptor molecules in the postsynaptic membrane.

Step 6

Step 6

The response of the postsynaptic cell (e.g., excitation or inhibition) depends upon the particular neurotransmitter and receptor combination. For example, the receptor for ACh is permeable to sodium. After binding to acetylcholine, the channel opens and sodium ions enter the postsynaptic cell, thereby generating an excitatory postsynaptic response.

Step 7

Step 7

Transmitters are inactivated or removed rapidly from the synaptic cleft, so that transmission is brief and accurately follows the presynaptic input signal. For acetylcholine, an enzyme in the synaptic cleft, AChE breaks down ACh into choline and acetate. The release of transmitter from the receptors causes the channels to close.

Step 8

Step 8

Many transmitters are rapidly cleared from the synaptic cleft by being taken up into the presynaptic terminal by special proteins called transporters. This is known as reuptake; reuptake not only cuts off the synaptic activity promptly, but also allows the terminal to recycle transmitter molecules.

Step 9

Step 9

The membrane needed for creating synaptic vesicles is also recycled, via endocytosis of the presynaptic membrane. The recycled vesicles are refilled with neurotransmitter molecules, and are ready for another round of synaptic transmission.