[Flash] Quá trình truyền xung thần kinh - Synaptic transmission - eduMedia

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Summary

This animation presents the major ionic and molecular events at the origin of the synaptic transmission of an action potential from the pre-synaptic to the postsynaptic neuron.
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Learning goals

• To understand how an action potential can be transmitted from a pre-synaptic to a postsynaptic neuron.
• To analyze the ionic and molecular phenomena involved in synaptic transmission.
• To illustrate the chronology of events that ensures synaptic transmission.
• To understand how degradation enzymes are at the origin of the cessation of synaptic transmission.
• To clarify the role of pre-synaptic transporters in the recapture of certain products of enzymatic degradation.

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A synapse between neurons is a narrow zone of contact between the pre-synaptic button of one of the neurons and the postsynaptic cell body of the other. When an action potential propagates along an axon, it is unable to cross the synapse. The passage of the nerve signal from one neuron to another is called synaptic transmission.  This progresses in the following manner:  

1. Arrival of the action potential at the synaptic button: the train of action potentials induces the temporary opening of voltage dependent calcium channels.
2. Entry of the calcium ions: calcium ions in the extracellular medium can now enter the synaptic button (also referred to as the synaptic knob). The calcium ions aid the process of exocytosis of neurotransmitter vesicles.
3. Exocytosis of neurotransmitter vesicles: the membranes of the vesicles fuse with that of the synaptic button. The contents of the vesicles are thus released into the  synaptic gap (the space between the pre-synaptic and postsynaptic structures).
4. Diffusion of the neurotransmitters: the release of the neurotransmitters leads to their diffusion into the synaptic gap. This phenomenon is simply a dispersion of the neurotransmitter molecules.
5. Fixation of the neurotransmitters:  the neurotransmitters attach themselves to the receptor channels of the postsynaptic cell.  Once so attached, the neurotransmitters induce the opening of these channels.
6. Entry of the sodium ions:  the receptor channels thus opened display a selective permeability to certain ions – ie those of sodium. The entry of the sodium ions induces the beginning of a depolarization called the postsynaptic potential. If this potential is above threshold value, an action potential will then be created in the postsynaptic neuron.
7. Degradation of the neurotransmitters: the receptor channels close, favoring the enzymatic degradation of the neurotransmitters. In effect, the synaptic gap contains enzymes that are able to degrade, and so inactivate, the neurotransmitters.
8. Closing of the receptor channels:  the degradation of the neurotransmitters is followed by the closure of the receptor channels.
9. Recapture of the neurotransmitters:  certain products of the degradation of the neurotransmitters are recycled with the aid of transporters located in the membrane of the pre-synaptic cell.  These products are then used to construct new neurotransmitters.
10. 1 Formation of neurotransmitter vesicles: new vesicles are formed. The pre-synaptic cell is now ready to carry out new transmissions.

The entire process takes place on only .5ms, a time period known as the synaptic delay.

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