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Nervous System ContentsNeurons and Nerves
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Neurotransmitters are chemicals that take a nerve signal across the synaptic gap (Figure 02a) between a sending neuron, and a receiving one. On the receiving neuron are receptors into which the neurotransmitters fit like a key in a lock. Once a neuro-transmitter is bound to its specific receptor, the likelihood of the receiving cell "firing" to send its own message is affected. The excitatory neurotransmitter-receptor systems make receiving cells more likely to fire, whereas the inhibitory systems make the
|firing less likely (see Figure 29a). It all depends on the type of neurotransmitter. An individual nerve cell can possess both kinds of synaptic connections (with a total of about 50000 synapses on the surface) to other nerve cells. Only if the excitatory charges (positive charge) exceed a threshold does the target neuron starting a nerve impulse of its own and is known as transduction. Figure 02b shows the various components in the synapse. The vesicle contains the neuro-transmitters in the axon. The receptor is located on the surface of the dendrite to pick up the neuro-transmitters. The |
|transporter is for recycling un-used neutrotransmitters back into the axon; while the glial cell provides nutrition and support for the neurons. |
Figure 02c shows the process of signal transmission across the synapse:
(not shown), which terminate the release once a neutrotransmitter drifts back upstream and hits one of these receptors.
- Release - As the action potential comes down the axon, the calcium influx triggers an exocytosis of vesicles that contain the neurotransmitters, which are release into the synaptic cleft.
- Bind - The neurotransmitters then drifts across , binds to the postsynaptic receptors.
- Transduction - Depending on the integration of the excitatory and inhibitory inputs, the receiving dendrite may fire a signal for further transmission.
- Reuptake - The neurotransmitter transporters remove the un-used neutrotransmitters in the synaptic gap back to the axon for re-use. This step is to prevent continuous stimulation of the postsynaptic neuron.
|There are other ways to turn the signal off. One is simple diffusion into the extracellular space. Another way is to break down the neuro-transmitters with enzymes. Then there are the presynaptic autoreceptors |
Since the neurotransmitters are more accessible than the neuron itself, it can be subjected to a lot of internal and external
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Neurotransmitters are chemicals that take a nerve signal