The traveling of signal in the nervous system between different neurons is mediated by the effect of a chemical substance released at the nerve terminal called chemical transmitter.
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In the sympathetic nervous system the chemical transmitter is adrenaline, noradrenaline or sometimes acetylcholine.
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When the chemical transmitter is adrenaline the nerve fiber is called adrenergic,
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when the chemical transmitter is acetylcholine, the nerve fiber is called cholinergic.
Release of the chemical transmitters
On the arrival of an action potential to the nerve terminal, vesicles containing acetylcholine or granules containing norepinephrine accumulate at the nerve ending facing the membrane.
Once they got closer to the membrane they open releasing their contents of the chemical transmitter through an exocytotic process.
- Calcium ion is essential for this step (through Ca++ channels).
The vesicle membranes are retained to return back to the nerve cytoplasm to be used for storage again. The released chemical transmitter will stimulate the specific autonomic receptors.
The interaction of chemical transmitter with the receptors will produce the post receptor events that will elicit the change in the function of the organ that contains that receptor.