| CLASSIFICATION OF NEUROTRANSMITTERS
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A classification of neurotransmitters based on chemical composition is shown in Table 40.1. Many are derived from simple compounds, such as amino acids (Table 40.2), but peptides are also now known to be extremely important. The principal transmitters in the peripheral nervous system are norepinephrine and acetylcholine (Ach) (Fig. 40.1).
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| Several transmitters may be found in one nerve
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Table 40-1.
Classification of neurotransmitters. |
| Body_ID: None |
| Classification of neurotransmitters |
| Body_ID: T040001.50 |
| Group | Examples |
| Body_ID: T040001.100 |
| | acetylocholine (ACh) |
| Body_ID: T040001.150 |
| amines | norepinephrine, epinephrine, dopamine, 5-HT |
| Body_ID: T040001.200 |
| amino acids | glutamate, GABA |
| Body_ID: T040001.250 |
| purines | ATP, adenosine |
| Body_ID: T040001.300 |
| gases | nitric oxide |
| Body_ID: T040001.350 |
| peptides | endorphins, tachykinins, many others |
| Body_ID: T040001.400 |
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| Body_ID: T040001.450 |
Neurotransmitters can be classified in several ways. The scheme shown relies on chemical similarities. All except the peptides are synthesized at the nerve ending and packaged into vesicles there; peptides are synthesized in the cell body and transported down the axon. 5-HT, 5-hydroxytryptamine; GABA, γ-amino butyric acid.
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Table 40-2.
Neurotransmitters of low molecular weight. |
| Body_ID: None |
| Neurotransmitters of low molecular weight |
| Body_ID: T040002.50 |
| Compound | Source | Site of production |
| Body_ID: T040002.100 |
| Amino acids |
| Body_ID: T040002.150 |
| glutamate | | central nervous system (CNS) |
| Body_ID: T040002.200 |
| aspartate | | CNS |
| Body_ID: T040002.250 |
| glycine | | spinal cord |
| Body_ID: T040002.300 |
| Amino acid derivatives |
| Body_ID: T040002.350 |
| GABA | glutamate | CNS |
| Body_ID: T040002.400 |
| histamine | histidine | hypothalamus |
| Body_ID: T040002.450 |
| norepinephrine | tyrosine | sympathetic nerves, CNS |
| Body_ID: T040002.500 |
| epinephrine | tyrosine | adrenal medulla, a few |
| Body_ID: T040002.550 |
| | | CNS nerves |
| Body_ID: T040002.600 |
| dopamine | tyrosine | CNS |
| Body_ID: T040002.650 |
| 5-HT | tryptophan | CNS, enterochromaffin gut cells, |
| Body_ID: T040002.700 |
| | | enteric nerves |
| Body_ID: T040002.750 |
| Purines |
| Body_ID: T040002.800 |
| ATP | | sensory, enteric, sympathetic nerves |
| Body_ID: T040002.850 |
| adenosine | ATP | CNS, peripheral nerves |
| Body_ID: T040002.900 |
| Gas |
| Body_ID: T040002.950 |
| nitric oxide | arginine | genitourinary tract, CNS |
| Body_ID: T040002.1000 |
| Miscellaneous |
| Body_ID: T040002.1050 |
| ACh | choline | parasympathetic nerves, CNS |
| Body_ID: T040002.1100 |
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| Body_ID: T040002.1150 |
Many neurotransmitters are of low molecular weight, and are simple compounds, often derived from common amino acids.
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| Figure 40.1 Transmitters in the autonomic nervous systen. Catecholamines and acetylcholine are transmitters in the sympathetic and parasympathetic nervous systems. Preganglionic nerves all release ACh, which binds to nicotinic (N) receptors. Most postganglionic sympathetic nerves release norepinephrine (NE), whereas postganglionic parasympathetic nerves release ACh, which acts at muscarinic (M) receptors. Motor neurons release ACh, which acts at distinct nicotinic receptors. E, epinephrine. |
| An early dogma of nerve function held that one nerve contained one transmitter. However, this is now known to be an oversimplification and combinations of transmitters are the rule. The pattern of cellular transmitters may characterize a particular functional role, but details of this also remain unclear. A major low-molecular-weight transmitter such as an amine is often present, along with several peptides, an amino acid, and a purine. Sometimes, there may even be more than one possible transmitter in a particular vesicle, as is believed to be the case for adenosine triphosphate (ATP) and norepinephrine in sympathetic nerves. In some cases, the intensity of stimulation may control which transmitter is released, peptides often requiring greater levels of stimulus. Furthermore, different transmitters may have a different timescale of action. Sympathetic nerves are good examples of nerves for which this is the case: it is believed that ATP causes their rapid excitation, whereas norepinephrine and the
neuromodulator, neuropeptide Y (NPY), cause a slower phase of action. In some tissues, NPY on its own may be able to produce a very slow excitation.
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