See this simplified diagram of a neuron:
Nerve impulses always flow in one direction - from the branched extensions called dendrites, down the neuron to the presynaptic terminals. The join between the presynaptic terminals of one neuron and the dendrites of another is called the synapse. The two neurons do not actually touch eachother but are separated by a space called the synaptic cleft. When a nerve impulse arrives at a presynaptic terminal it causes neurotransmitters to be released into the synaptic cleft. The neurotransmitters then bind with special "postsynaptic receptors" in the dendrites of the receiving neuron. When a postsynaptic receptor receives a neurotransmitter it can either cause a nerve impulse to travel down the neuron or it can inhibit a nerve impulse depending on the neurotransmitter released.
Neurotransmitters which propagate nerve impluses in the receiving neuron are called excitory neurotransmitters. Those which inhibit nerve impulses are called inhibitory neurotransmitters.
Neurotransmitters are sythesised in the cell body (the soma) and migrate down the axon to the presynaptic terminals. Here they are stored in little packets called vesicles which fuse with the synaptic membrane. When a depolarising current (the action potential) is received, these vesicles release their contents into the synaptic cleft.
Many different substances effect the transmission of nerve impulses across the synapse and many of these are falsely called neurotransmitters. To be a neurotransmitter a substance must:
The following is an incomplete list of neurotransmitters,
together with the sites in which they work:
|Group||Neurotransmitter||Region of Operation|
|Acetylcholine||Acetylcholine||Central Nervous System (CNS), Peripheral Nervous System (PNS) and Autonomic Nervous System (ANS)|
|Serotonin||Serotonin||CNS and PNS|
|Amino acids||Glutamate, Gamma Aminobutyric Acid (GABA), Glycine, Aspartate||CNS|
|Catecholamines||Norpinephrine, Epinephrine (Adrenalin)||CNS and Sympathetic Nervous System|
|Neuropeptides||Endorphins (Enkephalins and Dynorphins), Substance P||CNS|
|Nucleotides||Adenosine, Adenosine Triphosphate (ATP)||CNS, PNS and ANS|
|Nitric oxide||Nitric oxide||CNS|
Biochemistry of Neurotransmitters
Neuroscience for Kids - Neurotransmitters
Cerebral Institute of Discovery - Neurotransmitters
A Brief Introduction to the Brain: Neurotransmitters