Neurotransmitters are molecules that play an important role in neural communication. They are chemical messengers that carry information from neural cells to other cells. Amino acids, amines and peptides are the three major classes of neurotransmitters. There have been found around 100 different neurotransmitter molecules. However, the most work is done by only tenth of the transmitters. First neurotransmitter discovered was the acetylcholine which helped to understand the cholinergic system.
Neurotransmitter is determined as a molecule which needs to be synthesized and stored in presynaptic neuron. In addition, it needs to be released from the presynaptic axon terminal following stimulation. Also, it needs to produce a response in the post-synaptic cell.
Neurotransmitters can be roughly divided into two classes based on how they affect to the post-synaptic cell. Neurotransmitter can be either excitatory or inhibitory. Excitatory neurotransmitters have excitatory effects on the neuron. This means they increase the probability that the neuron will fire an action potential in the post-synaptic neuron. Glutamate is the most common excitatory neurotransmitter in the brain (AMPA receptor, NMDA receptor, kainata receptor). Inhibitory neurotransmitters decrease the probability of the post-synaptic action potential. GABA is the most common inhibitor. Other common neurotransmitters are acetylcholine (nicotinic receptor, muscaranic receptor) and serotonin.
Neurotransmitters can act through a second messenger neurotransmission. In the second messenger neurotransmission the first messengers cause target cell to release the second messengers. The first messengers are extracellular factors, often neurotransmitters or hormones. Serotonin is one neurotransmitter that acts as the first messenger. The type of the second messengers can vary a lot. cAMP is a one example of the second messenger molecule.
The brain has diffuse modulatory systems that are neurotransmitter systems which perform regulatory functions, i.e. modulate postsynaptic neurons in various areas of the brain by altering their activity, e.g. making them more or less excitable or synchronous. The diffuse modulatory systems tend to consists of a small core set of neurons that are located in the brain stem, and the activity of this small subset of neurons affects the activity of a much broader set of postsynaptic neurons.
The norepinephrine system located in the locus coerelus in the pons is one of the diffuse modulatory systems. The axons leaving from locus coeruleus innervate almost every part of the brain, i.e. all of the cortex, the cerebellum, the midbrain, the thalamus and so on. Activity in locus coeruleus has been linked to mood and depression, as well as attention and arousal.
Another of the diffuse modulatory systems is the serotonergnic system. The serotonin containing neurons are located within the raphe nuclei in the brain stem. These nuclei innervate most of the brain similarly to locus coeruleus neurons. Also similarly to locus coeruleus, the raphe nuclei seem to activate most during wakefulness and arousal.
The cholinergic system located in the basal forebrain complex is one of the two major diffuse modulatory cholinergic systems. This system has two major nuclei: medial septal nuclei and the basal nucleus of Meynert. The medial septal nuclei innervates hippocampus, whereas the nucleus of Meynert innervates the neocortex. Like the other systems in the brain stem, the cholinergnic system seems to be part of regulating arousal and sleep-wake cycles. It also seems to be important for learning and memory.