A neurotransmitter is a chemical compound that is sythesized and secreted by neurons in the nervous system to the synaptic cleft between adjacent neurons. There the neurotransmitter binds to receptors on the cellular membrane and induces either depolarization or hyperpolarization of the adjacent cell. This means it either excites or inhibits the neuron that it acted upon.
Most known neurotransmitters are polar compounds that do not readily penetrate the phospholipid bilayer of the cellular membrane. This is so due to the presence of hydroxyl group or amino group in its structures. In addition, for example in acetylcholine the charged group in its structure makes it polar compound that does not easily cross the phospholipid bilayer of the cellular membrane which is nonpolar and hydrophobic.
An exception to this trend is the neurotransmitter nitric oxide or NO. This molecule has low dipole moment with low polarity so that it can penetrate readily the phospholipid bilayer. It exerts its effect by activating an enzyme that catalyzes a second messenger called GMP which eventually leads to the dilatation of blood vessels.
NO is a very reactive compound due to its radical structure. It binds in the cell with water and oxygen and forms nitrate and nitrite compounds. NO is synthesized from the amino acid arginine by the effect of the enzyme nitric oxide synthase or NOS.
Other known neurotransmitters in the nervous system of humans function mainly by binding to receptors on the cellular membrane causing either openning or closing of ion channels which in turn can send or inhibit signals along the nervous system between neurons.
An example is substance P which is a neuropeptide that mediates the sensation pain. It probably functions by causing depolarization (excitation) of neurons so that pain signals are transmitted along the sensory pathway of the nervous system to the brain where this sensation is perceived as pain.
Neurotransmitter that functions the opposite way to this is also neurotransmitter of the enkephaline group in which it probably inhibit the transmission of pain signals along the nervous system by causing hyperpolarization of the cellular membrane potential. Thus cutting the transmission of pain signal to the brain. This neurotransmitter functions mainly as pain killer and it is part of a group of neuropeptides that act as neurotransmitters in the nervous system.
Other neurotransmitters such as the amino acid derivative GABA or gamma amino butyric acid is an inhibitory neurotransmitter in the central nervous system which also probably functions by cutting excitatory signals in the nervous system by causing hyperpolarization of the cellular membrane potential.
GABA is a type of unusual amino acid that is not incorporated into the structure of proteins in the body. However, it is also a polar compound that does not readily penetrate the phospholipid bilayer and it probably makes its effect by openning ion channels in neigbouring cells causing hyperpolarization of the cellular membrane potential. As a result a person would feel relaxed due to its effect.
Glutamic acid is another typical amino acid that is excitatory in its physiological effect on the nervous system. This amino acid is especially important in the medical syndrome that is called lateral amyotrophic sclerosis in which this excitatory amino acid accumulate in neurons of the body causing motor dysfunction of the nervous system.
The autonomic nervous system with its two divisions of the sympathetic and the parasympathetic nervous systems has two important neurotransmitters that mainly mediate its function. These neurotransmitters are acetylcholine and norepinephrine and epinephrine. Usually, acetylcholine excites neurons in the parasympathetic division of the autonomic nervous system such as its excitatory effect on the gastrointestinal motility. On the other hand, epinephrine and norepinephrine mediate the function of the sympathetic nervous system inducing situations like that obtained under stress and under anxiety.