The brain is made up of individual units: nerve cells and glial cells, which form the nervous system. The nervous system receives and interprets information about the internal and external environment of the body (Sensory System), makes decisions about this information (Integrating System) and organizes and carries out action (Motor System).
The neuron doctrine holds that neurons are the basic signaling units of the nervous system and that each neuron is the discretely bounded cell whose several processes arise from its cell body.
A neuron has four morphologically defined regions:
Synaptic transmission refers to the propagation of nerve impulses from one nerve cell to another. This occurs at a specialized cellular structure known as the synapse, a junction at which the axon of the presynaptic neuron comes into extremely close proximity to a location upon the postsynaptic neuron. The end of a presynaptic axon, where it is juxtaposed to the postsynaptic neuron, is enlarged and forms a structure known as the terminal button. An axon can make contact anywhere along the second neuron: on the dendrites (an axodendritic synapse), the cell body (an axosomatic synapse) or the axon (an axo-axonal synapse). The space between the presynaptic and postsynaptic terminal is known as the synaptic cleft. There are two types of synapses:
Neurotransmitters are chemicals that allow the movement of information from one neuron across the gap between it and the adjacent neuron. The release of neurotransmitters from one area of a neuron and the recognition of the chemicals by a receptor structure on the adjacent neuron typically causes an electrical reaction in the postsynaptic neuron.
Amino Acids: Some simple amino acids can function as neurotransmitters. For example, glutamate and glycine are both amino acids that are ordinarily used to build proteins. But in some neurons, they can also get packaged into synaptic vesicles for eventual release as neurotransmitters.
Glutamate receptors typically mediate an excitatory response in the postsynaptic neuron. In fact, there are certain types of glutamate receptors that are thought to be involved in the learning process that occurs in our brains. Meanwhile, glycine receptors typically mediate an inhibitory response, for example, in specific areas of the spinal cord.
Monoamines: The term "monoamine" simply indicates an amino acid that has been modified in a specific manner. For example, the amino acid tyrosine gets modified through chemical reactions and is turned into dopamine. Dopamine is a monoamine that acts as a neurotransmitter. When someone has Parkinson's disease, the neurons that produce dopamine in the brain slowly degenerate, and that is a major cause of the loss of motor skills observed as the disease progresses.
Dopamine can also get modified further (in neurons that do not use it as a neurotransmitter) to become epinephrine (also called adrenaline), norepinephrine or serotonin. All of these substances are referred to as "monoamines."
Peptides: A peptide is a short chain of amino acids, not quite long enough to be considered a full-fledged protein. It has been noted that these peptide neurotransmitters can have complex effects on the postsynaptic neuron.
Many neurotransmittesr are known to cause the opening of a ligand-gated channel. But, some neurotransmitters, especially many of the peptides, don't necessarily work in that fashion. They can work through their own receptors to alter the way the ligand-gated channels of other neurotransmitters will work. This is why, they are known as neuromodulators.
Within the cells, small-molecule neurotransmitter molecules are usually packaged in vesicles. When an action potential travels to the synapse, the rapid depolarization of the presynaptic membrane causes calcium ion channels to open and the intracellular level of calcium to increase. Calcium then stimulates the transport of vesicles to the synaptic membrane; the vesicle and cell membrane fuse, leading to the release of the packaged neurotransmitter, a mechanism called exocytosis.
The neurotransmitters then passively diffuse across the synaptic cleft to bind to receptors. The receptors are broadly classified into ionotropic and metabotropic receptors. Ionotropic receptors are ligand-gated ion channels that open or close through neurotransmitter binding. Metabotropic receptors, which can have a diverse range of effects on a cell, transduct the signal by secondary messenger systems, or G-proteins. Binding to the receptor leads to generation of excitatory or inhibitory postsynaptic potentials (EPSP / IPSP) through the closing and opening of specific ion channels in the postsynaptic membrane.
When a neuron is in its resting state, its voltage is about -70 millivolts. An excitatory neurotransmitter alters the membrane of the postsynaptic neuron, making it possible for ions (electrically charged molecules) to move across the neuron's membranes. This flow of ions makes the neuron's voltage rise toward zero (depolarization). If enough excitatory receptors have been activated, the postsynaptic neuron responds by firing, i.e. generating a nerve impulse, that causes its own neurotransmitter to be released into the next synapse.
An inhibitory neurotransmitter causes different ions to pass across the postsynaptic neuron's membrane, lowering the nerve cell's voltage to -80 or -90 millivolts (hyperpolarization). The drop in voltage makes it less likely that the postsynaptic cell will fire.
If the postsynaptic cell is a muscle cell rather than a neuron, an excitatory neurotransmitter will cause the muscle cell to generate a muscle twitch. If the postsynaptic cell is a gland cell, an excitatory neurotransmitter will cause the cell to secrete its contents.
While most neurotransmitters interact with their receptors to create new electrical nerve impulses that energize or inhibit the adjoining cell, some neurotransmitter interactions do not generate or suppress nerve impulses. Instead, they interact with a second type of receptor that changes the internal chemistry of the postsynaptic cell by either causing or blocking the formation of chemicals called second messenger molecules. These second messengers regulate the postsynaptic cell's biochemical processes and enable it to conduct the maintenance necessary to continue synthesizing neurotransmitters and conducting nerve impulses. Examples of second messengers, which are formed and entirely contained within the postsynaptic cell, include cyclic adenosine monophosphate (cAMP), diacylglycerol, and inositol phosphates.
Once neurotransmitters have been secreted into synapses and have passed on their chemical signals, the neurotransmitter molcules are cleared from the synaptic cleft, usually by either the presynaptic or the postsynaptic neuron.
Clearance of Neurotransmitters
Fate of the Vesicular Membrane The membrane of the neurotransmitter-containing vesicles, which have fused with the presynaptic terminal and released the neurotransmitter molecules, is retrieved back into the presynaptic cell by endocytosis.
Neurotransmitters are known to be involved in a number of disorders:
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