Neurotransmitter
Neurotransmitters are chemical substances that transmit information between nerve cells, neurons, and between neurons and effectors, muscles or glands. The transmission occurs across a narrow gap or cleft at the synapse, microscopic distance between a transmitting presynaptic cell and a receiving postsynaptic cell. Neurotransmitters are synthesized by neurons and stored in structures called synaptic vesicles that hold thousands of molecules of neurotransmitter. When a nerve impulse passes along a presynaptic axon, it reaches the axon bulb at the axon terminal and depolarizes the presynaptic membrane. This causes calcium ions to rush into the neuron through voltage gated channels. Stimulated by calcium ions, the synaptic vesicles then fuse with the presynaptic membrane and spill their neurotransmitter into the synaptic cleft. The neurotransmitter diffuses to the postsynaptic membrane on the other side of the synapse. Specialized receptor proteins that project from the outside of the postsynaptic membrane receive the neurotransmitter. This sets off the movement of specific ions such as sodium, potassium or chlorine which in turn initiates an action potential, electric nerve impulse, in the postsynaptic cell. Depending on the nature of the ions, the action potential generated may be excitory or inhibitory. At this point, enzymes in the synaptic cleft and the postsynaptic membrane swiftly break down the neurotransmitter, thereby making sure that its effect is quick and precise. Neurons receive chemical information from neighboring neurons using thousands of synapses.
Researchers have discovered dozens of chemical substances that serve as neurotransmitters, and expect to discover more. The major groups of neurotransmitters include acetylcholine, biogenic amines, amino acids, and neuropeptides.
Motor neurons in vertebrates release acetylcholine into the synapses that separate them from skeletal muscle cells. Here, the neurotransmitter is excitory and stimulates muscle contraction. On the other hand, acytlcholine may be inhibitory as in vertebrates and mollusks where it slows down the heart rate. Biogenic amines are neurotransmitters derived from amino acids. For example, the amino acid tyrosine gives rise to a group of neurotransmitters called catecholamines that includes epinephrine, norepinephrine and dopamine. The amino acid tryptophan, on the other hand, is the precursor of another biogenic amine called serotonin. Biogenic amines transmit impulses mainly in the central nervous system, but norepinephrine functions in the branch of the peripheral nervous system known as the autonomic nervous system. Dopamine and serotonin work mainly in the brain where they help control sleep, attention, learning, and mood. Imbalances of biogenic amines result in disorders including Parkinson's disease (insufficient dopamine) and schizophrenia (excessive dopamine). Certain drugs such as LSD and mescaline induce hallucinations when they bind to serotonin and dopamine brain receptors. Amino acids that act as neurotransmitters include gamma aminobutyric acid (GABA), glycine, glutamate, and aspartate. GABA partakes in inhibitory synapses in the brain.
Europeptides, short chains of amino acids, include substance P, a neurotransmitter that causes the feeling of pain, and endorphins that decrease the pain sensation. Neurochemists discovered the natural pain killers, endorphins, in the 1970s when they were studying the nature of opium addiction. They found that drugs bind to the same receptors as endorphins and cause narcotic effects.
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