Neurotransmitters
Neurotransmitters are special chemicals which help to pass nerve impulses on from one neuron to another. When a nerve impulse runs down the axon of a neuron, ultimately it arrives at the axon terminal, where the synaptic knobs are located. The synaptic knobs contain packets (vesicles) of specialized chemicals, the neurotransmitters. When the nerve impulse reaches the synaptic knobs, the vesicles spill out into a space located between the axon terminal and the next neuron's cell body or soma. This space is referred to as the synapse, or synaptic junction.
The neurotransmitters spread across the synaptic junction, until they reach the next neuron's soma, where they attach themselves to special receptors along the soma's membrane. This allows the nerve impulse to continue to be transmitted along this next neuron's axon.
Almost immediately, special enzymes also arrive in the synaptic junction, to degrade any leftover neurotransmitters lingering there. This prevents the neurotransmitters from continuously stimulating the next neuron, even after the need for nerve impulse transmission has passed.
A number of neurotransmitters exist, including acetylcholine, norepinephrine, ephinephrine (also called adrenaline), serotonin, glycine, dopamine, histamine, enkephalins, and endorphins. Some of these neurotransmitters are considered to be excitatory, meaning that they cause the chemical and electrical changes necessary to send a nerve impulse down a particular neuron. Other neurotransmitters are considered to be inhibitory, meaning that they cause the chemical and electrical changes necessary to temporarily prevent a nerve impulse from traveling down a particular neuron.
Neurotransmitters are responsible for a large variety of bodily functions, including muscle movements; learning and memory; regulation of moods, emotions, and sleep; transmission of sensations such as pain; and regulation of such basic functions as temperature and water balance in the body. A number of illnesses and diseases can occur due to problems with the neurotransmitters. Curare (remember those poison-tipped arrows in the movies?) is a poison which acts by filling the neurotransmitter receptor sites, preventing the true neurotransmitters from entering. This results in total paralysis. In fact, this process has been mimicked with drugs which are given to surgical patients, to purposely induce paralysis, preventing any muscle movement during surgery. Organophosphate insecticides, as well as nerve gas substances used in warfare, cause poisoning by interfering with the enzymes which should clear neurotransmitters from the synaptic junctions. The neurotransmitters continue to stimulate the neurons, resulting in uncontrollable muscle twitching, and ultimately leading to paralysis. The presence of excess dopamine in the brain has been identified as the cause of Parkinson's disease. The excess dopamine causes muscles to be over-stimulated, resulting in stiffness and uncontrollable movements. Too little of the neurotransmitters dopamine, serotonin, and norephinephrine has been identified as a cause for serious depression. Medications to treat depression interfere with the enzymes which normally degrade these neurotransmitters within the synaptic junction, allowing these neurotransmitters to continue stimulating the next neuron's soma.
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