A neurotransmitter is a chemical messenger that carries, boosts and modulates signals between neurons and other cells in the body. In most cases, a neurotransmitter is released from the axon terminal after an action potential has reached the synapse. The neurotransmitter then crosses the synaptic gap to reach the receptor site of the other cell or neuron. Then, in a process known as reuptake, the neurotransmitter attaches to the receptor site and is reabsorbed by the neuron.
Neurotransmitters play a major role in everyday life and functioning. Scientists do not yet know exactly how many neurotransmitters exist, but more than 100 chemical messengers have been identified. When neurotransmitters are affected by disease or drugs, there can be a number of different adverse effects on the body. Diseases such as Alzheimer's and Parkinson's are associated with deficits in certain neurotransmitters.
Types of Neurotransmitters
Neurotransmitters can be classified by function:
Excitatory neurotransmitters: These types of neurotransmitters have excitatory effects on the neuron; they increase the likelihood that the neuron will fire an action potential. Some of the major excitatory neurotransmitters include epinephrine and norepinephrine.
Inhibitory neurotransmitters: These types of neurotransmitters have inhibitory effects on the neuron; they decrease the likelihood that the neuron will fire an action potential. Some of the major inhibitory neurotransmitters include serotonin and GABA
Some neurotransmitters, such as acetylcholine and dopamine, can both excitatory and inhibitory effects depending upon the type of receptors that are present.
Acetylcholine – involved in voluntary movement, learning, memory, and sleep
Too much acetylcholine is associated with depression, and too little in the hippocampus has been associated with dementia.
Dopamine – correlated with movement, attention, and learning
Too much dopamine has been associated with schizophrenia, and too little is associated with some forms of depression as well as the muscular rigidity and tremors found in Parkinson’s disease.
Norepinephrine – associated with eating, alertness
Too little norepinephrine has been associated with depression, while an excess has been associated with schizophrenia.
Epinephrine – involved in energy, and glucose metabolism
Too little epinephrine has been associated with depression.
Serotonin – plays a role in mood, sleep, appetite, and impulsive and aggressive behavior
Too little serotonin is associated with depression and some anxiety disorders, especially obsessive-compulsive disorder. Some antidepressant medications increase the availability of serotonin at the receptor sites.
GABA (Gamma-Amino Butyric Acid) – inhibits excitation and anxiety
Too little GABA is associated with anxiety and anxiety disorders. Some antianxiety medication increases GABA at the receptor sites.
Endorphins – involved in pain relief and feelings of pleasure and contentedness
Communication at the synapses between neurons relies on chemicals called neurotransmitters. Secreted from a part of one neuron (the axon) into the synaptic gap between two others, neurotransmitters diffuse across this space and combine with specific proteins on the surface of the receiving cell, triggering an electrochemical response in the target cell.
Afterward, neurotransmitters are either destroyed or reabsorbed back into the neuron for storage and reuse. The release of neurotransmitters by a neuron has three main functions: 1) exciting a second neuron, thus causing it to depolarize; 2) inhibiting a second neuron, which prevents it from depolarizing; and 3) stimulating a muscle fiber to contract.
Neurotransmitters: Neural transmission (information being sent from neuron to neuron) works at both the electrical and chemical levels. When the space between two neurons is small enough the electrical signal can simply jump the gap and continue on its way. However, when the gap is too large, the signal must be converted from electricity to chemicals. These chemicals are neurotransmitters, which can be defined as chemicals released by neurons which carry information from one neuron to another. These chemicals are then read or received by a neuron's synapses.
During the 1950s it was discovered that the blood pressure medication “reserpine” causes depression in some people (Ayd, 1956). Reserpine was found to lower levels of the neurotransmitter norepinephrine in the brain, which suggests that depression may be related to a lowered supply of norepinephrine.
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