Neurotransmitters: The Body’s Chemical Messengers
Neurotransmitters refer to molecules that are used by the nervous system to transmit messages between neurons, or neurons to muscles.
When neurons communicate with each other, they do so by a synaptic cleft (synapse) which is the small gap between the synapses of neurons. Here, electrical signals that have traveled along the axon are briefly converted into chemical ones through the release of neurotransmitters, causing a specific response in the receiving neuron.
Classification of neurotransmitters
A neurotransmitter influences a neuron in one of three ways: excitatory, inhibitory or modulatory. therefore neurotransmitters can be classified as either excitatory, inhibitory or modulatory neurotransmitters.
Excitatory neurotransmitters function to activate receptors on the postsynaptic membrane and enhance the effects of the action potential, while inhibitory neurotransmitters function in a reverse mechanism.
Neuromodulators are not restricted to the synaptic cleft between two neurons. Therefore they can affect large numbers of neurons at once. Neuromodulators regulate populations of neurons, while also operating over a slower time course than excitatory and inhibitory transmitters
Most neurotransmitters are either small amine molecules, amino acids, or neuropeptides.
Key neurotransmitters include:
Acetylcholine is an excitatory neurotransmitter occurring throughout the nervous system and is the most well understood and studied. Acetylcholine has many functions ranging from the stimulation of muscles, including the muscles of the gastrointestinal system to vital organs. It is also found in sensory neurons and in the autonomic nervous system and has a part in scheduling the “dream state” while an individual is fast asleep.
Acetylcholine plays a vital role in the normal functioning of muscles. For example, the plant poisons, curare, and hemlock cause paralysis of muscles by blocking the acetylcholine receptor sites of myocytes. The well-known poison botulin works by preventing the vesicles in the axon ending from releasing acetylcholine, thus leading to paralysis of the effector's muscle.
Acetylcholine is released by motor neurons and neurons of the autonomic nervous system. It also plays an important role in the central nervous system in maintaining cognitive function. Damage to the cholinergic neurons of the CNS is associated with Alzheimer's disease.
Norepinephrine, also known as noradrenaline, is an excitatory neurotransmitter. It is a monoamine and the primary neurotransmitter in the sympathetic nervous system
It acts to increase the alertness of the nervous system as well as to stimulate the processes in the body. For example, it is very important in the endogenous production of epinephrine. Norepinephrine has been implicated in mood disorders such as anxiety, in which case its concentration in the body is abnormally high. Alternatively, an abnormally low concentration of it may lead to an impaired sleep cycle.
Also known as adrenaline, epinephrine is an excitatory neurotransmitter produced by the adrenal glands and released into the bloodstream. It prepares the body for the fight or flight reaction. That means that when a person is highly stimulated (fear, anger, etc), extra amounts of epinephrine are released into the bloodstream.
This release of epinephrine, increases the heart rate, the blood pressure and the glucose production from the liver (glycogenolysis). In this way, the nervous and the endocrine system prepare the body for dangerous and extreme situations.
Dopamine is a monoamine neuromodulator. Dopamine is considered a special type of neurotransmitter because its effects are both excitatory and inhibitory.
It is strongly associated with the reward mechanisms in the brain, and drugs such as cocaine, opium, heroin, and alcohol can temporarily increase its levels in the blood, leading to abnormal firing of nerve cells, which may sometimes manifest as intoxication, or several manners of consciousness/focus issues (such as not remembering where we put our keys, or forgetting what a paragraph said when we have just finished reading it, or simply daydreaming and not being able to stay on task).
However, an appropriate secretion of dopamine in the bloodstream plays a role in the motivation or desire to complete a task.
Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter. Abnormally low secretion of GABA may cause conditions like anxiety. Because it is an inhibitory neurotransmitter, GABA acts as a brake to the excitatory neurotransmitters, and thus when it is abnormally low this can lead to anxiety. It is widely distributed in the brain and plays a principal role in reducing neuronal excitability throughout the nervous system.
Glutamate is the primary excitatory transmitter in the central nervous system. Conversely, a major inhibitory transmitter is its derivative γ-aminobutyric acid (GABA), while another inhibitory neurotransmitter is the amino acid called glycine, which is mainly found in the spinal cord
Glutamate is usually ensured homeostasis with the effects of GABA. excessive levels of it can be toxic to the nerve cells and may lead to conditions like stroke.
Serotonin is also a monoamine inhibitory neurotransmitter that has been found to be intimately involved in emotion and mood. Adequate amounts of serotonin are necessary for a stable mood, and also to balance any excessive excitatory neurotransmitter effects in the brain.
Like norepinephrine, serotonin also regulates many processes in the body, such as carbohydrate cravings, the sleep cycle, pain control, and the digestion of food.
Insufficient secretion of serotonin may result in decreased immune system function, as well as a range of emotional disorders like depression, anger control problems, obsessive-compulsive disorder, and even suicidal tendencies.
Histamine is a monoamine excitatory neurotransmitter produced by basophils and is found in high concentrations in the blood. It is involved primarily in the inflammatory responses, as well as a range of other functions such as vasodilation, and regulation of the immune response to foreign bodies.
For example, when allergens are introduced into the bloodstream, histamine assists in the fight against these microorganisms causing itching of the skin or irritations of the throat, nose and or lungs. It also plays a role in the wake/sleep cycle, by increasing wakefulness.