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Neurotransmitters

Definition

Nerve cells arranged in networks communicate information with one another through neurotransmission which is the conduction of electrical messages from neuron to neuron. The electrochemical messages cross the synapses between neural cells by way of special chemical messengers called neurotransmitters. 

Neurotransmitters are chemicals found and produced in the brain to allow the transmission of impulses from one nerve cell to the next across synapses. They aid in the conduction of information throughout the body. These chemicals fit into specific receptor cells embedded in the membrane of the dendrite that either fuel up or excite action in the cells (excitatory) or stop or inhibit an action (inhibitory). Presence or absence of these neurotransmitters can directly or indirectly have an effect on neurons in a specific portion of the brain. Thus, the levels of these chemicals in the brain have an effect to a person’s behavior.

Mechanism of Nerve Impulse

Nerve cell conduction or communication is an extremely rapid process which involves the following:

  1. The impulse moves through the nerve in a long and slender cellular part called the axon.
  2. As the impulse travels through the axon it travels the presynaptic membrane. It is in this area that neurotransmitters are released in the free space called the synaptic cleft.
  3. The receptors located in the postsynaptic membrane of another nearby neuron pick up the free flowing neurotransmitters.
  4. The molecule is adapted in the next nerve cell and the impulse continues to the next nerve cell until the message is relayed throughout the body.

What happens after neurotransmitters sent the information?

After they served their purpose of being released into the synapse and relaying the messages to the receptor cells, they are transported back from the synapse to the axon to be stored for later use which is a process called reuptake. If the neurotransmitters will not undergo reuptake, they will be metabolized and inactivated by enzymes, primarily monoamine oxidase.

Major Neurotransmitters

Acetylcholine

This is the first neurotransmitter discovered which is found in the brain, spinal cord and particularly at the neuromuscular junction of the skeletal muscle of the peripheral nervous system. The mechanism of action of acetylcholine could either be excitatory or inhibitory. Acetylcholine is synthesized from dietary choline found in red meat and vegetables. Research and studies have linked the incidence of Alzheimer’s disease with decreased acetylcholine secretion in the neurons.

Physiologic Effects and Functions:

  1. Muscular stimulation – acetylcholine signals muscles to become active including the muscles of the gastrointestinal system.
  2. Controls the sleep and wakefulness cycle of a person. It has a part in scheduling the rapid eye movement (REM) sleep or dream.

Norepinephrine

The most predominant neurotransmitter in the nervous system is the norepinephrine. This neurotransmitter is also known as noradrenaline and is located primarily in the brain stem and sympathetic nervous system. The mechanism of action of norepinephrine is excitatory and plays a role in:

  1. Attention
  2. Memory and learning
  3. Sleep and wakefulness
  4. Mood regulation

The presence of norepinephrine places the nervous system into certain alertness and it increases a person’s heart rate and blood pressure.  Excessive amounts of norepinephrine have been strongly linked with several anxiety disorders. However, deficits of these chemicals contribute to memory loss, social withdrawal and depression.

Dopamine

Dopamine is located primarily in the brain stem. It is synthesized from tyrosine which is a dietary amino acid. This neurotransmitter is involved in the control of the following:

  1. Complex movements
  2. Motivation
  3. Cognition
  4. Regulation of emotional responses

Excessive amounts of dopamine in the frontal lobes of the brain have been strongly linked to the occurrence of the severe mental illness called schizophrenia. Thus, antipsychotic medications work by blocking the dopaminergic receptors and reduce dopamine activity. On the other hand, insufficient levels of dopamine in the motor areas of the brain can contribute to the aggravation of the clinical manifestations of Parkinson’s disease.

Gamma Aminobutyric Acid (GABA)

GABA is the major inhibitory neurotransmitter in the brain. The main function of this neurotransmitter is to modulate other neurotransmitter systems rather than to provide a direct stimulus. GABA is released and transported to act like a car brake to the excitatory actions of other neurotransmitters. Medications that increase the levels of GABA in the body are used to treat anxiety and induce sleep. Scant amount of this chemical is a contributing factor to development of anxiety disorders.

Glutamate

This neurotransmitter is an excitatory amino acid and plays an important part in regards to memory. If glutamate levels are too high, major neurotoxic effects will be noted. Glutamate is toxic to neurons at elevated levels and its excess will kill the helpful nerve cells. This chemical has been implicated in the brain damage caused by the following conditions:

  1. Stroke
  2. Hypoglycemia
  3. Sustained hypoxia or ischemia
  4. Some degenerative diseases such as Huntington’s Chorea or Alzheimer’s Disease

Serotonin

Serotonin is a neurotransmitter only found in the brain and plays a very important role in a range of various brain functions. It is derived from the dietary amino acid, tryptophan. Serotonin has been found to be strongly involved in the emotion and mood of a person. The mechanism of action of serotonin is mostly inhibitory and it has been linked with a wide variety of functions which includes the following:

  1. Control of food intake – insufficient levels of serotonin leads to an increased appetite for carbohydrates and starchy foods
  2. Regulation of sleep and wakefulness cycle – scarce amount of serotonin in the brain may lead to trouble sleeping
  3. Temperature regulation
  4. Pain control
  5. Sexual behaviors
  6. Regulation of emotions and mood

Low levels of serotonin have a role in the development of depression, problems in controlling anger, obsessive-compulsive disorder and suicide. Depressed people who are taking antidepressants manage the condition by preventing the reuptake of serotonin or inhibiting the scouring action of the neurons to excess serotonin. Mothers usually give their kids a glass of warm milk before bedtime to help their sons or daughter sleep. It is interesting to know that serotonin is a derivative of tryptophan, which is found in milk. Thus, a glass of milk before bedtime really helps!

Image courtesy of: http://www.drfrancescott.com

Source:

http://www.rnpedia.com/home/notes/medical-surgical-nursing-notes/neurotransmitters

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