Neurotransmitters are chemicals that are secreted by neurons (nerve cells) to carry messages to other neurons or other cells like muscles fibers. They are primarily used for stimulating a nerve impulse (action potentials), but they can also be inhibitory. They are extremely important in brain function, muscle contraction, emotion, and even sleep patterns.
Neurotransmitters function primarily within synapses between neurons (nerve cells). Neurotransmitters are required in muscle function because the presynaptic terminal on a nerve does not make contact with the muscle fibers with which it is interacting. Therefore, the nerve has synaptic vesicles that release neurotransmitters. These neurotransmitters then travel across the synaptic cleft where they bind with receptor sites. This bind between the receptor site and the neurotransmitter results in an action potential .
Neurotransmitters can generally be broken into two basic types: excitatory and inhibitory. The difference in the two lies in their effect on postsynaptic membranes. Excitatory neurotransmitters are involved in stimulating the brain while inhibitory help maintain balance in the brain. . They accomplish these goals through either depolarization or hyperpolarization. Excitatory neurotransmitters use depolarization, which promotes the generation of action potentials. On the other hand, Inhibitory neurotransmitters use hyperpolarization to slow and eventually inhibit the generation of action potentials .
Acetylcholine is a very important neurotransmitter being involved in skeletal muscle movement, smooth muscle regulation, and memory. When a nerve impulse (action potential) reaches the end of the neuron (the axon) acetylcholine released by vesicles into the synaptic cleft. It then travels (is "transmitted") across the synaptic cleft where it binds to receptors on the post-synaptic cell where it stimulates an action potential in the muscle or other cell. Once this process is complete, the acetylcholine molecules are recycled by being broken back down into acetic acid and choline. This breakdown allows acetylcholine to be synthesized without a substantial amount of additional help.
- Main Article: Dopamine
Dopamine is another common inhibitory neurotransmitter. Dopamine is related to the pleasure systems of the brain. Dopamine provides feelings of pleasure when doing naturally satisfying things such as eating. However, research has caused some scientists to believe that Dopamine may be involved in desire rather than simply being a "reward chemical." Additionally, Dopamine is also released in the presence of negative stimuli. Studies have also shown that Dopamine may be involved in predicting pleasurable activity. This means that Dopamine may also be integral in the decision making process
Serotonin could possibly be one of the most influential neurotransmitters in the human body. Some of the important functions that serotonin controls include, sleep, temperature regulation, libido (sexual desire), appetite, learning, and even memory .
Serotonin occurs naturally in the body. It exists as a monoamine neurotransmitter. It gets into the body through the digestion of foods containing an amino acid known as L-Tryptophan . This explains why serotonin is present in the intestinal membranes .
Endorphin is a common type of inhibitory neurotransmitter. The effects involved with endorphin are similar to the effects of the drugs morphine or heroine. Endorphin is also involved in the hibernation patterns of animals.  Endorphin has been known to cause what is known as "runner's high." This feeling is common among habitual runners  and has been compared to the feeling generated by marijuana use.  While there is no concrete evidence that Endorphin causes this feeling, this neurotransmitter is released during periods of moderate exercise.
Effect of Methamphetamine Use
Methamphetamine (or Meth) is a very dangerous stimulant. It is extremely addictive and damaging to users. The problems that surround meth use are directly associated to its effect on neurotransmitters. Meth use causes the brain to release increased amounts of Dopamine while causing less of it to be reabsorbed. This causes there to be more Dopamine in the brain than is necessary. This causes a feeling of euphoria. 
Chronic use of methamphetamine causes many problems for the brain. The problems are not only chemical, but also structural. Studies have shown that some of the emotional and cognitive problems come as a result of these structural problems. Long term use can result in the dopamine receptors being destroyed. This destruction of receptors makes it virtually impossible for users to feel any sort of pleasure. 
Serotonin's Effect on Mental Health
Serotonin has been considered an integral part of mental health, especially in the context of social behavior, for a long time. However, the precise purpose of serotonin was never entirely clear. Studies have shown that people tend to be more aggressive when they are hungry. It is now believed that this aggression comes from a lack of L-Tryptophan, an amino acid that can only enter the body through food . Serotonin has also been known to have a role in depression. Scientists believe that depression comes from an inbalance of 2 primary neurotransmitters, Serotonin and Norepinephrine. In 1988, new drugs called selective serotonin-reuptake inhibitors (or SSRIs). These drugs help improve the effectiveness of serotonin in the brain by making present serotonin useful to the brain for a longer period of time .
Table of transmitters
A scanning electron microscope picture of a nerve ending broken open to reveal vesicles (orange and blue) containing Neurotransmitters.
- Wile, Jay L., and Shannon, Marilyn M. The Human Body: Fearfully and Wonderfully Made!. Cincinnati: Apologia Educational Ministries, Inc., 2001. Print. (P.138,205)
- Neurotransmitters and its Types from Biotech Articles
- How do Excitatory Neurotransmitters differ from Inhibitory Neurotransmitters? Response from Answers.com
- Acetylcholine-What is It? by Nan Little and Published by Insight Journal.
- Dopamine from International Society for Complexity, Information, and Design
- Merriam Webster Dictionary
- Function of Serotonin
- Women's Health Serotonin
- The Reality of the "Runners High by UPMC
- CNN Health study links marijuana buzz to 'runner's high'
- Methamphetamine from the National Institute on Drug Abuse
- The Meth Epidemic from PBS Frontline
- Insight into Clinical Disorders Characterized by Low Serotonin Level from Medical News Today
- Serotonin and Depression from the University of Alabama in Birmingham