Neurotransmitter
Communication of information between neurons is accomplished by movement of chemicals across a small gap called the synapse. Chemicals, called neurotransmitters, are released from one neuron at the presynaptic nerve terminal. Neurotransmitters then cross the synapse where they may be accepted by the next neuron at a specialized site called a receptor. The action that follows activation of a receptor site may be either depolarization (an excitatory postsynaptic potential) or hyperpolarization (an inhibitory postsynaptic potential). A depolarization makes it MORE likely that an action potential will fire; a hyperpolarization makes it LESS likely that an action potential will fire.
Plants and herbs have been used around the world for millennia to alter brain function. In most places around the world today, plants are used daily to alter mental function. For example, the caffeine in coffee and black and green tea is a stimulant that helps people to wake up in the morning. Chamomile tea helps people to relax in the evening and is considered to be a depressant. Some people regularly use alcohol, nicotine, and illegal drugs such as cocaine; these drugs are derived from plants and have large effects on the human nervous system.
Some plants have chemicals that mimic the action of natural chemicals found in the body. These neuroactive plant chemicals can therefore cause the same behavior as the body’s. Communication of information between neurons is accomplished by movement of chemicals across a small gap called the synapse. Chemicals, called neurotransmitters, are released from one neuron, cross the synapse and may be accepted by the next neuron at a specialized site called a receptor.
At the synapse a variety of things can go wrong if certain other chemicals are present.
Some chemicals called blockers can occupy the receptor sites. Other chemicals called reuptake inhibitors prevent the neurotransmitter from being reabsorbed into the presynaptic neuron. Mimics are shaped very much like a given neurotransmitter and occupy the receptor site causing extra nerve firings.
Discovery
As you have learned in the other Sowing the Seeds of Neuroscience labs, the nervous system communicates using both electrical and chemical transmissions. But people didn’t always know this to be true. In 1921, an Austrian scientist named Otto Loewi discovered that the nervous system controlled the circulatory system using both electrical and chemical signals. Before his discovery, no one knew whether nerve cells used chemicals to communicate.
Otto Loewi hypothesized that nerves communicate using chemicals, but he wasn’t sure how to test his hypothesis. An experiment came to him in a dream. He woke up and immediately dissected the hearts out of two frogs and placed them in separate chambers full of saline solution. One heart (Heart #1) was still connected to the vagus nerve which controls heart rate. The other heart (Heart #2) was not connected to the vagus nerve. Otto Loewi electrically stimulated the vagus nerve of Heart #1, causing the heart rate to slow down. Loewi then removed some of the saline solution from Heart #1 and added it to Heart #2…and the heart rate of Heart #2 also slowed down!
From this experiment, Loewi learned that the nervous system controls heart rate using both electrical and chemical communication. Specifically, he learned that stimulating the vagus nerve released a chemical that slowed down the heart rate. We now know this chemical as the neurotransmitter acetylcholine. This discovery earned Otto Loewi the Nobel Prize in Physiology or Medicine in 1936 as well as the nickname “The Father of Neuroscience.”
Amazingly, the idea for his famous heart experiment came to Otto Loewi in his sleep. In Loewi's own words:
"In the night of Easter Saturday, 1921, I awoke, turned on the light, and jotted down a few notes on a tiny slip of paper. Then I fell asleep again. It occurred to me at six o'clock in the morning that during the night I had written down something most important, but I was unable to decipher the scrawl. That Sunday was the most desperate day in my whole scientific life. During the next night, however, I awoke again, at three o'clock, and I remembered what it was. This time I did not take any risk; I got up immediately, went to the laboratory, made the experiment on the frog's heart, and at five o' clock the chemical transmission of nervous impulse was conclusively proved."
~Quoted from Loewi, O., From the Workshop of Discoveries, Lawrence: University of Kansas Press, 1953.
Common Neurotransmitters
glutamate- excitatory
GABA- inhibitory in brain
acetylcholine- neuromuscular junctions
epinephrine (adrenaline)- arousal and reward
norepinephrine (noradrenaline)- CNS and symphathetic nervous system
Links
NIH- Understanding Neurobiology- Nerves in series
NIH- Understanding Neurobiology- Inhibitatory and Excitatory responses