Hi, welcome back to our lecture video on synapses. So far we've pretty much established that the transmission of information along a neuron occurs through an electrical process, but our similar electrical process is also found for the transmission of information at the synapses. While we now understand that chemical processes and not electrical processes are the primary events occurring at synapses, knowledge about the existence of chemical reactions at synapses was only discovered in the early 20th century.
Recall that Sherrington was instrumental in identifying the existence of a synapse. Although Sherrington concluded that there were different physiological processes occurring at the synapses, that caused the transmission of information at synapses to be slower than the transmission of information along an axon. At that particular time, Sherrington still assumed that the exchange of information at the synapse also occurred through electrical processes.
However, not all scientists at the time agreed with Sherrington's conclusions about the electrical processes at the synapse. More precisely, in 1905, A British scientist named T.R. Eliot conducted a study in which he applied the hormone adrenaline to the surface of the heart, the stomach, and the pupils of the eyes.
Eliot observed that the application of adrenaline to these organs caused effects that resembled the effects seen with the activation of the sympathetic nervous system. Based on these observations, Elliot proposed that the sympathetic nervous system works by releasing adrenaline or a similar chemical molecule. However, because Sherrington was a figure who was highly respected by his colleagues, at that time no one dared to challenge Sherrington's assumptions, and the results of Elliot's research were pretty much ignored by other scientists.
Otto Lowy, a German scientist, was one of the people who was interested in Elliot's ideas. One night in 1920, Louis awoke in the middle of the night from a dream. The dream he saw was about a procedure that would prove that neurons communicate through chemicals and not through electrical mechanisms. Since he woke up in the middle of the night, he decided to go back to sleep and to simply carry out his research idea the next day. He jotted down his ideas in a note.
before he went back to sleep. Unfortunately, when he woke up the next morning, he found that his notes were illegible. But Loewy was lucky because the next night he woke up again from the same dream.
Since he had learned from the previous night's mistake, this time Loewy went straight to his laboratory to perform the experiment he saw in his dream. In his research, Loewy dissected two frogs and took out their hearts. To maintain the heart's functions, each heart was kept in a container filled with fluid that mimicked a frog's bodily fluids.
Loewy then stimulated the vagus nerve, which is a nerve that's connected to the heart of the first frog's heart, resulting in the slowing down of that particular heart's rate. After that, he took the fluid from the container that housed the first heart and then transferred the fluid over to the container containing the second heart. This caused a similar decrease in the rate of the second frog's heart. Next, he tried to stimulate another nerve that caused the first frog's heart rate to increase. He then again moved fluid from the first container to the second container.
And Loewy once again observed a similar event. The second heart also experienced an increase in rate after fluid from the first heart's container was transferred over to the second container. Lowy concluded that if the transfer of fluid from the first container to the second container caused a change in the second heart's rate, then this must be caused by the transfer of chemical molecules from the first container to the second container.
According to Lowy, It is impossible for such a large effect to occur if it was simply due to the transfer of electrical ions. Chemicals that are released at the synapse and then affect the work of the postsynaptic neuron are known as neurotransmitter molecules. Currently there are about 100 chemical molecules that are believed to function as neurotransmitters. Most neurotransmitters consist of amino acids with a few exceptions.
One such exception is NO or nitric oxide which is a gas. In general, each neuron releases two or more neurotransmitters. However, no single neuron is capable of releasing all of the existing types of neurotransmitters.
This table shows several, but not all, classes of neurotransmitters. As stated earlier, most of the neurotransmitter molecules are amino acids or are, in one way or another, related to amino acids. Examples of amino acid neurotransmitters include glutamate, GABA, and glycine. Some examples of neurotransmitters from the amino acid family are acetylcholine, serotonin, dopamine, norepinephrine, and epinephrine.
Several other neurotransmitters are classified as peptides, which consist of a series of amino acids and are therefore larger in size. Included in this category are endorphins and substance P. Then, as mentioned earlier, there are also a few neurotransmitters that are gaseous, such as nitric oxide.
The raw materials needed to form neurotransmitter molecules are usually obtained from amino acids contained in the food we eat. As an example, acetylcholine is formed from acetate and choline. Acetate is obtained from acidic foods such as vinegar and lemon, while choline is obtained from milk, eggs, and nuts.
As another example, serotonin is formed from the amino acid tryptophan, which is abundant in soybeans. Other neurotransmitters like dopamine, epinephrine, and norepinephrine are formed from the amino acids phenylalanine and tyrosine, which are obtained from high-protein foods. Then, once neurotransmitters are synthesized, some of them are stored in synaptic sacs or synaptic vesicles, which are located at the presynaptic terminals. If you're still having trouble imagining what a synapse looks like, maybe these images can help you visualize it better. Image A, located at the top left of the screen, is a visualization of the synapse obtained through the use of a special technology called an electron micrograph.
The arrow on the left indicates the presynaptic terminal of a presynaptic neuron, while the arrow on the right points to the dendritic spine of a postsynaptic neuron. The synaptic gap that separates the two neurons is highlighted in yellow. Meanwhile, in figure B, the structures colored in blue are the presynaptic terminals of the presynaptic neurons, while the orange area is the cell body of the postsynaptic neuron. This marks the end of this subtopic's discussions.
At the next subtopic of the synapse chapter, we will continue with additional explanations on synapses. In particular, we will look at the sequence of events that occur at the synapse that enable the exchange of information among neurons. See you at the next video!