Synapses: The brain’s tiny communication centers

Don Partridge & Kevin Caldwell, UNM College of Medicine
 

Schizophrenia, ADHD, depression, addiction, recreational drugs – what do all of these have in common? It turns out that they are all involved with the chemicals that the brain uses to transmit information. The place where these chemicals are made and have their effects is a tiny structure called a synapse. In the century since synapses were first described, the study of these amazingly complicated microscopic structures have become central to understanding how the brain works. Although each nerve cell in the brain is a separate entity, two adjacent nerve cells approach each other so closely at a synapse that a moderate sized protein molecule would span the gap. Not only are synapses small, but they are numerous – so much so that a small piece of brain the size of a rice grain would contain 10 billion of them.
 
So, synapses are small and they are numerous, but why have so many neuroscientists – many earning Nobel Prizes for their efforts - devoted their careers to studying them? There are probably as many answers to this question as there neuroscientists, but we will concentrate on three special and fascinating features of these tiny structures.
 
First, the obligatory mode of communication within a single nerve cell is electrical. Like radio broadcasts, nerve cells change both the amplitude (AM) and the frequency (FM) of their electrical signals, but when it comes to the synapse suddenly these electrical signals are converted to chemical signals. In general, it is difficult to interfere with the electrical signals of nerve cells, but there are many ways to affect the chemical signals at synapses. Much of the pharmaceutical industry (both legal and illegal) is focused on modifying the chemical signals at synapses.
 
Second, many synapses are highly adapted to being able to change their chemical signaling in response to their past history. This property is especially interesting because it is thought to be the basis of those most important functions of the brain, learning and memory. As you read this, synapses throughout your brain are being modified as some become more effective and others less effective. Your brain will never be the same again!
 
Third, the operation of synapses can go awry, either because of some genetic error or because of some insult to the brain. This can lead to such diseases as schizophrenia, ADHD and depression and it is the basis of drug addictions. Better understanding how synapses work will help scientists to understand what has gone wrong in these diseases and hopefully then to devise better treatments.
 
Our discussion will focus on some of what we know about how synapses work and why they’ve been so intriguing to neuroscience researchers. First we’ll look at the input side of the synapse and how it receives an electrical signal and converts it to a chemical signal. Next we’ll discuss the chemicals used for this signaling and will use some familiar names – like adrenaline and dopamine – and some less familiar names – like GABA. Then we’ll address the problem of how the chemical signal is converted back to an electrical signal that either excites or inhibits the nerve cell on the output side of the synapse.
 
Once we have a bit of an understanding of how these little structures work, we’ll take a look at human brains to see where these synapses are located and how they’re used to communicate important information between different brain areas. Finally, we’ll tackle some of the important problems that neuroscientists study, such as the basis of learning and memory and diseases of the brain, such as depression, ADHD, or schizophrenia.
 

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