Neurons and What They Do
Neurons are the building blocks of the human nervous system. These highly specialized cells are responsible for communication throughout the nervous system, sending and receiving information from one part of the body to another. The nervous system is comprised of many billions of neurons — some estimates suggest the brain contains over 100 billion — and they come in many varieties, each with a highly specialized function.
There are three basic types of neurons. Sensory neurons are those specialized to communicate environmental information about such things as heat, sound, and light. Motor neurons are those that convey information to the muscles and glands. Interneurons represent the large group of neurons within the body, and they transmit information between other neurons. While these neurons may differ in function, they do have some essential things in common.
The Basic Structure of a Neuron
Like other cells, neurons have a cell body with a nucleus and other structures that keep them alive. Also like other cells, the nucleus contains genes that cause a neuron to become what it is in the first place. Early in life — mostly during the prenatal period — these genes also cause neurons to reproduce as the nervous system matures.
The remaining structures are involved in processing information. A neuron receives signals from other neurons at branches called dendrites and at smaller receptor sites on the cell body. It sends signals to other neurons via its axon terminals. In between is the axon, which, when conditions are right, generates a signal to its terminals. Many axons also have a myelin sheath, which serves a purpose similar to that of the insulation on an electrical wire.
So how exactly do neurons send information? Messages are usually received by the dendrites and then propagated down the axon via an electrical impulse known as an action potential. In some cases, such as when neurons are very close together, this electrical signal can pass almost instantaneously from one neuron to the next. While some neural communication occurs electrically, approximately 99 percent of all neural transmission happens chemically. Remember that neurons aren't directly connected to other neurons. A neuron's axon terminal and another's dendrite or receptor site “connect” via a synapse, which contains a minuscule gap across which chemical messengers, known as neurotransmitters, flow. Within the human brain in particular, a given neuron may synapse with thousands of other neurons, giving rise to many trillions of such interconnections — a level of complexity that staggers the imagination and helps you see why unraveling the workings of the nervous system is such a formidable task.
Neurotransmitters are minute biochemicals; scientists have identified more than 100 different neurotransmitters, but some estimate that there are perhaps several hundred different kinds. Some neurotransmitters stimulate the next neuron in line, while others inhibit it.
A “nervous breakdown” is not what it might seem. If your central nervous system were to break down like a car engine, you wouldn't run at all, and that would be the end of things. Instead, typically because of extreme stress, you lose the ability to think clearly and control your emotions; with proper care, this usually lasts for only a few days or weeks.
On a larger scale, an abundance or shortage of certain neurotransmitters in different areas of the brain affects our thought processes, emotionality, and overall level of arousal. One way psychologists know this is that alcohol and other “psychoactive” drugs — including medicinal ones — affect the brain through altering the levels of specific neurotransmitters. For example, “depressant” drugs alter certain neurotransmitter levels with the general effect of slowing down brain functioning, whereas “stimulant” drugs alter certain levels and speed it up. Another way neurotransmitters are implicated is through their association with certain disorders. For example, Parkinson's disease is associated with the degeneration of neurons that produce dopamine, a neurotransmitter that is essential to normal brain function.