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Overview of the Nervous System

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The Manual's Editorial Staff

Last full review/revision Jun 2020| Content last modified Jun 2020
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What is the nervous system?

The nervous system is your body's information processing and communication system. It receives messages, processes information, and then sends signals to the rest of your body telling it what to do.

The nervous system is made up of the:

  • Brain

  • Spinal cord

  • Nerves

The nervous system is involved in everything you think, say, and do.

The brain receives information from your eyes, ears, nose, and other sense organs. It processes information and generates thoughts and ideas. Then the brain sends messages to your body. For example, it tells your muscles how to move so you can walk, talk, and do the things you want your body to do. Your brain also controls a lot of what your body does without you thinking about it. For example, your brain automatically adjusts your breathing, heart rate, and blood pressure.

The spinal cord runs from your brain down your back through the hollow center of your spine What is your spine? Your spine is your backbone. It's actually a long line of 24 bones called vertebrae plus your tailbone (sacrum). The vertebrae start below your skull and go all the way down to your pelvis.... read more . Nerves in the brain send messages down your spinal cord. Other nerves in your spinal cord then relay those messages to your body. The spinal cord also carries messages from the body to your brain.

The central nervous system is the brain and spinal cord. The nerves outside the brain and spinal cord are called the peripheral nervous system.

How does your nervous system work?

Your nervous system is made of:

  • Nerve cells and their fibers

There are billions and billions of nerve cells in your brain, your spinal cord, and in clumps just outside your spinal cord.

Each nerve cell has a microscopic body:

  • The body of the nerve cell is responsible for processing nutrients and keeping the cell alive

Each nerve cell has fibers going to and from it:

  • Input fibers receive signals from other nerve cells or from receptors in your sense organs

  • Output fibers send signals to other nerves, to muscles, or to other organs

  • Signals travel only one way in a nerve cell

Sometimes nerve fibers are dozens of centimeters long. For example, a single nerve fiber may run from near your spinal cord all the way to your toe. Some nerve fibers that go to your skin or your organs have sensory receptors. For example, the receptors at the end of nerve fibers in your skin detect things that are sharp or hot.

Although nerve fibers and their signals act a lot like a wire carrying electrical signals, that's not exactly right. Nerve cells really send their signals using chemicals.

  • Chemical changes take place progressively along the length of a nerve fiber

  • When the chemical changes reach the end of the nerve fiber, they release other chemicals called neurotransmitters

  • The neurotransmitters drift across a microscopic gap where they hit the chemical receptors of another cell

  • The neurotransmitters trigger chemical changes in that other cell

  • If that cell is a nerve cell, then the progressive chemical changes continue down the fibers of that cell to pass the signal along

  • Other times the next cell is not a nerve cell—for example, if the next cell is a muscle cell, then the neurotransmitter causes chemical changes that make the muscle cell contract

Typical Structure of a Nerve Cell

A nerve cell (neuron) consists of a large cell body and nerve fibers—one elongated extension (axon) for sending impulses and usually many branches (dendrites) for receiving impulses. The impulses from the axon cross a synapse (the junction between two nerve cells) to the dendrite of another cell.

Each large axon is surrounded by oligodendrocytes in the brain and spinal cord and by Schwann cells in the peripheral nervous system. The membranes of these cells consist of a fat (lipoprotein) called myelin. The membranes are wrapped tightly around the axon, forming a multilayered sheath. This myelin sheath resembles insulation, such as that around an electrical wire. Nerve impulses travel much faster in nerves with a myelin sheath than in those without one.

Typical Structure of a Nerve Cell

One nerve cell sends just one kind of signal that can't carry a lot of information. However, when billions of nerve cells are interconnected like they are in your brain, they form a very powerful information processor.

What can go wrong with your nervous system?

Many problems can affect your nervous system, including:

Once nerve cells in your brain or spinal cord die, they can't grow back. However, nerve fibers sometimes can grow back if their cell body isn't hurt. It can take months for nerve fibers to grow back. Doctors sometimes can sew smaller nerves back together and get them to work again. Doctors can't repair the spinal cord or the brain.

How does getting older affect the nervous system?

As people get older, they have fewer nerve cells in their brain. It's harder for your body to make new nerve cells and fix damaged ones. Compared to younger people, older people are more likely to have:

  • Trouble remembering recent events or learning new things

  • Trouble using words

  • Slower reaction times

  • Less sensation in their skin

  • Clumsiness

  • Slower reflexes

The following may help keep your brain sharp longer:

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