Quick Facts: Overview of the Nervous System - Merck Manuals Consumer Version (2024)

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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.

Overview of the Nervous System

The nervous system is made up of the:

Brain
Spinal cord
Nerves

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

Your brain The Brain Your brain is like the central processing unit in a computer The brain receives information from your eyes, ears, nose, and other sense organs. It processes information, generates thoughts and... read more is like the central processing unit (CPU) in a computer

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.

Your spinal cord The Spinal Cord The spinal cord is a long tube made up of nerves that goes from the brain down your back through the hollow center of your spine. The spinal cord is like a thick electrical cable that carries... read more is a long tube of nerves like a thick electrical cable.

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.

Your nerves The Nerves A nerve is a bundle of nerve fibers from many nerve cells. The fibers are bundled together for strength and protection from injury. Your nerves are like signal wires Nerves send messages back... read more are like signal wires.

Each nerve contains fibers from many nerve cells The Nerves A nerve is a bundle of nerve fibers from many nerve cells. The fibers are bundled together for strength and protection from injury. Your nerves are like signal wires Nerves send messages back... read more . The fibers are bundled together for strength and protection from injury.

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

Typical Structure of a Nerve Cell

A nerve cell (neuron) consists of a large cell body and nerve fibers—1 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 2 nerve cells) to the dendrite of another cell.

Many large nerve cells have axons that are 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.

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:

Injuries that destroy nerve cells or cut nerves
Infections such as meningitis Meningitis or encephalitis Encephalitis Encephalitis is inflammation of your brain, usually caused by an infection. Encephalitis can be caused by a virus directly infecting your brain or when a virus or a misguided immune reaction... read more
Blockage or rupture of blood vessels that feed the nervous system, causing a stroke Stroke A stroke is a sudden brain problem that happens when a blood vessel in your brain either gets blocked or breaks open and bleeds. As with all organs in the body, the brain needs oxygen and nutrients... read more
Cancer of the brain Overview of Brain Tumors A brain tumor is a growth in your brain that may or may not be cancer. Brain tumors may start in your brain or may have spread to your brain (metastasized) from another part of your body. Brain... read more
Damage to the nerve fibers from diabetes Diabetes Diabetes is a disease in which your blood sugar (glucose) levels are too high. You get diabetes if your body's normal way of controlling blood sugar isn't working right. There are 2 types of... read more or multiple sclerosis Multiple Sclerosis (MS) In multiple sclerosis, patches of myelin (the substance that covers most nerve fibers) and underlying nerve fibers in the brain, optic nerves, and spinal cord are damaged or destroyed. The cause... read more

Once nerve cells in your brain or spinal cord die, they can't grow back. However, sometimes other nearby brain cells can learn to take over for dead brain cells. 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 age affect the nervous system?

As people get older, they have fewer nerve cells in their brain. 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:

Physical exercise
Limiting alcohol
Not smoking
Getting treatment if you have high blood pressure High Blood Pressure Each heart beat pushes blood through your arteries. Arteries are the blood vessels that carry blood from your heart to your body. Blood pressure is the pressure of blood in your arteries. Without... read more , high cholesterol High Cholesterol Cholesterol is a type of fat. Fats are also called lipids. Cholesterol is a type of lipid—an essential element contained in all human cells. However, excess lipids and other fatty substances... read more , or diabetes Diabetes Diabetes is a disease in which your blood sugar (glucose) levels are too high. You get diabetes if your body's normal way of controlling blood sugar isn't working right. There are 2 types of... read more

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