Brain and Nervous System
The Brain Is the Boss
When you’re taking a big math test, you know that your brain is hard at work. But your brain is doing a lot more than just remembering formulas. Those sweaty palms you get as the test starts? That’s your brain at work. The relief you feel when you know an answer’s right? That’s your brain too. And yes, your brain is even in charge when you take a minute to daydream about the big party on Friday night.
The brain may simply be the bossiest part of the body: It tells virtually every other part of your body what to do, all the time, whether you’re aware of it or not. It controls what you think and feel, how you learn and remember, and the way you move. It also controls things you might not think about — like the beating of your heart and whether you feel sleepy or awake.
The Brain & Nervous System in Everyday Life
If the brain is like a central computer that controls all the functions of your body, then the nervous system is like a network that sends messages back and forth from the brain to different parts of the body. It does this via the spinal cord, which runs from the brain down through the back and contains threadlike nerves that branch out to every organ and body part.
When a message comes into the brain from anywhere in the body, the brain tells the body how to react. For example, if you accidentally touch a hot stove, the nerves in your skin shoot a message of pain to your brain. The brain then sends a message back telling the muscles in your hand to pull away. Luckily, this neurological relay race takes a lot less time than it just took to read about it!
How the Brain Works
Considering everything it does, the human brain is incredibly compact, weighing just 3 pounds. Its many folds and grooves, though, provide it with the additional surface area necessary for storing all of the body’s important information.
The spinal cord, on the other hand, is a long bundle of nerve tissue about 18 inches long and ¾ inch thick. It extends from the lower part of the brain down through spine. Along the way, various nerves branch out to the entire body. These make up the peripheral nervous system.
Both the brain and the spinal cord are protected by bone: the brain by the bones of the skull, and the spinal cord by the set of ring-shaped bones called vertebrae that make up the spine. They’re both cushioned by layers of membranes called meninges as well as a special fluid called cerebrospinal fluid. This fluid helps protect the nerve tissue, keep it healthy, and remove waste products.
The brain is made up of three main sections: the forebrain, the midbrain, and the hindbrain.
The forebrain is the largest and most complex part of the brain. It consists of the cerebrum — the area with all the folds and grooves typically seen in pictures of the brain — as well as some other structures beneath it.
The cerebrum contains the information that essentially makes us who we are: our intelligence, memory, personality, emotion, speech, and ability to feel and move. Specific areas of the cerebrum are in charge of processing these different types of information. These are called lobes, and there are four of them: the frontal, parietal, temporal, and occipital.
The cerebrum has right and left halves, called hemispheres, which are connected in the middle by a band of nerve fibers (the corpus collosum) that enables the two sides to communicate.
Although these halves may look like mirror images of each other, many scientists believe they have different functions. The left side is considered the logical, analytical, objective side. The right side is thought to be more intuitive, creative, and subjective. So when you’re doing a math problem you’re using the left side, and when you’re listening to music you’re using the right side. Scientists think that some people are more “right-brained” or “left-brained” while others are more “whole-brained,” meaning they use both halves of their brain to the same degree.
The outer layer of the cerebrum is called the cortex (also known as “gray matter”). Information collected by the five senses comes into the brain from the spinal cord to the cortex. This information is then directed to other parts of the nervous system for further processing. For example, when you touch the hot stove, not only does a message go out to move your hand but one also goes to another part of the brain to help you remember not to do that again.
In the inner part of the forebrain sit the thalamus, hypothalamus, and pituitary gland. The thalamus carries messages from the sensory organs like the eyes, ears, nose, and fingers to the cortex. The hypothalamus controls body temperature, thirst, appetite, sleep patterns, and other processes in our bodies that happen automatically. It also controls the pituitary gland, which makes the hormones that control our growth, metabolism, digestion, sexual maturity, and how we respond to stress.
The midbrain, located underneath the middle of the forebrain, acts as a master coordinator for all the messages going in and out of the brain to the spinal cord.
The hindbrain sits underneath the back end of the cerebrum, and it consists of the cerebellum, pons, and medulla. The cerebellum — also called the “little brain” because it looks like a small version of the cerebrum — is responsible for balance, movement, and coordination. The pons and the medulla, along with the midbrain, are often called the brainstem. The brainstem takes in, sends out, and coordinates all of the brain’s messages. It is also controls many of the body’s automatic functions, like breathing, heart rate, blood pressure, swallowing, digestion, and blinking.
How the Nervous System Works
The basic functioning of the nervous system depends a lot on tiny cells called neurons. The brain has billions of them, and they have many specialized jobs. For example, sensory neurons take information from the eyes, ears, nose, tongue, and skin to the brain. Motor neurons carry messages away from the brain and back to the rest of the body.
All neurons relay information to each other through a complex electrochemical process, making connections that affect the way we think, learn, move, and behave.
Intelligence, Learning, and Memory
When you learn things, messages travel from one neuron to another, over and over. Then the brain creates connections (or pathways) between the neurons, so things become easier and you can do them better and better.
In young children, the brain is highly adaptable. In fact, when one part of a young child’s brain is injured, another part may learn to take over some of the lost function. But as we age, the brain has to work harder to make new neural pathways, making it more difficult to master new tasks or change established behavior patterns. That’s why many scientists believe it’s important to keep challenging your brain to learn new things and make new connections — it helps keep the brain active over the course of a lifetime.
Memory is another complex function of the brain. The things we’ve done, learned, and seen are first processed in the cortex, and then, if we sense that this information is important enough to remember permanently, it’s passed inward to other regions of the brain (such as the hippocampus and amygdala) for long-term storage and retrieval. As these messages travel through the brain, they create pathways that serve as the basis of our memory.
Different parts of the cerebrum are responsible for moving different body parts. The left side of the brain controls the movements of the right side of the body, and the right side of the brain controls the movements of the left side of the body. When you kick a soccer ball with your right foot, for example, it’s the left side of your brain that sends the message allowing you to do it.
Basic Body Functions
A part of the peripheral nervous system called the autonomic nervous system is responsible for controlling many of the body processes we almost never need to think about, like breathing, digestion, sweating, and shivering. The autonomic nervous system has two parts: the sympathetic and the parasympathetic nervous systems.
The sympathetic nervous system prepares the body for sudden stress, like if you see a robbery taking place. When something frightening happens, the sympathetic nervous system makes the heart beat faster so that it sends blood more quickly to the different body parts that might need it. It also causes the adrenal glands at the top of the kidneys to release adrenaline, a hormone that helps give extra power to the muscles for a quick getaway. This process is known as the body’s “fight or flight” response.
The parasympathetic nervous system does the exact opposite: It prepares the body for rest. It also helps the digestive tract move along so our bodies can efficiently take in nutrients from the food we eat.
Your eyes may watch as your best friend walks toward you — but without the brain, you wouldn’t even recognize her. Pepperoni pizza sure is delicious — but without the brain, your taste buds wouldn’t be able to tell if you were eating pizza or the box it came in. None of your senses would be useful without the processing that occurs in the brain.
- Sight. Sight probably tells us more about the world than any other sense. Light entering the eye forms an upside-down image on the retina. The retina transforms the light into nerve signals for the brain. The brain then turns the image right-side up and tells us what we are seeing.
- Hearing. Every sound we hear is the result of sound waves entering our ears and causing our eardrums to vibrate. These vibrations are then transferred along the tiny bones of the middle ear and converted into nerve signals. The cortex then processes these signals, telling us what we are hearing.
- Taste. The tongue contains small groups of sensory cells called taste buds that react to chemicals in foods. Taste buds react to sweet, sour, salty, and bitter. Messages are sent from the taste buds to the areas in the cortex responsible for processing taste.
- Smell. Olfactory cells in the mucous membranes lining each nostril react to chemicals we breathe in and send messages along specific nerves to the brain — which, according to experts, can distinguish between more than 10,000 different smells. With that kind of sensitivity, it’s no wonder research suggests that smells are very closely linked to our memories.
- Touch. The skin contains more than 4 million sensory receptors — mostly concentrated in the fingers, tongue, and lips — that gather information related to touch, pressure, temperature, and pain and send it to the brain for processing and reaction.
Things That Can Go Wrong With the Brain
Because the brain controls just about everything, when something goes wrong, it’s often serious and can affect many different parts of the body. Inherited diseases, brain disorders associated with mental illness, and head injuries can all affect the way the brain works and upset the daily activities of the rest of the body.
Here are some of the problems that can affect the brain:
Brain tumors. A tumor is an abnormal tissue growth in the brain. A tumor in the brain may grow slowly and produce few symptoms until it becomes large. Or a tumor can grow and spread rapidly, causing severe and quickly worsening symptoms.
Brain tumors can be benign or malignant. They usually grow in one place and may be curable through surgery if they’re located in a place where they can be removed without damaging the normal tissue near the tumor. A malignant tumor is cancerous and more likely to grow rapidly and spread.
Cerebral palsy. This condition is the result of a developmental defect or damage to the brain before or during a child’s birth, or during the first few years of life. Cerebral palsy affects the motor areas of the brain. A person with cerebral palsy may have average intelligence or can have severe developmental delays or mental retardation.
Cerebral palsy can affect body movement in many different ways. In mild cases of cerebral palsy, a person may have minor muscle weakness in the arms and legs. In other cases, there may be more severe motor impairment — a person may have trouble talking and performing basic movements like walking.
Epilepsy. Epilepsy is a condition of the nervous system that can lead people to have seizures. Partial seizures involve specific areas of the brain, and symptoms vary depending on the location of the seizure activity. Other seizures, called generalized seizures, involve a larger portion of the brain and usually cause uncontrolled movements of the entire body and loss of consciousness. Although in many cases doctors don’t know what causes it, epilepsy can be related to brain injury, tumors, or infections. The tendency to develop epilepsy may be inherited in families.
Headaches. Of the many different types of headaches, some of the more common are:
- tension headache (the most common type) is caused by muscle tension in the head, neck, and shoulders
- migraine is an intense, recurring headache with an unclear cause
- cluster headache is often considered to be a form of migraine
Migraines occur with or without warning and may last for several hours or days. The tendency to develop migraines may be inherited in families. A person who has migraines may get one after being exposed to a “trigger” (such as a particular food), and may feel dizzy, numb, nauseated, and sensitive to light, and may see flashing zigzag lines before their eyes.
Meningitis and encephalitis. These are infections of the brain and spinal cord that are usually caused by bacteria or viruses. Meningitis is an inflammation of the coverings of the brain and spinal cord, and encephalitis is an inflammation of the brain tissue. Both conditions may result in permanent injury to the brain.
Mental illness. Mental illnesses involve a wide range of problems in how people think and function. Experts now know that certain mental illnesses are linked to structural or chemical problems in the person’s brain. Some mental illnesses are inherited, but even though researchers know that these illnesses run in families, they often can’t pinpoint what causes them in the first place. Injuries to the brain and chronic drug or alcohol abuse also can trigger some mental illnesses.
Signs of chronic mental illnesses such as bipolar disorder or schizophrenia may first show up in childhood. Mental illnesses that can be seen in teens include depression, eating disorders such as bulimia or anorexia nervosa, obsessive-compulsive disorder (OCD), and phobias.
Head injuries. Head injuries fit into two categories: external (usually scalp) injuries and internal injuries. Internal injuries may involve the skull, the blood vessels within the skull, or the brain. Fortunately, most falls or blows to the head usually injure the scalp only, which is usually more frightening than threatening. An internal head injury is usually more serious because the skull serves as the protective helmet for the delicate brain.
Concussions are a type of internal head injury that causes the temporary loss of normal brain function. Repeated concussions can lead to permanent brain injury. Playing sports is one of the most common ways teens get concussions. The right protective gear, worn the right way, can help people avoid concussions. But even helmets don’t prevent all concussions. If you get hit on the head while playing sports and think there’s a chance you have a concussion, stop playing right away and let a coach or parent know what’s going on.
Reviewed by: Yamini Durani, MD
Date reviewed: October 2012