The brain is the most complex part of the human body. This three-pound organ is the seat of intelligence, interpreter of the senses, initiator of body movement, and controller of behavior. Lying in its bony shell and washed by protective fluid, the brain is the source of all the qualities that define our humanity. It is the crown jewel of the human body.

This fact sheet is a basic introduction to the human brain. It can help you understand how the healthy brain works, how to keep your brain healthy, and what happens when the brain doesn't work like it should.

The brain is like a group of experts. All the parts of the brain work together, but each part has its own special responsibilities. The brain can be divided into three basic units: the forebrain, the midbrain, and the hindbrain.

The hindbrain includes the upper part of the spinal cord, the brain stem, and a wrinkled ball of tissue called the cerebellum. The hindbrain controls the body’s vital functions such as respiration and heart rate.

The cerebellum coordinates movement and is involved in learned movements. When you play the piano or hit a tennis ball, you are activating the cerebellum.

The uppermost part of the brainstem is the midbrain, which controls some reflex actions and is part of the circuit involved in the control of eye movements and other voluntary movements. The forebrain is the largest and most highly developed part of the human brain: it consists primarily of the cerebrum and the structures hidden beneath it (see "The Inner Brain").

When people see pictures of the brain it is usually the cerebrum that they notice. The cerebrum sits at the topmost part of the brain and is the source of conscious thoughts and actions. It holds your memories and allows you to plan, imagine, and think. It allows you to recognize friends, read, and play games.

The cerebrum is split into two halves (hemispheres) by a deep fissure. The two cerebral hemispheres communicate with each other through a thick tract of nerve fibers that lies at the base of this fissure, called the corpus callosum. Although the two hemispheres seem to be mirror images of each other, they are different. For instance, the ability to form words seems to lie primarily in the left hemisphere, while the right hemisphere seems to control many abstract reasoning skills.

For some as-yet-unknown reason, nearly all of the signals from the brain to the body and vice versa cross over on their way to and from the brain. This means that the right cerebral hemisphere primarily controls the left side of the body, and the left hemisphere primarily controls the right side. When one side of the brain is damaged, the opposite side of the body is affected. For example, a stroke in the right hemisphere of the brain can leave the left arm and leg paralyzed.

Coating the surface of the cerebrum and the cerebellum is a vital layer of tissue the thickness of a stack of two or three dimes. It is called the cortex, from the Latin word for bark. Most of the actual information processing in the brain takes place in the cerebral cortex. When people talk about "gray matter" in the brain, they are talking about the cortex. The cortex is gray because nerves in this area lack the insulation that makes most other parts of the brain appear to be white. The folds in the brain add to its surface area and therefore increase the amount of gray matter and the volume of information that can be processed.

Each cerebral hemisphere can be divided into sections, or lobes, each of which specializes in different functions. To understand each lobe and its specialty, we will take a tour of the cerebral hemispheres.

Deep within the brain, hidden from view, lie structures that are the gatekeepers between the spinal cord and the cerebral hemispheres. These structures not only determine our emotional state, but they also modify our perceptions and responses and allow us to initiate movements that without thinking about them. Like the lobes in the cerebral hemispheres, the structures described below come in pairs: each is duplicated in the opposite half of the brain.

The hypothalamus, about the size of a pearl, directs a multitude of important functions. It wakes you up in the morning and gets the adrenaline flowing during a test or job interview. The hypothalamus is also an important emotional center, controlling the chemicals that make you feel exhilarated, angry, or unhappy. Near the hypothalamus lies the thalamus, a major clearinghouse for information going to and from the spinal cord and the cerebrum.

An arching tract of nerve cells leads from the hypothalamus and the thalamus to the hippocampus. This tiny nub acts as a memory indexer—sending memories out to the appropriate part of the cerebral hemisphere for long-term storage and retrieving them when necessary. The basal ganglia (not shown) are clusters of nerve cells surrounding the thalamus. They are responsible for initiating and integrating movements. Parkinson’s disease, which results in tremors, rigidity, and a stiff, shuffling walk, affects the nerve cells in the basal ganglia.

The brain and the rest of the nervous system are composed of many different types of cells, but the primary functional unit is a cell called the neuron. All sensations, movements, thoughts, memories, and feelings are the result of signals that pass through neurons. Neurons consist of three parts: the cell body, dendrites, and the axon.

The cell body contains the nucleus, where most of the molecules that the neuron needs to survive and function are manufactured. Dendrites extend out from the cell body like the branches of a tree and receive messages from other nerve cells. Signals then pass from the dendrites through the cell body and travel away from the cell body down an axon to another neuron, a muscle cell, or cells in some other organ.

The neuron is usually surrounded by many support cells. Some types of cells wrap around the axon to form an insulating myelin sheath. Myelin is a fatty molecule which provides insulation for the axon and helps nerve signals travel faster and farther. Axons may be very short, such as those that carry signals from one cell in the cortex to another cell less than a hair’s width away. Other axons may be very long, such as those that carry messages from the brain all the way down the spinal cord.

Scientists have learned a great deal about neurons by studying the synapse—the place where a signal passes from the neuron to another cell. When the signal reaches the end of the axon it stimulates the release of tiny sacs called vesicles. These vesicles release chemicals known as neurotransmitters into the synaptic cleft. The neurotransmitters cross the synapse and attach to receptors on the neighboring cell. These receptors can change the properties of the receiving cell. If the receiving cell is also a neuron, the signal can continue the transmission to the next cell.

Neurotransmitters are chemicals that brain cells use to talk to each other. Some neurotransmitters make cells more active (called excitatory) while others block or dampen a cell's activity (called inhibitory).

• Acetylcholine is an excitatory neurotransmitter. It governs muscle contractions and causes glands to secrete hormones. Alzheimer’s disease, which initially affects memory formation, is associated with a shortage of acetylcholine.
• Glutamate is a major excitatory neurotransmitter. Too much glutamate can kill or damage neurons and has been linked to disorders including Parkinson's disease, stroke, seizures, and increased sensitivity to pain.
• GABA (gamma-aminobutyric acid) is an inhibitory neurotransmitter that helps control muscle activity and is an important part of the visual system. Drugs that increase GABA levels in the brain are used to treat epileptic seizures and tremors in patients with Huntington’s disease.
• Serotonin is a neurotransmitter that constricts blood vessels and brings on sleep. It is also involved in temperature regulation. Low levels of serotonin may cause sleep problems and depression, while too much serotonin can lead to seizures.
• Dopamine can be excitatory or inhibitory and is involved in mood and the control of complex movements. The loss of dopamine activity in some portions of the brain leads to the muscular rigidity of Parkinson’s disease. Many medications used to treat mental health disorders and conditions work by modifying the action of dopamine in the brain.

The brain is one of the hardest working organs in the body. When the brain is healthy it functions quickly and automatically. But when problems occur, the results can be devastating. NINDS supports research on hundreds of neurological disorders. Knowing more about the brain can lead to the development of new treatments for diseases and disorders of the nervous system and improve many areas of human health.