The nervous system is a complex chain of organs, tissues and cells that occur throughout the human body. Parts of the nervous system can be divided to make them easier to understand. One division is the central nervous system (CNS), which includes the brain and spinal cord. The parts of the brain are characterized by their function, which is largely derived from the type, number and connections of neurons (nerve cells).

The cerebrum of the brain is divided into two cerebral hemispheres (right and left) that surround most of the other parts of the brain. The two hemispheres are largely separate, though they are linked by a group of nerves called the corpus callosum. Each hemisphere of the cerebrum is most involved in monitoring and controlling the opposite half of the body. For example, the right hemisphere is largely associated with the left side of body.

The cerebrum has several parts. The cerebral cortex is the wrinkled outer layer of the brain. The wrinkles provide a structural advantage by allowing more cells to fit into the available space. The cerebral cortex is largely made of gray matter, which consists of nerve cell bodies and extensions called dendrites. Much of the brain’s cognitive ability occurs in the cerebral cortex. Several sections called lobes make up the cortex and each exists on both the right and left hemispheres of the cerebrum. The lobes are the frontal (involved in planning future action and controlling movement), the parietal (involved with touch, sensing pain and spatial relationships), the occipital (involved with vision) and the temporal (involved with hearing, learning, memory and emotions).

Two clusters called basal ganglia are near the center of the cerebrum. They are among the many parts of the nervous system involved in regulating muscle movement. The basal ganglia also have a role in memory.

Two other portions of the cerebrum are in the temporal lobe. The hippocampus is deep inside the temporal lobe and is involved in memory, learning and regulating emotions. The amygdala is also a portion of the temporal lobe and is involved in regulating emotions and social behavior. It helps coordinate the hormonal response of other tissues.

The hemispheres of the cerebrum surround the diencephalon. It has two parts, the thalamus and the hypothalamus. The thalamus is a gateway that blocks or enhances information before it reaches the cerebral cortex. The hypothalamus is part of the autonomic nervous system, which regulates the actions of the internal organs that are not controlled by conscious thought (e.g., heartbeat, digestion).

Below the cerebrum and at the back of the brain is the cerebellum, which is involved in movement and learning motor skills. It also helps control the force and range of muscular contractions.

The brain stem is in the center of the brain, in front of the cerebellum and surrounded by the temporal lobe. The brain stem receives sensory data from the skin and muscles of the head and also controls the signals that trigger movement of the head and neck. The brain stem regulates a person’s level of arousal and awareness, and a severe injury to this part of the brain may lead to a coma. The parts of the brain stem are:

• Midbrain. The top of the brain stem is the midbrain. This area helps controls some sensory and motor function, especially movement of the eyes. It is also involved in coordinating reflex reactions to visual and auditory stimuli.
  
  
• Pons. This tissue, directly below the midbrain, conveys information from the cerebrum to the cerebellum. It is also involved in respiration, taste and sleep.
  
  
• Medulla oblongata. This part of the brain stem joins the pons to the spinal cord. It is involved in monitoring and controlling autonomic functions, such as digestion, breathing and heart rate.

There are other ways that the brain may be divided. Some physicians may refer to the hindbrain, midbrain and forebrain. The hindbrain includes the medulla, pons and cerebellum. The diencephalon and cerebrum make up the forebrain. The midbrain is the section above the pons in the brain stem.

The spinal cord is the other major portion of the central nervous system. It runs vertically along the middle of the back. The spinal cord is surrounded by a chain of dense, protective bones called vertebrae. The nerves of the spinal cord receive and process information from the senses as well as many internal organs. Some nerves also run from the brain through the spinal cord to muscles. These nerves control muscle movement in the limbs and trunk of the body. Clusters of nerves called ganglia occur at points along the spinal cord to allow information to be processed and responded to without traveling the entire distance to the brain.

The meninges are three layers of tissue that surround the brain and spinal cord. They are between the nerves of the CNS and the surrounding bones of the skull or vertebrae. The meninges contain veins that carry waste products away from the nerve cells.

The peripheral nervous system (PNS) includes the nerves of the body outside the central nervous system. These nerves run from the brain and spinal cord to the muscles, glands and vital organs. Some information received by the PNS is processed through ganglia. This allows the body to respond more rapidly by not involving the CNS.