Nervous System

The nervous system includes the brain, the spinal cord (Read about "The Brain" "The Spine") and the nerves that reach out to the rest of the body, acting as a two way street carrying information back and forth. The brain and spinal cord make up the central nervous system. The rest of the nervous system is called the peripheral nervous system.

The brain and the rest of the nervous system are composed of different types of cells, but the main one is a cell called the neuron. All sensations, movements, thoughts, memories and feelings are the result of signals that pass through neurons. The National Institute of Neurological Diseases and Stroke (NINDS) says neurons consist of three parts:

• Cell body - The cell body contains the nucleus, where most of the molecules that the neuron needs to survive and function are manufactured.
• Dendrites - Dendrites extend out from the cell body like the branches of a tree and receive messages from other nerve cells.
• Axons - Signals pass from the dendrites through the cell body and may travel 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 sheath. This sheath can include a fatty molecule called myelin, 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 cerebral cortex to another cell less than a hair's width away. Or axons may be very long, such as those that carry messages from the brain all the way down the spinal cord. The longest axon in the body is the sciatic nerve, which goes from the base of the spinal cord all the way down the leg. The space between an axon and the dendrites of another neuron is called the synapse. Cells communicate with each other by sending chemicals into the synapse that are picked up by the next cell and passed on.

The spinal cord starts at the base of the skull and runs down the middle of the spinal column, inside the vertebrae. At each vertebra, nerves exit the spinal cord and reach out to the rest of the body. If the spinal cord is cut, by an injury or disease, the messages stop at the point of injury and the functions of the nervous system below that point stop.

The brain and spinal cord make up the central nervous system. The peripheral nervous system is the immense network of nerves that send and receive information from the brain and spinal cord to all other parts of the body. The peripheral nervous system is divided into two major parts: the somatic nervous system and the autonomic nervous system. The autonomic nervous system is further divided into three parts: the sympathetic nervous system, the parasympathetic nervous system and the enteric nervous system.

The central nervous system is protected by the meninges and cerebrospinal fluid, but the peripheral nervous system does not have this kind of cushioning. Peripheral nerves are wrapped in myelin, however. Both parts of the nervous system are vulnerable to damage, and a wide range of diseases and complications can result.

Following the links below will give you more information on some conditions that affect the brain and nervous system, including mental health conditions.