The brain and nervous system direct and initiate the actions and vital functions of the human body. The nervous system is a complex network of cells that receives, processes and responds to stimuli from outside and inside the body. Information about external conditions is provided by the sensory organs (e.g., eyes, ears, skin, nose, tongue). Though people are often unaware of it, the nervous system also gets information about what is happening inside the body. This includes monitoring a person’s blood chemistry and adjusting it through the heart rate, breathing and digestion.

Much information that travels through the nervous system is processed in the brain. Some signals are processed in other areas, such as the spinal cord or areas called ganglia to provide a quicker response. Many responses are so rapid that a person is barely aware of them, such as blinking an eye to repel dust. Other responses occur in reaction to body functions, such as responses of muscles and gastric enzymes in the digestive system after a meal.

The nervous system responds to the information it receives and processes through effectors, such as muscles and glands. Muscles have many functions. Some are controlled consciously and move the limbs and trunk and others work automatically to help control the lungs and pump blood. Glands, another type of effector, are located throughout the body. They release hormones and other chemicals to initiate or stop a response by other tissues.

The nervous system can be classified in multiple ways. Anatomy is one of the most basic methods. The central nervous system includes the brain and spinal cord. The peripheral nervous system includes all nerves that are not in the brain and spinal cord.

The nervous system can also be categorized by whether the functions are consciously controlled. The somatic nervous system includes those parts of the nervous system under conscious control. They include the nerves that initiate movement, such as flexing a joint. The five senses are also part of the somatic system. The autonomic nervous system regulates body functions that are not entirely under conscious control. A person can modify some of the functions, such as breathing. Other processes, such as reflexes, blood pressure and digestion, occur without thought.

The scope of the functions of the brain and nervous system highlights the potential severity of conditions and diseases that affect them. Any malfunction in an area of the brain or nerves may affect other systems or abilities such as movement. Research has helped improve scientific understanding of the brain and nervous system and its diseases, such as Alzheimer’s disease, Parkinson’s disease and muscular dystrophy. While much has been learned about functioning and some of these conditions, research continues to focus on improving treatment.