The brain and nervous system receive, process and respond to stimuli from both outside and inside the body. The senses (e.g., touch, taste, sight, hearing and smell) provide information about the person’s environment. After the information is processed, the nervous system initiates a response that might include moving a limb, increasing hormone levels or increasing the person’s heart rate.

The nervous system is spread throughout the body, but it can be divided into two categories. The brain and spinal cord make up the central nervous system (CNS). The remainder, which include the nerves running to internal organs and extremities, is called the peripheral nervous system (PNS).

The nervous system functions in ways in which a person is both aware and unaware. Conscious thought and decisions to move are examples of nervous system processes of which a person is aware that occur in the CNS. Processes that involve no awareness include breathing, heart beating and digestion. Most of these are regulated through the PNS, though some also receive commands from the CNS.

Neurons are the basic cell of the nervous system. They receive and transmit information throughout the body. Each neuron consists of three main parts: the cell body, the dendrite and the axon. Nerve signals are received at the dendrites and travel through the axon to the next neuron or an effector, such as a muscle or gland. Before the signal can reach the effector it must cross a space called the synapse. This may be accomplished with electrical signals or through neurotransmitters, a type of chemical messenger.

The nervous system is complex and is susceptible to a number of different conditions and diseases. Some are caused by bacterial or viral infections, such as encephalitis, while others are caused by disruptions in chemical or electrical processes. Physical changes in the brain or neurons can also lead to some conditions (e.g., stroke, multiple sclerosis). Because of the importance of the brain and nervous system, any condition affecting these areas may produce extreme symptoms such as paralysis or loss of speech and life-threatening conditions that may affect breathing, cardiac function or digestion.