The brain, the seat of higher mental functions such as consciousness, memory, planning, and voluntary actions, also controls lower body functions such as the maintenance of respiration, heart rate, blood pressure, and digestion. The brain and spinal cord together form the central nervous system (CNS), where information is processed and responses originate. The approximately 100 billion neurons of the brain form the main control center of the body. The brain, a soft, wrinkled organ that weighs about 3 pounds, is located inside the cranial cavity, where the bones of the skull surround and protect it. Because neurons are extremely specialized cells that are essential to body function and almost never reproduce, neuroglia are vital to maintaining a functional nervous system.

Each neuron in the body is surrounded by anywhere from 6 to 60 neuroglia that protect, feed, and insulate the neuron. Neuroglia, also known as glial cells, act as the “helper” cells of the nervous system. Interneurons form complex networks within the central nervous system to integrate the information received from afferent neurons and to direct the function of the body through efferent neurons. Also known as motor neurons, efferent neurons transmit signals from the central nervous system to effectors in the body such as muscles and glands. Also known as sensory neurons, afferent neurons transmit sensory signals to the central nervous system from receptors in the body. There are 3 basic classes of neurons: afferent neurons, efferent neurons, and interneurons. Long transmitting processes called axons extend from the cell body to send signals onward to other neurons or effector cells in the body. Small tree-like structures called dendrites extend from the cell body to pick up stimuli from the environment, other neurons, or sensory receptor cells. The cell body is the roughly round part of a neuron that contains the nucleus, mitochondria, and most of the cellular organelles. Neurons look quite different from other cells in the body due to the many long cellular processes that extend from their central cell body. Neurons, also known as nerve cells, communicate within the body by transmitting electrochemical signals.

The majority of the nervous system is tissue made up of two classes of cells: neurons and neuroglia.

Somatic nervous system labeled