The nervous system has two principal cell types: neurons and glia. Neurons (like wires) conduct electrical signals and are organized into circuits to perform specific functions. They have a unique cellular architecture: small cellular extensions (dendrites) receive chemical and electrical signals from other neurons; a longer extension (the axon, which can be up to a meter in length) sends electrical signals down its length to one or more nerve terminals. Nerve terminals contain neurotransmitters, molecules released by arriving electrical signals that modify the electrical activity of adjacent neurons. Neurons have considerable energy needs; indeed the brain, which is 2% of body weight, consumes 15—20% of the body's daily energy intake. Glial cells, which constitute about 60% of the brain's cell mass, provide physical and metabolic support for neurons, and insulate axons and nerve terminals, to ensure privacy in electrical signaling. The glial cells found in peripheral nerves serve the same functions.
The nervous system is broadly divided into two parts: the central and peripheral nervous systems. The central nervous system (CNS) consists of the brain, retina, and spinal cord, and contains complex neuronal circuits that control body functions (e.g., blood pressure, breathing, hunger, movement). The peripheral nervous system consists of groups of neurons that mostly lie outside of the CNS, and either supply sensory information to the CNS, or send CNS commands to effector cells, such as muscle and gland cells.
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