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The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS comprises the brain and spinal cord, whereas the PNS consists of all neural structures outside the CNS. The CNS receives, integrates, and responds to sensory information and generates motor output to coordinate behavior and maintain homeostasis (see Image. Major Divisions of the Nervous System). The brain is an organ of nervous tissue responsible for sensation, movement, emotional responses, communication, cognition, and memory. The skull, meninges, and cerebrospinal fluid (CSF) provide protection. Nervous tissue exhibits extreme fragility, rendering it susceptible to injury from relatively minor forces. Additional protection is conferred by the blood–brain barrier, which restricts the entry of potentially harmful substances circulating in the blood. The spinal cord, a vital component of the CNS, resides within the vertebral column. Primary functions of the spinal cord include transmitting motor commands from the brain to the peripheral body and relaying sensory information from peripheral receptors to the brain. The vertebrae, meninges, and CSF provide protection.