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The nervous system divides into the central nervous system (CNS), which contains the brain and spinal cord, and the peripheral nervous system (PNS), which includes nerves traveling between the CNS and the rest of the body. The CNS processes information and coordinates responses, while the PNS transmits motor commands and sensory information between the CNS and body tissues.

The spinal cord extends down the vertebral canal below the brain and facilitates signaling between the brain and PNS. Beyond relaying information, the spinal cord independently controls basic functions such as reflexes without requiring input from the brain, enabling rapid responses to stimuli.

The nervous system comprises neurons, which generate and transmit neural signals, and glial cells, which support neural functioning. Neurons communicate at junctions called synapses using neurochemicals and ions. Glial cells roughly equal the number of neurons in the brain and include astrocytes, microglia, oligodendrocytes, and ependymal cells in the CNS.

Peripheral nerves transmit motor instructions from the CNS to skeletal muscles, controlling movement, and to internal organs, regulating autonomic functions like digestion. Nerves also carry sensory information from receptor cells in the skin and other tissues back to the CNS for analysis and processing.

The brain subdivides into the cerebrum, cerebellum, and brainstem. The cerebrum, the largest region, processes visual information and coordinates movements through highly interconnected areas. The cerebellum coordinates muscle movement, while the brainstem connects the brain to the spinal cord and controls vital functions such as breathing and swallowing.

The autonomic nervous system, part of both the CNS and PNS, controls smooth muscles, cardiac muscles, and glands governing involuntary actions. It regulates functions including digestion, heart rate, and breathing through the sympathetic nervous system fight flight response and parasympathetic divisions that balance these involuntary processes.

The nervous system detects the outside world through sensory receptor cells in the skin and other tissues that transmit information to the CNS. The CNS processes this sensory input and generates appropriate motor responses, enabling organisms to interact with their environment and escape danger through coordinated movement and autonomic adjustments.