The nervous system senses the world, decides what to do and sends commands to muscles and glands. It is organized by location and by function, which lets you trace where a problem lies.
Central nervous system
The brain and spinal cord form the central nervous system (CNS). Bone, the meninges and cerebrospinal fluid protect them while they integrate information and make decisions.
Peripheral nervous system
The peripheral nervous system (PNS) is everything outside the CNS: 12 pairs of cranial nerves, 31 pairs of spinal nerves and their branches and ganglia. Sensory (afferent) fibers carry signals in; motor (efferent) fibers carry commands out.
Somatic division
The somatic system controls the skeletal muscles we move on purpose. It also carries touch, pain, temperature and position sense from the skin and joints.
Autonomic division
The autonomic system runs the organs without conscious effort. The sympathetic branch prepares the body for stress (faster heart, wider pupils, more glucose); the parasympathetic branch supports rest and digestion (slower heart, saliva, gut movement).
At the bedside
A neurological exam follows this map: mental status and cranial nerves test the brain, then strength, reflexes, sensation and coordination test the spinal cord and peripheral nerves. Matching all the findings to one site is how you localize the lesion.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).