The two main divisions of the nervous system are the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord, while the PNS includes all the nerves that branch out from the spinal cord and extend to every other part of the body. Together, these two divisions transmit signals that control your ability to move, breathe, think, digest food, and feel sensations.
The Central Nervous System
The central nervous system is the command center. It has three core jobs: receiving sensory information from the body, processing that information, and sending out instructions in response. Your brain handles the processing side, interpreting everything from the pressure on your fingertips to the sound of a conversation, then deciding what to do about it. It regulates thoughts, emotions, movement, and organ function, telling your lungs to breathe and your stomach to digest without you ever having to think about it.
The spinal cord acts as the communication highway between the brain and the rest of the body. When your brain creates a message, it sends that signal down the spinal cord, which relays it outward through the peripheral nerves to reach your muscles, glands, or organs. Signals travel in both directions constantly. Sensory information flows up to the brain, and motor commands flow back down. In some cases, the spinal cord can handle responses on its own through reflex arcs, like pulling your hand away from a hot surface before your brain even registers the pain.
How the CNS Is Protected
Because the brain and spinal cord are so critical, the body wraps them in multiple layers of protection. Three membranes called the meninges surround the entire CNS. The outermost layer, the dura mater, is a tough sheet of connective tissue that attaches directly to the inside of the skull. Beneath it sits the arachnoid mater, named for its spiderweb-like appearance. The innermost layer, the pia mater, clings tightly to the surface of the brain and spinal cord like shrink wrap.
Between the arachnoid and pia layers is a fluid-filled space containing cerebrospinal fluid, which cushions the brain and spinal cord against physical impact. The meninges also anchor the brain in place so it doesn’t shift around inside the skull, and they provide a support framework for blood vessels that deliver oxygen to nervous tissue.
The Peripheral Nervous System
The peripheral nervous system is everything outside the brain and spinal cord. It includes 12 pairs of cranial nerves (which emerge directly from the brain and handle functions like vision, hearing, and facial movement) and 31 pairs of spinal nerves that branch off from the spinal cord and reach the limbs, torso, and skin. These nerves carry sensory information inward toward the CNS and motor commands outward to muscles and organs.
The PNS itself splits into two functional divisions: the somatic nervous system and the autonomic nervous system.
Somatic Nervous System
The somatic nervous system handles voluntary movement. Every time you decide to walk, type, or pick up a glass of water, somatic nerves carry signals from your brain to the skeletal muscles that perform the action. This system also processes sensory input from the skin and muscles, letting you feel touch, temperature, and pain. Although it primarily deals with voluntary control, it also mediates involuntary reflex arcs, those automatic responses that happen before conscious thought kicks in.
Autonomic Nervous System
The autonomic nervous system controls the processes you don’t consciously manage: heart rate, digestion, breathing rate, salivation, sweating, and pupil dilation. It operates largely below the level of awareness, though some functions like breathing can be deliberately overridden. The autonomic system breaks down further into two branches that work in opposition to each other.
The sympathetic branch triggers your “fight or flight” response. It raises your heart rate, diverts blood to your muscles, and puts your body on high alert when you perceive a threat. The parasympathetic branch does the opposite, sometimes called “rest and digest.” It lowers your heart rate and pumping force, increases your rate of digestion, constricts your pupils, and reduces the workload on your lungs during periods of calm. These two branches constantly balance each other, ramping systems up or dialing them back depending on what the situation demands.
There is also a network of nerves embedded in the walls of the digestive tract, sometimes called the enteric nervous system. It contains both sensory and motor nerves that help regulate gut function. While it can operate somewhat independently, it is generally classified as part of the peripheral nervous system rather than a separate third division.
Different Support Cells in Each Division
Both divisions rely on support cells (called glial cells) that maintain the environment around neurons, but each division has its own specialized types. In the CNS, oligodendrocytes insulate nerve fibers to speed up signal transmission, astrocytes maintain the chemical environment neurons need to function, and microglia act as the brain’s immune cells, protecting against injury and disease. Ependymal cells line the fluid-filled cavities of the brain and spinal cord.
In the PNS, Schwann cells perform the same insulating job that oligodendrocytes handle in the CNS. Satellite cells surround nerve cell bodies in clusters throughout the peripheral system and help regulate their chemical surroundings. A third type, enteric glial cells, supports the nerves in the digestive tract specifically.
What Happens When Each Division Is Damaged
Problems in the CNS and PNS tend to look quite different, which reflects their distinct roles. CNS disorders include stroke, epilepsy, multiple sclerosis, Parkinson’s disease, and Alzheimer’s disease. Symptoms often involve changes in cognition, coordination, speech, or vision: persistent headaches, memory loss, tremors, seizures, muscle rigidity, or slurred speech.
PNS problems, by contrast, tend to show up as tingling, numbness, weakness, or pain in specific areas of the body. Carpal tunnel syndrome is a common example, where a compressed nerve in the wrist causes tingling and weakness in the hand. Peripheral neuropathy, which can result from diabetes or autoimmune conditions, causes numbness or burning sensations that often start in the feet and hands. Guillain-Barré syndrome is a more serious PNS condition where the immune system attacks peripheral nerves, leading to progressive muscle weakness.
Some conditions affect both divisions simultaneously. ALS, for instance, degrades motor neurons in the brain and spinal cord as well as the peripheral nerves that connect to muscles, causing progressive loss of voluntary movement throughout the body.